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 Mehtapress 2015 Print - ISSN : 2319–9814 Journal of Online - ISSN : 2319–9822 SpFauclle pEapxerploration Full Paper WWW.MEHTAPRESS.COM

Liviu Popa-Simil Novel nuclear power sources with application

LAAS- Los Alamos Academy of Sci- to space propulsion ences, Los Alamos, NM 87544-0784 E-mail: [email protected] Abstract

Received : March 13, 2015 Nuclear power is still a genuinely new domain with respect to space exploration. The field Accepted : April 27, 2015 is still in its infancy, despite of over 60 years of peaceful exploitation of nuclear power on Published : May 13, 2015 Earth, and many successful applications in space. Nuclear power sources are very compact and may be used for local power generation or as a source for propulsion in advanced thrusters. Fission batteries experience drastic limitations with respect to the energy that *Corresponding author’s Name & might be stored on board a spacecraft, due to criticality issues. Fusion batteries remove Add. this impediment, but fusion energy is in an early stage of technological research. Today, the best propulsion systems are inertial, jet based electric thrusters, providing a owerful Liviu Popa-Simil source of efficient propulsive energy, but even with the development of these thrusters, LAAS- Los Alamos Academy of Sci- travel throughout the solar system within reasonable time scales is unattainable. In the absence of other superior, physics-based propulsion concepts, nuclear-energy-powered jet ences, Los Alamos, NM 87544-0784 propulsion may take us to the limits of our solar system, but no further. E-mail: [email protected] Keywords

Nuclear power; Jet propulsion; Electric thrusters; Fusion battery; Fission battery; Isoto- pic battery, Nano-nuclear; Direct energy conversion; radiation guiding.

INTRODUCTION der to act as space energy concentrators. These individual devices might be applied jointly, re- This paper presents ideas for novel nuclear energy sulting in complex systems with outstanding capabili- sources based on new nano-material structures and dis- ties and performances. Ultimately, they might pro- cusses their applicability to space propulsion. Special vide for comfortable space travel inside the solar sys- emphasis is given to direct energy conversion processes, tem, resulting in relatively acceptable travel times i.e., the direct conversion of kinetic energy frames, even between future space colonies or space into elecric current. outposts at the edge of the solar system. However, Nuclear energy as we know it today is a technology because of propulsion effectiveness issues all of these that requires significant development before it will systems using these advanced concepts are not suit- reach mathurity. In this regard, recently developed able for human crews in interstellar travel, i.e., one micro-nano-hetero-structures are giving a novel per- cannot go outside the solar system. spective to nuclear power, making possible the devel- opment of compact, solid state nuclear power systems. NOVEL NUCLEAR POWER SOURCES Additionally, nano-materials may permit high effi- ciency light shielding, robust self-repairing of materi- The evolution of nuclear power sources is primarily als structure from radiation damage; or enable the con- based on new developments in materials technology. struction of active radiation guiding materials. Radia- Among these, is the direct conversion of nuclear en- tion guiding materials, conceptually, may be used as ergy into electric energy, utilizing compact solid-state antenna coating for directed energy harvesting, for devices. These direct energy conversion devices rely active energy shielding, or as harvesting devices in or- on nuclear decay, fission, or fusion, and are generi- Full Paper JSE 4(1) 2015

Figure 1 : The physical principle of direct energy conversion is depicted cally called batteries. coated by another insulator layer I. This quadruple The operation principle of such a solid-state battery is structure is repeated, and the number of these layers shown in Figure 1, resembling a capacitor that is is large enough to ultimately bring the particle to rest charged from the kinetic energy of moving within this lattice. The plot in the left of Figure 1 and is discharged directly as electricity. shows the novel type of capacitor with its two con- The solid-state battery consists of layered materials ductive semi-conductor layers, C and c that are con- whose electronic shells are crossed by fast moving nected to the (external) load resistor R , where layer c L nuclear or atomic particles. Such a particle could be a plot is negatively charged with respect to C. When neutral or , a fission product, an alpha this kind of novel energy conversion is applied to fis- particle obtained from fusion or nuclear decay, a beta sion, e.g. a nuclear reactor, it may be possible to elimi- particle (, ), a (gamma) , or a nate the complete thermo- electric energy conversion . As soon as the particle interacts with the bat- system, and thus reduce the size of the power station tery material, it gets into a dynamic ionization state[1], to the nuclear reactor itself together with only an in- shown by the cylindrical gray tube. The particle inter- terface needed for electrical output. The chart (right acts with the electronic shells by knocking-off , of Figure 1) shows the performance of various power shown by the red arrows that are directed in the direc- sources both in terms of power and energy density. It tion of motion of the particle. These ejected electrons is obvious that nuclear power sources are delivering further interact with other electrons, ejecting them from an energy output three orders of magnitude higher their orbits, as indicated by the green arrows. The final than any other known energy source. result is a bulk electron current that has less energy Figure 2 shows the voltage and current distribution than the initial knock-off electron energy. inside the super-capacitor. The red layers, possibly If this process takes place inside a repetitive multi- made of gold, have a thickness of several tens nanom- layered capacitor, as shown in Figure 1, left side, it eters and are the high electron conductor nano-layers. might amplify the polarizing process thus enhancing It is these layers that are generating the electron shower. the conversion of electron kinetic energy into elec- These electrons, then, tunnel through the dielectric tricity. Such a “super capacitor” can be formed from material and are stopped at the blue layers, which are nano-material layers containing a conductor C of high made of low electron density material, like aluminum. electron density with a thickess of several 10 nm. In- Their thickness has to be large enough to reduce the tense electron currents will be generated during the tunneled electron curent, I , (close) to zero, and thus d sudden deceleration process from the deposited kinetic this layer gets negatively charged. Furthermore, the energy of the moving particle. The knock-on electrons insulator layers need to be thick enough to withstand may tunnel up to 100 nm inside the conductor mate- the breakdown voltage V that is higher than the op- bk rial, followed by a thin layer I, of insulator material. erational voltage V . The capacitor foils are connected op Electrons tunnelthrough this layer with little interac- in parallel to the external load resistor R (Figure 1, L tion, and are eventually stopped into a layer of con- upper right), extracting the current I at a voltage op ductive material, c, of low electron density. This layer V . The power density depends on the power deposi- op generates only a very small electron shower and is tion of each particle in the respective layer as well as

