SSM perspective
Background
The Swedish Radiation Safety Authority (SSM) follows the research on fuel performance closely. One aspect that is currently being studied in several research projects is the risk of release of fragmented fuel into the primary coolant in case of an accident. This risk depends on complex conditions were one is the possibility and size of a rupture of the fuel rod cladding tube.
The work presented in this report is part of a larger endeavor to update the computer codes that SSM disposes of through Quantum Technologies AB. The work is a direct continuation of the development of cladding rupture criteria and fuel fragmentation models in previous projects. The present report analyses test results and deduces a model that can be used to predict fuel cladding burst opening sizes.
Results
In this project, cladding ruptures that have occurred in tests with simulated loss-of-coolant accident (LOCA) conditions are analysed regarding their dimensions and based on that an empirical model is proposed. In the analysis of test data, the most influential parameters are identified and their influence in the model considered. It is also concluded that there are several phenomena that affects the rupture dimensions and some are not easy to consider in computational analyses.
Relevance
With this project, SSM has gained insight into which parameters that are important when estimating the risk of dispersal of fuel from cladding tubes that rupture under typical LOCA conditions. SSM has also gained insight into how such a model can be used in a computer code and the uncertainties that it can include.
Understanding of fuel fragmentation and dispersal is used to further enhance the safety of nuclear fuel in accident conditions. With better understanding, more actual analysis can be performed and possible needs for revised limitations can be determined. Furthermore, this project is part of the international development work and enables active participation in international contexts
Need for further research
The continued development of models for analysing rupture behavior in nuclear fuel is necessary. A continuation is to implement the model for burst dimensions and couple it to previously developed models for fuel fragmentation. More tests are also needed to understand the impact of stochastic phenomena and to further expand the database that the empirical model is built upon. On a longer time scale much research and development remains to fully understand the behavior of high burn up fuel.