Study of the Influence of Axial Power Distribution on Dryout

Axial power distribution is one of the parameters that influence the occurrence of the dryout in nuclear fuel assemblies. Experimental data indicate that this influence is quite substantial, ranging from few to above ten percent of the total power. Thus accurate prediction of the dryout power for various power distributions has important implications on the economy and safety of nuclear power plants. The difficulty with capturing the influence of that parameter stems from the fact that during reactor operation practically unlimited number of power shapes can occur. This fact makes it very difficult to investigate the effect experimentally, and an analytical approach is needed. Various methods have been proposed in the past to capture the effect of non-uniform power distribution on dryout. These approaches can be divided into several categories, where the two main ones are as follows: (a) methods based on introduction of a shape factor, which is calculated from the known shape of the power distribution; (b) methods using certain integral parameters, such as the boiling length and the annular flow length, which are expressed as functions of axial power distribution. In the present approach a simplified annular flow model is used, in which the dryout occurrence is based on the prediction of the disappearance of the liquid film. The dependence of the dryout power on the axial power shape is obtained in a general analytical form. Based on this analytical solution, a new set of terms that govern the dryout power in channels with various axial power distributions is proposed.