FP 19 JSE 4(1) 2015 . Full Paper the number of particles stopped in that layer and, natu- trons and holes that are bound together. rally, the efficiency of the energy conversion. The upper-right of Figure 3 depicts the dimensions The energy conversion efficiency of these new mate- and properties of super-cpacitor material in order to rials will be larger than the efficieny obtainable from be able to sustain the indicated power density (com- any of the current nuclear power plants, also includ- pare measurements in Figure 3 upper right). For ex- ing thermo-electric convertors (TEC) with a conver- ample, consider a planar-layered structure of a vol- sion efficiency lower than 10%. ume of 1 mm3, where edge effects are neglected. The The direct energy conversion structure has several con- structure is supposed to be made of silica and gold- structive variations, as shown in Figure 3. It can be aluminum (bi-layer), and thus should sustain a voltage built from planar depositions (Figure 2), but there is a of more than 10 kV. The admissible current is 0.5 A disadvantage in that most of the heterogeneous struc- or higher. That is, a total power of 5 kW/mm3 might tures deposited in such nano-layers possess the ten- be achieved, which is about three orders of magnitude dency to separate and precipitate, forming nano-beads higher than the largest power density achievable in at higher temperatures. This kind of structure modifi- nuclear reactors (maximum of about 1 kW/cm3). cation limits the operation temperature of planar nano- Though metamaterials have the capacity to store and structures, but there exist other structures (see below) carry a huge electric power density, as shown in Fig- were conceived that might successfully operate at ure 3 upper right, in practice the generated power higher temperatures. density is still drastically limited by the energy con- Care must be taken when the dimension of the struc- version efficiency. Any energy not converted into elec- ture is decreased down to the nanometer range (Fig- tricity being removed from the structure becomes ure 3 upper-left), because quantum effects start to domi- waste heat, generating the corresponding heat flow. nate and the local material properties strongly differ For example, by assuming a conversion efficiency of from those of the bulk, giving rise to spherical charge 90% and a resulting heat flow that is equivalent to a reflection on a grapheme foil creating power density of 100 W/cm3, the maximum power that are collective excitation within a material that density is limited to 1 kW/ cm3. behaves like a fundamental particle, i.e. a or The lower-left picture of Figure 3 shows a bi-material a dropleton which comprises a small number of elec- nano-beaded structure embedded into an amorphous

Figure 2 : Shows the detailed structural assembly inside the prpoposed super-capacitor

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Figure 3 : Various fabrication techniques for nano-hetero structures for direct nuclear energy conversion

Figure 4 : The principle of radiation channeling or guiding radiative energy in solid structures silica insulator, where the beads are connected in se- This structure presents the advantage of operating at ries by knock-on electron discharge induced by the higher temperatures of up to 1,000 K, where the power moving particle, representing another variant of density of the heat flow may reach 0.5 kW/cm3, of- metamaterial structure. fering the possibility of higher electric power density,

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Figure 5 : Radiation robust materials

Figure 6 : Isotopic battery power sources structure and specific energy dependence of the employed radioisotope along with a supplementary thermo-mechano-electric ture. Though operating at lower temperature, this conversion that may add up to 150W/ cm3, further structure may offer very good energy efficiency. improving the overall efficiency. The lower right of Figure 3 depicts a loaded-multi- Radiation guiding devices wall nano-tube construction that operates for a large Since the 1970s it has been observed that radiation solid angle (moving particle geometry) being another may be channeled inside crystal structures[2]. More re- constructive variant of direct energy conversion struc- cently nano-structures are used[3] to channel radiation,

