Developing Fuel Management Capabilities Based On Coupled Monte Carlo Depletion in Support of the MIT Research Reactor Conversion

Abstract

Pursuant to a 1986 NRC ruling, the MIT Reactor (MITR) is planning on converting from the use of highly enriched uranium (HEU) to low enriched uranium (LEU) for fuel. Prior studies have shown that the MITR will be able to operate using monolithic U-Mo LEU fuel while achieving neutron fluxes close to that of an HEU core. However, to date, detailed studies on fuel management and burnup while using LEU fuel have not been performed. In this work, a code package is developed for performing detailed fuel management studies at the MITR that is easy to use and is based on state-of-the-art computational methodologies. A wrapper was written that enables fuel management operations to be modeled using MCODE, a code developed at MIT that couples MCNP to the point-depletion code ORIGEN. To explicitly model the movement of the control blades in the MITR as the core is being depleted, a criticality search algorithm was implemented to determine the critical position of the control blades at each depletion timestep. Additionally, a graphical user interface (GUI) was developed to automate the creation of model input files. The fuel management wrapper and GUI were developed in Python, with the PyQt4 extension being used for GUI-specific features. The MCODE fuel management wrapper has been shown to perform reliably based on a number of studies. An LEU equilibrium core was modeled and burned for 640 days with the fuel being moved in the same pattern every 80 days. The control blade movement and nuclide concentrations were shown to be in agreement with what one would intuitively predict. The fuel management capabilities of REBUS-PC and the MCODE fuel management wrapper were compared by modeling the same refueling scheme using an HEU core. The element power peaking factors for the two models showed remarkable agreement. Together, the fuel management wrapper and graphical user interface will help the staff at the MITR perform in-core fuel management calculations quickly and with a higher level of detail than that previously possible.