Thin-walled VVR-M5 fuel elements for research reactors

New VVR-M reactor fuel elements were developed in two stages. In the first stage their geometric parameters were optimized, and the specific heat-transfer surface increased by a factor of 1.8 as a result. Several lots of assemblies of this type (VVR-M3) were in service at the VVR-M reactor of the Institute of Nuclear Physics in Leningrad between 1973 and 1980. In the second stage the design concentration of U 235 in the core was optimized (at 125 g/liter), leading to a large increase in the power production of each assembly and in the reactor reactivity margin. The characteristics of VVR-M assemblies are presented in a table. In particular, the VVR-M5 fuel elements have undergone development and testing, and have been in service for more than 5 years. These elements allow raising the capacity of pool reactors to 30 MW and afford a twofold increase in the core space available for experimental devices. This is achieved with a much lower fuel consumption