Transient behaviour of high burnup fuel (KOTO): KOTO summary report

There is a clear economic incentive for the industry to continue the trend of pursuing higher fuel discharge burnups, which translates into higher neutron fluences, often together with longer residence times in the reactor. Testing and irradiation experience demonstrates that it is feasible in terms of reliability and safety to operate fuel beyond the recent burnup limits in normal conditions. Still until now, the acceptance criteria for transient and accident conditions have broadly based on early data that are often from much lower burnups, and high burnup effects must not be overlooked as regards postulated accident behaviour. The project supports assessing the licensing criteria and evaluating the consequences of efforts for improved fuel utilisation. Statistical methods for fuel analyses have been elaborated for extensive applications. A new development featuring advanced thermal hydraulics coupled with a fuel accident performance model, the FRAPTRAN-GENFLO code, is opening an unparalleled capability for realistic transient simulation. Well-placed international participation is providing data from representative test conditions and burnup ranges relevant to model development and validation for the near future. During the project period, the Finnish nuclear utilities were granted a discharge burnup limit extension from 40 to 45 MWd/kgU in assembly average for the fuel types in use.