FP 22 Full Paper JSE 4(1) 2015 and to make devices that control direction of neu- RADIATION ROBUST MATERIALS trons and X rays. Figure 4 (upper-left) shows the principle of radiation Structural materials utilized in nuclear reactors deliver guiding in nano-structures. A portion of the incoming very good mechanical performance by having a small particle flowis trapped inside the nano-structure that neutron absorption cross section, but are sensitive to may be a nano-tube, nano-wire, or patterned struc- radiation damage that makes them swallow and ture. The trapped particles then interact with electric embritter. and magnetic fields inside the structure, and subse- New types of sintered materials have been developed quently are smoothly diverted at grazing angle, thus to obtain material properties that remain constant for being driven forward along the tube. large radiation doses. These materials must have an The figure in the middle (upper part) presents a view optimized microstructure with regard to the type of along the guiding tube, seen from behind the neutron, radiation damage and the specific application. There along the displacement direction. The structures in exist immiscible solid solutions, as shown in Figure 5 blue and yellow depict magnetic fields with different at bottom left, that offer good mechanical properties that remain almost independent of the radiation level. polarities produced by the nano-structures. The inter- To demonstrate this behavior, the mechanical behav- action of the neutron momentum with the structure ior of stainless steel SS316 is shown and compared to material is collective-repetitive that is, many nano-structured composite material in the diagram at from the guiding tube structure interact only wealy the lower right of Figure 5. The upper right of Figure with the moving particle, diverting and turning it, and, 5 shows several other types of composite nano-mate- only occasionally, scatter it into the opposite direc- rials that are supposed to also possess good robustness tion without significantly modifying its energy, i.e., with respect to radiation damage. the scattering process is elastic. This process resembles the focusing/defocusing that occurs in FODO ISOTOPIC BATTERY (FOcusing/ DefOcusing) structures, employed in stor- age rings of particle accelerator systems. An isotopic battery directly converts the kinetic enegy The upper-right figure zooms inside the channeling of nuclear decay products into electric energy which tube, showing the atomic bonds, illuminated by the is then stored in a nano-hetero-structure. The battery touch of the (dipole) magnetic moment of the moving is powered by a micro-layer of radioactive material particle. comprising (mainly) alpha or beta emitters. The chart at the bottom shows the associated wave Figure 6 (upper right) shows the schematic diagram of length for (blue line) and (red double an isotopic battery. The battery is made of a nano- line) plotted versus energy. The energy that is guided beaded structure embedded in amorphous silica, plated through the tube is shown by the pink ellipsoid for X on one side with gold and on the other side with a rays, whereas the red segment shows the energy for layer of aluminum (or other materials of similar func- neutrons. In addition, the domain in which X and tionality). If alpha decay is used as the energy source, gamma rays have to operate for the next generation the layer thickness should be about 20 microns. The of guiding tubes is also shown. alpha-emitter itself has a thickness of a few microns There exist many applications for this type of device, incorporating a gold substrate acting as electron shower for instance, in radiation shielding, or concentrators generator. (with or without imaging capabilities). Nuclear reac- A battery can be made, upper right figure, from mul- tor shielding may be made of absorption blankets. tiple layers containing intermediary voltage plots, thus Active control of criticality can be achieved by using enabling customized voltages for a variety of applica- blankets made of nano-guides to which a NEMS tions. The bar chart below plots power densities from (Nano-Electro-Mechanical System) device is attached, alpha radiation (red bars on the left axis) for different in order to provide electronic commands in selecting isotopes together with lifetime (blue bars). The pre- the particle route, i.e., back toward the reactor core ferred power sources are based on alpha and beta de- for increasing reactivity. Alernatively, particles can cay and preferablly do not generate gamma or X rays, as is, however, the case for 238Pu or 90Sr. The lowest be made to move toward an absorber to increase trans- footprint of such a battery, or, in other words the mutation as well as to reduce power. The response smallest area (for normal conversion efficiency) is time of these NEMS is in the micro-second range, pro- about 100 x 100 microns and its thickness is some 50 ducing an outstanding degree of control. microns. The radioisotope is contained between two

