SSM perspective
Background
Quantum Technologies AB (QT) has for many years developed computational models for the thermomechanical behaviour of nuclear fuel in events such as LOCA and RIA. The research projects have been based on evaluations of tests on nuclear fuel in Halden Reactor Project (HRP) and Studsvik Cladding Integrity Project (SCIP). The models developed by QT enable analyses and interpretations of the tests, including the analysis of new phenomena such as fuel fragmentation and its impact on the damage process.
In this research project, a newly developed version of FRAPTRAN has been used for more realistic analyses of the events experienced by several fuel rods during a LOCA. In collaboration with Ringhals AB and Vattenfall Nuclear Fuel AB (VNF), QT has analysed 50 fuel rods with high burnup and how they could behave in a LOCA. Thermal-hydraulic boundary conditions (scenario and heat transfer coefficient) have been taken from a work done at Chalmers University of Technology as part of a previous research assignment for SSM (SSM2014-1043-15). VNF has selected rods and produced power histories for the rods to use as initial values in the calculations of the LOCA process.
Results
The results show that even though several of the rods in the study reach conditions that result in fragmentation and relocation of the fuel, only a few of these rods experience cladding failure. The fragmentation and relocation of the pellets leads to several effects on the fuel, primarily a temperature increase which in turn drives oxidation and cladding temperature. Cladding failure is obtained in this analysis due to oxidation and stress conditions in the cladding in the later stage of the event. The choice of scenario has an impact on the calculated damage process; the rods that are expected to be damaged are so in the later part of the event. In the current work, a “cladding damage index” has also been produced which shows that in the course of the event several rods are close to damage in the earlier part of the event. None of the rods experience damage in the early part of the event in the current scenario but minor changes to the scenario can lead to other causes of damage and possibly more severe consequences of the event.
It is noted that no significant effect is seen on the rods with an initial linear heat generation rate of less than 15 kW/m, suggesting that the behaviour of the fuel is largely dependent on its power. Power level seems to be a more important parameter than e.g. burnup.
All in all, it can be concluded that for a realistic case, which however has some conservative boundary values, there does not seem to be extensive damage to the fuel, nor the consequences with the release of fission gases and pellet fragments. Fragmentation in high burn-up fuel can occur without causing damage. By understanding which phenomena are most significant, strategies can also be developed to mitigate the process based on these phenomena.
Relevance
This research project has provided an insight into the conditions for the nuclear fuel in Ringhals in a LOCA, taking into account phenomena studied in recent years (fuel fragmentation). The research project has provided further in-depth input to SSM’s work in the supervision of the safety assessment for the reactors at Ringhals. This work and its conclusions can complement the previous review in SSM2014-1043-4 ”Granskning inför beslut om godkännande av förnyad SAR och ansökan om provdrift vid 3300 MW termisk effekt för Ringhals 4” and specifically the expectations expressed by SSM in Action 9 to investigate the extent of the impact on the fuel in the analyses for loss of coolant for the power-increased reactor. This current research project indicates that the operation of the Ringhals reactors is such that only a limited amount of fuel rods risk damage during a LOCA. However, only one LOCA scenario has been studied here and small changes in the scenario can lead to other effects in the fuel. It should also be considered that the fuel rods in the study were chosen by Ringhals.
Need for further research
As the fuel is developed and tested under new conditions, new phenomena are observed. Analyses and interpretations of the tests are very valuable in understanding the phenomena present in the tests, and what may be driving an event. Based on this knowledge, limits can be determined and analyses improved. It is important that the tests continue and that SSM has support for analyses and interpretations of the tests.