Building and Testing a 3D Thermomechanical Finite Element Model of Fuel in the Annular Core Research Reactor

Abstract

The Annular Core Research Reactor (ACRR), located at Sandia National Laboratories(SNL) in Albuquerque, New Mexico, is part of the Training, Research, Isotopes, General Atomics (TRIGA) reactor family and has performed over 14,000 operations since 1979. Originally designed for a fast reactor safety program, it is primarily used for radiation testing of electronics. The ACRR core is made up of 236 fuel elements with annular ceramic composite Beryllium Oxide-Uranium Dioxide (BeO-UO2) fuel pellets stacked in Niobium cans surrounded by 304 Stainless Steel cladding and arranged in an annulus around a 9" dry central cavity. The ACRR’s fuel elements were originally designed to have fluted Niobium cans with equal radius inward and outward flutes to contain the pellets and limit the heat transferred from the fuel. However, imaging in the spring of 2024 revealed that, although within the dimensional tolerance, the cans were manufactured with significantly smaller inward flutes. In the manufactured case, there is significantly more play; the pellets can move around and come into direct contact with a much larger area of Niobium, resulting in greater heat transfer to the cans and cladding. An initial 2D finite element model indicated that there was a possibility for the Stainless Steel cladding to exceed its safety basis limit temperature of 500°C. As such, it was decided that a complete 3D model of an ACRR fuel element should be constructed to provide more robust results and consider various internal fuel configurations. This work uses MOOSE, an open source, C++ based, parallel framework developed at Idaho National Laboratory and Gmsh, an open source mesh generator, to build the model. The model is complete with temperature dependent material properties, volumetric heat generation, and thermomechanical interactions. The model was constructed in a stepwise manner and demonstrated to run throughout an ACRR pulse.