Fuel Cycle Developments and Safety Analyses of Transition Core Plan for Supporting MIT Reactor Low Enriched Uranium Conversion

摘要

The Massachusetts Institute of Technology Reactor (MITR) is a research reactor in Cambridge, Massachusetts designed primarily for experiments using neutron beam and in-core irradiation facilities. At 6 MW, it delivers neutron flux and energy spectrum comparable to power light water reactors (LWRs) in a compact core using highly enriched uranium (HEU) fuel. In the framework of non-proliferation policy, research and test reactors have started a program to convert HEU fuel to low enriched uranium (LEU) fuel. A new type of LEU fuel based on a high density alloy of uranium and molybdenum (U-10Mo) is expected to allow conversion of U.S. high performance reactors (USHPRRs) like the MITR. The principal part of the Preliminary Safety Analysis Report (PSAR) has been completed for the MITR LEU conversion. A transition core plan, from 22 fresh LEU fuel elements (i.e., beginning-of-life) gradually to 24 of them arranged in an equilibrium configuration, is expected to serve as an appendix chapter in the PSAR. The current study presents the fuel cycle development, which eventually leads to the transition core plan. The results confirm the equilibrium state, where both shim bank movement (i.e., core reactivity) and fissile materials stabilize, can be achieved by fixed pattern fuel management. Fission density has been evaluated for a number of fully discharged LEU fuel elements, using both conservative and best-estimate approaches. There are adequate margins to the planned qualification fission density limit of three different MITR U-10Mo plate configurations. The fuel cycle calculations also generate power profiles at each core state. A steady-state thermal-hydraulic safety analysis has thus been performed, where onset of nucleate boiling (ONB) is considered as the safety criterion. The results confirm significant margins to ONB at all analyzed transition and equilibrium fuel cycle states.