Rim Structure Effects on Fuel Rod Performance with Burn up Extension Using FRAPCON

Extending the authorized fuel burn up limit of Light Water Reactor (LWR) is one of the most promising methods to enhance the commercial competitiveness of nuclear power plants. The benefits with high burn up include reduced maintenance and fuel cycle costs, less refueling operations thus leading to higher capacity factors, and reduction of the volume of spent fuel discharged normalized to the energy produced. However, to ensure the integrity of fuel rods with high burn up fuel, there are still a number of issues that need to be remedied. For example, the formation of High Burn up Structure (HBS) or rim structure is possibly the most significant restructuring processes at the rim of pellets in-pile with burn up extension in LWR and the effect of HBS on fuel thermophysical or mechanical properties is a key requirement to ensure successful implementation of extended burn up. There is independently experimental evidence to support that HBS concomitantly affects thermal and mechanical properties of fuel. But it is still not well understood how such HBS affects fuel thermo-physical or mechanical properties. In this work, the impact of the HBS formation on the fission gas behavior of fuel rods was evaluated under normal operating conditions. The fuel performance code FRAPCON-4.0 was used to simulate fission gas related properties of full-length fuel rods with the HBS formation via the FRAPFGR model and without rim structure via the Massih model under normal operating conditions. It is found that as the burn up extends from the current limit of 62 to proposed new limit of 75 GWd/MTU, the plenum pressure and fission gas release have only modest increase with flat power history profiles and without the HBS effects considered. However the increase is more pronounced when the power history profiles have higher peaking factors and when the contribution of HBS formation is taken account. It is recommended to keep the power history profiles as flat as possible, both radially and axially, in NPP operations to ensure the integrity of fuel rods such that the benefits of burn up extension can be realized.