Safety Aspects of Nuclear Battery Reactor Design

Abstract

This paper describes the inherent safety features of a small, solid-state, passively cooled reactor power supply known as the Nuclear Battery. This reactor is intended to produce approximately 2.4 MW of thermal power for a lifetime of about 15 full-power-years, with a very high level of safety and reliability. The distinctive safety features of the Nuclear Battery concept include: The TRISO-coated particle fuel provides the primary containment envelope against the release of fission products under all conceivable accident conditions; The intrinsic negative temperature coefficient of the solid graphite moderator and fuel provides self-regulation of the reactor power level to match the available capacity for heat removal; The solid graphite moderator does not undergo a phase change over the full range of temperatures encountered in reactor accidents or upsets. Furthermore, the moderator temperature responds slowly to power changes as a result of its large heat capacity; Burnable poisons are used to reduce the variation of the core reactivity during the reactor lifetime. The small residual reactivity variation is compensated by changes in fixed shim devices at intervals of a few years and by regular movements of a control rod of low reactivity worth to maintain a constant operating temperature. Minimizing the available excess reactivity ensures that the theoretical possibility of a prompt criticality accident can be eliminated once the reactor has reached its normal operating temperature and power level; Decay heat removal from the reactor is provided passively by conduction and natural convection losses; Primary heat transport occurs passively through natural circulation in multiple, independent and redundant heat pipes; The reactor core is maintained in an inert helium environment at a low pressure of about one atmosphere and is enclosed within a steel containment vessel.