Nuclear Power Systems for Space Applications

Aristotle University of Thessaloniki

Master Program in Computational Physics

Department of Nuclear and Elementary Particles Physics Laboratory of Atomic and Nuclear Physics

Professor: Christos Eleftheriadis

Dimitrios Chatzipanagiotidis Acknowledgments

The presented work is for my thesis in master degree in Computational Physics at Aristotle University of Thessaloniki. This effort combines my two favorite fields of interest, Nuclear Physics and Space Physics.

Firstly, I want to thanks my supervisor, Professor Christos Eleftheriadis, who help and support me all those years from my bachelor degree until now for my thesis with his unique way of teaching and explain Nuclear Physics.

Also, I want to thanks Professor Pavel Tsvetkov of Texas A&M University Engineering as he offered to help and guide me for my thesis about RTG.

Finally, I would like to thanks my family and my friends for supporting me in my life all these years.

Dimitrios Chatzipanagiotidis, Thessaloniki, October 2019

Abstract

As the solar energy for space applications put some significant limitations about missions beyond Jupiter and planetary surface rovers, the use of nuclear energy is the future of space exploration. Plutonium 238 is the most desirable fuel for Radioisotopes Thermoelectric Generators (RTG) but is limited on the planet and the production is a highly cost procedure. New techniques of Plutonium 238 production are developed in order to enable new space missions. Other radioisotopes can be used instead of Plutonium 238, like Strontium 90, but the shielding considera- tions or the half-life make them undesirable for space applications. Although Plutonium 238 offer great designs of power sources still we have some limitations about the amount of power that can be produces by those devices and the profile of a space mission. The next step is the energy from fission reactions. Small fission reactors have been designed form the early years of space exploration but not used due to the safety restrictions but also because RTG provided enough power for the missions of that era. The last years the demands of further exploration of planets and moons or deep space missions with more precise instrumentation and advanced propulsion systems leading to designs of higher power sources that use fissionable radioisotopes like Ura- nium. The design of space fission reactors requires precise simulation with Monte Carlo techniques to predict the power and the life time of the energy source. One major problem of these designs is the movable parts that control the fission reactions as increase instability that is undesirable in space missions. A solution to this problem is the design of subcritical cores with external neutron source that increase the safety and doesn’t require movable control parts. There are 3 main fuels that considered for the design of a subcritical core and evaluated in this thesis. A suitable neutron source discussed as well. Also, the energy conversion systems are very important and the two concepts are a steady no movable conversion via thermocouples and a movable based on stripling cycle machine.

Index

0 Introduction ...... 1 1 History of RTG ...... 4 1.1 Early Steps ...... 4 1.2 First Flights of RTG ...... 5 1.3 Outer Planet Missions ...... 7 1.4 Future of RTG ...... 10 2 Physics of RTG ...... 13

2.1 Main Idea of RTG ...... 13 2.2 Radioactivity ...... 14 2.3 Thermal power production from radioisotopes ...... 17 2.4 Heat Conversion Systems ...... 20 3 Plutonium 238 ...... 24

3.1 Discovery of Plutonium ...... 24 3.2 RTG with Plutonium 238 ...... 26 3.2 Artificial Plutonium ...... 32 4 Proper Radioisotopes for RTG ...... 37

4.1 Basic Characteristics of Radioisotopes ...... 37 4.2 Limits of Radioisotopes ...... 38 4.3 Fission Radioisotopes and Space Fission Reactors ...... 40 5 Power requirements for space applications ...... 47 5.1 Space applications with Nuclear power systems ...... 47 5.2 Power Requirements ...... 49 5.3 Specific Missions Overview ...... 51 5.3.1 Opportunity rover ...... 51 5.3.2 Deep Space 1 ...... 52 5.3.3 InSight Mission ...... 53 5.3.4 Juno ...... 54 5.3.5 Psyche ...... 55 5.3.6 Europa Clipper ...... 56 6 Subcritical Reactor with External Source ...... 58 6.1 Basic concept of subcritical reactor ...... 58 6.2 External neutron source ...... 58 6.3 Subcritical Module ...... 62

Introduction Space exploration is an exciting journey for humanity. The attempt of this journey provides benefits to society all these years from the first rocket launch, Bumper 2 in 1950 at altitude of 250 miles (higher than the International Space Station’s orbit) to measure the atmosphere’s temperature and cosmic rays. Space exploration has a great contribution on creation of new indus- tries, evolution of technology, research in fundamental physics and peaceful connections with other nations in order to satisfied this global curiosity for the unknown. Space probes are spacecrafts with instruments that designed to take measurements and collect data from Lunar, planets and moons of our solar system and space environment. These probes need energy and commercial batteries cannot power them due to limited power that can store or the big weight that have. The probes near the Sun can collect energy with solar panels all the time and be functional like satellites on Earth, Moon, Venus, Mars etc. Another power source that space probes use is Radioisotope Thermoelectric Generators (RTG) that convert thermal energy, which is produced from the radioactive decay of radioisotope, to electricity. Some probes must operate in the dark on the night side of the planets or distant reaches of the Solar system where solar cells cannot power the systems because solar flux drops as inverse square of distance from Sun. Solar cells can be destroyed when pass through radiation belts around the Earth, like Van-Allen belts. Space missions to planets with high density atmos- pheres, like Venus, cannot use solar panels because light cannot penetrate the atmosphere. At space, where temperature is minus 240 Co, electronics and storage batteries must be heated and the high thermal energy that isotopes produce make them suitable for that purpose.

The technology of RTG have been successfully employed to power lighthouses in distant areas and remote weather and environment monitoring stations. Weather stations in Antarctica was powered by RTG, but the safety concerns led to the removal of these units by the early of 1990s.

Solar flux reduction with the distance

The most proper radioisotope as fuel for an RTG so far is Plutonium 238 but the amounts of it on Earth are extremely small for this purpose. Until now, RTG use artificial Plutonium 238 that is expensive and difficult to produce. The current Plutonium 238 inventory is not enough to power scheduled NASA missions after 2018. With the end of Cold War, the processes of production resources for Nuclear Weapons, that was connected with Plutonium production, reduced significantly. There are attempts for design future programs for Plutonium 238 production from De- partment of Energy. Because of limited Plutonium 238, research has been carried out to improve the conversion of thermal energy, that radioisotope produce, to electrical. This will lead to the reduction of Plutonium 238 required as the power per mass unit will be increased. One way is to use more efficient thermoelectric materials and the other is to design better dynamic system that convert the thermal energy to mechanical work to drive an alternator to produce electricity.

Plutonium 238 pellet in high temperature

Other radioactive isotopes have evaluated as a fuel for RTG like Strontium 90 that used mainly by the Soviet Union but Plutonium 238 is by far the best choice. Also, fission radioisotopes described as the best candidates for higher power production in space by controlled fission reactors.

General Power Production

In this project will be a general overview of RTG physics and technology, the suitability of using Plutonium 238 as a fuel, missions to space and other different types that had designed until now. Also, there will be an overview about fission reactors for extensive space applications that needs more power than RTGs can provide. Different profiles of space missions and their energy requirements will discuss in order to obtain and design of a specific nuclear power system.

1 History of RTG 1.1 First steps

Nuclear physics and particularly radioactivity of actinides became a field with intense research during World War II with Manhattan project and the years after during Cold War for the design of nuclear weapons and other military purposes like power surveillance satellites. The energy decay and the type of particles become well understood and was recognized that radioscopes can produce power. North American Aviation Corporation proposed radioisotopes for power production in space with radioactive cell-mercury vapor system. In 1949, Research and Development Corporation (RAND) published the first study for radioisotopes power systems and 1952 reported an extensive discussion about their use in space applications In 1955, the Department of Defense (DOD) having understood the importance of nuclear power in military satellites requested the Atomic Energy Commission (AEC) to perform research for a nuclear power unit for an Air Force satellite system concept and developed the first System for Nuclear Auxiliary Power (SNAP) that used Cerium 144 to boil Mercury and drive a small turbine, but was never used in space. High radiation levels of beta and gamma rays produces several problems as the high penetration ability make the shielding of electronics difficult. At the same period researchers Kenneth Jordan and John Birden, finding difficult to generate electricity via steam turbine, invented at AEC’s Mound Laboratory in Miamisburg in Ohio an alternative method for conversion of heat that radioisotopes produce, to electrical energy. Two metals with high different electrical conductivities could generate electricity directly from a heat source. This thermoelectric method remains the conversion way in radioisotopes thermal generators until now. The first RTG with Jordan and Birden’s conversion method was SNAP 3 in 1958 with 2.5 watts electric for 280 days with Po 210 (alpha emitter) as a fuel and was constructed for

1 demonstrations. The president of U.S., Eisenhower was interesting in use nuclear power for military satellites and ask from NASA to design missions that could use RTG after a demonstration on his desk in the Oval office in January 1959.

1.2 First flights of RTG

The U.S. Navy’s Transit satellite program was the first RTG flight in space. This satellite was able to provide accurate location information for ships and developed from John Hopkins Applied Physics Laboratory (APL). Under Transit program developed many technologies that GPS system use today. APL wanted to use Plutonium 238, instead of Strontium 90 (beta decay) that need heavy shielding, for fuel but was unavailable due to AEC restrictions. Eventually AEC provide Plu- tonium 238 for the construction of SNAP-3B (pic 1). The Plutonium isotopes composition of heat source was 80% Pu 238, 16% Pu 239, 3% Pu 240, and 1% Pu 241. The thermal power production was 52.5 watts from 92.7 grams of Plutonium and designed to last five years. The power conversion system used 27 connected pairs of Lead and Telluride in temperature of 783 kelvins (hot junction) and 366 Kelvins (cold junction) respectively and had efficiency of 5 to 6 % with 1.3 electric watts per kg.

Picture 1.2.1: SNAP-3b

Transit-4A that powered from SNAB-3B, launched on June 1961 and operated for 15 years. For the next series of Transit program SNAP-9A developed from AEC and others which considered as the primary spacecraft power source with 26.8 electric watts, an order of magnitude greater than the SNAP-3B. The unit was a cylinder 26.7 cm tall with 25.4 cm radius sealed with Magnesium and Thorium and weight 12.3 kg. As a fuel used Plutonium 238 with 525 thermal watts and could provide power for 5 years. A fraction of heat that produced was used for keeping electronic instruments at temperature near 293 Kelvin. The lunch of SNAP-9A in 1963, was the first flight of RTG with specific and established security protocols for radiation safety in case of failure into Earth’s atmosphere. Although the Transit program was for Department of Defense, NASA was

1 invited to participate in safety studies. On July 1964, Transit 5B-N mission was aborted at altitude of 1000 miles over the South Pole. The Plutonium 238 fuel burned up into particles with diameter of about 25 nm and dispersed widely something that confirmed with measurements from bal- loons.

After the success of first SNAP program NASA requested the AEC for an RTG with 50 watt electric power for the first weather satellite system capable to make night and day measurements at different levels of atmosphere. Until then, all weather satellites powered by solar panels. The studies for this project create the opportunity to test an RTG on a NASA spacecraft for the first time. SNAP-19B used Plutonium in a ceramic form, PuO2, in capsules in contrast with the Plutonium metal form of SNAP-9A. With this modification, in case of accident and dispersion of the fuel, the particles will be too large to inhaled from human. On May 1968, Nimbus-B1 launched but an error caused it to veer of course. A destruction signal sent at altitude of 100000 feet and in September the unit was found and sent to Mount Laboratory for reuse. In 1969, Nimbus III launched and placed into orbit successfully and operated for 2.5 years. This was the last time that NASA used RTG to power satellite that orbits the Earth because with new launch safety process for RTG, solar panels was less expensive.

