atw Vol. 65 (2020) | Issue 4 ı April Serial | Major Trends in Energy Policy and Nuclear Power Research in Support of European Radio- 198 isotope Power System Development at the European Commission’s Joint Research Centre in Karlsruhe Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings 1 Introduction The urge to discover the unknown, to explore the unexplored and to broaden our knowledge beyond the limits of the present is inherent to human nature. One of the most interesting and fascinating fields of science is the exploration of the cosmos, either from Earth using telescopes or by sending automated probes to other planets and into the vastness of space. scientific instruments on the Moon enables long-lasting missions [20]. (Figure 1), and they were used for Unfortunately, there is a global many of the most famous and exiting shortage of this isotope, the efforts exploratory endeavours, such as associated with its production are the Pioneer missions to Saturn and high [21] and there are currently no Jupiter [9,10], the Viking missions to facilities for its synthesis in Europe. Mars [11], and the Voyager 1 & 2 An alternative is the americium spacecraft, which travelled beyond isotope Am-241, which is more easily the boundaries of our solar system available, since it is produced through and are still delivering scientific decay from Pu-241 and can be results from the interstellar medium, extracted isotopically pure from more than 40 years after their launch existing stocks of civil plutonium in [12,13]. More recently, the radio- France or the United Kingdom via isotope- powered missions Galileo, chemical extraction [22]. Therefore, Ulysses and Cassini-Huygens were ESA has decided to study the use of exploring Jupiter, the Sun and Am-241 for its RPS development Saturn/Titan, respectively. These [5,16,17]. However, Am-241 has some more recent missions were performed disadvantages compared to Pu-238, in collaboration between the National such as a lower power density of | Fig. 1. Aeronautics and Space Administra- 0.114 W/g and slightly higher radia- Apollo astronaut photo of a SNAP-27 RTG on the Moon. Photo: NASA. tion (NASA) and the European Space tion levels. In addition, the oxide Agency (ESA) [14]. Historically, this shows chemical instability at high A basic requirement to operate auto- was the only way for Europe to gain temperatures, the experience with mated spacecraft successfully over the access to space nuclear power systems. Am241 is limited and additional course of an exploratory mission, is In 2005 a European Working Group research and safety assessment is the reliable supply of long-lasting on Nuclear Power Sources for Space needed before it can be used for space power. If independence of solar radia- identified RPS as a “key enabling applications. tion is required, e.g. when travelling technology for future European activi- Within the ESA research pro- SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER into deep space or to the dark side of ties in space” [15], and suggested the gramme, the UK’s National Nuclear planetary bodies, nuclear energy establishment of an European safety Laboratory (NNL) is exploring the becomes advantageous compared to framework for space nuclear power cost effective production of Am-241 other potential sources of energy. The sources and the development of and the University of Leicester (UoL) utilisation of nuclear power for appli- the technical capabilities to perform is developing a European Radio- cations in space had been considered nuclear powered missions indendently isotope Heater Unit (RHU) and Radio- since the early beginnings of space [16,17]. As a consequence, a research isotope Thermoelectric Generator flight in the late 1940s, and the first and development programme was (RTG) [16,23,24]. To support these Radioisotope Power System (RPS) in launched by ESA for the production of efforts, the European Commission’s space was already launched by the European RPS to satisfy thermal Joint Research Centre (JRC) in Karls- U.S. Navy in 1961, onboard the Transit management and electrical power ruhe is investigating methods to 4A navigational satellite [1,2]. Since needs for spacecraft [16,17,18,19]. stabilize americium in the oxide then RPS have enabled some of the In the past, most RPS for space form and to establish a safe and most spectacular missions in the missions were based on the plutonium reliable pelletizing process [20,25]. history of space exploration [1-7], isotope Pu-238 [1-5], a radionuclide In collaboration with UoL, safety mostly performed by the USA but also which is superior to other isotopes, relevant properties and behaviour of by the former Soviet Union, China and because of its high specific power of Americium oxide are assessed under Europe (via cooperation with the 0.567 W/g, low radiation, compa­ representative conditions for storage USA). RPS were used on satellites for tibility