Loss of flow accident analysis of the MIT research reactor HEU-transitional cores using RELAP5-3D

The MIT Research Reactor (MTTR) is in the process of evaluating a transitional core conversion strategy for converting from HEU to LEU fuel. The objective of this study is to analyze the Loss of Primary Flow (LOF) accident of the LEU-HEU mixed core. All-HEU and all-LEU cores at 6 MW are analyzed as reference cases for seven transitional core configurations, which consist of various combinations of HEU and LEU fuel elements. The thermal-hydraulic calculation was performed using the RELAP5-3D code. A lumped average and a single hot channel were modeled in each core configuration with radial peaking factors of 2.0 and 1.76 for HEU and LEU fuel element, respectively. Four natural convection valves and two anti-siphon valves were modeled for natural convective heat removal during the LOF transient. Two different hot channels of a full and a half channel configurations were evaluated because the unique design of the MTTR fuel element can form these two types of flow channel geometries. RELAP5-3D calculation results suggest that the proposed transitional conversion strategy is feasible and the thermal hydraulic safety margin of the all-HEU and all-LEU cores can be maintained when operating at full power of 6 MW if the highest radial peaking is adjacent to a full coolant channel, however, it is necessary to operate at slightly reduced power during the transition phase if the highest radial peaking of the HEU element is adjacent to a half channel. (authors)