Picture 1.2.2: NIMBUS III first RTG flight for NASA

1.3 Outer planet missions

After the succession of NIMBUS III, NASA though of use SNAP-19 for two important mission of the 70s, Mars and Jupiter exploration. Pioneer 10 and 11 missions that launched on 2 March 1972 and 6 April 1973, was the first missions that pass the asteroid belt and make direct observation of Jupiter. The modification on SNAP-19 with Sn-Te thermoelectric elements increase efficiency with power output 40.3 watts electric, 50% more than NIMBUS III. Pioneer 11 used Jupi- ter’s gravity assist for a new trajectory towards Saturn to provide the first information from local measurements on this planet in 1979, before a new trajectory to pass our solar system. Pioneer 10 and 11 missions continued to transmit data far beyond Pluto something that wasn’t in the first schedule of the mission. The modification of SNAP-19 for Viking mission was designed for the high temperature environment near Mars and his surface. The SNAP-19 version for Viking 1 and 2 was slightly larger than this of the Pioneer’s with power output 42.56 watts electric. Viking 1 was launched on 29 August 1975, reached Mars orbit on 19 June 1976 and landed on his surface on 20 July 1976 and Viking 2 was launched on 9 September 1975 and landed on his surface on 3 September 1976.

. Picture 1.3.1: Viking 1

One of the goals of Apollo program was to place scientific stations on the Moon’s surface that could transmit data for a long period for measurements of the fluctuation of solar magnetic field, moon’s atmosphere and internal composition. Bendix Aerospace Systems Division developed the ASLEP (Apollo Lunar Surface Experiment Package) for these measurements and AEC requested to develop an RTG to power the systems to be functional on 14 day and night of the Moon. Ad- ditional safety features added to SNAP-27 for the trip to the Moon and for the deployment on the Moon’s surface by astronauts.

Picture 1.3.2: Heat source deployment of SNAP-27

The power output pf SNAP-27 was 63.5 watts electric with 3.2 watts electric per kg, 10% higher than SANP-19, and used Pb-Te thermocouples for energy conversion system. From 1969 up to 1972 five units placed on Moon’s surface and operated without problems until 1977 when were intentionally shutdown.

Picture 1.3.3: Deployed SNAP-27 on the lunar in Apollo 16 mission

When NASA starts the ambitious program for robotic missions in the future request from AEC a higher power RTG up to several hundred watts electric. This unit became known as MHW-RTG (Multi Hundred Power-RTG). Department of Energy also request an MHW-RTG for two

1 communication satellites that constructed from Lincoln Laboratory at MIT. LES (Lincoln Experi- mental Satellite) 8 and 9 launched together on March 1976 with two MHW-RTG each to power them and were deployed to orbit altitude of 36,000 km. In 2004 LES 8 turned off because of control difficulties and LES 9 is operating for almost 30 years. The development of this unit moti- vates NASA to design a Grand Tour mission of the planets of our Solar System. The Voyager 1 and 2 spacecrafts, launched on 5 September and 20 August 1977, are the greater explorers in the history of space missions. It passed from all the giant planets, their 48 moons, measured magnetic fields and rings. Voyager 2 passed Pioneer 10 and became the most distance human made object in space. Each Voyager spacecraft powered from 3 MHW-RTG.

The development of MHW-RTG was a big advancement in RTG technology. With a cylinder shape with 58.3 cm height contained 24 capsules of PuO2 with 3.7 cm diameter that each produced 100 watts thermal in a metallic shell of Iridium and Graphite for resistance to mechanical impact loads. A part of his succession was the 312 Silicon-Germanium (Si-Ge) thermoelectric couples of the power converter that could work in higher temperatures (up to 1000 OC) than Pb-Te thermocouples. The MHW-RTG units that used on LES (Lincoln Experimental Satellites) 8 and 9 had power 154 watt electric at the start of the mission with 39.7 kg and 3.9 We per kg. LES 8 AND 9 launched on 15 March 1976 Later, for Voyager, were used 3 RTG improved at 4.2 We per kg with mass 37.7 kg and 158 watt electric at the start of the mission. Also, another characteristic that contribute to the succession of this unit was that the higher temperature (about 1300 Kelvin) operation giving higher heat rejection that required smaller radiators hence lower mass.

Picture 1.3.4: PuO2 fuel capsules in MHW-RTG

In 1989-90 the Galileo mission to Jupiter and Ulysses mission to Sun used a developed model named GPHS-RTG (General Purpose Heat Source) that could provide 300 watts electric, the

1 power of two MHW-RTG by using 572 Si-Ge thermoelectric couples. The Cassini spacecraft, launched in 1997 to orbit Saturn, used 3 GPHS that performed so well leading to the extension of the mission.

Picture 1.3.5: General Purpose Heat Source In 2006 launched New Horizon probe with the mission to explore Pluto and his satellite Charon. Due to the limited availability of Plutonium 238, the GPSH that used produced about 245 watts electric at the beginning of the mission as the Plutonium was aged at 21 years. In 2014 continuing his mission made a flyby of Kuiper Belt Object, MU69 (Ultima Thule).

The last RTG that used in space application was on Curiosity rover, launched on November 26, 2011 reaching Mars on August, 2012. The MMRTG (Multi Mission RTG) that used was an effort of NASA and DOE and built by Rocketdyne and Teledyne. The energy conversion system that used was telluride thermoelectric technology, like the one use by SNAP-19, because it was proved that can work both in space and the surface of a planetary.

1.4 Future of RTG

Until now the U.S.A has flown 41 RTG on 25 missions for scientific stations on the Moon, robotic explorer on Mars, Earth orbital satellites, interplanetary missions to Jupiter and Saturn and deep space exploration at the edge of our Solar System. The last mission that use RTG is Curiosity rover on Mars which was launched on November 2011. The former U.S.S.R. also used RTG to power terrestrial and space missions. The use of RTG is tightly connected with the inventories and production of Plutonium 238 which is by far the best radioisotope with the most desirable characteristics as a heat source. In 2013 DOE (Department of Energy) restarted the production of 238Pu assuming that will be able to produce 1.5 kg per year by 2025. If the number will be correct

1 new generation of outer solar system missions can be considered and other missions like sample of Mars return to Earth, orbiters to Uranus and Neptune etc. A new technique that developed at Oak Ridge National Laboratory (ORNL) increased the production of Plutonium. This automation technique allows the human resources of the laboratory to work in other tasks and with more safety as the previous procedure was time limited for humans due to radiation exposure.

Picture: 1.4.1 Plutonium pellets production at Oak Ridge

Also, NASA after the cancelation of ASRG (Advanced Stirling Radioisotope Generators) program in 2013, which was a movable Stirling machine conversion system four times more efficient than thermocouples, continued funding movable conversion systems through other private com- panies.

The evaluation of other radioisotopes is important but their limitations about the power density that could provide or their life time make them less desirable. One of the best options except 238U, is 90Sr, a beta decay radioisotope that used by widely by U.S.S.R. with half- life of 28.8 years.

As the needs for power increased the last years for space applications and technology the attention has moved for the design of nuclear power devices that is based on fission reactions providing power at 10s-100s kwatts. Although, this technology is more complicated than RTG and more research is needed by complex Monte Carlo simulations.

References

Nirmal Singh “Radioisotopes Applications in Physical Science” Published by InTech (2011)

Robert L. Cataldo, Gary L. Bennett “U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future” NASA (2011)

Richard R.Furlong, Earl J. Wahlquist “U.S. space missions using radioisotope power systems” Nuclear News (1999)

David Dickinson “Speeding Up Plutonium Production for Space Exploration” Sky and Telescope (2019)

2 Physics of RTG 2.1 Main idea of RTG Radioisotope thermal generator is a device that generate electricity using the heat that produced from a specific quantity of a radioisotope with a conversion system. The desirable characteristics of a heat source for that concept Is small size of the device and big power output. These two characteristics compound the electrical power density (electric watts/mass). This quantity is governed by the physical properties of the radioisotope, that is responsible for the thermal power density (thermal watts/mass), and the efficiency of the conversion system that produce a percentage of thermal power to electrical. Energy density (Joule/gram) of the nuclear energy sources is higher than this of the chemical energy sources by many orders of magnitude as we can see at the next table:

Type of source Energy per gram [Joule/g] Alkali metal 422.6 Carbon-Zinc 130.7 Ni-Cad 117.8 NiMH 288 Li Ion 460 Pu-238 2.19 109

Thermal power density is determined from two basics characteristics of the radioisotope, the type of the decay (energy and type of the particle) and the activity which is the number of decayed particles per time and varies inversely with the life time of the radioisotope.

2.2 Radioactivity In nature, radioactive elements emit subatomic particles in order to create more stable forms 4 (lower mass). The types of decay are alpha (alpha particle 2 He ), beta (electron or positron and neutrinos), gamma (high energy photon). Also, there is spontaneous fission where a heavy nucleus separates to two lighter nuclei. These elements have several isotopes (different neutron number) with different decay rates. The unit to measure these rates is Becquerel (Βq) which is a decay per second and Curie (Ci = 3.7 1010 Bq ).

A number of radioactive nucleus that have just produced (t=0) has activity given by the equation:

A0 =  0 [Bq] (2.2.1)

, where 0 is the number of radioactive nucleus the time that produced and  is the decay constant which is characteristic of the radioisotope and is connected with half-life time with the equation:

ln2  = [sec−1] (2.2.2) t1/ 2

, half time is the time that radioactive nucleus will be the half of the initial number .

The reduction of the decay rate is exponential with time and governed from the decay constant λ:

−t A =  0 e [Bq] (2.2.3)

Picture 2.2.1: Exponential reduction of radioactivity

A specific mass m of an element has a number of nucleus given by the equation:

m [gr] N N = A [nucleus] (2.2.4) 0 AM [gr]

23 , where  A = 6.022 10 is the Avogadro number and AM the atomic mass of the element. The radioactivity of specific mass m of a radioisotope is:

ln 2 m [gr] N A A0 = [Bq] t1/ 2[sec] AM [gr]

23 m [gr] A0 = 4.17 10 [Bq] (2.2.5) t1/ 2[sec] AM [gr]

These decays are spontaneous and due to the law of nature that create more stables forms of nucleus, in order to have less mass-energy, there is a release of energy given by Einstein’s equation:

2 Q = (mi − m f )c

2 , where mi ,m f the mass of the nucleus in Mev/c before and after decay respectively and c is the speed of light. The equations of the three types of decay are:

A A−4 Alpha decay Z X →Z −2Y + a

A A − A A + Beta decay Z X →Z +1Y + e + ve or Z X →Z −1Y + e + ve

A * Gamma Decay Z X → X + 

Spontaneous fission A X →A1 Y + A2 Y Z Z1 1 Z 2 2

The value of Q expressed as the kinetic energy of Y nucleus and the decay particle. The mass of Y nucleus is way bigger than this of the decay particle and due to momentum conservation, the decay particle takes almost the whole energy that released from the decay. In alpha decay the energy that release is expressed as the kinetic energy of alpha particle that emitted and in gamma decay as the energy of gamma photon that emitted. In beta decay, there are two particles that take and share the kinetic energy that occurs from Q value of the reaction and the energy spectrum has the following continues form:

Picture 2.2.2: Beta decay spectrum

In spontaneous fission two new lighter nuclei formed from an initial unstable heavy nucleus. The lighter nuclei called fragments of the fission and caries about the 80% of the Q value of the decay and share it through momentum conservation.

