Comparative safety assessment of HEU and LEU core of MNSR under large reactivity insertion accidents

Highlights: • The power excursion characteristics of MNSR against various large RIA were simulated for HEU and LEU. • The code MERSAT was employed to model the MNSR and conduct the safety analysis. • The results indicated the inherently safety feature of MNSR under such extreme accident scenario. - Abstract: This work presents the results of thermal hydraulic and safety analysis of low enriched MNSR core compared with the original one of high enriched core, under large reactivity insertion accidents. The PIE leading to the considered accident scenario is a combined effect of inner and outer irradiations sites flooding by jointly inadvertent full ejection of control rod. The resulting excess reactivity insertion varies between 5.5 mk (0.65 $) and 7.15 mk (0.85 $). The code MERSAT has been employed to model the MNSR and conduct the safety analysis. The excess reactivity values are estimated using MCNP code and experimental data. The results demonstrate that even in case of the highest reactivity insertion of 7.15 mk (0.85 $) corresponding to about 85% of delayed neutron fraction, the prevailing conditions remain within the safety limits for both HEU and LEU cores due to the high self limiting features. Although the maximum clad temperature arrived about 143 °C with the consequent of initiating subcooled boiling, the DNB limit was not exceeded in any time of the accident. It is rather anticipated that void formation can supported the inherent safety feature via negative void effect of reactivity. Thus, from the viewpoint of safety requirement the proposed LEU fuel presents a possible replacement of the current HEU fuel of MNSR reactors