Optimization Design For The Leu Core Of The MNSR

Miniature Neutron Source Reactor(MNSR) is a kind of mini research reactor, using high enrichment uranium(HEU) as fuel. But because the HEU fuel has the risk of nuclear safety, and in according to the requirements of nuclear non-proliferation principle,our country should gradually complete the Low Enrichment Uranium(LEU) core design,refueling, and loading work. In this paper, simulation, calculation and optimization design have been done by using MCNP, and the main contents and results can be stated as below.(1)Arming at lowing the enrichment of the uranium fuels for MNSR, a series of design and analysis work have been carried out, mainly including establishment of high enrichment uranium model, optimization design of the low enrichment uranium model without changing the structure of HEU core, and fully new core structure design of the low enrichment uranium model, named New Low Enrichment Uranium(NLEU).(2)In this paper, the HEU core model of MNSR is established by using MCNP. And the parameters such as, HEU core’s central control rod worth, the critical loading quality,top beryllium shim reactivity and axial neutron flux density distribution, are consistent with the experimental data. And this comparison verifies the correctness of the model.(3)In this paper, the LEU core model is established on the basis of HEU core model.UO2 fuel with the enrichment degree of 12.5% is chosen as the fuel pellets in the LEU core, and Zr-4 alloy as the fuels cladding, without changing the rest core structure, size and materials. And by comparing and analyzing the parameters, such as neutron flux density in the inner irradiation channel, excess reactivity, central control rod worth and so on, the conclusion can be stated as, the LEU conversion of MNSR is feasible, and the HEU core of the existing MNSR can be directly replaced by LEU core.(4)In this paper, aiming to solve the problems of the LEU core, the decrease of Φ/P ratio, thermal share ratio and central control rod worth, the NLEU core is fully new designed. UO2 fuel with the enrichment degree of 19.75% is chosen as the fuel pellets inthe NLEU core. In the NLEU core, compared to the LEU core, the 235 U loading mass is reduced by 10.35%, Φ/P ratio and the thermal neutron flux density in inner irradiation channel are higher, and the thermal neutron share is improve form the 42.65% to 45.40%.The results have shown that the NLEU core can optimize the performance, save the cost and be more benefited to the MNSR applications.