Most Russian research and test reactors use HEU fuels consisting of UO2-aluminum dispersions fabricated as thin-walled tubular elements of various enrichments and configurations. A Russian program was started in the 1990s to further reduce the enrichment of fuel used in Russian-origin research reactors that are located outside of the Russian Federation. This work has been led by three Russian organizations (NIKIET, Bochvar All-

Russian Scientific Research Institute for Inorganic Materials [VNIINM], and Novosibirsk Chemical Concentrates Plant [NZKhK]) with the collabo­ration of several other organizations and customers (i. e., research reactor operators) and has resulted in the development of LEU fuels.

The initial phase of this program created UO2-Al LEU fuel assem­blies for conversion of all existing Russian-origin research reactors that are located outside of the Russian Federation. The aim was to reduce the enrichment of uranium in the fuel elements without changing fuel element geometry. LEU fuel assemblies of several designs have been developed (Figure 2-3):

• VVR-M2 fuel assembly. This assembly has a tubular geometry and contains a UO2-aluminum dispersion fuel meat with a density of 2.5 gU/cm3. These fuel assemblies have undergone a full cycle of design, testing, and licensing and are currently being manufactured at the fuel production facility at NZKhK in Novosibirsk. This fuel is being supplied to Russian — origin research reactors in Hungary, Vietnam, and Romania.

• IRT-4M fuel assembly. This assembly has a square geometry and contains a UO2-aluminum dispersion fuel meat with a density of 3.0 gU/cm3. This fuel, which is fully licensed, is the highest-demand fuel for Russian-origin research reactors located outside of the Russian Federation. This fuel is being supplied to Russian-origin research reactors in the Czech Republic, Uzbekistan, and Libya.

• VVR-KN fuel assembly. This assembly has a hexagonal geometry and is being developed for use in a Russian-origin research reactor in Ka­zakhstan. It will replace a 36 percent enriched assembly that is now in use. Three assemblies have been manufactured and are now being irradiated in the reactor. Conversion studies and fuel qualification activities for this reac­tor are proceeding in close cooperation with the reactor operator, producing good results.

• MR fuel assembly. Design work is about to begin to develop a UO2-aluminum dispersion fuel for this tubular fuel assembly. The fuel meat (which currently has an enrichment of 36 percent) is expected to have an enrichment of 19.5 percent with a density no less than 3.5 g U/cm3. It is expected to take about a year to complete this design work and manufac­ture fuel assemblies for testing. The 19.5 percent enriched fuel will be used in the Russian-origin MARIA research reactor in Poland.

The transition to these LEU fuel assemblies has proceeded using the same fabrication technologies and equipment for producing HEU fuel. However, the use of LEU fuels can reduce reactor “performance” (i. e., re­duce neutron flux densities in the core and reflector regions) by up to about 15 percent and shorten fuel replacement cycles. Consequently, the develop-

ment of higher-density LEU fuels is needed to maintain reactor performance and fuel cycle length and also to increase fuel robustness by allowing an increase in cladding thickness.

The development of higher-density fuels is being carried out in a second phase of the Russian program to reduce fuel enrichments. Work is proceed­ing on a UMo dispersion LEU fuel with a density of about 5 gU/cm3.[33] Test irradiations of this material have been carried out to burnups of 40-60 percent. Design efforts are under way for two fuel assembly types: IRT-3M (which has a tubular geometry) and IRT-U (which has a pin geometry).

The third phase of the reduced enrichment program is envisaged to involve the development of completely new fuel designs for future reac­tors. These new designs should be safe, reliable, easy to fabricate, and economically efficient compared to current designs. UMo monolithic LEU fuels manufactured in the form of pins appear to be a promising future de­sign concept. These could be arranged in geometries to mimic the tubular, square, and hexagonal geometries of current-generation fuel assemblies that are used in Russian-origin research reactors.