Pre‐Orbital Criticality Safety for the Proposed NEPSTP Mission

In December 1991, the Strategic Defense Initiative Organization (SDIO) proposed investigating whether launching a Russian Topaz‐II space nuclear power system could be done safely and within budget constraints. Functional safety requirements developed for the potential US space application of the Topaz‐II mandated that the reactor remain subcritical when immersed in water. Topaz‐II is an epithermal, enriched‐uranium‐fueled, NaK‐(liquid metal alloy with 22% sodium and 78% potassium) cooled, and zirconium hydride‐moderated reactor. A radial beryllium reflector containing 12 rotatable control drums surrounds the core. We prepared a computer model of the Topaz reactor that explicitly represented all major reactor components. Initial analyses indicated that in several water‐immersion scenarios, the reactor would not remain subcritical. After additional calculations, modifications were proposed that would assure subcriticality under such conditions. This paper describes the analyses and the proposed modifications.