Static and Dynamic Neutronic Analysis of a Burst-Mode, Multiple-Cavity Gas Core Reactor, Rankine Cycle Space Power System

Abstract

Static and dynamic neutronic analyses have been performed on an innovative burst-mode (hundreds of megawatts output for a few thousand seconds) Ultrahigh-Temperature Vapor Core Reactor (UTVR) space nuclear power system. This novel reactor concept employs multiple neutronically coupled fissioning cores and operates on a direct closed Rankine cycle using a disk magnetohydrodynamic generator for energy conversion. The UTVR includes two types of fissioning core regions: (a) the central Ultrahigh-Temperature Vapor Core (UTVC), which contains a vapor mixture of highly enriched UF4fuel and a metal fluoride working fluid and (b) the UF4 boiler column cores located in the BeO moderator-reflector region. The gaseous nature of the fuel, the fact that the fuel is circulating, the multiple coupled fissioning cores, and the use of a two-phase fissioning fuel lead to unique static and dynamic neutronic characteristics.Static neutronic analysis was conducted using two-dimensional Sn transport theory calculations and three-dimensional Monte Carlo transport theory calculations. Circulating-fuel, coupled-core point reactor kinetics equations were used for analyzing the dynamic behavior of the UTVR. In addition to including reactivity feedback phenomena associated with the individual fissioning cores, the effects of core-to-core neutronic and mass flow coupling between the UTVC and the surrounding boiler cores were also included in the dynamic model.The dynamic analysis of the UTVR reveals the existence of some very effective inherent reactivity feedback effects that are capable of quickly stabilizing this system, within a few seconds, even when large positive reactivity insertions are imposed. If the UTVC vapor-fuel density feedback is suppressed, the UTVR is still inherently stable because of the boiler core liquid-fuel volume feedback; in contrast, suppression of the vapor-fuel density feedback in “conventional” gas core cavity reactors causes them to become inherently unstable. Because of the strength of the negative reactivity feedback in the UTVR, it is found that external reactivity insertions alone are inadequate for bringing about significant power level changes during normal reactor operations. Additional methods of reactivity control, such as variations in the gaseous fuel mass flow rate, are needed to achieve the desired power level control.