Fuel rod analysis and rationalization of fuel rod integrity criteria for Super LWR

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The Super LWR is a thermal spectrum SCWR, recognized as a Generation-IV reactor concept, being developed by the University of Tokyo. Fuel rod behaviors of the Super LWR are analyzed and the principles for rationalizing the fuel rod integrity criteria are developed.The FEMAXI-VI, developed by Japan Atomic Energy Agency, is applied to the fuel rod analyses. The maximum cladding surface temperature for normal operating conditions is evaluated first with subchannel analyses and a statistical thermal design procedure since the cladding surface temperature is one of the key boundary conditions for the fuel rod analyses.The fuel rod behavior is analyzed for normal operating condition. The maximum pellet centerline temperature increases towards EOC of each cycle since the axial power distribution shifts to the upper part of the fuel rod where the cladding temperature is high. The fuel rod internal pressure linearly increases towards EOC due to FP gas release. Even though PCMI can occur, change of the cladding outer diameter is governed by thermal expansion of the cladding. The mechanical strength requirement for the cladding is evaluated and the principles for reducing it are developed. Designing efforts in reducing FP gas release and PCMI are in the tradeoff relationship since increasing the initial pellet grain size reduces FP gas release but increases pellet swelling rate. Placing the gas plenum in the lower part of the fuel rod is effective in reducing the stress on the cladding. Eliminating the constraint that the fuel rod internal pressure is kept below the coolant pressure is also effective.The principle of rationalizing the criteria for abnormal transients is developed. Fuel rod analyses show that allowable limits to the maximum fuel rod power, for overpower transients, and maximum cladding temperature, for loss-of-cooling transients can be determined to assure the fuel integrities.