Isotopic Inventories in Thermal Neutron Spectrum Reactor Designs Suitable for Direct Nuclear Thermal Propulsion

Abstract

Advancement of U.S. scientific, security, and economic interests through a robust space exploration program requires high performance propulsion systems to support a variety of robotic and crewed missions beyond low Earth orbit. Past studies, in particular those in support of the Space Exploration Initiative (SEI), have shown nuclear thermal propulsion systems provide superior performance for high mass high propulsive delta-V missions. The recent NASA Design Reference Architecture (DRA) 5.0 Study re-examined mission, payload, and transportation system requirements for a human Mars landing mission in the post-2030 timeframe. Nuclear thermal propulsion was again identified as the preferred in-space transportation system. An extensive nuclear thermal rocket technology development effort was conducted from 1955-1973 under the Rover/NERVA Program. Both graphite and refractory metal alloy fuel types were pursued. Recent activities have included parallel evaluation and design efforts of engine concepts based on both fuel types. Isotopic inventory changes in a propulsion reactor can influence multiple aspects of engine operation. Two primary areas of concern are changes in engine reactivity and in the radiation environments external to the engine. The magnitudes of the impacts vary depending on the reactor type (fast neutron spectrum or thermal/epithermal neutron spectrum), engine operating times, cooling times since last engine operation, and engine design and operating mode. Engines can provide direct nuclear thermal propulsion only or provide some combination of direct propulsion and electrical power generation. Operating times are usually short for engines employed solely for direct nuclear thermal propulsion. Reactivity losses are usually low and are due to a combination of fissile depletion and fission product absorption. These small reactivity losses due to depletion can be accommodated by control drum rotation, but drum rotation also results in core power distribution changes that can lower engine performance. Engines providing electrical power generation will operate for longer periods and reactivity losses due to fissile depletion and fission product absorption will be higher. Engine systems employing fast spectrum reactors show lower sensitivity to fission product buildup than those employing thermal neutron spectrum reactors. Regardless of engine type or operating mode, fission product and heavy metal isotopes are the primary neutron and gamma sources and establish the radiation environment and biological dose rates near the engine. This paper addresses isotopic changes in a representative thermal neutron spectrum engine. Fission product and heavy metal isotopes important to neutron absorption are identified and ranked at selected times during engine operation. Fission product and heavy metal isotopes important to dose contributions around the engine are identified and ranked at selected times during and after operation.