Radioisotope power systems (RPSs) have transformed our ability to explore the solar system. RPSs have been in existence for almost seven decades. Most missions have utilized ²³⁸Pu as the radioisotope of choice to generate electrical power and to produce heat for the operation and thermal management of spacecraft systems. In Europe, for the past decade ²⁴¹Am has been selected for RPS research programs. This paper hypothesizes that the inclusion of small quantities of relatively short-lived radioisotopes such as ²³²U and ²⁴⁴Cm, particularly when dealing with long-lived radioisotope ²⁴¹Am, could have beneficial implications for future RPS designs. This paper focuses on the thermal output implications and impact on system-level design. The authors recognize that the selection of any new or modified radioisotope heat source material will require extensive research on fuel form stability, the radiological impact, cost of production, containment, and launch safety considerations.

The European Space Agency is funding the research and development of ²⁴¹Am-bearing oxide-fuelled radioisotope power systems (RPSs) including radioisotope thermoelectric generators (RTGs) and European Large Heat Sources (ELHSs). The RPSs’ requirements include that the fuel’s maximum temperature, T_max, must remain below its melting temperature. The current prospected fuel is (Am_0.80U_0.12Np_0.06Pu_0.02)O_1.8. The fuel’s experimental heat capacity, C_p, is determined between 20 K and 1786 K based on direct low temperature heat capacity measurements and high temperature drop calorimetry measurements. The recommended high temperature equation is C_p(T/K) = 55.1189 + 3.46216 × 10² T − 4.58312 × 10⁵ T⁻² (valid up to 1786 K). The RTG/ELHS T_max is estimated as a function of the fuel thermal conductivity, k, and the clad’s inner surface temperature, T_{i cl}, using a new analytical thermal model. Estimated bounds, based on conduction-only and radiation-only conditions between the fuel and clad, are established. Estimates for k (80–100% T.D.) are made using C_p, and estimates of thermal diffusivity and thermal expansion estimates of americium/uranium oxides. The lowest melting temperature of americium/uranium oxides is assumed. The lowest k estimates are assumed (80% T.D.). The highest estimated T_max for a ‘standard operating’ RTG is 1120 K. A hypothetical scenario is investigated: an ELHS T_{i cl} = 1973K-the RPSs’ requirements’ maximum permitted temperature. Fuel melting will not occur.

E3S Web of Conferences, open access proceedings in environment, energy and earth sciences