Experimental studies of GCFR safety physics parameters in the ZPR-9 critical assemblies

Physics parameters related to the safety analysis of the Gas-Cooled Fast Reactor (GCFR) have been measured in a series of critical assemblies constructed on ZPR-9. These measurements represent the first full-scale experimental study of GCFR physics. The critical assemblies were modelled after the 300 MWe GCFR Demonstration Plant designed by General Atomic Company. The specific integral parameters studied were the coolant (helium) depressurization worth, the 238U Doppler effect, control rod worths and the reactivity worth of materials of importance to GCFR safety. An extensive study was made of the effects of steam entry in a GCFR core. The results showed that the helium depressurization worth is small and considerably overpredicted (by approximately 35 percent) by standard methods, and that the GCFR spectrum was harder than corresponding LMFBR spectra as evidenced by the decreased 238U Doppler effect and control rod worths and central reactivity worths. Steam entry in the critical assembly was shown to result in a positive reactivity insertion and cause a large change in the magnitudes of the safety related physics parameters. The results provide a basis for validating the methods used for GCFR safety analyses