The paper analyzes a possibility for large-scale production of high-purity plutonium suitable for radioisotope thermo-electrical generators (RITEG) of spacecrafts in a light-water power reactor with supercritical coolant parameters. Neptunium and americium fractions of minor actinides from trans-uranium radioactive wastes were used as the starting materials. The starting materials were placed in central fuel assembly (FA) of the reactor core. It was shown that significant plutonium amounts with low content of 236Pu (below 2 ppm) and high content of 238Pu (above 80%) could be produced in central (Np,Am)-FA surrounded by the assemblies filled up with natural or radiogenic lead. These assemblies play a role of a protective barrier against high-energy fission neutrons emitted by the neighboring UO2–FA which are able to increase content of 236Pu through threshold 237Np(n,2n)236Pu reaction. The paper presents numerical evaluations for production rates of the RITEG-suitable plutonium in central (Np,Am)-FA surrounded by such protective lead-containing assemblies.