Loss of Coolant Accidents (LOCA) are among the most demanding accidents that can happen in a Light Water Reactor (LWR). The lack of cooling and the drop in pressure impose large stresses on the nuclear fuel which would increase the risk of fuel rod damage and the subsequent release of active material. But LOCA is also an accident that the nuclear power plant is designed to withstand with a limited release of radioactivity to the surroundings. Resent research has shown that nuclear fuel that has been irradiated to high burnup can fail at lower temperatures than prescribed by current design criteria. One benign phenomenon is the fragementation of pellets and axial relocation of fuel fragments. Fuel fragments that move within the rod can accumulate at positions where the cladding is strained and cause an increased load there. Models that describes fuel fragmentation and relocation and enable analysis including those phenomena has been developed by Quantum Technologies AB and implemented in FRAPTRAN-1.5 (see SSM report 2015:37).
This report describes the validation of the models against tests performed in Halden and in Studsvik, the latter commissioned by NRC. The objective for SSM in this project is to gain insight into the course of events in a LOCA and how these can be implemented in analytical tools.
Results and conclusion
The comparisons between analysis and tests in this report show good agreement and improvement of the calculations when using the fragmentation and relocation models. The results indicate that without modeling fuel relocation there can be a significant underestimation of cladding temperature and local oxidation in case of a cladding damage. This implies that there is a need to include fuel fragmentation in analytical verification of high burnup nuclear fuel in LOCA.
The present work also identifies needs for further development of the analytical tools, for example; failure criteria, fragmentation mechanisms including constraint from the cladding and mechanical effects from fission gases. This development in turn needs further data from tests on nuclear fuel, both for the development and for the validation of the new models.