FP 23 JSE 4(1) 2015 . Full Paper conversion-foils, each about 20 microns thick. This between the conversion structures. The necessary con- geometry matches the power (left side) obtainable from dition for such a structure to operate is to meet the 238Pu and 210Po, producing basically the same energy criticality conditions and to have an adjustable system output. Such a configuration is equivalent to more for the neutron flux. For any configuration, in order than 50,000 chemical batteries, having a weight, in to meet the criticality conditions, structures with a the range of 1 to 3 chemical batteries weight, depend- diameter larger than 1 m are required, where all of ing on the structure used inside. them need to be comprised of nano-structures, which In the future this type of battery might be a candidate represents a substantial challenge for the manufacturer. to replace the actual thermo-electric converter ele- ments, operating by converting heat into electricity. FUSION BATTERY These devices are also known as RTG (Rradioisotope Thermoelectric Generator), delivering a conversion Recent advances in physics make it possible to use efficiency less than 10%. Moreover, even the novel long-range nuclear reactions, triggered by low energy power sources based on the Stirling or Brayton en- quantum changes, in order to induce high-energy quan- gine-generator can only reach conversion rates up to tum reactions. This effect was discovered experimen- 30%, but due to the presence of moving parts, have tally in the 1920s, and since that period more than lower mechancal reliability. A special feature of iso- several hundred successful systems were fabricated. To topic batteries is that their power output cannot be understand the phenomena presented in Figure 8, a adjusted at will, since the energy is generated from more thorough understanding of quantum and higher- natural radioactive material. dimensional physics is necessary to properly account for entanglement, quantum – nuclear active environ- FISSION BATTERY ments, and higher multi-dimensional spaces. It was experimentally proved[4] that fusion reactions This is a new device that uses the same direct nuclear do occur in quantum-nuclear active environments energy conversion for electricity generation. The fis- under special conditions. The upper-left side of Fig- sion products deliver about 170 MeV per fission act, ure 8 shows a Pd–D lattice that is self-excited by ran- equivalent to 25 pJ. The fission battery has the advan- dom temperature effects. Alternatively, the lattice tage of producing 20 times more energy (per moving might be excited using electro-magnetic fields apply- particle) than a source powered by alpha decay. ing resonant frequencies in the THz range. In this en- Figure 7 shows the functional diagram of a solid-state vironment, a He nucleus spontaneously emerges from fission battery. This device, which is similar to an iso- the fusion of two deuterium nuclei, mediated by the topic battery, has 235U or 239Pu based fuels embedded palladium lattice. The process of fusing two deute-

Figure 7 : Sketch of solid-state fission battery

FP 24 Full Paper JSE 4(1) 2015 rium nuclei into a single helium nucleus, as shown by ity. In our case, if these fibers of space with special the picture underneath, releases about 22.4 MeV per properties, as entanglement or teleportation emitted fusion reaction, where 22 MeV is due to the kinetic by the palladium and deuterium nuclei, should be energy of the ejected helium nucleus, and about 300 touching each other, as happens at points C1 and C2, keV is attributed to the recoil kinetic energy of the as shown in Figure 8, those nuclei are supposed to be palladium nucleus. These nuclei produce over 10,000 in a special state of communion, similar to the one dpa (displacements per atom) in the region, thus de- obtained in head-on collisions of accelerated particles stroying the quantum-nuclear resonance of their envi- breaking through the Coulomb barrier. In this case, ronment, as indicated by the two stars in the upper- they simply ignore the electric field by communicat- left picture. If this structure is embedded into a direct ing and interchanging their quantum states, taking a energy conversion structure, as shown before, the pal- new stand in accordance with the requirements of the ladium and helium energy may be harvested and deliv- quantum-nuclear active environment. As soon as the ered as electricity, as depicted by the lower-left image. stand has been taken, the new manifestation takes ef- A very simplistic explanation is expressed graphically fect and new nuclear particles and states appear, in in the upper right picture[5]. Self-excited nuclei have a the form of fusion, transmutation, and even fission[7]. special state of that develops along some fibers In our case, we are interested in generating those quan- of space,[6] outside the force-field of the electric shell tum environments leading to fusion, because the ki- (a nucleus has an apparent radius of a few fm, e.g. 12C netic energy of the resulting fusion products will be has a radius of about 2.8 fm), and extends far beyond. harvested in the nano-hetero structure customized for As demonstrated by special entanglement communi- this process. Such a device is shown in Figure 9, and it cation experiments, a state of quantum correlation may is called the fusion battery[8]. be maintained for hundreds of kilometers. It is also The fusion battery is made of a quantum-nuclear ac- known that entanglement is continuously created and tive microstructure, which is surrounded by direct destroyed and that the kinematic equilibrium depends nuclear energy conversion (DNECE) structures that on amost all properties of the environment in which harvest the kinetic energy and deliver it as electricity. it takes place. Communication experiments also cope A fusion battery contains a deuterium tank from with teleportation of quantum states and non-local- which the gas is loaded in a controlled manner into

Figure 8 : Physical principles of long-range nuclear fusion and transmutation

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Figure 9 : Modular diagram of the proposed fusion battery for stage 4: building the functional device the structure, and together with the excitation con- Since the energy density obtained from matter-anti- trol, the reaction rate and hence the power output is matter annililation is about three orders of magnitude controlled. The system is surrounded by cooling equip- higher than from fission processes, harvesting annihi- ment based on internal helium cooling for the active lation energy becomes an extremely difficult task com- structure that is taking out the heat to the external parable to the storage of antimatter. For instance, the heat exchanger, keeping the temperature within oper- reaction of only 1 gram of matter-antimatter releases ating limits. For high temperature systems a thermo- the energy of about 0.1 MtTNT. At present, the en- mechano-electric system might be included in the cool- ergy obtained by annihilation of all the antimatter ing system, producing some extra energy. The struc- yearly produced at Fermi National Laboratory in US ture is compact and, by computer control, delivery of is a little bit over 0.05 kWh. energy is regulated. Other applications Annihilation battery Figure 10 briefly discusses several different, but very Previously discussed sources of energy convert the bind- important applications of nano-engineered structures. ing energy of a nucleus, released after a nuclear state Shown are: a body (or volume) radiation shielding (left change, into electricity.These nuclear sources are about figure); same as the for protecting the electronics six orders of magnitude more compact than corre- against detrimental radiation events is depicted. The sponding chemical energy process that convert the right presents the (already discussed) fission battery, binding energy of their electronic shells. The next stage using advanced control based NEMS (Nano-Electro- of energy conversion is related to the existence of Mechanical System), together with radiation guides and DNECE (Direct Nuclear Energy Conversion in Elec- matter as a condensed state of energy and its symmetri- tricity) structures. cal partner (with respect to vacuum energy), called anti-matter. By burning 235U, one obtains 1 GWDay/ RESULTS AND DISCUSSION kg, about 3 GWDay/kg for deuterium, and about 1,200 GWDay/kg for annihilating hydrogen. Although the above-presented types of nuclear devices

