Predicting the growth of deposits on the fuel rods in a VK-50 reactor

A newly developed procedure for predicting the growth of deposits on the fuel rods of a water-cooled water-moderated reactor under coolant boiling conditions is described. The results obtained from an experimental validation of the procedure carried out for fuel assemblies at different stages of their operation in a vessel-type boiling-water reactor are presented. It is shown on the basis of experimental data that the deposits forming on the fuel rods in a boiling-water reactor consist mainly of copper and iron. Copper exists in dissolved form and precipitates in pores between the particles of iron compounds, and the thickness of deposits is determined by the particles of iron corrosion products themselves. The corrosion products incipience and growth processes were investigated, and the effect of deposit formation from fine iron particles on fuel rod claddings operating under coolant boiling conditions was predicted theoretically and revealed experimentally. Relatively large particles moving along a fuel rod cannot penetrate into the laminar sublayer due to the effect of Magnus force on them. Based on the results of theoretical and experimental investigations, recommendations on decreasing the content of iron corrosion product particles in transient operating modes of boiling water reactors are worked out. The method of very fast decrease of pressure at low levels of reactor power worked out on a VK-50 reactor makes it possible to remove relatively large particles of shutdown corrosion products to the coolant purification system while keeping them from depositing on the fuel rods. With the use of this routine operation, matters concerned with radiation safety and durability of fuel assemblies in boiling light-water reactors are solved in a more efficient manner.