Comparative study on HTGR designs for passive decay heat removal

•The sensitivity of peak temperatures does not depend on the reactor location.•Reactor design parameters' relationships for safe core are the same for two reactors.•The major parameter for practical-size above-ground RB is a core temperature.•Reactor power is the most influential factor for a reasonable-size underground RB.

The design parameters of a small high-temperature gas-cooled reactor (HTGR) for passive decay heat removal and located either above-ground or underground have been studied in previous works. In the present work, sensitivity analyses on peak temperatures of both the core and reactor building (RB) after reactor shutdown was conducted for both situations. It was found that the sensitivity of peak temperatures was quite similar for the two situations of the reactor, i.e., above- and below-ground. Therefore, a parametric survey on design parameters for long-life small prismatic HTGRs having features for successful decay heat removal using fundamental heat transfer mechanisms was performed for both reactor concepts. Comparison of the two conditions showed that a passively safe, low-power, underground reactor operating at high initial core temperature could have a practical size of RB. Conversely, for the RB size of an above-ground reactor with passive safety features to be reasonable, it could operate at relatively high power but would need a low initial core temperature. The appropriate sizes of reactor core and RB were evaluated for reactors with powers of 50 MWth, 100 MWth and 300 MWth having different maximum power densities and operating at temperatures of 1123 K, 973 K, and 573 K.