Submersion criticality safety of fast spectrum space reactors: Potential spectral shift absorbers

Compact, fast spectrum, nuclear reactors are being considered to support NASA's future space exploration sometime in the next decade. In order to secure launch approval, these reactors should remain sufficiently subcritical when submerged in seawater or wet sand and subsequently flooded, following a launch abort accident. In such an accident, the neutron spectrum in the reactor is thermalized, typically increasing reactivity, and potentially making the reactor supercritical. Incorporating “Spectral Shift Absorbers” (or SSAs), which have significantly higher absorption cross-sections for thermal versus fast neutrons, could offset the reactivity increase. It has always been the assertion that the worst-case submersion accident involves a fully flooded reactor; however, this work shows that, depending on the type and amount of SSA in the reactor, a submerged but unflooded reactor could be more reactive. A screening of the existing nuclear database for potential SSAs yielded 28 elements and nuclides, which are examined in detail as additives to a representative homogenous space reactor core by varying the SSA-to-U²³⁵ atom ratio. The effect of placing a thin coating of different SSA materials on the outside surface of the reactor core is also investigated. Nine SSAs (boron-10, cadmium, cadmium-113, samarium-149, europium-151, gadolinium, gadolinium-155, gadolinium-157, and iridium) are recommended for further consideration in actual space reactor designs.