with cladding materials and on Earth, operations in space as navigation, meteorology and commu- chemical stability as oxide. Pu-238 well as hypothetical accident and nication [8], they have powered has a half-life of 87.7 years which post- accident environments [16,26], Serial | Major Trends in Energy Policy and Nuclear Power Research in Support of European Radio isotope Power System Development at the European Commission’s Joint Research Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings atw Vol. 65 (2020) | Issue 4 ı April and the compatibility with the clad- used when onboard power is needed Energy source Energy density, MJ/kg ding material is tested. Complementa- for no more than a few weeks, or as 2 H + O 13 33 ry work is performed to develop a rechargeable energy buffer to supply 2 2 199 qualified welding methodology of the peak loads or to bridge periods with- N2H4* + O2 9 75 safety encapsulation. out sunlight. For missions, which 2 Li + O2 12 2 require continuous power supply for Li-ion battery 0 9 2 Energy Supply in Space an extended time, only solar or In order to operate spacecraft, a nuclear energy sources are feasible, Fission of U-235** 8 2 · 106 reliable source of power in the form of since the payload associated with Decay of Pu-238*** 3 3 · 105 electricity and heat is required. chemical fuels or batteries would Decay of Am-241*** 7 1 · 104 Electricity is needed to power onboard simply become too high. electronic systems such as navigation Solar energy is principally un- | Tab. 1. and manoeuvring systems, onboard limited, as long as the solar cells, used Energy densities of typical energy sources. computers, lighting, robotics, scien- to convert the radiation energy into *Hydrazine, **total fission, ***over 20 years mission time tific instruments and communication electricity, are not degrading, and as systems. In some cases, spacecraft are long as they can be adjusted in the controlled fission of fissile isotopes, equipped with electric propulsion direction of the sunlight and the such as U235 or Pu-239, and Radio- systems, and electricity for life support spacecraft is not in the shadow of isotope Power Sources or Systems is needed if the mission is manned. planetary bodies or too far away from (RPS), which obtain their energy from Heat is needed in cold environments the sun. However, their effective area, the spontaneous decay of radioactive to keep sensitive spacecraft com- the conversion efficiency and the isotopes. Both types can generate ponents at minimum operational or intensity of the solar radiation deter- heat for temperature control and/or survival temperature, e.g. during mine the power density of solar cells. electricity via additional energy lunar nights. This intensity decreases inversely with conversion systems. Primary energy sources can be the square of the distance from the While nuclear reactors are chemical, solar or nuclear. Some of sun, as shown in Figure 2, and if the generally suited for applications, these are limited with respect to their distance becomes too large solar which need significant power levels power or energy density, and the energy becomes unpractical. For above 10 kW, RPS are employed profile of each individual mission example, while the solar constant is whenever a limited amount of solar- determines which energy sources can 1.367 kW/m2 at the semi-major axis of independent power, up to 5 kW, is be utilized. Table 1 shows typical Earth, at one Astronomical Unit (AU) needed for a longer time period. RPS energy densities for different energy distance to the sun, it decreases are compact, long-lived, reliable, sources. to only 51 W/m2 or 3.7 % at the semi-­ robust, radiation resistant, solar- Chemical energy sources in the major axis of Jupiter (5.2 AU). There- independent, maintenance-free, and form of solid or liquid fuels can release fore, solar arrays are not an option for they have energy densities, which are a large quantity of energy in a very all research missions to the outer solar several orders of magnitude above short time, but they are limited with system, but also not if a system is to be chemical power sources (Table 1) respect to total energy density. For in- operated during long periods of dark- [1,2,3]. Figure 3 shows qualitatively stance, chemical fuels for propulsion ness, for example during the lunar the different regimes of power levels can be used whenever high thrust is nights, which last 14 days. and durations, where different energy needed for a short time, e.g. as rocket Nuclear energy has the highest sources are applicable. fuel to overcome the gravity field of power densities of all possible on- Space power systems, which are earth, but they have shortcomings if board energy sources, and can deliver based on the decay heat of radio- long-lasting power or long accelera- reliable power over very long time isotopes, can be distinguished into tion times are needed, e.g.