Behavior of Nb2O5 doped/unpressurized UO2 fuel under transient

This paper describes the results of reactivity initiated accident (RIA) experiments using unpressurized UO2 fuel pellets containing 0.30 wt% Nb2O5 additive. From the result of postpulse-irradiation examination (PIE) such as visual inspection, dimensional measurement and metallo/ceramography, fuel failure threshold and failure mechanism were explored. The results obtained were: (1) Failure threshold of the Nb2O5 doped fuel was almost equal to that of NSRR standard fuel rod data, which were adopted in the Licensing Guideline for RIA, Japan. (2) Failure mechanism of the Nb2O5 doped fuel under unpressurized condition was cladding melt followed by enbrittlement of thinned cladding wall. No significant difference in the failure mechanism existed between the doped and undoped fuels. Bonding is not peculiar to the doped fuel, namely occurred at lower value of deposited energy (276 cal/g·fuel) in the doped fuel than that (341 cal/g·fuel) in the undoped fuel. (3) No measurable difference in the threshold for mechanical energy release existed between Nb2O5 doped fuel rod and NSRR standard fuel rods. (4) Axial elongation in the unpressurized doped fuels was significantly greater than that in prepressurized doped ones. (author)