Abstract
Engineering design studies to determine the feasibility of conversion of the High Flux Isotope Reactor (HFIR) from highly enriched uranium (HEU) to low-enriched uranium (LEU) fuel are ongoing at the Oak Ridge National Laboratory. This activity is sponsored by the Reactor Conversion Program under the auspices of the US Department of Energy National Nuclear Security Administration’s Office of Material Management and Minimization. HFIR is a very high flux, pressurised light water–cooled and moderated, flux trap–type research reactor with a core made of involute shaped U3O8/Al cermet fuel plates and coolant channels. HFIR currently operates at a thermal power of 85 MWth and supports key national and international missions in neutron scattering, isotope production, and materials irradiation, including neutron activation analysis.
Advanced multiphysics computational fluid dynamics models have been developed in COMSOL to simulate 1) best-estimate (nominal) and 2) safety basis (worst case) steady state operating conditions for both the current HEU and the proposed LEU U/Mo alloy fuel designs. Several important physics—for example, spatially dependent nuclear heat deposition, multilayer heat conduction, conjugate heat transfer, turbulent flows (using Reynolds Averaged Navier Stokes turbulence models), structural mechanics (thermal-structural interactions and fuel swelling) and oxide layer build-up—were coupled and solved for HFIR’s inner and outer fuel elements. Multiple uncertainty factors were considered in the safety basis models, which include the uncertainties in power density distributions, variation of average fuel concentration in the neighbouring fuel plates, flux peaking for fuel zones extending beyond nominal boundaries, and hot streak (axial track overloading) and hot spot (fuel segregation and non-bond) factors. In the safety basis models, different power-to-flow safety limits were established using the incipient boiling (Bergles-Rohsenow correlation), flow excursion (modified Saha-Zuber correlation), and burnout (Gambill correlation) criteria.
Alternate design features of the new LEU fuel were evaluated using these multiphysics models and led to a new baseline LEU design that is currently being optimised for performance. The new LEU design combines a burnable absorber in the inner element side plates, a relocated and reshaped (but still radially contoured) fuel zone, and axial grading of the bottom of fuel region to reduce the power spike near the coolant exit, which often is the location of the lowest thermal margin.
