Thermal-hydraulic simulation of loss of flow accident for WWR-S research reactor

A thermal-hydraulic model has been developed by RELAP5 code to simulate the thermal-hydraulic behavior of a WWR-S research reactor under loss of flow accident (LOFA). The reactor power is 2 MW with downward flow direction and different types of fuel bundles of different power densities and different coolant flow-rates. The simulation is performed for two scenarios; protected and unprotected LOFA. In the protected LOFA scenario; Scram is triggered as soon as the coolant flow rate reaches 85 % of its nominal value while in the unprotected LOFA scenario; the reactor continues operation during pump cost-down and natural circulation flow. Once the flow is low enough, the buoyancy force increases in the core leading to flow inversion phenomenon and establishment of a natural circulation mechanism within the core coolant channels in both scenarios. In the protected LOFA scenario, the coolant remains subcooled by a vast margin during transient while bulk boiling is predicted at the upper part of the core for the unprotected LOFA scenario. The heat fluxes leading to the onset of nucleate boiling and the critical heat flux are predicted and the safety margins are determined. The results for both scenarios are analyzed and discussed.