Modelling and simulation of an unprotected loss of flow accident (LOFA) in MTR research reactors

Unprotected loss of flow accidents (ULOFAs) in research reactors pose significant safety concerns, particularly in scenarios where adequate flow of coolant is lost, and no emergency cooling systems are available to mitigate the situation. This paper presents a computational model to simulate a LOFA in research reactors using the Finite Volume Method (FVM). The study investigates the thermal-hydraulic behavior during a LOFA event, focusing on the reactor core’s temperature evolution, coolant velocity, and pressure distribution. The proposed model integrates flow and heat transfer equations, solving them numerically within a discretized control volume framework. A set of boundary conditions representative of a typical research reactor is used to evaluate the transient response to LOFA. The model provides insights into the thermal response of the reactor under adverse conditions, offering a tool for reactor safety analysis and design improvement. The average channel fuel and coolant temperatures compared with those reported in the literature and the agreement was reasonable. Although the cladding temperature at low flow rates may exceed the design limit, the coolant temperature remains well below its saturation temperature of 115 °C; therefore, no nucleate boiling is predicted in the present ULOFA simulation. The evaluation of CHF and OFI indicates that the coolant channel is OFI-limited at low mass fluxes (<500 kg/m2s) and CHF-limited at higher mass fluxes (>500 kg/m2s). A future study that couples neutronic calculations with the current thermal-hydraulic model could provide further valuable insights.