Modelling heat transfer and dissolved species concentrations within PWR crud

The crud formed on PWR fuel tends to contain “chimneys” penetrating down to the metal surface. This provides an opportunity for wick-boiling to occur. This is associated with the generation of very high concentrations (many tens of times the bulk values) of dissolved species within the crud, and associated significant increase in the local saturation temperature. These both have consequences for cladding corrosion, and the accumulation of boron in the crud contributes to the phenomenon termed “axial offset anomaly”. We present here detailed, coupled two-dimensional models of these processes, which allow the three linked phenomena, heat conduction, advection and diffusion of species in the water, and diffusion of the water itself, to be modeled and their aggregate effect to be seen.

Research highlights▶ PWR fuel can be coated with porous crud. ▶ This crud also can contain chimney-like large through-thickness pores. ▶ Together these change the heat transfer mechanism to predominantly wick boiling, not convection. ▶ Dissolved species concentrations are increased by factors of several tens over bulk coolant values by the wick boiling process. ▶ Clad temperatures beneath the crud can be raised considerably.