Analysis of reactivity insertion transients in the ETDR - a small Gas Cooled Fast Reactor

In the framework of the EU FP6 Gas Cooled Fast Reactor (GCFR) project, transient analysis is performed to assess the safety behaviour of the Experimental Technological Demonstration Reactor (ETDR) in its current design stage. Different hypothetical scenarios are investigated, in particular the overpower transient due to progressive or rapid control rod withdrawal. For this purpose, a detailed thermal-hydraulics / 3D neutron kinetics TRACE/PARCS coupled model was developed in the framework of the FAST code system being used at the Paul Scherrer Institute (PSI). Each fuel subassembly is modelled by a heat structure and an associated 1D channel for the coolant. The key static neutronic parameters obtained with PARCS under diffusion approximation agree well with corresponding ERANOS transport calculations. A consistent set of point-kinetic parameters was derived from ERANOS calculations for point kinetics model. The study examines 3D effects through a systematic comparison of point and spatial kinetics for symmetric and asymmetric control rod withdrawal transients. The clad temperatures obtained with the two models, viz. 0D and 3D, reveal local differences of maximum 100 deg. C in case of asymmetric control rod withdrawal, with deviations of ∼50 deg. C for their peaking values. Plant protection parameters, decay heat removal strategy and grace time before clad melting were examined for a wide range of reactivity insertions. (authors)