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Figure 10 : Applications of advanced nano-engineered nuclear materials

Figure 11 : Near Earth strategic applications of nuclear power systems have many applications in terrestrial power systems, ergy sources that could make use of the above-cited these devics are most outstanding in extreme environ- fission or fusion batteries or an isotopic battery that ments (including underwater applications). These new delivers continuous power. In the absence of such compact, silent, high power sources may also allow power supplies, designs have to be based on solar en- more advanced propulsion methods, providing long ergy that requires extremely large solar panels. Apart operation hours, and thus a seamless presence in all from their size, these huge structures are easy to de- strategic locations in space should be achievable. tect and annihilate and therefore are less useful for military space applications. Directed energy devices in low earth orbits In Figure 11 several space projects are presented, all Right now there are numerous space missions under of them requiring compact high power sources in design that require powerful, portable solid-state en- outer-space. The upper left figure presents altitude

FP 27 JSE 4(1) 2015 . Full Paper versus range for various weapon systems, currently for about 1 year[10] is required, delivering an energy used in local conflicts. Also presented is the capability output of about 2 PJ (PetaJoule), which corresponds of airborne or space systems equipped with directed to a power source of about 200 MW. Only nuclear energy devices to disable their use, inflicting minimal power can deliver this amount of energy, which is damage to the enemy but enforcing peace talks or ne- equivalent to a solar array of 1 km2, which poses a gotiations. real (but impossible to meet) challenge for any space Low and high terrestrial orbit vehicles require mega- station. In this process 150 MW turn into low grade watt level power on board, power that only a nuclear heat at temperatures of about 400 K. Getting rid of source can deliver in a compact manner. The center such a large amount of waste heat for the period of 1 picture shows a scenario for neutralizing an intercon- year, poses a major technical challenge, and requires a tinental missile using kinematics impactors, launched heat sink of about 1 km2 surface area in combination as missiles by a BMD (Ballistic Missle Defense) system with a heat exchange agent (fluid). In space, the effi- that is located at the Pacific and west coast of the U.S. ciency of a directed energy device is critical and effi- and Canada. These missiles have a certain failure prob- ciencies over 90% are desired. An alternative to ability, or may cause an unintended deflection of the would be a linear accelerator, which exhibits high ef- flight direction of the incoming (target) missile, and ficiency, but the capacitance of a 500-m asteroid is thus require at least two launches to secure a 90% hit less than 1 mF, and in less than 1 s its electric charge is probability. The window of opportunity for missile deflecting the beam. A single charged beam will be- launching is less than 5 minutes, in order to have in- come divergent due to space charge built-up. A pos- terception at mid range. Instead, by using directed- sible solution could be to use blended dual beams at energy devices from outer-space platforms poses prac- energies assuring optimal penetration depth. The tem- tically no constraint on the annihilation shot, i.e., a perature of the target volume would be brought up to total flight time of about 20 minutes is available for about 5,000 K and a mass jet would be locally created, defense. to both provide thrust and modify the asteroid trajec- In the center of Figure 11 a directed-energy device is tory, (hopefully) away from a collision course to a presented, carrying a high power , ion beam ac- near miss. celerator, or microwave pulsed power device as part of an active denial system. The power level require- Nuclear powered propulsion ment is over 10 MW for few minutes action, which Since the 1950s a new type of propulsion involving would be very difficult to achieve using solar panels. the use of nuclear power[11, 13] has been extensively re- The top-right of Figure 11 shows dummy satellites (in searched, but, in spite of the magnitude and excellence orbit around the Earth) that, by using a direct energy of this research work[14], nuclear propulsion still does conversion antenna, are remotely receiving substan- not match the performance of actual chemical pro- tial amounts of energy through a neutral beam scooted pulsion sytems[15]. from a directed energy platform, for activating the From the exceptional research and the inventions pre- satellite, enabling a specific mission, or switching it sented[15], only a few examples will be mentioned. The back in stand-by mode. technologies described here are all in the conceptual Another very effective weapon in space is the molted phase, but have some collateral experimental and com- metal or jet directed toward the target. This puter simulation successes. type of weapon overlaps with the advanced jet pro- Jet propulsion today is the most advanced and reliable pulsion system, which uses a more dispersed jet, but propulsion scheme we know, but it relies on chemi- almost employs the same physical principles applied cal fuel and delivers maximal exhaust speeds of some in reverse. 4,500 m/s because of the limited chemical reaction Another most valuable and challenging developments energy (10 eV per molecule or atom). Chemical pro- of the future is the steering of asteroids. Not only pulsion requires bulky fuel tanks. Chemical propul- under the aspect of deflecting asteroid trajectories away sion is also dangerous, because the combination of two from the Earth orbit. As an alternative, the trajectory highly reactant materials like liquid hydrogen (LH) of an asteroid that is bound to collide with the Earth and liquid oxygen (LO) requires a very high level of might be modified by changing the course of other technologic performance (e.g. maintenance intensive asteroids in order to deflect the otherwise impacting cyrogenic fuel pumps). Other fuels exhibit a high level asteroid[9]. Calculations show that to vaporize a large of toxicity, increasing the technological challenge even asteroid with a diameter of about 500 m, the continu- further. ous use of an IR (infrared) laser beam of 50 MW power To provide the reader with a picture of the compact-

