Behaviour of high burnup PWR fuels during simulated reactivity-initiated accident conditions

Fuketa, Toyoshi; Sugiyama, Tomoyuki ; Umeda, Miki ; Tomiyasu, Kunihiko; Sasajima, Hideo 

By using pulse-irradiation capability of the NSRR, JAEA continues to perform experiments simulating fuel behaviour during reactivity-initiated accidents (RIAs). The recently-tested fuels include those irradiated in European PWRs and have burnups of 67 to 79 MWd/kgU. Cladding materials of these tested rods are Zirlo, M5, MDA and NDA. The tests resulting in the PCMI failure indicate that the fuel enthalpy at failure correlates closely with the thickness of the hydride rim, and in turn with the oxide layer thickness. Fission gas release during the RIA transient is highly dependent on the peak fuel enthalpy, and a higher enthalpy causes a larger fission gas release. Pellets with a larger grain size may have a suppression effect on the release. When DNB occurs during the transient, the large cladding deformation is caused by the increase of the rod internal pressure in combination with the decreased yield stress of the cladding at an elevated temperature. In the phase of the PCMI, the deformation is driven only by solid thermal expansion of fuel pellets.