COMPUTATIONAL STUDY OF ZIRCONIUM HYDRIDES MORPHOLOGY AT WIDELY VARIED COOLING RATES

Computation study of the average zirconium hydride length on the cooling rate was performed using the precipitate nucleation and growth model. The cooling rate was varied in the range equal to six orders between typical values for the spent nuclear fuel dry storage conditions to values typical for laboratory tests modeling the dry storage. The calculations showed that as the cooling rate decreases, the hydrides concentration decreases, and their average length increases linearly on a double logarithmic scale. These dependencies have no limit if hydrides were abscended in the sample before the cooling began. If there were hydrides in the sample before the start of cooling, then they will grow and new hydrides will not nucleate in the limit of low cooling rates. For spent nuclear fuel dry storage, these results mean that if hydrides remain in the fuel claddings at the initial storage period, then hydrides morphology and hydrogen embrittlement at the end of the storage period are similar values gained under laboratory conditions with sufficiently slow cooling. If hydrides in fuel claddings are completely dissolved at the beginning of dry storage, then their length will be significantly greater than in laboratory tests at the end of the storage. Therefore, if the threshold values for the circumferential stresses are exceeded in fuel claddings, the hydrogen embrittlement can be expected to be higher than after faster cooling in typical laboratory studies. In this case, the hydrogen embrittlement assessment should be performed in a conservative approach assuming that radial hydrides have an average length equal to the thickness of the fuel cladding.