Assessment of Local Temperature Reactivity Response in Multi-Module HTGR Special Purpose Reactor

Abstract

HolosGen, LLC (HolosGen) is proposing a highly innovative microreactor concept targeting both civilian and military applications. It consists of an advanced helium-cooled gas reactor using a decoupled and electronically controlled turbojet-type turbine and compressor to achieve a highly condensed reactor that fits into commercial ISO container. The broader scope of this project is to pursue one path to enabling autonomous or semi-autonomous operations through passive systems. To wit, we wish to characterize the local reactivity response to temperature to facilitate the design of passive variable flow controllers that would respond to local temperatures and adjust coolant flow rates that induce a desired reactivity response. The purpose of this milestone is to perform a quantitative assessment of the local temperature reactivity response for the Holos-Quad core reactor concept. Full core Monte Carlo models of the Holos-Quad capturing local temperature distributions, and corresponding SAM models using the local power distributions were developed. A methodology to determine the local fuel and moderator temperature coefficients of reactivity was developed and applied to quantify the local reactivity responses. The results of the analysis indicate that the fuel temperature reactivity is approximately a factor of two greater than the moderator temperature coefficient. This effect is also approximately spatially uniform. The highest worth regions are neither at the periphery or center of the core, but in the regions farthest from the subcritical module boundaries. Collectively this area accounts for about 1/3 of the doppler feedback. Preliminary studies on the control via the conventional mechanical shim suggest that ~50 pcm is needed to perform maneuvers that encompass a range of 20% rated power. To cover 90% rated power would require closer to 300 pcm changes in reactivity. This suggests global temperature changes of ~20K or local temperature variations up to 50K. Therefore, for small power maneuvers in the range of 5% to 10% rated power, the local reactivity control via passive flow controls would be possible if temperature changes of 20K to 50K could be achieved. This initial study along with associated preliminary results for the controllability of the reactor seem to indicate at this point that the development and use of local passive variable flow controllers is feasible from a control standpoint to support small power maneuvers. Future work will include refining the temperature response to smaller regions in the highest worth regions, examining the coefficients at different burnups, and a closer examination of the effect on heat transfer of adjusting the coolant flow rate to produce a change in fuel temperature and moderator temperature.