Simulation of flow instability for forced circulation in plate-type fuel assemblies of pool research reactors

Flow excursive instability for forced circulation in a fuel assembly of IVV-2M type is numerically modeled with the help of two-phase quasi-dynamic sub-channel code REMOL. Similar plate-type assemblies are widely used in pool research and test reactors. To validate the models employed by the code a pressure drop versus flow rate curves (S-curves) are calculated for a uniformly heated narrow channels at near atmospheric pressure. Comparison of the calculated and measured S-curves is given for rectangular channel (2.03 mm in channel gap, 2.54 mm in width, 483 mm in length) with various heat fluxes (in the range from 60 to 220 kW/m2). Various approaches to a mathematical description of the assembly were used for simulation of the flow instability. Results of the numerical calculations are compared with the experimental data obtained from the in-pile test with the real IVV-2M assembly in Impulse Graphite Reactor (IGR). This test imitated reactivity-initiated accident resulting in slow power increase to such level that the part of the hottest fuel element melted. (author)