Simulation of Bubble Dynamics in Sub-Cooled Boiling on Fuel Clad in PWRs

The crud deposition on nuclear fuel assembly cladding generally increases the resistance to heat transfer, which may result in deterioration of thermal performance, degradation of the fuel cladding, and an axial power shift, i.e. Axial Offset Anomaly (AOA). Crud formation continues to elude prediction. An operational difficulty, of not being able to accurately determine power safety margin, then arises. In some cases, this condition has required decreasing the core power by as much as thirty percent, hence, resulting in considerable loss of revenue for the utility. The specific purpose of this study is to examine bubble dynamics, flow characteristics of the surrounding fluid, and its impact on the formation of the curd. The presence of a bubble on the clad surface affects the flow field around it , particularly in forming a stagnant flow region behind the bubble. The temperature difference between the bubble and the bulk coolant surrounding it causes vaporization at the bubble-clad interface and condensation at its apex. Pure water is thereby moved into the bubble through vaporization resulting in the concentration of solutes in the water at the bubble/wall surface region, which may cause their precipitation on and/or attachment to the clad surface, thereby initiating crud deposition. We investigate analytically and numerically, the growth of a bubble in the boundary layer and the influence of the bubble on the flow. Because of the small bubble size, a spherical model of the bubble is selected for our research. A two-step calculation is applied to this model. In the first step, bubble growth is estimated analytically with omission of the effect of the bulk fluid velocity, a reasonable approximation. In the second step, the flow field around the stationary bubble is obtained through numerical methods. Some parameters in PWR operating condition have been determined approximately e.g. size of the bubble, boundary layer thickness, flow velocity and drag forces on the bubble.