Effect of blisters and rims on radial hydride precipitation in fuel elements under dry storage conditions

Abstract

Hydrogen enters in Zry clads during fuel element life in service; when hydrogen exceeds the terminal solid solubility, circumferential hydrides are precipitated. In the presence of thermal gradients, produced for instance by oxide spallation, localized concentration of hydrides -known as blisters -can be formed. Due to the fact that zirconium hydrides have a lower density that zirconium alloys, stresses are produced around blisters. These stresses may overcome the necessary threshold to precipitate radial hydrides. Moreover, cracks inside blisters may act as initiators of delayed hydride cracking (DHC). In the dry storage facilities developed for Atucha 1, Zry -4 clads of spent fuel elements (SFE) must withstand environmental conditions which include maximum temperature of 200°C and internal SFE pressure of ~ 75 bar (value at end of cycle). During dry storage, if blisters are present, stresses around them are superposed to circumferential stresses generated by the internal pressure due to fission gasses helping precipitation and/or growth of radial hydrides and DHC process. In order to assess this possibility, in the present work sections of Zry-4 cladding were hydrided. Blisters have been grown by thermal gradient under different conditions of temperature and shapes of thermal contact in order to obtain different blister dimensions and fraction of radial hydrides. After blister formation, some specimens were subjected to Hydride Reorientation Test (HRT) by applying an internal pressure of 75 bar at 200°C. Finally, tubes were sectioned and metallographically analyzed looking for radial hydrides. In some samples radial hydrides were observed, which effectively grew by the internal pressure of Zrycladding during HRT.