FP 28 Full Paper JSE 4(1) 2015 ness of nuclear energy compared to chemical fuel Fig- plication of nuclear power on Earth has fewer con- ure 12, upper left, shows the energy equivalent of 2 straints than in space applications, and most of the GWDay in a jar that contains pure 235U or 239Pu. The technical solutions used on Earth will prove impracti- upper-right side shows a chart depicting the difference cal in space. Very important limitations with respect between energy and impulse (thrust). In order to in- to the total mass of fissile fuel that can be packed into crease thrust, mass must be ejected at high speed, but an operational nuclear structure are revealed in refer- the (nonrelativistic) energy of the ejected mass is grow- ence[16]. The upper part of Figure 13 pictures various ing faster than the thrust, since it is proportional to nuclear power structures that represent an optimum the square of the exhaust velocity. Therefore, ejecting to reach both criticality and to allow a continuous additional mass becomes more efficient only up to a mode of operation. It is clear that by replacing mate- limit, after which the jet speed has to be increased. rials of low neutron absorption cross section by those This leads to the conclusion that there is an optimal of high neutron affinity, one may reduce the mass of mass-speed ratio. the structure and maintain its sub-criticality for a ra- The picture on the lower-right of Figure 12 shows tio of 235U in 238U as encountered in depleted uranium. that, based on the rocket equation, the maximum ve- The jar, as pictured, is meant to contain natural ura- locity of the rocket is limited by the exhaust velocity nium that is, this mixture includes only 0.7% 235U. By v of the fuel, and thus there is a need to increase the contrast, if highly enriched uranium were placed in exhaust velocity in order to obtain the optimal per- such a geometry, the hazard of a criticality accident formance. would rise substantially. The figure to the right of the Under practical circumstances there is a limited jar depicts the waste fuel storage, followed by the amount of energy on board, symbolically represented CANDU reactor picture, which is operated by natu- by the uranium in the jar (upper right of Figure 12). ral uranium and heavy water. Next, a light water re- The exhaust speed (and also the mass flow rate) are actor (PWR) is presented. The right-most picture shows adjusted in order to maximize performance (i.e., mini- the principle of operation of nuclear detonation de- mizing fuel consumption) for a given flight trajectory. vices. Underneath these figures, a table of critical masses This translates into a complex, time dependent opti- for various heavy metals actinides used in nuclear mization procedure. power is given, being meant to give a complete image There exist limitations for the usage of nuclear power of the complexity of this domain as well as the chal- in space that are not directly obvious. The relation- lenge of getting it ready for space power applications. ship between the energy stored in the actinide fuel Figure 13 shows (in blue, data points are marked by and its criticality is easy to understand in terrestrial squares) a plot of the total mass of a nuclear structure nuclear power, but the problem of how to maximize (right ordinate, logarithmic scale) versus enrichment the stored plutonium and, at the same time, minimize level. The left ordinate gives the total mass of the fis- the total mass of the storage so far was not of interest, sile material (in red, linear scale) as a function of en- but will become important with regard to space ap- richment level marked by blue dots with error bars. plications. The residual heat generated by nuclear fuel The straight dashed lines that are a fit of the total is a major problem in space applications. Figure 13 nuclear mass data, show the domains of the heavy water shows a chart depicting the use of nuclear power for (D O) moderator, light water moderator (H O), and 2 2 terrestrial applications. It is well known that the ap- for weapons fuel (W). The dashed red curve, repre-