2.3 Thermal power production from radioisotopes

As we see above a radioisotope emits particles. The moving particle when pass through matter deposit his kinetic energy with different ways that depend on the energy and type of particle (charge, mass), the density and element of matter. The deposition of energy increases the temperature of the medium which is relevant with the amount of energy that deposited. Alpha particles are heavy ions and the interaction with matter is strongly described by the Bethe-Bloch stopping power equation. An alpha particle with kinetic energy about 5 MeV loses his energy in about 9.32 μm in a medium with atomic number Z=90. The range of a charged particle is a function of the electron density of the medium that travels. So less dense materials have lower stopping power. For example, an alpha particle that we see above travels for 40.6 mm in air. Alpha particles due to the large mass and their positive charge (Z=2) deposits his energy in a small distance, so the energy Q from decay can’t escape from a relatively small amount of a radioisotope and the whole value of Q convert into thermal energy.

In beta decay, we have two moving particles, electron/positron and neutrino. Neutrinos are extremely weakly interactive with matter and escape from the radioisotope decreasing the amount of energy that deposited into the medium. Thus, the percentage of Q value of beta decay that converted into thermal energy for the increase of temperature is lower than this of the alpha particle. From the spectrum of beta decay (pic 2.2.2), we can see that large number of electrons/positrons caries a small percentage of Q value so the rest escapes from the medium with

1 neutrinos. There is simple rule that describe the percentage of Q value that caries the electron/positron and simplifies the calculations for the energy deposition. This rule use as average electron/positron energy the 1/3 of the Q value. But the ionization profile produced by the average spectrum of beta particles is different which leads to errors that propagates in the rest calculations for the design of a nuclear heat source. At the next table described the difference between the average beta particles energy spectrum and the simplification of 1/3 of Q rule for three beta radioisotopes:

Energy Q (keV) Max Energy Difference Isotopes Half life 1/3 Average (keV) (%) Rule Spectrum S 35 87.51 days 167.47 55.8 53.1 +5 Sr 90 28.8 years 546 182 167 +9 Y 90 2.67 days 2280 760 945 -20

The full beta spectrum is more accurate for the calculation of the distance that electrons deposits their energy. In the next table are shown the ranges of beta particles into SiC from the three ways that the electrons energy can be described:

1/3 Rule Average spectrum Full beta spectrum Isotopes (mm) (mm) (mm)

S 35 10.6 0.02 0.08 Sr 90 55.1 0.12 0.40 Y 90 344 1.6 3

The range of the beta particles is important for the region that the conversion system will be placed in order not to have damage from the energy deposition.

Positron from beta + decay is anti-particles of electron. A positron after the deposition of his energy can encounter an electron and the two particles annihilate to produce two gamma rays with energy at least the rest of mass energy of the particles. Gamma rays have great penetration ability and can easily escape from the medium and interact out of it. So, beta + radioisotopes need excessive shielding something that make them unsuitable as a fuel for RTG.

In gamma decay, photons have high penetration ability and at relatively small amount of mass we have energy loses and shielding problems as we describe above. So, gamma emitters are unsuitable as a fuel for RTG as the thermal energy that produce is very low and the shielding requirements are high. In spontaneous fission, the fragments due to the large mass and high atomic number deposit their kinetic energy at extremely small distance (smaller than alpha particles). After the fission process the fragments have excess numbers of neutrons and in order to become more stable decay them. Neutrons due to their zero charge interact with matter at long range so additional shielding required.

The thermal energy that produced from the deposited energy of the moving particle in a radioisotope with specific mass per second (thermal power) using equation 2.2.5 can be described by the following equation:

m [gr] P = 4.17 1023 Q [eV ] [eV / sec] Thermal Depositied t1/ 2[sec] AM [gr]

, where QDepositied is the value of energy Q released from the decay, multiplied by a factor k that describe the percentage of Q that don’t escape from the radioisotope’s mass. The value of k factor for alpha decay and spontaneous fission is almost 1, for beta particles is about 0.33 (rule of 1/3). We can write the same equation in order to calculate the thermal power of a specific mass in watts, as unit of half-time in years, as this order of time scale is important for an RTG design, and QDeposited in MeV:

m [gr] P = 2115 Q [MeV ] [watts] Thermal Depositied (2.2.6) t1/ 2[years] AM [gr]

By using the same equation for mass of 1 gr we can have the equation of the thermal power density: 2115 P = Q [MeV ] [watts / gr] Thermal density Depositied (2.2.7) t1/ 2[years] AM [gr]

We can calculate the electric power density by multiplying the thermal power density with the efficiency factor η of the conversion system which describe the percentage of the thermal power that converts into electrical power:

P =  P Electrical density Thermal density (2.2.8)

Equations 2.2.7 and 2.2.8 describe the most important characteristics for the design of an RTG.

2.4 Heat conversion systems

The electrical power that can be produced by radioactive decay of an RTG is governed by the efficiency of the conversion system that transform thermal energy to electrical. There are two conversion methods that RTG use to convert thermal energy to electricity, static conversion and dynamic conversion.

The static conversion systems don’t need mechanical parts in contrast with the dynamic conversion systems. The common method that RTG use until now is static conversion despite the fact that dynamic conversion can be more efficient but is not the first choice due to low reliability of the construction. Thermoelectric converters can operate for several decades due to the compact structure with no vibrations from moving parts and have great radiation resistance. They don’t require start up devices and can produce electricity directly when connected to the heat source. The low conversion efficiency (lower than 10%) is the only disadvantage of thermoelectric converters that requires more fuel for a specific power output in comparison with a dynamic system.

Static systems include thermoelectric, thermophotovoltaic and thermionic conversion devices. Thermoelectricity was discovered in 1825 by German scientist, Thomas Johann Seebeck. When two different metals (conductors or semiconductors) are connected to a closed circuit and their junctions are in different temperature an electrical voltage is produced. The electrons (va- lence electrons) in the hot metal are more energetic than those in the cold, so there is a movement of them to the cold metal until an electric field appears from the positive ions that formed in the hot metal due to the missing electrons (pic 2.3.1). This phenomenon is called Seebeck effect.

Picture 2.3.1: Seebeck effect

The average energy of the electron in a metal is given by Fermi-Dirac Distribution:

2 3  5 2     E = E 1+    (2.5.1) average 5 F 12  E    F  

, where EF is the Fermi energy at 0 Kelvin and T the temperature of the metal. As we can see the average energy of the electrons increase with the temperature. Fermi energy is characteristic of the metal. The voltage that generated per unit of temperature is called Seebeck coefficient:

V S = [Volts / Kelvin] (2.5.2) T

, where ΔV is the potential difference across a metal due to a temperature difference ΔT. In this system can occur energy losses due to thermal conductivity k of the thermoelectric material and from Joule heating effect which is determined from the electrical resistivity of the thermoelectric material. These two effects with Seebeck coefficient can be described in Z factor with the equation:

S 2 Z = kR

The higher is Z factor of a radioactive material the more proper is for the design of a conversion system with Seebeck effect.

Thermoelectric couples produce low voltage (μVolts per kelvin temperature difference) so in order to have the amount of voltage we need several thermocouples must be connected in parallel arrangement and if we want to increase current in series arrangement. The parallel-series connection ensures the function of thermocouples in case of circuit failure in a single thermo- couple. The best option of thermocouples is Bismuth Telluride (Bi-Te), Lead Telluride (Pb-Te), Tellurides of Antimony, Germanium and Silver (TAGS), Lead Tin Telluride (PbSn-Te), and Silicon Germanium (Si-Ge). All thermocouples except Bi-Te have been used in space flights. More materials are still being investigated for the design of a thermoelectric material that has higher efficiency, less weight and more stable performance for a longer operating lifetime.

In dynamic conversion system, the thermal energy first converts into mechanical energy that moves an alternator to produce electrical energy. The heat source evaporates a fluid that drive a turbine to produce electricity via Rankine, Brayton and Stirling cycles. The research for this conversion system starts in 1950 to power future missions that requires larger power. The dynamic conversion system is designed to produce up to some KWatts of electrical power.

Picture 2.3.3: Dynamic system conversion More intense research for dynamic conversion systems started at decade of 90s. Eventually it became apparent that the Stirling engine is the best option for power production for RTG due to the life time that has which is about two decades.

The waste heat that produced at conversion systems must be rejected to the environment. In space that the environment has very low temperatures, a fraction of the wasted heat used to heat the equipment of the spacecraft.

References

Robert C. O’Brien “Radioisotopes and Nuclear Technologies for Space Exploration” Ph.D. Department of Physics and Astronomy, University of Leicester (2010)

Antonio Sanchez-Torres “Radioisotope Power Systems for Space Applications” Ministry of Science and Innovation of Spain (2009)

M. Ragheb “Radioisotopes Power Production”

Nirmal Singh “Radioisotopes Applications in Physical Science” Published by InTech (2011)

Mark Prelas, Matthew Boraas, Fernando De La Torre Aguilar, John-David Seelig, Modeste Tchakoua Tchouaso, Denis Wisniewski “Nuclear Batteries and Radioisotopes” Springer International Publishing Switzerland (2016)

Arman Molki “Simple Demonstration of the Seebeck Effect” Science Education Review, 9(3), 2010

3 Plutonium 238 3.1 Discovery of Plutonium The element Plutonium was discovered in 1941 by Glenn T. Seaborg (Nobel Prize in Chemistry 1951) at Berkeley, California. Seaborg and his colleagues bombarded Uranium 238 (U) with Deu- terium (D); using a 60-inch cyclotron accelerator, they produced elemental Plutonium 238(Pu) with a chain reaction as follows:

2 238 238 1 D+ 92 U→ 93 Np + 2n

beta− 238 238 − 93 Np ⎯ ⎯ ⎯ ⎯ ⎯ → 94 Pu + e + ve t1/ 2 =2.117 days

Plutonium 238 is an unstable radioactive element and decays according to the following sequence,

alpha 238 234 94 Pu ⎯ ⎯ ⎯ ⎯ ⎯ → 92U + a t1/ 2 =87.7 years

1 with energy release of Q = 5.592MeV . Through momentum conservation, the kinetic energy of alpha particle is Ta = 5.485 MeV , and of Uranium is TU = 0.093 MeV .

Plutonium was named after the planetoid Pluto, in similar way as Uranium named after planet Uranus, and Neptunium after planet Neptune. Seaborg published his work at the journal Physical Review in March 1941, but was soon removed as it was recognized as a fissile element, making it suitable for the construction of nuclear weapons. Seaborg moved to University of Chicago where he continued the production of Plutonium at Met Lab for the secret project Manhattan, during World War II, for the construction of atomic bomb.

