Comparative analysis on behavior of high burnup PWR fuels pulse-irradiated in reactivity-initiated accident conditions

Suzuki, Motoe; Sugiyama, Tomoyuki ; Udagawa, Yutaka  ; Nagase, Fumihisa ; Fuketa, Toyoshi

Two cases of the Reactivity-Initiated-Accident-simulating pulse irradiation experiments conducted in the NSRR using a high burnup PWR fuel are analyzed by using the RANNS code. One case was performed in a stagnant coolant water of room temperature and at atmospheric pressure, and the other in the stagnant coolant of high temperature at 7 MPa pressure. The two cases resulted in failure. Behavior in a fast transient of the fuels is investigated by comparing the two experiments. Specifically, the temperature of rod, pellet-clad mechanical interaction, and stress/strain of cladding etc. are calculated and the results are discussed in comparison with the experimental observations. The metallography of cladding shows that the cladding failed by macroscopic shear gliding from a tip of a crack. The calculated plastic strains of claddings at failure reasonably coincide with the observed local strains if the latter strains are averaged over the total circumference.