Development of a Multiscale Analysis Methodology for Analysis of Transients in a Small Lead Cooled Advanced Reactor

Abstract

In the recent years there has been growing interest in small modular reactors (SMRs). Before this type of reactors are deployed it is necessary to assess their safety with the newest available tools. This thesis focuses on one SMR, the SEALER reactor, which is a 3 to 10 MWe lead cooled reactor intended for remote communities or mines. The designer of the SEALER reactor has previously identified a possible issue during a loss of flow transient. At the beginning of the transient, the mass flow at the pumps undergoes a fluctuation that could lead to reverse flow if amplified. The first part of the thesis was to perform an uncertainty and sensitivity analysis using an existing lumped-parameter model. Two types of transients were studied: unprotected loss of flow and unprotected overpower. Results show that, for both transients, temperatures remain well below safety limits for the entire parameters space. However, it is also found that reverse flow at the pumps is possible by changing some parameters in a realistic way. It was therefore decided to develop a more realistic model to study the same transients, which constitutes the second and main part of the thesis. The new model uses CFD for simulating the flow of coolant in the entire primary circuit. The complex components (fuel channels, pumps and steam generators) are replaced with a simple geometry and appropriate heat and momentum sources/sinks. The CFD simulation is coupled with a custom-made code for solving heat transfer in the fuel pins and to point kinetics for neutronics. To demonstrate the viability of the model, a validation exercise was performed to ensure that the CFD part is able to reproduce experimental data with important features, like temperature stratification and a jet into a plenum. Results from the new model confirm the mass flow fluctuation if a small pump flywheel is used. For a flywheel of reasonable size, transients are slow without any mass flow fluctuation, and temperature variations are small.