In the same year, Plutonium 239 produced by bombarding U 238 with neutrons. The reactions are:

238 239 n+ 92 U→ 92 U +

beta− 239 239 − 92U ⎯ ⎯ ⎯ ⎯ ⎯⎯ → 93 Np + e + ve t1/ 2 =23.5 min utes

beta− 239 239 − 93 Np ⎯ ⎯ ⎯ ⎯⎯ → 94 Pu + e + ve t1/ 2 =2.35 days

Plutonium 239 is a radioactive element and decay with the following reaction:

alpha 239 235 94 Pu ⎯ ⎯ ⎯ ⎯ ⎯⎯ → 92U + a t1/ 2 =24110 years

Until now 23 radioactive isotopes of Plutonium have been identified with mass number range from 228 to 247. Nine of theses isotopes have very short life; less than 1 sec. The isotopes with the longest life time are Pu 244, 242, and 239. All the others isotopes have half-life times less than 7000 years. The next table shows some important isotopes and their properties.

Properties of Plutonium isotopes

Melting Point Boiling Point Density 641 Co 3232Co 16-20 grams/cm3 Isotopes of Pu 238 239 240 241 242 244 Half-life 87.74 24110 6563 14.4 376,000 80,000,000 (years) Radioactivity 634 109 2.29 109 8.4 109 3.8 1012 0.14 109 0.67 106 (Bq/gram) alpha alpha Decay Mode alpha alpha beta alpha fission fission Decay Energy 5.592 5.246 5.255 0.021 4.983 4.889 (MeV)

3.2 RTG with Plutonium 238 Plutonium 238 produces alpha particles that have low free mean path and deposit their energy into the radioisotope’s mass, so the ionizing radiation effects on spacecraft system is minimal and making it safer to handle and work around. The half time of Plutonium 238 is suitable for typical space missions because of small gradual decline in power output (~50% at 88 years) due to the nature of radioactive decay. The high thermal power density (watts per gram) of Plutonium 238 allow the construction of small and light weight heat resources. The thermal power density of Pu 238 can be calculated by using the equation 2.2.7 with the data from table of Plutonium 238 properties above:

2115 P = Q [MeV ] [watts / gr] Thermal density Depositied t1/ 2[years] AM [gr]

2115 PPu 238 = 5.592[MeV ] [watts / gr] Thermal density 87.74[years] 238.049[gr]

PPu 238 = 0.566 [watts / gr] Thermal density

The most advanced RTG system that use Pu 238 as heat resource is the Multi Mission Radioi- sotope Thermoelectric Generator (MMRTG).

Picture 3.2.1: MMRTG

The main characteristics of MMRTG are shown at the next table:

MMRTG Characteristics Values Mass 45 kg Diameter 64.2 cm Length 66.8 cm GPHS Units 8 Pu 238 mass 3.478 kg Conversion thermocouples 768 PbTe/TAGS couples Hot junction temperature 530 Co Cold junction temperature 200 Co Thermal Power 1.975 kwatt Electrical Power 0.123 kwatt Electrical power density (whole device) 2.8 watt/kg Conversion efficiency 6.25 % Lifetime at least 14 years

This power unit is design to operate in space (vacuum) and within an atmosphere such as on a surface of a planet. The remaining thermal power of 1875 watts is rejected to the environment or space via fins held at temperature of 210 Co. An active heat exchanger is used to heat electronics and mechanical subsystems that require a specific temperature to operate. It consists of 8 General purpose heat source (GPHS) with total thermal power of about 2 kwatts. The conversion method that used at MMRTG is thermoelectricity with 768 PbTe/TAGS thermoelectric mod- ules. The first flight of an MMRTG was 26 November 2011 as a power resource for Curiosity rover that send it to Mars from NASA’s Mars Science Laboratory.

Picture 3.2.2: Curiosity rover on Mars use MMRTG as a power source

Because Plutonium-238 is an extremely limited resource the need for higher-efficiency systems that would reduce the amount of Pu-238 required for a given electric power output is essential. A dynamic conversion system based RTG is currently under development by Department of Energy and NASA. The Advanced Stirling Radioisotope Generator (ASRG) is design to use free piston Stirling converters connected each to a GPHS unit to produce electricity. The efficiency of that converter is greater than this of the thermoelectric method.

Picture 3.2.3: ASRG

The main characteristics of ASRG are shown at the next table:

ASRG Characteristics Values Mass 32 kg Wide 46 by 39 cm Length 76 cm GPHS Units 2 Pu 238 mass 0.984 kg Source operating temperature 640 C0 Heat rejection 60 C0 Thermal Power 0.5 kwatt Electrical Power 0.13 kwatt Electrical power density (whole device) 4 watt/kg Conversion efficiency 26% Lifetime at least 17 years

Each general-purpose heat source (GPHS) is designed to produce 250 watts thermal power with weight of about 1.43 kg and dimensions 9.72 cm × 9.32 cm × 5.31 cm. Before Plutonium 238 placed into capsules (“pellets”) mixed with dioxide to form ceramic plutonium-dioxide (PuO2) with 82.2% Pu 238. This modification offers excellent chemical stability and high melting point at 2450 Co (pure Plutonium at 641 Co). This ceramic form has low dissolution in water. In case of

1 fracture this form tends to break into large macroscopic particles which is more safe than micro- scopic particles. The cylindrical pellets produced with the compression of 151 gr PuO2 at density of 9.89 gr/cm3, diameter and length of 2.75 cm with 62.5 watts thermal power. Each fuel pellet encapsulated in Iridium alloy clad of 0.73 mm thickness. The clad constructed from two pieces, the vented cup and the shielding cap. The vented cap has a hole that is filtered by porous Iridium that allows the Helium produced from radioactivity to escape. The shielding cup is made of 0.127 mm thick Iridium and prevent the alpha particles to escape as a 5.59 MeV alpha particle has range of about 0.825 10-2 mm into Iridium [SRIM 2008 code].

Picture 3.2.2: Iridium fuel clad

Fuel clads inserted into a Graphite impact shell (GIS) in pairs that allows their precise positioning and separated by a porous fine weave pierced fabric (FWPF). This floating membrane allows the Helium produced from the radioactivity to escape. One other purpose of the Graphite impact shell is to provide protection from impacts caused by atmosphere re-entry or accidents cases. Both ends of the Graphite impact shell closed by a cap vented with holes. Each Graphite impact shell surrounding by a Carbon Bonded Carbon Fibre sleeve and two disks of 1 mm thickness and thermal conductivity of 0.059 W/m K providing insulation of the fuel from the worst-case re-entry conditions. This sleeve also allows the Helium from radioactivity to escape. The two Graphite impact shells inserted into a single square block aeroshell made from fine weave pierced fabric that provides protection from high temperatures during atmosphere re- entry events and fire accidents while providing very good thermal conductivity between the conversion system and the heat source. The two Graphite impact shells locked into the aeroshell

1 with caps. The General-purpose heat source module is design to withstand impacts at velocity of 55 m/s and temperature of 970 Co.

Picture 3.2.3: General purpose heat source

Plutonium 238 has 87.74 years half-life so according to that we can calculate the thermal power density reduction per year by starting with the equation 2.2.5 which is the radioactivity of a specific mass at the time of the radioisotope production (t=0):

23 m [gr] A0 = 4.17 10 [Bq] t1/ 2[sec] AM [gr]

The radioactivity at a specific time t, A(t), is given from the above equation multiplying by e−t :

m [gr] A(t) = 4.17 1023 e−t [Bq] t1/ 2[sec]  AM [gr]

So, the thermal power of a specific mass in watts, as unit of half-time in years and QDeposited in MeV after time t is given buy equation:

m [gr] P = 2115 Q [MeV ] e−t [watts] Thermal Depositied (3.2.1) t1/ 2[years] AM [gr]

, with  = ln2/t1/ 2

Now we can calculate the thermal power density reduction for 1 kg of Plutonium 238 per year:

P = 566.25 e−0.007898t [watts / kg] [1/ year] Thermal density per year

The reduction of power output is not governed from the fuel decay only but also from the conversion system that loose efficiency due to the time of operation. Models of power degradation for MMRTG have developed based on data from existing RTG as showed in the next picture:

Picture 3.2.4 : MMRTG power degradation with time

3.3 Artificial Plutonium

Plutonium does occur naturally at very low concentrations and can be observed with sensitive modern techniques. Plutonium and other heavy elements are formed in Universe due to high energy death of massive stars, Supernovae explosion, where rapid neutron capture process take place. These elements that formed with this process and have lifetime smaller than Earth’s age (4.5 billion years) has decayed in lighter more stable elements closest to the Iron-56 peak (pic 1) with a-decay, beta decay or fission so we can’t find them on Earth.

Picture 3.3.1: Biding Energy per nucleon The small quantities that we trace can be occur by natural nuclear reactions here on Earth. Uranium 238 is responsible for Plutonium production in nature. Uranium primary decay alpha particles, but there is another reaction with low probability, spontaneous fission. At spontaneous fission 2 or 3 neutrons released and react with other Uranium 238 nucleus and produce Pluto- nium 239 with the reactions that we describe above. Today, the quantities of Plutonium that we found on Earth results mainly from human activi- ties. After World War II the tests of nuclear weapons release tons of Plutonium mainly at the north hemisphere in the atmosphere that is still there. Nuclear industry has produced about 1300 tons of Plutonium (mainly Pu 239) so far with transmutation of Uranium 238 when capture one neutron, in nuclear reactors (pic 2).

Picture 3.3.2: Plutonium production in reactor The isotopes of Plutonium that we use are 239 and 238. Plutonium 239 can be used in nuclear weapons and as a fuel, for the production of electric power, in fast nuclear reactors because

1 undergoes a fission chain reaction when concentrated enough. Plutonium 238 was selected by Department of Energy (DOE) to power space probes when the light of the Sun isn’t sufficient and has been used for more than 50 years. Radioactive thermal generators (nuclear batteries), which are using Plutonium 238 as a fuel, has been used to create electricity for several hardware on science experiments on the Moon, on robotic exploration like Curiosity on Mars and Voyager 1 and 2 which traveling at the edge of our solar system.

As we saw above, in nature, the quantity of Plutonium is very small, so space probes using artificial Plutonium 238. The method of production is complicated and expensive. In nuclear thermal power reactors that use enriched Uranium 238 (3-5% U 235) produced Neptunium 237 through these reactions. The first two is for Uranium 237 formation and the last one is for the Neptunium 237

238 237 n+ 92 U→ 92 U + 2n

235 236 236 237 n+ 92 U→ 92 U +  ⎯⎯ → n+ 92 U→ 92 U +

beta− 237 237 − 92U ⎯ ⎯ ⎯ ⎯⎯ → 93 Np + e + ve t1/ 2 =6.7 days

After the separation of Neptunium 237 from byproducts that formed in fission process, Nep- tunium 237 place into targets that put into reactors with high neutron flux where the following reactions take place:

237 238 n+ 93 Np→ 93 Np +

beta− 238 238 − 93 Np ⎯ ⎯ ⎯ ⎯ ⎯ → 94 Pu + e + ve t1/ 2 =2.117 days

The percentage of Neptunium conversion to Plutonium is about 10-15% because fission reactions take place at neutron irradiation process, as Neptunium 238 has a large cross section in thermal neutrons and 85% of it created is fissioned. The produced Plutonium had an isotopic composition of 81% Plutonium 238, 15% Plutonium 239, 2.9% Plutonium 240, and very small

1 amounts of plutonium-241 and plutonium-242. The extraction and separation of Plutonium 238 from all these byproducts needs chemical processing.

