gamma-Zr-Hydride Precipitate in Irradiated Massive delta- Zr-Hydride

During examination of A Zircaloy-2-clad fuel pin, which had been part of a test fuel assembly in a boiling water reactor, several regions of severe internal hydriding were noticed in the upper-plenum end of the pin. Examination of similar fuel pins has shown that hydride of this type is caused by moisture from the UO//2 fuel. The reason for initiating this particular study was the severe fragmentation of the cladding, which was observed in the regions of excessive hydriding. This letter briefly describes the metallographic observations, the results of X-ray diffraction studies and the conclusions which were drawn concerning these particular hydrided regions. A possible mechanism for the fragmentation was brought to light during the metallographic examination. X-ray diffraction, which was used to identify the phases present, also led to conclusions concerning the resistance of the hydrides to irradiation-induced swelling.