Current Status of the Post Boiling Transition Research in Japan Integrity Evaluation of Nuclear Fuel Assemblies after Boiling Transition and Development of Rewetting Correlations

Development of rewetting correlation formula was the key to predict fuel-cladding temperature after Boiling Transition (BT). Japanese BWR utilities and vendors performed some tests of rewetting and made two rewetting correlation formulas. The effect on fuel integrity after BT depends on temperature of fuel rod and time of dryout. Main cause of losing fuel integrity during BWR's Anticipated Operational Occurrences (AOO) after BT is embrittlement of the claddings due to oxidation. Ballooning of fuel rod is excepted because its pressure boundary isn't broken. In Japan, the Standards Committee of Atomic Energy Society of Japan (AESJ) is making post BT standard. This standard provides guidelines based on the latest knowledge to judge fuel integrity in case of BT and the validity of reusing the fuel assembly that experienced BT in BWRs.