Non-monotonic albedo behavior in heavy reflectors: a benchmark study of IPEN/MB-01 with MCNP5 and Serpent

The neutronic performance of heavy reflector materials plays a central role in neutron economy, leakage reduction, and spectral behavior in thermal research reactors. In this study, the critical loading configurations of the IPEN/MB-01 reactor with carbon steel and nickel reflectors were reexamined using the Monte Carlo codes MCNP5 and Serpent, based on benchmark experimental configurations reported in the literature. The analysis adopts albedo not as a fixed boundary condition, but as a thickness-dependent design parameter for reflector assessment. Reflector performance was investigated through the combined evaluation of effective multiplication factor, energy-dependent albedo, absorption probability, elastic scattering probability, and spatial neutron flux distributions. Albedo values were determined for thermal, epithermal, and fast neutron groups, while flux maps were used to interpret the transport mechanisms associated with increasing reflector thickness. The results show a non-monotonic dependence of total albedo on thickness, with an initial reduction up to about 2 cm followed by a progressive increase as spectral hardening becomes dominant. Thermal albedo decreases with reflector thickening, whereas epithermal and fast contributions increase. Nickel exhibits superior reflector performance, with lower absorption, higher scattering effectiveness, and total albedo values that exceed the reference water configuration beyond 4.5 cm, while carbon steel approaches a slightly lower asymptotic limit. The integrated analysis of albedo metrics and flux distributions demonstrates that reflector behavior is governed by the interplay between material properties, thickness, and local neutron spectrum. These findings support the use of albedo-based methodology as a practical framework for reflector selection and optimization in reflected reactor systems.