Analyses of operational transients in the TRADE experimental program

The TRiga Accelerator Driven Experiment (TRADE) consisted of the coupling of an external proton accelerator to a target in a subcritical core configuration of the TRIGA reactor at the ENEA Casaccia Research Center. In the frame of the safety-related evaluations of the TRIGA reactor a large number of experimental transients induced by reactivity insertions were performed in a critical configuration. Power and fuel temperature experimental transients were analysed by several tools and calculated results showed a reasonable agreement with experiments. Meaningful discrepancies in the simulated temperature trends were found systematically in the 300-600 kW power range. In the paper experimental results in this power range are reported and analysed by means of the TIESTE-MINOSSE code. Investigations lead to suppose the existence of the clad superheating phenomenon at the boiling onset (i.e. boiling delay). On the basis of the studies performed in the past about this subject, the code was improved to take into account this phenomenon. The new simulation results showed a reasonable agreement with the experimental fuel temperature. Results also clearly indicate the local origin of this phenomenon.