Because of the interest in enhancing the reliability of CANDU reactors and in using enriched U 0 2 fuel, it is important to understand the chemistry of the interaction of the graphite in the CANLUB coating with fission products.

This thesis suggests that the stabilization of cesium-zirconium-iodinecarbon compounds (i.e., Cs(Zr_xI_yC)) by "organic carbon" in CANLUB graphite coatings is the chemical step involved in preventing stresscorrosion cracking (SCC) of fuel sheathing. The coating works by removing corrosive reactants, such as l2, ZrI₄ and Csl, in the form of inert Cs(Zr_xI_yC) compounds, so that the SCC reactions cannot occur. The active ingredient in CANLUB coating that leads to the formation of the compounds is likely to be the ethyl cellulose left after standard curing. Unlike iodine and iodides, Zr_xI_yC-type compounds are not SCC agents, because the vapour pressure of iodine over Zr₆I₁₂C is very low even at 320°C. The beet evidence we have is that the chemical states of Zr, I and C present in the neutron-irradiated graphite discs and CANLUB coating are the same as the well-characterized Zr₆I₁₂C standard. The stability of the Zr₆I₁₂C compound under neutron irradiation is demonstrated. This is the first indication that fission-product iodine can be immobilized through chemical interaction with CANLUB graphite.

Kinetic experiments to measure the reaction rate of ¹³¹I with CANLUB indicate that when CANLUB graphite is present, 1311 is retained. The time required for ¹³¹I to reach the steady state (i.e., matching the natural ¹³¹I decay curve) is shorter in the presence of ZrI₄ compared to I₂. This suggests that the sheathing would be more susceptible to cracking when ZrI₄ is present, because ZrI₄ facilitates the reaction of ¹³¹I with unoxidized zirconium and CANLUB.

The preliminary results reported here also appear to support the hypothesis that the CANLUB graphite coating reduces the rate at which oxygen can react with the fuel sheathing. X-ray photoelectron spectroscopic characterization of Zircaloy sheathing obtained from extended burnup Bruce-type elements BDL-406-XY (555 MW.h/kgU) and BDL-406-AAH (731 MW.h/kgU) indicates that the zirconium oxide formed on the non-CANLUB-coated sheathing is more stoichiometric than that formed on the CANLUB-coated sheathing. Particulate UO₂ adhering to the CANLUB-coated sheathing is also found to be consistently more surface oxidized than that adhering to the non-CANLUB-coated sheathing. Therefore, CANLUB may contribute to the onset of UO₂ oxidation at extended burnup.