The MIR. M1 reactor has had a long-running research program focused on HEU minimization. In addition, further work is being undertaken as part of the contract (described previously) that was recently signed with the United States to study the feasibility of converting MIR. M1 from HEU to LEU.

If MIR. M1 is converted from HEU to LEU, several key performance characteristics will need to remain the same to allow the reactor to continue to fulfill its main missions. The thermal neutron flux to the experiments cannot be degraded, and the reactor power (100 MW) and campaign dura­tion (30 days) will also need to remain constant.

Two fuel types were considered as candidates for converting the MIR. M1 reactor: (1) a UMo dispersion LEU fuel (described in Chapter 2), and (2) a uranium dioxide (UO2) dispersion LEU fuel (the existing technology). A uranium silicide fuel type was considered at an earlier stage but was ruled out because the technology for producing UO2 and UMo dispersion LEU fuels is better understood in Russia.[73]

UMo dispersion LEU fuel is the most likely candidate for conversion of the MIR. M1 reactor. Recent calculations have shown that to retain the re­quired performance characteristics after conversion, the density of uranium in the core will need to be higher than is possible technologically for UO2 LEU fuel but that is obtainable using UMo dispersion LEU fuel.

UMo dispersion LEU fuel has been tested extensively in Russia. Dif­ferent material compositions (e. g., additions of silicon to the aluminum matrix) as well as different fuel fabrication technologies have been tested both with and without coatings. The results have been positive, particularly when the fuel is coated with titanium nitride. Four tests on full-scale as­semblies have been performed so far—primarily to validate the conversion of the research reactor in Tashkent, Uzbekistan—and the findings have been reported by Russian scientists at conferences on enrichment reduction (Chernyshov et al., 2002). Post-irradiation materials science studies have been performed and are still ongoing.

MIR. M1 staff has found that changes in the thermal loading will require the fuel assemblies to be changed slightly from the original HEU design. Preliminary analysis has shown that using UMo dispersion LEU fuel is feasible if the fuel meat thickness is increased from 0.56 mm to 0.94 mm. Under this scenario, the annual fuel consumption for LEU would be four times higher than for HEU, but the number of fuel assemblies used would decrease by a factor of approximately 1.75.

Overall, it appears that the quality of the core can be improved by us­ing UMo dispersion LEU fuel and changing the fuel meat thickness. The next stage of the feasibility analysis will involve verification using precision programs. However, some outstanding problems remain to be solved for the UMo dispersion LEU fuel before adopting it for use in MIR. RIAR (working in collaboration with Argonne National Laboratory) expects to complete the feasibility study for MIR. M1 by the end of 2011.

Argus

V. A. Pavshuk

The Argus reactor at the Kurchatov Institute in Moscow is one of three HEU-fueled research reactors at the Institute to be included in the U. S.- Russia conversion feasibility study agreement.[74] The Argus reactor is a 20 kW light-water cooled and moderated solution reactor with a core volume of 22 liters of UO2SO4 solution containing 1.71 kg of 90 percent enriched uranium. The reactor is used for neutron radiography, neutron activation analysis, and production of isotopes and nuclear filters.