Prediction, analysis and solution of the flow inversion phenomenon in a typical MTR-reactor with upward core cooling

Research reactors of power greater than 20 MW are usually designed to be cooled with upward coolant flow direction inside the reactor core. This is mainly to prevent flow inversion problems following a pump coast down. However, in some designs and under certain operating conditions, flow inversion phenomenon is predicted. In the present work, the best-estimate Material Testing Reactors Thermal-Hydraulic Analysis program (MTRTHA) is used to simulate a typical MTR reactor behavior with upward cooling. The MTRTHA model consists of five interactively coupled submodels for: (a) coolant, (b) fuel plate, (c) chimney, lower plenum, suction box and cold leg, (d) flap valve and (e) natural circulation flow. The model divides the active core into a specified axial regions and the fuel plate into a specified radial zones, then a nodal calculation is performed for both average and hot channels with a chopped cosine shaped heat generation flux. The reactor simulation under loss of off-site power is performed for two cases namely: two-flap valves open and one flap-valve fails to open. The simulation is performed under a hypothetical case of loss of off-site power. Unfortunately, the flow inversion phenomenon is predicted under certain decay heat and/or pool temperature values below the design values. In most cases, the flow inversion phenomenon is accompanied by boiling which is undesirable phenomenon in this type of reactors as it could affect the fuel-clad integrity. The model results for the flow inversion phenomenon prediction are analyzed and a solution of the problem is suggested.