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Evaluation of maximum fuel temperature in HTTR

In order to ensure the thermal integrity of fuel in the high temperature engineering test reactor (HTTR), it is necessary that the maximum fuel temperature in the normal operation is to be lower than a thermal design limit of 1495°C. In the core thermal and hydraulic design of the HTTR, the maximum fuel temperature was estimated to be 1492°C, which satisfied the thermal design limit. However, the estimated temperature was derived by using hot spot factors with a large safety margin for the consideration of uncertainties in the design stage without the HTTR practical operating data, and thus there is no doubt that the estimated temperature includes excessive conservativeness. In order to obtain the maximum fuel temperature with appropriate conservativeness, the maximum fuel temperature has been re-evaluated on the basis of the HTTR operating data. In this paper, the random factors of the hot spot factors are revised by using the HTTR first fuel fabrication data, and the new maximum fuel temperature is estimated. As a result, the estimated maximum fuel temperature can be reduced to 1424°C. The reduction of the maximum fuel temperature leads to a larger thermal margin in nuclear and fuel designs.