Chromium-Coated Cladding Effects in the Context of 10 CFR 50.46c

摘要

A major initiative in the nuclear industry is thedevelopment of accident tolerant fuel (ATF) designs. Onesuch design involves a chromium layer applied to theouter surface of a zirconium alloy cladding for thepurpose of corrosion resistance during normal operationas well as resistance to high temperature oxidation duringloss-of-coolant accidents (LOCAs). The NuclearRegulatory Commission is nearing the completion of the10 CFR 50.46c rulemaking which will revise theacceptance criteria for LOCA analyses. While theprimary focus of the rulemaking is to transition toperformance-based LOCA acceptance criteria, whichwould facilitate transitions to cladding materials otherthan zirconium, the supporting research into the newcladding embrittlement mechanisms for zirconium alloysis most notable for its consideration of the influence ofresident hydrogen on oxygen diffusion into the cladding.To address the effect of resident hydrogen, a hydrogenbasedequivalent cladding reacted (ECR) limit isproposed under 10 CFR 50.46c which would reduce theallowable amount of cladding which could react during aLOCA transient as the fuel burns via normal operation ina reactor core. The chromium coating, with expectationsof reduced corrosion and hydrogen absorption duringnormal operation, has the potential to mitigate thisconcern significantly.Furthermore, the effect of the coatings could enableperformance-based limits to be established based on thefailure mechanisms of coated cladding. The existing1204°C (2200°F) peak cladding temperature (PCT) limitfor zirconium alloys is intended to protect against adeparture from normal oxidation kinetics (“runawayoxidation”) which can lead to fuel failure and rapidembrittlement. Different failure mechanisms exist for achromium-coated cladding with less limiting oxidationbehavior, improving the margins between accidentconsequences and appropriately defined analysis figuresof-merit. Increases in the PCT limit along with reducedexothermic reaction during the postulated accidentimprove physical and analytical safety margins.The integration of fuel rod design, nuclear design, andLOCA analysis methods under the FULL SPECTRUM™1LOCA (FSLOCA™) Evaluation Model (EM) [1] wasshown to be aligned with the requirements of the 10 CFR50.46c rulemaking in [2]. Adjustments have been madeto the method described in [2] to accommodate thebehavior and physical limits of chromium-coatedcladding within the same analytical framework, and theresulting increase in safety margins is demonstrated.