Reactivity control schemes for fast spectrum space nuclear reactors

Several reactivity control schemes are considered for future space nuclear reactor power systems. Each of these control schemes uses a combination of boron carbide absorbers and/or beryllium oxide reflectors to achieve sufficient reactivity swing to keep the reactor subcritical during launch and to provide sufficient excess reactivity to operate the reactor over its expected 7–15 year lifetime. The size and shape of the control system directly impacts the size and mass of the space reactor's reflector and shadow shield, leading to a tradeoff between reactivity swing and total system mass. This paper presents a trade study of drum, shutter, slat, and petal control schemes based on reactivity swing and mass effects for a representative fast-spectrum, gas-cooled reactor. For each control scheme, the dimensions and composition of the core are constant, and the reflector is sized to provide $5 of cold-clean excess reactivity with each configuration in its most reactive state. Reactivity insertion behavior is analyzed for each control scheme, along with the submersion behavior following a launch abort. The advantages and disadvantages of each configuration are discussed, along with optimization techniques and novel geometric approaches for each scheme.

Research highlights
► Several reactivity control schemes for fast spectrum space reactors are compared.
► Control system geometry directly effects the size and mass of the shadow shield.
► Control shutters provide the overall lowest mass reactor system.
► Control drums result in the least reactivity increase when submerged.