Behaviour of small-sized BWR fuel under reactivity initiated accident conditions in comparison with standard BWR fuel

This report describes the results of reactivity initiated accident (RIA) experiments performed on small-sized BWR fuel for which the outer diameter of 11 mm is smaller than 12.3 mm of a conventional BWR. Evaluation of the fuel behaviour was based on in-core instrumentation data and data obtained from post-pulse irradiation examination. The principal matter of consideration was to determine the fuel failure threshold and the failure mechanism. The results obtained were: (1) The failure threshold of the small-sized BWR fuels was not less than 260 cal/g fuel (220 cal/g fuel in enthalpy) and no measurable differences in failure thresholds were observed between the small-sized BWR, the conventional BWR and the NSRR standard fuels. The failure mechanisms of the small-sized BWR fuels were either fuel melting/brittle fracture or cladding rupture depending upon the fuels whether lined by Zr or not. (2) No measurable difference in the threshold for mechanical energy release were found between the small-sized BWR fuels and the conventional BWR (and NSRR standard) fuels. (orig.)