A Comparison of the Radiation Shielding Requirements of Select Radioisotopes for Potential Use in Radioisotope Power Systems

Supplies of ²³⁸Pu, one of the most widely used radioisotope power system fuels, have steadily decreased since the end of the Cold War. This has prompted the recent investigation of several radioisotopes as alternative fuels. In this work, the radiation shielding requirements of seven candidate radioisotope power system (RPS) fuels with relatively high thermal heat generation from alpha and beta decay were compared to ²³⁸Pu. The candidate radioisotope battery fuel isotopes include ²⁴¹Am, ⁹⁰Sr, ²⁴⁴Cm, ²²⁷Ac, ²²⁸Ra, ²²⁸Th, and ²³²U. The amount of each fuel was standardized to 100 W_Th, and the SCALE suite of codes, ORIGEN and MAVRIC, were utilized to perform the radiation transport simulations for each type of penetrating ionizing radiation (i.e., gamma, neutron, bremsstrahlung) emitted to the surrounding environment. Several types of shielding were considered, including graphite, depleted uranium, and mixed metal and graphite composite shields. ²⁴¹Am, and ²³⁸Pu required no shielding to reduce the dose below 100 mrem/hr on contact. Ac-227 and Sr-90 required an intermediate amount of shielding (approximately 8 kg and 40 kg, respectively) to reach the same dose threshold, and the other isotopes required a large amount of shielding (varying from 100-1000 kg of total mass). The isotopes that require heavy shielding (i.e., ²⁴⁴Cm, ²²⁸Ra, ²²⁸Th, and ²³²U) are recommended for applications where weight is less of a mission constraint.