Design of separate-effects In-Pile transient boiling experiments at the TREAT Facility

The cladding-to-coolant heat transfer during rapid power excursions, such as reactivity-initiated accidents, remains a crucial area of uncertainty in nuclear reactor safety. This uncertainty impacts the ability to accurately predict fuel performance behavior for these conditions. Improving our understanding of transient cladding-to-coolant heat transfer will enhance our ability to model design-basis accidents and could increase design and safety margins for the current commercial fleet and advanced reactors. To address these issues, the Critical Heat Flux Static Environment Rodlet Transient Test Apparatus experiment was designed and uses a novel approach with a borated stainless-steel heater rodlet that can replicate cladding temperatures experienced during a design-basis reactivity-initiated accident. The design of the rodlet allows for separate-effect testing that eliminates complexities with a fuel and cladding specimen. The rodlet and experiment is instrumented to provide temperature and thermal–hydraulic conditions throughout the transient. The experiment results show that the current correlations in RELAP5-3D are conservative and predict much higher temperatures than measured during the experiments. The data from the experiments will be used to improve our models and understanding of boiling behavior, specifically critical heat flux, under transient conditions.