Transient behaviour of high burnup fuel (KOTO): Coupling of FRAPTRAN Fuel Rod for Transient Analysis with GENFLO Thermal Hydraulic Code (KOTO & READY)

Reactor analyses are becoming more and more challenging. Due to pursuing higher fuel discharge burnups, fuel designs and operational conditions are subject to constant upgrading. Advances in hardware have removed many of the limitations on detail of analyses, and best-estimate type applications have become a commonplace, a practice now increasingly adopted even in safety cases. At the same time there is a regulatory trend extending the range of events the licensee is to consider. Neutronics, thermal hydraulics, and fuel behaviour are closely interlinked during a reactor transient and cannot be generally separated in a realistic description. However, attempts to combine these into coupled models have been, even at their best, impracticably heavy to use. At VTT, in consent with the USNRC, an in-house general flow model GENFLO has now been coupled with the NRC's FRAPTRAN fuel performance code. The combination takes benefits of a fast-running non-iterative thermal hydraulic model and an updated fuel performance code validated for burnups of up to 65 MWd/kgU. A code description and results of two types of analyses are given. One is a hypothetical large break loss-of-coolant accident (LBLOCA) in a VVER reactor, the other is an instability transient in a Boiling Water Reactor (BWR), for which system conditions were separately available. A systematic validation and international peer review will follow.