Pre-conceptual high temperature gas cooled microreactor design utilizing two-phase composite moderators. Part II: Design space and safety characteristics

Abstract

This work expands on a pre-conceptual microreactor design based on a High Temperature Gas-Cooled Reactor (HTGR) platform generated in the companion paper of this study. Here, power, coolant inlet temperature, and coolant outlet temperature are varied to outline an operable design space associated maximum allowable component temperatures and coolant pressure drop for a given microreactor design. Calculations show that beryllium- and hydride-based composite moderators have a smaller operating design space than graphite because of a lower allowable moderator temperature under the conservative upper limits employed for this study. Power below 6MWth was considered to be undesirable for all designs because of a limited inlet/outlet temperature range. Reactivity Temperature Coefficients (RTC) are calculated to illustrate an inherent safety feature of HTGRs and are fed into a Depressurized Loss of Forced Coolant (DLOFC) without active intervention accident simulation, considered to be a bounding case for fuel temperatures in a HTGR. RTC calculations showed that all designs would expect to have a negative isothermal coefficient through their entire operating cycle and DLOFC calculations showed no case exceed their maximum temperature limits. An additional accident that simulates the release of fission products is performed to compare expected minimum Emergency Planning Zone (EPZ) size for all designs. EPZ calculations showed a marginally smaller achievable size for the composite moderators because of a lower concentration of relevant nuclides due to a more thermal spectrum