Calculation of the effect of boron as impurity on the reactivity of the Miniature Neutron Source Reactors using the MCNP code

Highlights: • Miniature Neutron Source Reactors (MNSRs) are modeled using the MCNP-4C Code. • The effect of boron as impurity in MNSRs materials is studied completely. • Purifying the side reflector in MNSRs by only 30 ppm can save 6 fuel rods. - Abstract: In order to estimate the saving of enriched uranium load in Miniature Neutron Source Reactors (MNSRs) a typical MNSR was modeled and simulated for the calculation of the effect of boron as impurity on the initial excess reactivity of MNSRs for the actual highly enriched fuel (UAl₄–Al, about 90% w/o) used in these reactors. Boron is present in 4 materials composing MNSRs, namely: the Side Reflector (SR), the Bottom Reflector (BR), the fuel and the fuel clad. Boron proves to be a very important impurity in the materials of MNSRs being only a small concentration (about 30 ppm) sufficient to save about 6 fuel rods and about 40 ppm would be sufficient to achieve the same result in the fuel, while the bottom reflector would be less important (75 ppm are required) followed by the fuel cladding (80 ppm). No particular importance should be given to the content of natural boron in the irradiated samples whether in the internal irradiation sites or in the external irradiation sites.