Spontaneous stabilization of HTGRs without reactor scram and core cooling; Safety demonstration tests using the HTTR: Loss of reactivity control and core cooling

Takamatsu, Kuniyoshi  ; Yan, X. ; Nakagawa, Shigeaki  ; Sakaba, Nariaki ; Kunitomi, Kazuhiko 

In this study, an all-gas-circulator trip test was analyzed as a loss of forced cooling (LOFC) test with an initial reactor power of 9 MW to demonstrate LOFC accidents. The analytical results indicate that reactor power decreases from 9 MW to 0 MW owing to the negative reactivity feedback effect of the core, even if the reactor shutdown system is not activated. The total reactivity decreases and then gradually increases in proportion to xenon reactivity; therefore, the HTTR achieves recritical after an elapsed time, which is different from the elapsed time at reactor power peak occurrence. After the reactor power peak occurs, the total reactivity oscillates several times because of the negative reactivity feedback effect and gradually decreases to zero. Moreover, the new conclusions are as follows: The minimum reactor power and the reactor power peak occurrence are affected by the neutron source. The greater the strength of the neutron source, the larger the minimum reactor power.