Figure 12 : Nuclear propulsion

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Figure 13 : Total fissile fuel mass limited due to criticality issues on board the space shuttle senting a logarithmic fit, approximates the fissile mass As shown in the lower chart of Figure 14, the ex- needed. In other words, an upper practical limit of pected power consumption of such a device is lower the energy stored would be under 5 GWYear, which than 10 MW. This value is also confirmed by the pre- is equal to the mass of a 1 GW PWR reactor to be vious nuclear programs from the 1950-1970s, where e transported into space, containing, however, highly test power was below 1 MW. With this power avail- enriched uranium (HEU). Such a space vehicle will be able, at a working temperature well over 2,000 K, the difficult to launch into a LEO due to its huge mass mass flow is small and so is the available thrust, but and might have no practical use. the duration of the generated Isp (specific impulse [1/ On the other hand, observing terrestrial applications, s]) might be much larger than for chemical propulsion the higher the operation temperature of nuclear power (several minutes only), resulting in an operation mode sources, the lower is their total power, as can be seen basically defined as continuous operation. from the chart in Figure 14 (lower right). An expo- The novel structures relying on nano-technologies de- nential fit line has been obtained for operation tem- liver electric power that can be optimally used in high peratures between 500 K and 1,000 K. efficiency electric thrusters, delivering the required Similar to Moore’s law used in indus- speed and flow rate. In these systems the velocity dis- try, there is no law of physics to enforce the variation tribution is narrower, and differs from the previously of the power with operation temperature. This effect presented Maxwell distribution. is the result of human perception on the safety of To explore the relationship between ion speed and nuclear power operation that sees in high tempera- operation power, an ideal nuclear reactor is assumed ture material a higher hazard, due to proximity to to be available, either fusion or fission based, that may meltng and evaporation points. Because of the deliver power at will, but of coursetotal energy is fi- Maxwellian speed distribution (upper-left of Figure 14), nite. The main question is how this energy can be for nuclear jet propulsion to become competitive to used in order to provide maximum propulsion per- chemical propulsion, higher operation temperatures formance, in particular, maximizing the vehicle delta are necessary in order to obtain higher molecular ve- v. The calculations are using both energy conserva- locities in the exhaust jet. The upper-right gives the tion and the rocket equation. An energy range be- numerical data for hydrogen, the most common fuel tween 0.1 GWDay and 100 GWDay is used to pro- for space propulsion. The table lists the parameters vide an almost uniform exhaust velocity. for several chemical rocket fuel combinations. It can Ion propulsion seems to be among the most effective be seen that the required operating temperature for a (with respect to fuel mass) propulsion systems, but thermal nuclear device has to be higher than 2,000 K concerning energy effectiveness (beam energy), the to render it superior over chemical propulsion. answer tends to become complicated and a tradeoff

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Figure 14 : Nuclear thermal propulsion benchmark figures

Figure 15 : The dependence of the final speed of a 10-ton shuttle on total jet mass and total jet energy between ejected mass and associated energy becomes plotted versus jet mass with total energy as parameter necessary. (see box above chart). The system considered is a nuclear-based electric power These data were used to calculate the potential (but source that powers an accelerator like jet engine pro- unrealistic) travel time for a round-trip of a space ve- ducing a mono-energetic jet. Under these circum- hicle from Earth to Alpha Centauri, starting from LEO stances, optimum performance is obtained when the (Low Earth Orbit). ejected mass is 2.7 times the payload mass, but good For a 10-ton vehicle returned and parked in LEO, ten results can be achieved in the range of 1 to 10 payload additional tons of fuel (hydrogen) are needed for the masses. braking maneuver when coming back from Alpha Figure 15 shows the spacecraft speed increase (left or- Centauri, which results in a 20-ton vehicle mass dur- dinate) as well as the exhaust velocity (right ordinate) ing the ballistic flight phase. Hence, the mass of a ve-