At the next picture, there is the sequence of Plutonium 238 production.

Picture 3.3.3: Production and preparation of Pu 238

Now days NASA needs Plutonium 238 to power future missions designed to explore deep space, new Mars rover and Europa, moon of Jupiter, exploration for possible extraterrestrial mi- crobial life because it has a subsurface ocean. Department of Energy (DOE) made Plutonium 238 at Savannah River Site from 1960 until 1980 because the reactor operations there ended. Since then, NASA have used existing inventories of Plutonium 238 and supplemental purchases from Russia to support exploration missions. In 2012, DOE and NASA initiated efforts for capability to produce Plutonium 238 using the existing reactors at Oak Ridge National Laboratory and Idaho National Laboratory after 30 years.

References

Robert C. O’Brien “Radioisotopes and Nuclear Technologies for Space Exploration” Ph.D. Department of Physics and Astronomy, University of Leicester (2010)

Antonio Sanchez-Torres “Radioisotope Power Systems for Space Applications” Ministry of Science and Innovation of Spain (2009)

John W. Poston, Gregory A. Lyzenga "Do transuranic elements such as plutonium ever occur naturally?" Scientific American (1998)

Institute for Energy and Environmental Research "Physical, Nuclear, and Chemical Properties of Plutonium"

Nasa "What is Plutonium-238?" rps.nasa.gov (2012)

Harold J. Groh, W. Lee Poe, and John A. Porter "Development and Performance of Processes and Equipment to Recover Neptunium-237 and Plutonium-238" Semantic Scholar (2000)

D. Thomas Rankin, William R. Kanne, Jr., McIntyre R. Louthan, Jr., Dennis F. Bickford, and James W. Congdon "Production of Pu-238 Oxide Fuel for Space Exploration" ResearchGate (2000)

Tibor S. Balint “Comparison of Power System Options Between Future Lunar and Mars Missions” International Lunar Conference (2005)

Thomas E. Hammel, Russell Bennett, Pratt, Whitney Rocketdyne “Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) and Performance Prediction Model” 7th International Energy Conversion Engineering Conference2009, Denver, Colorado (2009)

Los Alamos National Laboratory "Periodic Table of Elements"

4 Proper Radioisotopes for RTG 4.1 Basic characteristics of Radioisotopes

Radioisotope is the most important component of an RTG due to their role of the energy production and the life of the energy source. The basic characteristics that determine which radioisotope is proper for the design of a nuclear energy source are:

• Decay energy • Half time • Type of radiation

These 3 characteristics must be chosen carefully in order to determine the necessary electrical power levels, the life time, the mass of the energy source device and shielding requirements for a specific space mission.

Half-life of the radioisotope connected with the duration of the mission as the radioactivity decreases with time but also has an important role to the power production of the source as the decay constant determine the radioactivity of the radioisotope. Decay energy determine the power of the source but also the shielding requirements as the more energetic is the decay particle the more penetrates into the matter. The type of radiation (alpha, beta, gamma, fission fragments) is highly determine the shielding of the source as beta particles and gamma rays have greater penetration ability than alpha

1 particles and fission fragments. Also, beta and gamma rays have more complicated energy deposition mechanism than alpha so the energy production calculation is more complex. Another significant characteristic for the selection of radioisotope is the abundances of it on Earth. If is not in desirable quantities the procedure, the time and the cost of production are also very important. There is a big number of radioisotopes which are natural or made in accelerators or produced in fission reactions into nuclear energy reactors. One of the most important criteria is safety. Because RTG devices have applications in space missions in case of failure in atmosphere or re-entry cases the fuel forms must be compact.

4.2 Limits of Radioisotopes

The first limit that will set is about the half-life of radioisotope because the time scale of the space mission usually is at this specific area of years:

10.1 Watt thermal/gr

At the next table are shown some radioisotopes with those limits:

Proper Radioisotopes Half Life Power Density Radioisotope Type of Radiation (years) (Wth/gr) Tritium (3H) Beta- (e-) 12.3 0.26 Krypton(63Kr) Beta-(e-), gamma 10.7 0.62 Strontium (90Sr) Beta- (e-) 28.8 0.44 Ruthenium (108Ru) Beta- (e-) 1 33.1 Cesium (137Cs) Beta-(e-), gamma 30.1 0.42 Promethium (147Pm) Beta-(e-), gamma 2.6 0.33 Plutonium (238Pu) Alpha, gamma 87.7 0.56 Curium (244Cm) Alpha, gamma 18.1 2.84

In previous chapter Plutonium 238 was analyzed as the most ideal radioisotope to power an RTG. At early days of RTG programs, before Plutonium 238 recognized as the ideal choice and after the use of short life time Ce 144 (290 days) and Polonium 210 (138 days), Strontium 90 was recovered from wastes at Hanford Site from Plutonium productions and evaluated as fuel for RTG. Strontium 90 with half-life 28.8 years is a long live stable and insoluble fuel form that used widely in terrestrial power devices. Strontium 90 is a high energy beta decay radioisotope (Q=0.546 MeV) with 0.44 watts thermal power and require heavy shielding because of the brems- strahlung radiation that beta particles create. As we see in previous chapter the Q value of beta decay is the energy released that shared to electron and neutrino, so the calculated energy deposit into the medium (fuel) is a fraction of about 1/3 of the Q value. The beta decay of the Strontium 90 produce Yttrium 90 which is unstable to beta decay with half-life 64 hours and decay energy of 2.28 MeV. Because of the short half-life of Yttrium 90 an extra amount of energy is deposited to the initial fuel mass of Strontium 90 by this decay.

90 푏푒푡푎− 90 − 38푆푟 → 39푌 + 푒 + 푣푒 푡1/2=28.8푦푒푎푟푠

90 푏푒푡푎− 90 − 39푌 → 40푍푟 + 푒 + 푣푒 푡1/2=64ℎ표푢푟푠

The fuel in RTG must have chemical stability, mechanical strength and thermal conductivity and therefore the form of Sr fuel is SrTiO3 (Strontium titanate) with 55% pure Sr 90. While Sr has melting point at 777 oC, Strontium titanate has at 2040 oC. For terrestrial devices the mass of the shielding is not a problem as it is for space applications that required minimal payload weight. An advantage of Strontium 90 over Plutonium 238 is that about 5% of the fission reactions produce Sr 90 and something that make it available in large quantities. In the past was developed more than 100 power systems that used Sr 90 for terrestrial applications. The main concept was the power of monitoring devices that collected data about mete- orological and oceanographic studies by the U.S. Navy. Also, U.S Air Force used strontium RTG for seismic stations in Alaska. A big number of Strontium RTGs used by Soviet Union for remote navigation beacons and devices at the Soviet ocean bottom.

Another interesting radioisotope at the early days of RTG development was Curium 244 because it was expected to produce in desirable quantities from U.S program about breeder nuclear reactors. With 18.1 years half life time and 2.84 watt thermal power (5 times greater than Pu 238) it was a good candidate for RTG design. Although, due to the spontaneous fission of Curium 244 the shielding requirements for the neutrons and gamma rays that produce, increase significantly the weight of the power device. The production of Cu 244 is difficult because requires neutron captures starting with Plutonium 239.

An isotope with significant decay energy and relatively satisfying half-life time is Cesium 137. The problem is that the daughter nucleus of Cesium 137 is Barium 137 in excitation that undergoes gamma ray decay with energy of 0.661 MeV. These gamma rays required excessive shielding and the weight of the power device overcomes the limits.

At the end, Plutonium 238 is by far the most suitable radioisotope fuel for long duration space exploration. The technology based on Pu 238 power systems has been refined over the past 50 years. The basic issue of Pu 238 is the limited availability and the high cost of production. In the past the production of Pu 238 was part of by product from nuclear weapons production that ended in the 1990s. During the 2000s U.S. was supplied with Pu 238 from Russia but the quantities was limited.

4.3 Fission Radioisotopes and space fission reactors

If the power requirement is much higher than an RTG can provide, power systems using fission reactions are the best candidates. Fission fuels in space can be used for general purpose power generator, thermal propulsion, electric propulsion and surface power systems. In a fission reactions power system (FPS) the thermal power determined by the rate of the fissions that can be controlled in the design. In a radioisotope decays power system (RPS) the thermal power determined by the type and quantity of radioisotope and cannot be controlled. A significant advantage is that the fission reactor can be launched without being in function so there will be no radiation risks and can be get started by remote control when it will be in space. The basic fuel for an FPS is Uranium and the principals of his fission reaction are well known from the reactors that we use for energy production and unfortunately from weapons programs. In nature we can find Uranium consisting from three isotopes, 99.27% 238U, 0.72% 235U, 0.006% 234U. In a fission reactor we want the reaction rates to be sustained by the neutrons that produced from the reactions. We don’t have to calculate energy deposits from alpha decays of Ura- nium as the half-life time for 235 isotopes is 703.8 million years and for 238 is 4.46 billion years. From the isotopes of Uranium above the one that have the biggest cross section to fission process by neutron is 235U. The reaction is the following:

235 139 94 푛 + 92푈 → 56퐵푎 + 36퐾푟 + 3푛 The energy released from this reaction is 211.3 MeV. This energy shared as kinetic energy between the 2 fragments of the fission and the neutrons that produced. There is additional energy release from secondary radiation as beta decays and delayed neutrons and gamma rays from the fragments that are rich in neutrons after fission.

Energy Contribution Source Values (MeV) Kinetic energy of fragments 169.1 Kinetic energy of neutrons 4.8 Gamma-rays energy 7.0 Beta particles energy 6.5 Neutrinos energy 8.8 Delayed gamma-rays 6.3 Captured Neutrons (biding energy) 8.8 Sum 211

Neutrons after several collisions and 4.8 MeV energy deposited into the fuel have energy under of 0.1 eV and are capable to react with Uranium 235 nucleus to induce fission process. The fragments due to their mass and charge interact strongly with the medium (Uranium fuel) and deposits their whole energy in a very small distance. For Barium with kinetic energy 68.46 MeV the range is 4.5 μm and for Krypton with kinetic energy 100.64 MeV the range is 5.8 μm (SRIM 2008 CODE). The first design of fission reactions-based power device that launched was the SNAP-10A with 40 kwatts of thermal power and 500 watts of electric power. On April 3, 1965, was launched from Vandenberg Air Force Base and at 700-mile orbit was initiated from remote control. The reactor was shut down after 43 days of successful operating due to a high voltage failure in an electrical system. The fuel was fully enriched Uranium (91.5% 235U) mixed with Zirconium and Hydrogen (9.27% 235U, 0.85% 238U, 1.78% H, 88% Zr). Each fuel element, of the 37 that used, was 32.6 cm long with 3.07 cm diameter and 1.38 kg of fuel. The gap between them was filled by Hydrogen for better heat transfer. The total amount of Uranium 235 was 4.73 kg. The reactor could be functional at a steady power using moving control drums of Be by adjusting the neutrons leakage.

Picture 4.3.1 SNAP-10A Reactor Core

The maximum temperature of the core was 858 K, at hot junction 774 K and across the converter ΔT=132 K. The electric power production was made by silicon-germanium SiGe thermoelectric generator on a cone shell with dimensions of 2.54m height and 1.27m diameter. The heat of the reactor transferred by Sodium-Potassium NaK at temperature of 565.5 oC. The conversion system was shielded from the reactor with LiH reinforced with stainless steel into a stainless- steel casing with 96.8 kg weight.

