Abstract: A Russian design of Lead- cooled Fast Reactor (LFR) with a mixed uranium-plutonium nitride fuel is chosen as a case study. In order to demonstrate the inherent safety considered in this proposed design in the framework of Generation IV of nuclear reactors, a precise simulation of core has been attempted using MCNP lattice features. The material cross sections have been developed by NJOY & MAKXSF at different temperature levels. Meanwhile, certain static and dynamic parameters such as core effective multiplication factor (k_eff), group-wise and effective delayed neutron fractions are derived. Thermal reactivity feedbacks are calculated by changing the core composition and layout in the MCNP and inspecting its effect on k_eff. A complete systemic model comprising neutronic, thermal hydraulic (for hot channel) and feedbacks sub-systems has been developed. The power reactivity coefficient and reactivity margin (indicative of the maximum reactivity available to insert into the reactor core) are derived afterwards and it is demonstrated that the BREST reactor is equipped with inherent safety, and its reactivity margin stands well below the value of b_eff. The reactor, therefore, does not undergo prompt-criticality phenomena in available reactivity insertion accidents. Meanwhile, certain transient analyses are taken into account to verify the reactor intrinsic safety. Besides, a stability analysis through the formation of state transition matrix for the system describing equations and calculation of its eigenvalues which represent the system poles has been conducted. A set of poles with negative real parts stands for a dynamically stable system which is also a measure of the inherent safety.

Keywords: Inherent safety, Lead-cooled fast reactor, Reactivity margin, Thermal reactivity feedback