Sheath Protection Mechanism Afforded by CANLUB

Abstract

CANDU fuel elements can fail by stress corrosion cracking (SCC) of the Zircaloy-4 cladding during power ramps. Such failures result from the combined actions of stress concentrations and corrosive fission product. To combat the problem the CANLUB graphite coating was developed to reduce the operating stresses in the cladding below the critical level for SCC. However, it became evidence that the observed beneficial effects of CANLUB coatings on fuel performance stemmed primarily from the interaction of the coat with fission products, rather than from lubrication of the fuel-cladding interface. Therefore the coating must interact with corrosive specks, either physically (us a barrier) or chemically. It was concluded, however, that the graphite coating is porous and ineffective as a barrier. Hence its effectiveness in preventing SCC must result from chemical interaction with the corrosive species.

Because of the interest in enhancing the reliability of CANDU reactors and in using enriched UO₂ fuel, it is important to understand the chemistry of the interaction of the graphite in the CANLUB coating with fission products. It was first suggested by the authors that carbon could form compounds such as Zr₆I₁₂C at reactor temperatures, and it was suggested that these compounds could immobilize corrosive iodine species (4). Unlike iodine and iodides, Zr_xI_yC-type compounds are relatively stable. Detecting Zr_xI_yC compounds in spent fuel bundles proved to be very difficult, but X-ray photoelectron spectroscopy (XPS) has shown that minute quantities of the stable metal iodide(s) (probably CsZr_xI_yC-type compounds) were present on inter-pellet graphite discs from an irradiated fuel element. This was the first indication that fission product iodine can be immobilized through its chemical interaction with CANLUB graphite. This paper gives an overview of the latest results on CANLUB development.