Evaluation of coated fuel particle temperature during reactivity insertion events

In the High Temperature Gas-cooled Reactors (HTGRs), no adiabatic increase of fuel temperature occurs unless extremely large reactivity is added within a short time even at abnormal condition. Then, in the acceptance criteria of the High Temperature Engineering Test Reactor (HTTR), the maximum fuel temperature is restricted instead of the fuel enthalpy. As one of the studies of fuel behavior under abnormal conditions, temperature transient in coated fuel particle is calculated during reactivity insertion events, and the following results are obtained. (1) Since there is thermal diffusion from fuel kernel to coating layers, the fuel kernel temperature does not increase excessively even under the event of large reactivity insertion rate such as rapid control rod withdrawal accident of HTTR. (2) The maximum fuel kernel temperature is affected by the gap width between the buffer and the inner PyC layers, the composition of gas contained in the gap and the surface area of gap. (author)