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 where one is the possibility and size of a rupture of the fuel rod cladding tube.
This present report describes improvements to the FRAPTRAN-1.5 version extended by Quantum Technologies AB, regarding models for high-temperature cladding creep and rupture. The project is one part of a larger endeavour to update the computer codes that SSM disposes of through Quantum Technologies AB. It was deemed important to do a calibration of the aforementioned models since a previous project showed a need to scale the cladding high temperature creep rate significantly.
Results
In this project, selected model parameters have been calibrated against open literature data. The results shows that it is possible to get results that better reproduce the measured cladding burst times, temperatures, stresses and strains for the considered tests.
The report also discusses the impact of corrosion on cladding high temperature deformation and rupture. It summarizes the mechanisms through which oxygen and hydrogen affect the creep and rupture behaviour, and indicates issues that need further investigation.
Relevance
With this project, SSM has gained a computer code with an improved capability to predict cladding high-temperature rupture behaviour. SSM has also gained insight into the calibration of a model. This is of great importance when reviewing safety analyses for nuclear fuel, especially for assessing assumptions and motives for uncertainties. 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 high-temperature creep and rupture behaviour in nuclear fuel is necessary. After this project, it is clear that there is a need for validation against tests of modern cladding materials and for continued development of the effects of corrosion and hydrogen in the cladding. On a longer time scale much research and development remains to fully understand the behaviour of high burnup fuel.