Yuanzineng kexue jishu (Mar 2023)
Design Calculation and Experimental Verification of Nigerian MNSR with LEU Core
Abstract
The Nigerian miniature neutron source reactor (MNSR) is a research reactor developed and constructed by China Institute of Atomic Energy. The high enriched uranium (HEU) fuel is used in the reactor core fuel. In order to prevent nuclear proliferation, the Chinese government is committed to actively assisting other countries completing the conversion of low enriched uranium (LEU) of the research reactor, then the work of Nigerian MNSR with LEU core is started. In order to carry out the conversion of LEU smoothly, Nigerian MNSR with LEU core model was constructed using MCNP, WIMSD and CITATION codes, and main parameters of the Nigerian MNSR with LEU core were calculated, such as critical mass, central control rod reactivity efficiency, upper beryllium reactivity efficiency in outer ring, thermal neutron fluence rate in inner and outer irradiation tubes, core reactivity efficiency, irradiation tubes reactivity efficiency of core radial and axial in-homogeneity coefficients, fuel temperature coefficient, moderator temperature coefficient, effective delayed neutron fraction and prompt neutron lifetime, etc. Then, the zero-power experiment was completed on MNSR zero-power experiment device, the reactivity efficiencies of experimental components such as the central control rod, upper beryllium, fuel elements in outer ring, inner irradiation tube and outer irradiation tube were measured using the classical periodic method in reactor physics experiments. The experimentally measured value of the reactivity efficiency of the central control rod is measured as 742 pcm, the maximum backup reactivity of the Nigerian MNSR with LEU core is 400 pcm, and the minimum shutdown depth is 342 pcm, which meets the safety requirements for MNSR shutdown. The value of the reactivity efficiency of the upper beryllium reflector layer is 1 750 pcm. According to the operating experience, if the reactivity is consumed at 50 pcm per year, the Nigerian MNSR with LEU core can continue to operate for about 35 years without replacing the core. The zero-power experimental measurement data are in accordance with the design calculation data whose relative deviation between them is basically whin 5%. On the basis of these experimental and calculation data, the Nigerian MNSR with LEU core safety function experiment was successfully completed. The neutron fluence rate increases to a peak of about 2.2×1012 cm-2·s-1, and then gradually decreases. Due to the temperature negative feedback and characteristics of the Nigerian MNSR with LEU core, when the control rod is stuck in the reactor and a large reactivity is accidentally introduced, the reactor can be automatically adjusted and in a controllable safety level. The results of the experiment confirm the good safety performance of the Nigerian MNSR with LEU core, marking the completion of the Nigerian MNSR from HEU to LEU, which has contributed to the prevention of nuclear proliferation in the international community.
