The first experimental ATF fuel assemblies were loaded into the reactor in January 2019 and a first round of testing was completed in October the same year.
Tvel said each fuel assembly contains 24 fuel elements with four different combinations of cladding and fuel matrix materials, while fuel pellets were made of traditional uranium dioxide, and uranium-molybdenum alloy with increased density and thermal conductivity.
Zirconium alloy with a chromium coating and chromium-nickel alloy were used as rod cladding material, the state-owned company said.
After each irradiation cycle, several fuel rods of each type were extracted from the reactor core for further post-irradiation examinations, Tvel said.
Tvel previously said that fuel assemblies are being tested in the MIR reactor under conditions as close as possible to reactor operational conditions.
MIR is a pool/loop-type research reactor, designed mainly for testing fuel elements, fuel assemblies and other core components of different types of operating and under-development nuclear power reactors.
The global nuclear industry sees accident tolerant fuels as having the potential to transform the safety and operation of nuclear plants and the goal is to begin deploying them in the early to mid-2020s.