Picture 4.3.2 SNAP-10A device

While USA has sent to space only one fission reactor (SNAP-10A), former Soviet Union has sent over 30 fission reactions-based low power systems from 1967 to 1988 in RORSAT (Radar Ocean Reconnaissance Satellites) on Cosmos missions. The two known reactors were ROMASKA and TOPAZ I with their characteristics shown at the next table.

Reactors Characteristics Characteristics Romaska Topaz I 235 235 Fuel UC2 (90% U) UO2(96% U) Mass of 235U (kg) 49 12 Neutron Spectrum Fast Thermal Reflector Be BeO Control mechanism B4C Rotating Be/B4C drum Coolant None NaK Core Temperature (K) 2173 - Hot Junction (K) 1253 1875 ΔΤ Across Converter (K) 315 1000 Thermal power (kW) 40 150 Conversion System Thermoelectric Thermionic Electrical power (kW) 0.8 10

The latest news that are relevant with a power system based on fission process is a collabora- tion between NASA and DOE (Department of Energy) for the design of compact low cost, scalable fission power system for science and exploration with power from 1 kWe to 10 kWe. This attempt enables decadal survey missions and reduces the problem of dependence on Plutonium 238.

The reactor core is a solid Uranium-Molybdenum alloy as the properties of these materials are well known and the procedure of construction and operation is simple. The fuel consists of 92% Uranium (highly enriched to 93% 235U) and 8% Molybdenum with 32 kg weight. This quantity of fuel offers thermal power of 4 kwatts. The reflector that surrounding the fuel consisting of Beryl- lium-Oxide (BeO) which at the working temperature and neutron energy spectrum has high re- flectance.

In the center of the core is placed a control rod to keep low the neutron flux. This rod is B4C, a ceramic that has big cross-section in absorbing neutrons without forming long live radio nuclides. The startup of the reactor achieved, when the space probe reaches outer space, by a mechanism that slowly remove the control rod. This option allows the association of power system’s life time strict to the timeline of the mission. This design can accept different control rod options connected with movement of the mechanism that removes it. The simplest mechanism, with only one movement, starts the reactor and occur natural degradation of the core temperature as fuel spent with predictions to 3 Kelvin per year (for the 32 kg of fuel) with 0.1% fuel burn up over 15 years. A more complex removal mechanism is able to control the reactivity as the mission needs and provide constant power for several hundred years. The calculation shows that the power of the start of this device 1 ampere hour at 28 Volt (DC).

Picture 4.3.3 Reactor Core Design

The pipes of heat transfer are made by Hayne 230 alloy in stainless steel rings and contains Sodium as a working fluid. The selection of Hayne 230 is ideal due to the compatibility with So- dium and the high temperature strength. The heat from core vaporizes the sodium liquid into the pipes that are connected with 8 high efficiency Stirling convertors capable of producing 125 We each (1 kWe total). After the release of the heat by sodium vapor condensed to liquid phase and the wick pumps it back to evaporate again. The thermal transport is passive and no power needed for pumping.

Picture 4.3.4 Stirling convertor Between the core and the energy conversion system there is a shielding of LiH (Lithium hydride) and tungsten for protection from neutrons. Lithium hydride remains theoretically as the most efficient neutron shield material. The rejection of the waste heat from Stirling convertors achieved using titanium water heat pipes which have numerus successful designs and tests. The

1 heat transferred to a radiator fin where rejected to space. The power system could have fixed radiators for space probes or deployable radiators for planet surface application.

Picture 4.3.5 1kWe Power system

The total weight of the power device is 400 kg with performance of 2.5 watts electric per kg. Also, there is a study for thermoelectric conversion system but as the conversion efficiency is much lower required more thermal power from the core.

There is evaluation of higher power output reactors with the same design, with some differ- ences, up to 10 kwatts electric. For the minimum power system (1 kWe) the heat pipes are out- side of the core (perimeter) and for the others designs must be into the core and some additional pipes must be added. Other characteristics for the higher energy power system are shown in the next table. Also, we can see from the table that power density (We/kg) increases as the mass of the power device increase.

Different Power Reactors Electric Power (kW) 1 3 5 7 10 Thermal Power (kW) 4.3 13 21.7 30.3 43.3 Radiator (m2) 3.2 9.6 13.5 17.1 20 Mass (kg) 400 751 1011 1246 1544 Power Density (We/kg) 2.5 4 4.9 5.6 6.5

References

Robert C. O’Brien “Radioisotopes and Nuclear Technologies for Space Exploration” Ph.D. Department of Physics and Astronomy, University of Leicester (2010)

Abolhasan Khajepour, Faezeh Rahmani “An approach to design a 90Sr radioisotope thermoelectric generator using analytical and Monte Carlo methods with ANSYS, COMSOL, and MCNP” ResearchGate (2017)

Marc A. Gibson, Steven R. Oleson, David I. Poston, Patrick McClure “NASA’s Kilopower Reactor Development and the Path to Higher Power Missions” ResearchGate (2017)

Marc A. Gibson, Lee Mason, Cheryl Bowman, David I. Poston, Patrick R. McClure, John Creasy, Chris Robinson “Development of NASA’s Small Fission Power System for Science and Human Exploration” Semantic Scholar (2015)

Zara Hodgson “Nuclear reactors in space!” NNL Technical Conference (2015)

Susan S. Voss “Snap Reactor Overview” Kirtland Air Force Base (1984)

Gary L. Bennett “A view at the Soviet Space Nuclear Power Program” The 24th intersociety Energy Conversion Engineering Conference (1989)

Michael Houts “Space Reactor Design Overview” NASA, Presentation

5 Power requirements for space applications 5.1 Space applications with Nuclear power systems As is mentioned in introduction solar panels have a limit of power production due to the decrease of Sun’s irradiation with the distance by a factor 1/r2. Solar panels can be used almost efficiently up to Jupiter. For space missions beyond Jupiter solar panels are incapable to power probes and their instruments so nuclear power is needed. A solar panel power system uses batteries for energy storage as the power that provide is not continues. Batteries at very low temperatures (lower than -40 oC) become unfunctional. The radioisotope thermoelectric generator (RTG) can provide power all the time and solved this problem as there is no need for battery. Also, missions to the inner planets close to the sun would be difficult to powered by solar panels as the high temperature of the panels, due to the intense solar flux in that area, decrease their efficiency. Closer to 0.1 AU the solar panels are useless as the temperatures reaches at 1000 oC. The charged particles trapped in a planet’s orbit (Van Allen type belts) could damage the solar panels. For example, Jupiter has strong magnetic field and trapped more particles in orbit than Mars that has very weak magnetic field and so on no radiation belts. This effect the construction parameters of the solar panels considering higher mass for extra shielding. Nuclear power system sets free the space probe from additional moves like the orientation moves towards the sun that need a solar panel power system in order to achieve the higher power generation. These moves add extra mechanical parts that creates vibration, power con- sumption and complexity in the design. As the power demand increase over 10s of kwatts the

1 solar arrays meet serious problems in fabrication and due to the large mass adds maneuvering difficulties. For planet surface applications, like Mars rovers, the solar panels are capable of power the instruments up to a limited specific power but there is a problem with the dust that could cover the panels and prevents the interaction with the sunlight as it happened in May 2018 when a dust storm cover the panels of Opportunity rover (landed on Mars in January 2004). Lunar exploration with rovers can be much more efficient with nuclear power systems as the duration of the nights are 14 Earth days. Lunar has negligible magnetic field so the solar winds (charged particles) reach the surface unimpeded causing damage to solar panels resulting a higher degradation on power production. Also, the relatively high impact rate of meteoroids on Lunar can cause damage to solar panels. Nuclear power systems for surface missions can reduce volume and deployment complex systems and have less sensitivity to possible changes of the environment or topographical restrictions as the equatorial region where the sunlight flux is maximum. In the future when the Lunar exploration from rovers will be replaced by humans the requirements for energy power levels will be increase by a factor 100 in compare with those of robotics. In situ resources utilization (ISRU) is connected with human exploration and these resources on Lunar will likely be found into craters which are permanently shadowed and dark. Also, the technology required for the process of material and object manufacturing is energy intensive. Ion thrusters is a new consideration of propulsion for deep space exploration us it used already for Earth orbit satellites for orientation and positioning. The benefits are that need small amount of fuel and that enables small course corrections that allows missions with reduced risks, complexity, costs and more options of exploration of our solar system. The disadvantage of this engine is the low acceleration value (0.09 Newtons for Deep Space 1), so to reach a high speed needs a lot of time. The more power we have the more acceleration we can achieve with this engine and for that reason a nuclear power source can give a shorter timeline of a specific mission. Also, a nuclear power system allows the acceleration beyond the point where Sun’s irradiation can't harvest by solar panels.

So, space applications that need nuclear power systems (NPS) have the following characteristics:

• Probes beyond Jupiter’s orbit • Rovers on planets and moons • Human exploration • In situ resources utilization (ISRU) • Ion thrusters

5.2 Power requirements The power levels for probes beyond Jupiter can vary as the problem that solved by nuclear power systems is mainly the distance and not a threshold of power demanding. So, a probe with minimal power requirements can’t use solar panels and a nuclear power system is necessary. The last rover that sent to Mars, Curiosity, use nuclear power system with 110 watts electric power production. The power is not the only parameter for the function of rover while is important the time that this power generated. For example, Curiosity have an energy production of 2.5 kwatts hours with power 110 watts while Opportunity has maximum 0.84 watts hours with power 140 watts (per Sol day) as the time that solar panels of Opportunity produce power is limited due to the day-night cycle and dust cover. Human exploration and life sustenance on Mars or Lunar need advanced technologies and facilities to achieved. For the first human visits the need for power is about 10s of KW electric and for the establishment of bases is about 100s of kW electric. In situ resources utilization missions are connecting with planetary surface processing and mining to provide essential elements. For example, the production of oxygen on Mars by an ISRU unit called MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment) needs 0.3 kW electric to produce 10 gr of oxygen per hour from carbon dioxide (CO2). Another example is an ISRU plant that will produce liquid oxygen on Mars, a propellant fuel, at the quantity of 4400 kg. For a production rate of 0.45 kg per hour the power need is about 16 kW electric. For an ISRU unit that provides water from Mars we need thermal power to melt the ice and preserve the water in liquid form. In the next chart is shown a study for this production (the power values of the chart don’t include the power needs for pumping the water and the function of other instruments for the procedure):

Energy requirements and calculated time for producing water (5 CO from-80 Co ice)

(“Mining” Water Ice on Mars An Assessment of ISRU Options in Support of Future Human Missions

Stephen Hoffman, Alida Andrews, Kevin Watts July 2016)

The development of more powerful ion engines is a key to future exploration missions. Until now (August 2017) the most powerful ion engine that tested is X3, produced 5.4 Newtons of force with power input of 102 kwatts electric. In the next chart we can see the performance of HiPEP ion engine for different provided power levels:

HiPEP thrust variations

(The High Power Electric Propulsion (HiPEP) Ion Thruster, Michael J. Patterson NASA, Christian B. Carpenter SpaceX) The NEXT (NASA Evolutionary Xenon Thruster) ion engine tested for a duration of 51000 hours (6 years) continuously without failure, proving the capability of missions with a specific required duration. The performance of NEXT engine is from 1 kwatt to 1000 kwatts with 50 to 366 mN force. Studies have shown that a high-performance ion engine (like X3) with a power input up to 700 kwatts electric can reach the values of velocity of the known chemical propulsions systems but with the advantage of smaller fuel mass.

