FOCUS-TYPE FUEL ASSEMBLY FOR PWRS

In recent years the development efforts for Siemens PWR fuel assemblie s were mainly concentrated on reducing the fuel cycle costs and increa sing the operational reliability of the fuel assemblies. The first obj ective was aimed at increasing the average discharge burnup to > 50 MW d/kgU and increasing the critical heat flux. The high envisaged burnup required to develop a corrosion resistant cladding tube outside the Z ry-4 range. The decision was made to use a Duplex cladding tube consis ting of a corrosion optimized outer layer on a Zry-4 base material. A ZrSnFeCr alloy with reduced tin content was chosen for the outer layer . The critical heat flux could be increased by introducing mixing vane s on the spacer grids within the active length. To reach the second ob jective, reliable avoidance of spacer grid damage during core loading and unloading and reduction of fuel rod defects by debris fretting, th e spacer grid corners were improved and a debris separation grid was d eveloped. These design improvements were introduced into the new FOCUS -type fuel assembly. The name FOCUS stands for ''Fuel assembly with Op timized Cladding and Upgraded Structure''.