Reactivity Control Schemes for Fast Spectrum Space Nuclear Reactors

Abstract

Several different 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, 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. The advantages and disadvantages of each configuration are discussed, along with optimization techniques and novel geometric approaches for each scheme.

Advanced Shield Development for a Fission Surface Power System for the Lunar Surface

“…Optimization includes adjusting the thickness and positions of the various layers of shield material as discussed previously. Also, particular efforts are focused to attenuate radiation only in the direction where shielding is required, similar to a shadow shield concept [4]. For example, the 2 Mrad gamma dose requirement to the SmCo magnets requires more tungsten above the core than elsewhere for other dose requirements; thus, the tungsten layers above the core are tailored specially for this dose requirement.…”

“…For space reactor systems, a radiation shield is necessary to protect the payload (whether electronic or human) from excessive radiation doses. This radiation shield is typically the most massive component of a space reactor system, which directly impacts the launch cost and the affordability of the system [4,5]. Thus, there is a need to optimize the reactor's radiation shield in order to obtain the most massefficient system.…”

Optimization of the reactivity control drum system of ELECTRA

“…Noting that a similar research project carried out for a space reactor design (Craft et al, 2011) showed that 90-120º absorber angle with 1-2 cm thickness provides efficient and high reactivity, we investigated optimal drum configurations around this range.…”