Fuel CRUD Redeposition Following On-Line NobleChem™ Application

摘要

Corrosion and CRUD induced fuel cladding failure remains a concern to fuel reliability and remains an area of contention to the goal of zero fuel failures. Fuel cladding CRUD deposits, and its constituents, have proven to be detrimental, at some operating plants. Deposit formation on heat transfer surfaces, i.e. fuel cladding, has been extensively investigated and is known to be controlled by neutronic, thermo-hydraulic, radiolysis, and chemistry environmental conditions. Any shift or change of these parameters may have unforeseen consequences for the CRUD deposition process and hence, fuel reliability. AREVA developed a CRUD deposition model and code to assess and predict the CRUD deposition on BWR cores over their lifespan. Continued refinements of the model are implemented in the code as changes to the chemistry conditions and plant operations occur. Recently, the model was expanded with the PEZOG module to address the platinum enhanced zirconium oxide growth in response to the move of the industry to the On-Line Noble Chem™ technology. Operational transients such as down powers, control rod moves, or changes to core flow were traditionally the main origin of CRUD releases. The change from normal water chemistry to hydrogen water chemistry introduced CRUD releases in response to the loss of hydrogen, i.e., shifting between reducing and oxidizing bulk water chemistry conditions. The introduction of noble metals injection may further complicate the corrosion product release and deposition behavior. While hydrogen injection affects bulk water chemistry, noble metal injection may stress the surface oxide and CRUD layers by changing the local redox conditions. BWR operating experience indicates that injections of noble metals can cause a significant release of corrosion products, in particular of the insoluble corrosion product fraction. The initial noble metals application typically results in a much more severe corrosion product release than any subsequent application. While the magnitude of the corrosion product releases appear to decrease with continued annual noble metals injections; plants have not yet achieved their lower pre-On-Line NobleChem™ levels. The effect of CRUD redeposition following OLNC applications has been investigated in regards to CRUD thickness evolution over the fuel's lifetime. Simulations of the CRUD release and re-deposition processes related to OLNC applications for US plants are presented.