Experimental validation of effectiveness of rod-type burnable poisons on reactivity control in HTTR

Goto, Minoru  ; Shiozawa, Shusaku; Fujimoto, Nozomu; Nakagawa, Shigeaki  ; Nakao, Yasuyuki*

In block type high temperature gas-cooled reactors (HTGRs), insertion depth of control rods (CRs) into a core should be retained as shallow as possible to keep fuel temperature below limit through a burnup period. Using burnable poisons (BPs) to control reactivity is considered as a method to resolve this problem as in case of light water reactors (LWRs). BPs design method for LWRs has been validated by experimental data, however, that for HTGRs have not been yet, because there was not burnup characteristics data of HTGRs required for the validation. The High Temperature engineering Test Reactor (HTTR) is a block type HTGRs and it uses BPs to control reactivity. The HTTR has been operated up to middle burnup, and thereby the experimental data was expected to show effect of the BPs on the reactivity control. Hence, in order to validate the BPs design method, we investigated whether the BPs have functioned as designed. As a result, the CRs insertion depth has been retained shallow within allowable range, and then the BPs design method was validated.