Also, an important factor is the power density of a power device. The cost of sending a specific mass to space is about thousands of dollars per kg. So, we need light mass devices with high power production giving good value of power density.

5.3 Specific Missions Overview There is a selection and overview of some different kind of probes and mission profiles with solar arrays for power system that have ended, are in progress and designed for future launch.

5.3.1 Opportunity rover

Opportunity rover was launched on July 2003 and touched down to Mars surface on January 2004. The profile of mission was the characterization of rocks and soils that can provide hints of past water activity on Mars. The rover performed in-site geological analysis using a robotic arm capable to place instruments on rocks and soil of interest. The instrumentation of the rover consists of a panoramic camera (Pancam), a Miniature Thermal Emission Spectrometer (Mini-TES), a Mössbauer Spectrometer (MB), an Alpha Particle X-Ray Spectrometer (APXS), a Magnets, Micro- scopic Imager (MI) and a Rock Abrasion Tool (RAT). These instruments provided information about mineralogy of rocks and soil, abundances of elements, temperature of the atmosphere, magnetic and non-magnetic particles and structure of terrain.

Picture 5.3.1.1: Deep Space 1 (NASA/JPL)

The rover has dimensions of 1.49 m height, 2.28 m width and 1.58 m length with 185 kg mass. Opportunity rover using 6 wheels had covered about 45 km on Mars surface before a dust storm cover his solar panels and forced it to hibernation, having exceeded the initial operation timeline for over 55 times (14.75 Earth years). The last signal of Opportunity rover received on June 2018 when a big dust storm covered the solar panels driving it to shut down.

The solar panels that power Opportunity produce 140 watts at maximum operating conditions. This energy transferred to lithium ion batteries for storage and distribution to instruments and enables the rover to work when the sunlight is not enough.

5.3.2 Deep Space 1 The first probe that rely on ion thruster as primary propulsion system was Deep Space 1 and was launched on October 1998 covering over 258 million km and made flybys of the asteroid Braille and the comet Borelly. Researches add two extended missions increasing the first timeline 3 times. Ion engine was operated for 1.85 years making it the longer propulsion system in function by far. The main reason of the mission was to test 10 new advanced technologies and 2 instruments of exploration, a Miniature Integrated Camera Spectrometer (MICAS) and Plasma Experiment for Planetary Exploration (PEPE). Deep Space 1 was retired on December 18 2001. The body of the spacecraft with the instruments had dimensions of 2.5 m high, 2.1 m deep and 1.7 m wide with 486 kg total weigh including 81.5 kg Xenon gas and 31 kg hydrazine as fuel for the ion engine.

Picture 5.3.2.1: Deep Space 1 (NASA/JPL)

The ion thruster with operational power of 1300 watts produced 0.09 Newton of thrust at maximum level and with 500 watts 0.02 Newton at minimum. The xenon ion engine had 30 cm diameter consisted of an ionization chamber where xenon gas ionized by injected electrons. The positive xenon ions accelerated by 1280 volt and ejected from the bottom of the spacecraft creating thrust. The power source was consisting of 2 solar arrays of 4 panels with dimensions 1.13m x 1.6m (11.75 m deployed), mass of 27.7 kg each and batteries for energy storage. The total power at the beginning of the mission was 2500 watts (2100 watts for ion engine) at 100 volts decreasing as the probe moved away from the Sun. The power density of the solar arrays was 45.12 watts/kg.

5.3.3 InSight Mission

InSight lander was launched on May 2018 and after about 195 days landed on Mars on No- vember 2018 in order to examine the interior structure of Mars. These data will help us to un- derstand the formation and evolution of the rocky planets in general and study the levels of marsquakes and meteor impacts on the Red planet by detecting seismic waves. Three basic instruments will provide the data. Two of them, a seismometer (SEIS) for internal activity information and a heat probe (HP3) for measure the heat flowing out of the planet, deployed on the surface by a mechanical arm. The heat probe then hammers itself under the surface in depth of 3 to 5 m. The third instrument is a radio science instrument (RISE), placed into the lander, measures how much the North Pole of Mars wobbles as it orbits the sun. The InSight mission will run for two Earth years (about one Mars year).

Picture 5.3.3.1: InSight (NASA/JPL)

The spacecraft after deployment have dimensions of 6 m span (with solar arrays), 10.8 m height and 1.56 width of deck. The total mass of the spacecraft was 694 kg at launch including the lander with 358 kg. The power source that enables the instruments function is two circular solar arrays with 2.15 m diameter generating up to 700 watts (1300 watts on Earth) and 200-300 watts on a dusty day. The first day of solar function sets a record of the power production than any previous vehicles on Mars, producing 5888-watt hour of energy. The solar arrays can withstand winds up to 75 m/s. Two rechargeable lithium batteries with capacity of 25 amp hours powering the instruments during the night and have total mas of 17.8 kg.

5.3.4 Juno

Juno probe was launched on August 2011 and reached Jupiter on July 2016 and placed in orbit 5000 km above the top clouds of the biggest planet of our solar system. The main goal of the mission is to obtain information about Jupiter’s gravity, magnetic fields, dynamics and composition of his atmosphere in order to answer questions about the formation and evolution of the planet. The scientific instruments for this purpose are gravity/radio science system (Gravity Science), six- wavelength microwave radiometer for atmospheric sounding and composition (MWR), vector magnetometer (MAG), plasma and energetic particle detectors (JADE and JEDI), radio/plasma wave experiment (Waves),ultraviolet imager/spectrometer (UVS), infrared imager/spectrometer (JIRAM) and a color camera (JunoCam) for detailed glimpse of Jupiter’s poles. The end of the mission is scheduled on July 2021 with deorbit into Jupiter’s atmosphere.

Picture 5.3.4.1: Juno (NASA/JPL)

The spacecraft body have dimensions of 3.5 m high and 3.5 m in diameter. The total mass of the probe is 3625 kg including 1280 kg of hydrazine and 752 kg of oxidizer for propulsion purposes. Juno is powered from 3 arrays of solar panels with dimensions 2.9 m wide and 8.9 m long producing 460 to 490 watts at Jupiter while on Earth produced 14 kwatts as the sunlight is 25 times greater than this on Jupiter. Juno is the further probe that powered by solar panels. Also, two lithium-ion batteries with 55 amp hours capacity and 16 kg weight used to store electrical power when the spacecraft is in eclipse or when solar arrays don’t face on Sun. At the end of the

1 mission it’s estimated that the power will be 420 watts. The mass of the solar arrays is 340 kg providing a power density of 1.41 watts/kg at Jupiter. 5.3.5 Psyche

The Psyche mission is an observation of a unique metal asteroid orbiting the Sun at 3 AU between Mars and Jupiter. The mission will answer the questions of how was formed, how is connected with other planets and if it comes for an outer world of our solar system. The instrumentation of the spacecraft consists of a multi-spectral imager for surface geology composition, gamma ray and neutron spectrometer for element surface composition, a fluxgate magnetometer for magnetic field characterization and a radio science instrument for gravity field mapping. The mission launch is programmed for 2022 and estimated that the probe will arrive at 2026 for a 21-month observation in orbit. The propulsion system will be a SPT-140 Hall thruster (ion engine) with maximum power input at 4.5 kwatts producing 280mNewtons of force carrying 450 kg of Xenon fuel. The body of spacecraft has dimensions of 3.1 m long by 2.4 m wide and with the solar panels 24.7 m long by 7.3 m wide.

Picture 5.3.5.1: Psyche spacecraft (NASA/JPL)

The solar power system will generate about 20 kwatts close to Earth and 2.4 kwatts at Psyche consisting of 2 wings of five panels with total surface 75 m2. The efficiency of solar panels is up to 29.5 % with power density of 150 w/kg (at Earth).

5.3.6 Europa Clipper The Europa Clipper mission have the best possibilities to find life beyond Earth in our solar system with the examination of Europa, an icy moon of Jupiter. Europa is the best candidate as it assumed that beneath the frozen surface there is an ocean of water and hydrothermal activi- ties. The tasks are to characterize the ice shell and any subsurface water, examine the composition and chemistry of the ocean for habitability and give information about geology, past and current activity. These evidences will be provided by instrumentation of the spacecraft: plasma instrument for magnetic sounding (PIMS), interior characterization of Europa using Magnetome- try (ICEMAG), mapping imaging spectrometer for Europa (MISE), Europa imaging system (EIS), radar for Europa assessment and sounding (ocean to near-surface REASON), Europa thermal emission imaging system (E-THEMIS), mass spectrometer for planetary exploration (MASPEX), ultraviolet spectrograph (UVS) and surface dust mass analyzer (SUDA). The launch is programmed on June 2023 and estimated to reach Jupiter in 6 years via Venus- Earth-Earth gravity assist. It would make 45 flybys of Europa in duration of 3.5 years. The dimensions of the body would be 6.4 m long and 3.5 m wide.

Picture 5.3.6.1: Europa Clipper spacecraft (NASA/JPL)

The power system that selected is 8 solar panels with dimensions 2.2 m by 4.1 m with total area of 72 m2. The power level at Jupiter will be 500 watts at the end of the mission. Also, an Li- Ion battery with 180 amp hours capacity will be used for peak power levels.

References

Ugur Guven, Gurunadh Velidi “Usage Of Nuclear Reactors In Space Applications-Space Propulsion and Space Power Concepts” Research Gate (2011)

Valerie J.Lyons, Guillermo A.Gonzalez, Michael G.Houts , Christopher J.Iannello, John H.Scott, Subbarao Surampudi “Draft Space Power and Energy Storage” NASA (2010)

José M. Davis, Robert L. Cataldo, James F. Soeder, Michelle A. Manzo, Roshanak Hakimzadeh “An Overview of Power Capability Requirements for Exploration Missions” Glenn Research Center, Cleveland, Ohio (2005)

Stephen Hoffman, Alida Andrews, Kevin Watts ““Mining” Water Ice on Mars: An Assessment of ISRU Options in Support of Future Human Missions” NASA (2016)

Michael J. Patterson, Christian B. Carpenter “The High Power Electric Propulsion (HiPEP) Ion Thruster” Research Gate (2004)

Thomas W. Kerslake “Electric Power System Technology Options for Lunar Surface Missions” NASA, Glenn Research Center (2005)

John R Brophy “NASA’s Deep Space 1 ion engine (plenary)” Research Gate (2014)

Tibor S. Balint “Comparison of Power System Options Between Lunar and Mars Missions” 1st International Lunar Conference, Ontario, Canada (2005) “Small-RPS Enabled Mars Rover Concept” Jet Propulsion Laboratory

Z. Khan, A. Vranis, A. Zavoico, Frederick,, S. Freid, B. Manners “Power System Concepts for the Lunar Outpost: A Review of the Power Generation, Energy Storage, Power Man- agement and Distribution (PMAD) System Requirements and Potential Technologies for Development of the Lunar Outpost” Space Technology and Applications International Forum (2006)

Gabriel Farkas “Power Sources and Systems of Satellites and Deep Space Probes” ESA, Space for Education, Education for Space

