Burn-up characteristics and criticality effect of impurities in the graphite structure of a commercial-scale prismatic HTGR

Highlights: • We evaluate the criticality effect of impurities in graphite block for HTGR by whole core burn-up calculations. • We confirm the boron equivalent can be also employed for burn-up calculations. • We conclude fine purified grade graphite IG-110 is not necessary from the viewpoint of criticality. - Abstract: This study investigates the burn-up characteristics and the criticality effect of impurities in the graphite structure of commercial-scale prismatic High Temperature Gas-cooled Reactor (HTGR), and thereby reconsiders the necessity of high-grade graphite material. In an HTGR, the core is filled with the graphite, and the impurities in the graphite have a non-negligible poison effect on the criticality. To account for the effect of the reflector blocks deployed adjacent to the fuel blocks, GTHTR300, commercial-scale HTGR, employed fine purified grade graphite material IG-110. Ideally, the fuel blocks should also employ IG-110; however, for economic purposes they are constructed from an un-purified grade graphite material IG-11. The poisoning effect of the impurity (which behaves like ¹⁰B burn-up and is expressed in boron equivalents) decreases exponentially and eventually saturates at 1% of the initial boron equivalent. However, the reactivity worth of the fuel and reflector blocks with 0.03 ppm boron equivalents (equivalent to 1% of IG-11) is negligible (i.e., k/kk′). Because the poisoning effect of the impurity mimics that of naturally occurring boron, it was evaluated in whole-core burn-up calculations with the impurities represented by naturally occurring boron. According to the results, the criticality of the commercial-scale HTGR is unaffected by the impurity levels (even in the un-purified grade IG-11) because the impurities burn cleanly until the End of Cycle (EOC). Therefore, the economy of electricity generation by HTGRs can be improved by using the un-purified grade IG-11 instead of the fine purified grade graphite IG-110.