Analysis of a research reactor under anticipated transients without scram events using the RELAP5/MOD3.2 computer program

Abstract

Simulations for two series of anticipated transients phics. without scram (ATWS) events have been carried out for a small, hypothetical, research reactor based on the High Flux Australian Reador HIFAR using the RELAPS/MOD3.Z computer program. The first series simulated the ATWS events associated with pump transients and the second series simulated the ATWS events associated with heat exchanger transients. In the first series, two cases were simulated: (1) the non-availability of one of the two primary pumps and (2) the non-availability of one pump and 50 % operation of the other pimp. In the second series, three cases were simulated: (1) the non-availability of one of the three heat enchanters (2) the non-availability of two of the three heat enchanters and (3) the non-availability of all the three heat enchanters. To carry out the above simulations, modifications were incorporated to the critical heat flux (CHF) correlations of the RELAP program to account for the annular geometry of the fuel elements. The results of the simulated plump transients indicated that the mass flow rate would decrease to 270 Kg/s in the first transient and to 133 Kg/s in transient 2. The maximum rise in the coolant temperatures in the primly circuit would be 50C and the maximum rise in the central fuel element temperature would be 23[]C. The results of the first two cases of the heat exchanger transients indicated that the maximum rise in the coolant temperatures in the primary circuit would be 18.5[]C and the maximum rise in the central fuel element temperature would be 6[]C. However, during the simulation of the non-availability of all the three heat enchanters, primary coolant temperature started increasing rapidly. Void generation occurred in the central fuel element annuli at 408.5 seconds into the transient with saturation conditions being attained in the annulus. The phase change resulted in the expulsion of fluid from the annulus. Consequently, temperature in the second fuel ring exceeded 800K, resulting in its failure.