California Institute of Technology Jet Propulsion Laboratory jpl.nasa.gov

map

6 Subcritical Reactor with External Source 6.1 Basic concept of subcritical reactor The idea of a subcritical reactor from external neutron source developed for a safe design of a reactor that will produce energy from fission reactions reducing significantly the risk of core meltdown or the high precision of calculations and design for a reactor that must be kept near critical mode. Another advantage is that there is no need for complex mechanical parts that control the reactor from become supercritical, like control rods that absorbing neutrons, making this design more suitable for space applications and easier to developed. The main idea is that the neutrons needed for fission reactions of Uranium produced from a source with constant rate unlike the case of a near critical mode reactor where the neutrons produced exclusively from fission reactions. The produced neutron from fission reactions are limited so the number of fission reactions governed mainly by the source. So, the main parts of the power source are the subcritical module, the external neutron source and the energy conversion system. 6.2 External neutron source The external neutron source is consisting of two elements, the radioactive that decays particles and the target that release neutrons form reactions with the produced particles. Beta radioisotopes are unsuitable for neutron production as the decayed electrons or positrons cannot participate in nuclear reactions. Gamma radioisotopes are able to produce neutrons with (γ,n)

1 reactions but gamma rays are penetrating and could easily escape from the geometry of neutron source making the flux of neutron production unstable. The best candidate is an alpha decay radioisotope producing neutrons with (a,n) reactions with the target that has low atomic weight (Li, Be, B, O, C) with exothermic process. The alpha particles hit the target forming a compound nucleus that emitting a neutron in order to go to a ground state. Experiments shows that using Beryllium as a target we have the highest neutron production as the Q value of the reaction is 5.702 MeV and is the highest among other targets as 10B (1.06 MeV), 13C (2.215 MeV), 17O (0.587 MeV). The reaction is:

9 12 푎 + 4퐵푒 → 6퐶 + 푛

The kinetic energy of the alpha particles determines the neutron production in this reaction as the cross section for a (a,n) reaction depend on that energy. The energy level of the compound nucleus that formed from the reaction must be high enough in order to release a neutron so as the energy of the alpha particles increases, we have higher probability for the (a,n) reaction.

Picture 6.2.1: Neutron production with alpha particles energy

The alpha decay observed intensively and steady for nucleus with mass number bigger than 206Pb and those elements have long half-life. We know that the energy of the alpha particle decreases as the half-life increase so short half-life alpha emitters are more capable for neutron production. As the specific design of fission reactor is proposed for space applications the neutron source must provide steady flux of neutrons for several years especially for deep space missions or planetary future missions. The half-life of the alpha emission radioisotope determines the life of the mission but also the neutron flux. In order to satisfy these demands we have to consider to use radioisotopes with half-life at minimum 10 years. Also, there is a limitation about the maximum of the half-life, about 50 years, because the lower energy alpha particles of the long-lived radioisotopes produce a smaller number of neutrons. Also, the long live radioisotopes have small reactivity that means that the density of alpha particles emitted is small leading to lower neutron production. At the next table are shown neutron sources based on Beryllium as a target with different alpha emitters for a specific mass:

Neutron Sources with Be target

Half-Life Neutron Yield Radioisotopes (years) (n/g s)

Californium (252Cf) 2.56 2.32 1012 Lead (210Pb) 22 1.24 108 Radium (226Ra) 1620 1.1 107 Thorium (228Th) 1.91 1.08 1010 Uranium (232U) 69 3.131 108 Plutonium (238Pu) 86.4 4.66 107 Americium (241Am) 458 5.89 106

The higher neutron yield is calculated for 228Th and 252Cf as alpha emitters but the half-life of these elements adds limitations about the life of a space mission putting them out of the selection. The next higher magnitude of order is 108 and produced by 210Pb and 232U. The half-life of 210Pb at 22 years also put limitations so 232U is the most proper selection for the source with 69 years half-life. Also, we have to mention that the 232U decay chain produce 6 more alpha particles with high energies from 5.423 MeV up to 8.785 MeV that can contribute significantly to neutron

1 production. The first daughter nucleus of 232U is 228Th with half-life of 1.9 years. The following decays have much lower half-life so an ‘’aged’’ source of about 5-6 years will produce additional neutrons. The fact that 232U has the most alpha emissions in his decay chain than others elements that mentioned above give us one more reason to select it for the design of a neutron source.

Picture 6.2.2: 232U decay chain

The calculations of neutron production and their energy spectrum can be done by using complicated particle transport equations and solved by Monte Carlo method. Also, there is an empir- ical equation (Anderson and Hertz 1971) that can calculate the maximum number of neutrons for a 9Be target source in specific alpha particles energy range:

4.05 6 푁푛푒푢푡푟표푛푠 = 0.08 퐸푎푙푝ℎ푎 (푝푒푟 10 푎푙푝ℎ푎 푝푎푟푡. ) (4.1 < 퐸푎푙푝ℎ푎 ≤ 5.7 푀푒푉)

2.75 6 푁푛푒푢푡푟표푛푠 = 0.8 퐸푎푙푝ℎ푎 (푝푒푟 10 푎푙푝ℎ푎 푝푎푟푡. ) (5.7 < 퐸푎푙푝ℎ푎 ≤ 10 푀푒푉)

Picture 6.2.3: Neutron spectrum for 232U9Be

The way that the target element and the alpha emitter combined to create an efficient neutron source is a dense homogenous mixture with a ratio of 200 to 1 (target-alpha emitter). In order to achieve these two desirable characteristics, the mixture is made of ceramic metallic compound. Into the source (with 9Be target) may occur other reaction that can produce more neutrons. The neutrons from (a,n) reactions can participate to (n,2n) reactions with neutron threshold energy at 1.86 MeV leading to multiplications of neutrons. The reaction is:

9 8 푛 + 4퐵푒 → 4퐵푒 + 2푛

The (n,2n) multiplication reactions depend on the geometry of the source as the free mean path of the neutrons have to be less than the 9Be thickness so the volume to surface area ratio should be ideal (sphere) in order to have the minimum loss of neutrons. From the picture 6.3.3 we can see that the minimum energy of neutrons is 0.23 MeV and the maximum 13.5 MeV. With a simple run at WISP (free Geant4 graphical user interface) we determine that the maximum mean free path is about 60.24mm for 13.5 MeV neutrons. Also, neutrons can produced from (γ,n) reaction as Beryllium is have a considerable cross section to this reaction at gamma energy of 1.66 MeV.

6.3 Subcritical module

The fuel selection is very important for the design of the subcritical module. Uranium is the basic element of a fission reactor. The percentage of 235U determines the mass and the volume of the reactor as it has the higher fission cross section creating strong connection with the power production. Higher percentage offers smaller and lighter reactors. In order to have efficient conversion from thermal energy to electrical the temperature of the core must be higher than 1000 oC, putting a limit on the melting point of the core. In chapter 4 we describe a nuclear reactor for space applications that use movable parts like the control rod in the center which by absorbing neutrons determine their flux. The advantage of the subcritical module is that don’t use movable parts that are undesired for space applications due to the vibrations that add in the system. Also, the thermal power production will be steady unlike the case of the control rod. A desirable and tested choice as fuel is Uranium Nitride (UN), an alternative choice to the convectional UO2 that used in fuel rods of nuclear power plants. The thermal conductivity of UN

1 is 23 W/mK while the UO2 has 2.3 W/mK and the melting point is at 2903 K, much higher than Uranium (1405 K). Also, the density of Uranium in UN is higher with value of 13.51 g/cm3 of UN 3 than that of UO2 with value 9.66 g/cm , providing a more compact core. Also, Uranium Carbide (UC) evaluated as a fuel in this thesis with thermal conductivity of 5.98 W/mK and melting point at 2623 K.

Picture 6.3.1: Subcritical module dimensions

Subcritical Module fuels for k=0.97 (±0.01) and volume of 10291.21 cm3

Density Fuel Mass U Mass U-235 U-235 mass Total mass Fuel (g/cm3) (kg) (kg) Enrichment (kg) (kg) Uranium Ni- 14.3 147.16 139.03 66 91.75 169.64 tride (UN) Uranium Ox- 10.97 112.89 99.41 82 81.86 135.37 ide (UO2) Uranium Car- 13.63 133.52 133.52 26 34.71 156.1 bide (UC)

As we can see from the table above the UN fuel offer the higher Uranium-235 mass at the same volume which is the fissile radioisotope that produce the energy on the reactor. So, with UN we can have a more compact reactor with more fuel storage. It is notable that UC fuel has a very low enrichment of Uranium 235 as Carbon offers a high capability on decreasing neutrons energy (higher than Nitrogen) making it a good moderator in convectional nuclear reactors. The fission cross section of Uranium 235 increases as the energy of the neutron decreases and that’s why a good moderator like Carbon offers small enrichment of Uranium 235 as the fission process that release extra neutrons has higher probability in UC fuel. As a reflector of neutrons Beryllium 7 is chosen as it has high cross section for elastic collisions with neutrons and low absorption cross section that is important as it’s undesirable to lose neutrons in shielding material. Also, the selection of Beryllium 7 can offer a good moderation of neutrons energy. The thickness of Beryllium shield was calculated with WISP, a graphical user interface of GEANT4 Monte Carlo particle transport code.

Picture 6.3.2: Calculation of neutrons free mean path in Beryllium

(WISP-Geant4 Graphical User Interface)

The maximum neutron energy into the core can be obtained by the energy spectrum of the prompt neutrons from Uranium 235 fission.

Picture 6.3.3: Prompt neutrons energy spectrum

We can see that the maximum energy is at ~10 MeV with very low probability and the high probability neutrons around at 1-3 MeV. So, the thickness of Beryllium that selected in simulation was 4cm.

References

Sanbing Wang, Chaohui He “Subcritical space nuclear system without most movable control systems” Journal of Nuclear Science and Technology (2015)

Tora Murata, Keiichi Shibata “Evaluation of The (α, n) Reaction Nuclear Data for Light Nuclei” Journal of Nuclear Science and Technology (2014)

Ryan D. Bechtel “Uranium-232 beryllide neutron source” Master Thesis, Georgia Institute of Technology (2007)

Osamah N. Oudah “Determine Neutron Yield from Beryllium Compounds Bombarding with Alpha Particles” International Journal of Scientific and Technology Research, Volume 4 (2015)

Odile Petit1, Cédric Jouanne1, Olivier Litaize2, Olivier Serot2, Abdelhazize Chebboubi2,Yannick Pénéliau2 “FIFRELIN – TRIPOLI-4® coupling for Monte Carlo simulations with a fission model. Application to shielding calculations” EPJ Web of Conferences 153, (2017)

B. Ebbinghaus, J-S. Choi, T. Meier “A Modified Nitride-Based Fuel for Long Core Life and Proliferation Resistance” ANS International Winter Meeting (2003)

B. Szpunar and J. A. Szpunar “Thermal Conductivity of Uranium Nitride and Carbide” International Journal of Nuclear Energy Volume (2014)

J. K. Shultis, R. E. Faw “An MCNP Primer” Dept. of Mechanical and Nuclear Engineering Kansas State University Manhattan (2010)

R.G. Williams III, C.J. Gesh, R.T. Pagh “Compendium of Material Composition Data for Radiation Transport Modeling” Pacific Northwest National Laboratory, DOE (2006)