Part 3: Evaluating a small modular high temperature reactor design during control rod withdrawal and a depressurised loss of coolant accidents

With safety, economics and financial risk playing a large role within the deployment of small modular reactors, it is vital that passive safety features are implemented and the maximum operational temperatures within the design are well understood. This article within the series evaluates the designed 10 MWth U-Battery concept's safety features with respect to a control rod withdrawal and a depressurised loss of coolant accident. A high temperature reactor's passive safety features depend on the Tristructural-isotropic (TRISO) fuels mechanical and thermal integrity and through experimental data this is shown to remain intact up until ~1600 °C (1873 K). This article provides the methodology of how to perform both transients, with the control rod withdrawal being undertaken using point kinetics and the depressurised loss of coolant accident with thermal feedback for both transients being provided via a 1D heat transfer model. Overall, this work has shown that during the control rod withdrawal the fuel temperature rises by 110 K and at this point the excess reactivity is compensated by the negative temperature coefficient of the fuel. During the depressurised loss of coolant accident, the maximum fuel temperature reached 1455 K after 60 h. This concludes that during both transients the temperatures maintained well below the maximum fuel operating temperature.