HFIR staff has developed a reference fuel design, but there is signifi­cant work remaining to evaluate its safety. HFIR’s conversion plan requires maintaining the fuel plates’ involute shapes, the overall core geometry, thermal/hydraulic system parameters, and key neutron fluxes, particularly in the flux trap region. To create a “proof of concept” reference LEU fuel design, state-of-the-art HEU-validated neutronics analyses were coupled to the original HFIR thermal design analysis. The current analysis uses the Nuclear Energy Agency’s Monte Carlo depletion interface code VESTA (MCNP/ORIGEN) that accounts for the zirconium interlayer between the UMo fuel and cladding (see Figure 2-1 in Chapter 2).

Current calculations indicate that essential neutron fluxes as well as fuel-cycle length can be preserved using UMo monolithic LEU fuel if the reactor power is increased from 85 to 100 MW. The fuel plates will need to be radially contoured (see Figure 3-6) and axially contoured on the lower edge to avoid flux peaking at the edges of the fuel.

Increasing the reactor power to 100 MW will require changes in the thermal/hydraulics safety basis, and new safety limits and protective sys­tem setpoints[70] must be derived from a revised thermal analysis. Transient analyses must also be reevaluated as well as fission product release, trans­port, and consequence analyses.

The return to 100 MW operation will also increase the heat flux from the fuel plates. However, ORNL plans to maintain the current primary coolant inlet temperature, flow rate, and pressure (pressure is constrained to 475 pounds per square inch atmospheric [psia] because of the embrittled vessel). Consequently, the safety limits and associated protection system setpoints will need to be changed, which will require the identification of additional safety margin, either through analysis or changes in fuel design. There are several resources that could be used to find the addi­tional required safety margin: (1) use a modern multidimensional physics analysis to evaluate the safety margin and demonstrate its adequacy; (2) revise the manufacturing uncertainties included in the safety analysis; and (3) revise the approach to the consideration of uncertainties (statistical versus multiplicative).

ORNL plans to begin the revised thermal/hydraulic analysis by per­forming a modern multidimensional analysis to analyze the safety margin at higher operating power. This analysis will use a COMSOL[71]-based, three-dimensional, detailed multiphysics LEU model to replace the existing HFIR steady-state heat transfer code. At present, ORNL staff is working to develop the integrated multiphysics modeling tools and place them into production. The new COMSOL model will require validation against the old (HEU) data, new (LEU) data, and separate effects testing, plus accep­tance by the regulator (the U. S. Department of Energy).