Meeting fuel temperature limits in an HTR-module reactor during depressurized core heat-up

The thermal behaviour of an HTR-Module Reactor is discussed for the design basis event of core heat-up after fast depressurization taking into account the most unfavourable initial state and uncertainties of input data. The reactor is designed to retain its fission products inside the fuel coatings even if all active components for decay heat removal and reactivity control should fail. To meet this goal maximum fuel temperatures during core heat-up should not exceed the technological limit of 1620deg C, for which the integrity of the fuel coatings has been proven experimentally. Two-dimensional thermal-hydraulic calculations show that the maximum fuel temperature during core heat-up is expected to be 1472deg C taking into account nominal full power operation as an initial state, a sudden depressurization in the beginning of the event, and nominal input data. The most unfavourable initial state is the steady state operation close to the scram set points, i.e. 105% power and increased cold and hot gas temperatures. Accounting for this leads to a maximum fuel temperature of 1522deg C. Relevant uncertainties of input data are those of decay heat production, power distribution and core thermal conductivity and specific heat capacity. Their individual standard deviations can be combined to an integral uncertainty margin of ± 86 K which covers two standard deviations. Hence the maximum fuel temperature taking into account unfavourable initial state and uncertainties is 1608deg C. (orig.)