Evaluation of Deposition and Release of Crud on Fuel Surfaces under Boiling and Sub-cool Boiling Conditions

Latest experiences with water chemistry in Japanese nuclear power plants and world wide water chemistry information have been compiled to establish road maps on research and development plans for water chemistry of nuclear power plants. As a result of discussion on R and D plans on fuels, it was confirmed that anomaly offset alignment (AOA) was still a future subject in Japan, though it was one of the major issues related nuclear fuels under long period cycle operation. In Japan, experience with AOA in operating plants is not reported yet. However, evaluation model of crud and born deposition on fuel cladding under sub-cool boiling should be developed as a technical basis of future AOA mitigation. A theoretical model, micro-layer evaporation and drying-out (MED) model, was developed for predicting iron crud deposition on boiling surfaces of BWR fuel rods and it was extended for applying to metallic ion deposition. Release of iron crud from fuel surface was explained by shear stress caused by two phase flow. That of metallic ions was explained by dissolution rate from deposits into water, which was affected by chemical forms of the deposits. The MED model has been extended to evaluate deposition of curd and metallic ions on sub-cool boiling surfaces. Process of growth and collapse of bubbles was expressed as several non-linear equations, which was solved numerically to estimate time from bubble generation to collapse, total evaporation volume and concentration factor of micro-layer on fuel surface. The model can be applied to evaluate deposition of chemicals on fuel surface and also local pH. (author)