Assessment of the integration of a switched nuclear isomer material with a kinematic Stirling engine

Abstract

Presently, power systems are limited by the source and energy density of the chemical fuels that are currently in use. In response to this challenge, nuclear energy sources are being developed. Metastable nuclear states, or isomers, have an intrinsic energy density that is many orders of magnitude larger than chemical fuels. Through isomer depletion (or switching), these materials can be transformed from a long-lived half-life energy-storage state to a shorter-lived half-life energy-releasing state. The abundance of energy released by switched nuclear isomer (SNI) materials introduces unique engineering challenges for the power conversion system. Once an SNI material has been switched, its heat output decays with time. In contrast to hydrocarbon-based fuels, which can be throttled, generating constant power from an SNI system requires sufficient material based on duration of use and system efficiency. Despite these challenges, SNI materials could be used to replace chemical fuels for relatively high-power applications providing months of power from a compact energy source.The purpose of this work is to explore the coupling of an SNI heat source to a kinematic Stirling engine in order to assess the potential of such a system. First, the characteristics of a reference SNI material are discussed in the context of the conversion efficiency and power density of the power conversion system that it is coupled to. Second, the integration of an SNI with a kinematic Stirling engine is explored at two power levels, 6 kWe and 60 kWe, in order to explore the feasibility and efficiency of such a power system. In order to carry out this study, a second-order Stirling engine model (the Stirling Engine Trade-space Tool, or SETT) has been developed. The potential of the SETT tool to predict performance trends has been shown through comparison of the predicted to the measured performance for two baseline, well-documented Stirling engines: the GPU-3 (nominally 6 kW) and Mod II (nominally 60 kW) engines. The SETT model is then used to optimize the design of the integration heat exchanger that is used to thermally couple a Stirling engine to the SNI and evaluate the performance characteristics of this energy conversion system. The resulting model is also used to show that the kinematic Stirling engine has advantages related to its inherent ability to modulate its output through speed or inventory control, allowing it to effectively deal with the time varying rate of heat transfer that is inevitably provided by the SNI. This work shows that the kinematic Stirling engine is an attractive power conversion system to use with SNI material as it can achieve high conversion efficiency, high power density, and provides inherent modulation capability.