Abstract
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 Zry-cladding during HRT.
Highlights
Dry storage is currently being considered as a method of managing commercial nuclear fuels before delayed reprocessing or final disposal
The hydride reorientation (HR) phenomenon usually involves the dissolution of circumferential hydrides and the formation of zirconium hydrides oriented perpendicular to the hoop, hereby referred as radial hydrides [1]
Section B-B of specimen 0.8-Cu-b, without Hydride Reorientation Test (HRT) (Figure 9.b) shows lower density of radial hydrides compared with specimens 0.8-Cu-a/HRT and 0.8-Cu-c/HRT (Figure 9.a and 9.c, respectively) which would indicate that the HRT is effective in producing hydride reorientation
Summary
Dry storage is currently being considered as a method of managing commercial nuclear fuels before delayed reprocessing or final disposal. Numerous studies have been carried out in order to calculate the minimum necessary stress to produce precipitation of radial hydrides during cooling which takes place during dry storage. In relation to this “reorientation” process (from circumferential to radial position) the first point to be considered is the maximum temperature to which the cladding has been subjected; because this determines the hydrogen concentration in solution (TSSP or TSSD, depending on the thermal history) from which precipitation of radial hydrides will occur. This assumption must be corroborated with proper studies in Atucha 1 irradiated cladding
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