DEPRESSURIZATION ACCIDENT ANALYSIS FOR THE HTTR BY THE TAC-NC

The two-dimensional thermal analysis code TAC-NC is modified from the analytical code TAC-2D in order to calculate temperature transients in the case of loss of forced cooling accidents of the HTTR (High Temperature engineering Test Reactor) such as a depressurization accident. The TAC-NC code includes a special function to calculate heat transfer by natural convection between hotter and colder regions in the pressure vessel during the depressurization accident. Verification of the TAC-NC code was carried out by the comparison of the analytical results with the experimental ones of an air ingress test. Analytical results of simulated core temperature were in good agreement with experimental results. Temperature transients during a depressurization accident were evaluated by the TAC-NC code for the HTTR. The maximum fuel temperature decreases rapidly after the reactor scram and increases slightly after that due to decay heat. The maximum fuel temperature, however, remains below the initial maximum fuel temperature since most of the core decay heat is absorbed in the large thermal capacity of graphite in the core and reflector.