FP 31 JSE 4(1) 2015 . Full Paper hicle that returns from a LPO (Low Planetary Orbit) The development of new thrusters in the form of elec- at Alpha Centauri is about 20 tons, resulting in a total tric, plasma, or accelerator based devices is pushing mass of 40 tons. However, additional fuel is needed the limits for space travel. These new devices may al- for the braking phase when arriving in the Alpha low for active flight trajectories that are beyond bal- Centauri system, driving the total mass up to 80 tons. listic flight, and will substantially shorten the flight Therefore, the total vehicle mass is brought up to 160 times to neighboring planets. tons (LEO), because a similar acceleration phase is also Advanced nuclear power, will remain the best availabe needed at departure from Earth. This calculation is energy source for space, and if it will be augmented not really realistic, even for an automatic probe, since with advanced propulsion systems, it will make pos- for such a mission a dry mass, including the reactor sible the travel with human crews outside the solar and thrusters, is not sufficient, but the assumption system. should be good enough to be correct within an order Eventually, propellantless propulsion or field propul- of magnitude, and thus provides a crude estimate. sion (a term introduced and discussed already in 1960 by W. R. Corliss 22) (hopefully) might become reality. CONCLUSIONS ACKNOWLEDGEMENTS Nuclear batteries – isotopic, fission, and fusion - are among the very few energy sources that are compact Many thanks go to Dr. Dennise B. Pelowitz for her enough to assure extended space travel and might pro- valuable advice and for the revision of the paper as vide the possibility of colonizing our solar system from well as numerous corections and improvements, and Venus to Jupiter, even including Pluto in the next also to Professor Jochem Hauser for his encourage- two centuries. The first step will be Mars, the nearest ment, advice and corrections sugested in order to im- and most life-friendly planet. prove both style and content of this paper. Novel radiation channeling devices open the way for more advanced shielding for biological life, electron- REFERENCES ics and other sensitive devices, as well as for radiation concentrators for energy harvesting or imaging. [1] J.F.Zigler; Stopping of energetic light ions in Elemental The new developments of quantum-nuclear active en- Matter, Journal of Applied Physics, 85, 1249-1272 vironments, based on long-range nuclear reactions via (1999). entanglement, may open the way for better under- [2] P.B.Hirsch, H.A., R.B.Nicholson, D.W.Pashley, standing of advanced concepts and power production. M.Whelan; Electron microscopy of thin crystals, Butterworths, London, (1965). Fusion-based devices are the only ones that do not [3] S.Bellucci; Nanotubes for particle channeling, radiation have energy storage limitations in space, as is the case and electron sources, Nuclear Instruments and Meth- for fission and radioisotope based devices, delivering ods in Physics Research, B(234), 57–77 (2005). the best solution of generating power when required [4] E.Storms; The of the energy-active state in Pd-D, and producing an almost unlimited amount of energy, Infinite energy, Workshop on the loading of hydro- thus providing the most suitable power source for in- gen/deuterium in metals, Characterization of Mate- terplanetary travel. rials and Related Phenomena, Asti,(Oct 11-13), 77 Isotopic batteries are invaluable devices to power all (1995). kind of satellites, but, because of the half-life time of [5] L.Popa-Simil; “Roadmap to fusion battery – a novel type the respective isotopes, are the major cause for the of nuclear battery and potential outcomes and applica- tions”, ILENRS-12, 1(1), 20 (2012). limited life span of the satellite. [6] Gerard ‘t A.J.Hooft, Gerhard Mack, K.Pronob, Mitter, It was shown that those new devices developed by Raymond Stora; Quantum fields and quantum space using nano-technologies in combination with nuclear time, Physics, B(364) (1996). technology, have many military and strategic applica- [7] Kun Yan; The Tendency analytical equations of stable tions. The opportunities created by this novel, pow- nuclides and the superluminal velocity motion laws of erful technology, which, by itself is not detrimental, matter in geospace. Prog.Geophys, 21, 38-47 (2006). may, however, become dangerous to mankind if ap- [8] Popa L.Simil; Method and device for direct nuclear en- plied by humans of insufficient moral qualities. In ergy conversion in electricity in fusion and transmuta- other words, it is indispensable that the advancement tion processes, Patent Application US20130121449 A1, of technology is accompanied by a similar advance- 80 (2013). [9] P.C.D.Woskov; Vaporize the rock—no drilling required, ment of our civilization level that must take place at Energy Futures, MIT Energy Initiative(Spring), planetary scale. (2012).

FP 32 Full Paper JSE 4(1) 2015 [10] M.Gannon; Asteroid-targeting system could vaporize [18] S.S.B.Anghaie, T.Knight; Direct energy conversion fis- dangerous space rocks, space.com, http:// sion reactor gaseous core reactor with magnetohydrody- www.space.com/19812-asteroid-targeting-vaporize- namic (MHD Generator, DE-FG03-99SF21894, Final space-rocks.html(Feb. 15), (2013). Report, (2002). [11] Jr.J.A.Angelo, D.Buden; Space nuclear power, Malabar, [19] JPL, Prometheus project final report, NASA-JPL, 982- Florida,: Orbit Book Co., Inc., (1985). R120461, (2004). [12] L.Dudinski; Space nuclear power, in short course, [20] De S.Keyser; Orbital selected to build and launch STAIF, Albuquerque, (2004). NASA’s New millennium space technology 8 satellite, [13] General electric company, SP-100 Project Integration, Space Fellowship, (2005). http://spacefellowship.com/ Meeting Notes,. Long Beach, CA July 19-21, (1988). news/art1294/orbital-selected-to-build-and-launch- [14] G.L.Bennett; Space nuclear power: Opening the final nasa-039-s-new-millennium-space-technology-8- frontier, 4th International Energy Conversion Engineer- satellite.html. ing Conference and Exhibit (IECEC), 26-29 June 2006,. [21] Popa L.Simil; Meta-material based nuclear structure San Diego, California, AIAA 2006-4191(Metaspace En- applications in beamed thrust and space energy harvest- terprises, Emmett, Idaho, U.S.A.), (2006). ing, Aerospace Research Central, http://arc.aiaa.org/ [15] Popa L.Simil; The evolution of nuclear power genera- doi/abs/10.2514/6.2011-6039(47TH AIAA/ASME/ tion for mars, in Mars: Prospective Energy and Material SAE/ASEE JOINT PROPULSION CONFER- Resources, V.Badescu, Editor, Springer: Bucharest, ENCE), (2011). (2009). [22] W.R.Corliss; Propulsion Systems for Space Flight, [16] Popa L.Simil; Intergalactic nuclear power systems, 2007 McGraw-Hill, (1960). International Congress on Advances in National Power Plants (ICAPP ’07), Societe Francaise d’Energie Nucleaire (SFEN)(CD-ROM), 7099 (2007). [17] A.H.Shamamian; Coherent radiation of the modulated electron bunch, SPIE - International Conference on Charged and Neutral Particles Channeling Phenom- ena, 5974 (2005).

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