STUDY OF REDUCED-ENRICHMENT URANIUM FUEL POSSIBILITY FOR RESEARCH REACTORS

Having analyzed the results obtained in the work, it is possible to conclude that the flux density of fast and thermal neutrons in the shell of fuel elements in EFA in REU-zone decreased on average by 5% for UO2 fuel and by 7% for U9%Mo fuel. Change of neutrons flux density during the cycle does not exceed 4% for both fuel types. On average the fuel burnup in reactor core during the cycle for UO2 and U9%Mo increased by 2.8%. It is 1% less that in HEU-zone, which is conditioned by higher initial loading of 235U in fuel assembly with REU fuel.


Introduction
Research nuclear plants (RNP) -research reactors (RR), critical (CEF) and sub-critical experimental facilities (SEF) -played an important role in obtaining fundamental and applied knowledge in the field of nuclear physics. Being the source of neutrons, RNP are considered by researchers a unique instrument for investigations in various fields of science and engineering [1]. According to IAEA data 692 RNP of different types and with different power were built all over the world during the period of nuclear physics development [2].
In 1993 cooperation on the development of reduced enrichment fuel for RR exported by Russia started between Russia and the United States. This cooperation within the Reduced Enrichment Program for Research and Test Reactors is being performed now. The purpose of the Reduced Enrichment Program for Research and Test Reactors is to decrease the risk of nuclear materials distribution, as such materials are applicable for nuclear weapon creation; and increase of physical security of nuclear process facilities by removing enriched fuel from reactor cores of RR.
In 2010 the work on technical-economical evaluation of the conversion possibility of RR operating on highly-enriched uranium started. Conclusion of executive agreement between State Atomic Energy Corporation "Rosatom" and the United States Department of Energy was the basis for this. The main agreement requirements were the following: power of reactors should remain unchanged ; neutrons flux densities in experimental reactors can decrease insignificantly; the required value of neutrons flux densities decrease should be approved by operating organizations; geometry and dimensions of fuel assemblies should remain unchanged (it is desired to keep the size of nuclear fuel elements); economical costs caused by the conversion should be calculated accurately and should not be excessive; fuel component of operating expenses should remain unchanged or can be increased insignificantly [3].
The Russian company TVEL developed nuclear fuel based on reduced uranium for the only Polish RR "Maria", which passed reactor testing successfully.
At present full production of new fuel types with reduced enrichment uranium for research reactors has been developed and mastered at TVEL. The company has stopped exporting fuel assemblies with highly-enriched uranium since 2013. Research reactors of Russian design in such countries as Vietnam, Hungary, Libya, Uzbekistan, Ukraine and Czech Republic were switched to reduced enrichment uranium.
Works on the possibilities to switch Russian RR to reduced enrichment uranium continue, at the same time researches on the determination of the optimum composition of such fuel are carried out. The

Study technique and results
Technological operability possibility of conversion of reactor on dispersion UO 2 and U9%Mo fuel was considered in the work. In both cases the fuel was dispersed into aluminum matrix, and uranium enrichment was 19.7%. Neutron-physical calculations for evaluation of the reactor parameters were carried out using the MCU-RR program based on the Monte-Carlo method. Change of the nuclide fuel composition, reactor core reactivity, capacity of operating and loop fuel assemblies were calculated using the program MCU-RR with the connected program module BURNUP. In calculations with burnup accumulation of fission products absorbing neutrons only weekly and moderately and reactor poisoning by Xe-135 were considered.
For accurate comparison of the results the calculations were performed: at equal reactor power; at equal reactor lifetime; providing steady conditions of off-line refueling of the reactor operation; providing equal reactivity margins at the end of the cycle; providing equal duration of annual planned-preventive maintenance; providing equal reactor shutdown duration between the cycles.
The calculation results are presented in Table 1. For UO 2 fuel in the reactor core with REU it is suggested to use another type of fuel assembly compared to HEU reactor core. Fuel assembly constructively consists of 6 coaxially placed ring-shaped fuel elements fixed at the top and in the bottom. Each fuel element us a three-layer tube in which the fuel layer is covered by aluminum alloy (САВ-6) on both sides.
For U9%Mo fuel in the reactor core with REU it is supposed to use conventional construction fuel assembly with increased core thickness and higher density of uranium fuel composition.