Theoretical and Experimental Investigation of Nuclear Transmutation Pathways for Long-Lived Radioactive Waste Generated in Proposed Nigerian Research Reactors

The safe management of long-lived radioactive waste containing actinides and fission products is a critical challenge for any nation pursuing civilian nuclear energy. For Nigeria's proposed research reactor program under the Nigerian Atomic Energy Commission, understanding the transmutation of long-lived nuclides through neutron capture and fission reactions is essential for waste inventory minimization strategies. This research theoretically investigates nuclear transmutation pathways for key long-lived radioisotopes including technetium-99, iodine-129, americium-241, and neptunium-237 under the neutron flux and energy spectra of both the proposed 30 kW low-enriched uranium miniature neutron source reactor and a conceptual 10 MWth research reactor. Neutron cross-section libraries from ENDF/B-VIII.0 and JEFF-3.3 were benchmarked against experimental activation data from the University of Texas TRIGA reactor to assess their applicability to Nigerian reactor neutron spectrum conditions. A Monte Carlo N-Particle transport code model of both reactor configurations was developed and validated. Activation calculations spanning 10 to 100 year irradiation scenarios with fuel reloading were performed to quantify transmutation efficiency for each target nuclide. Original contributions include the derivation of an analytical approximation for the time-dependent effective transmutation cross-section accounting for the spectral softening caused by target nuclide accumulation during long irradiation campaigns. Results demonstrate that thermal neutron irradiation of technetium-99 in a 30-year irradiation campaign in the proposed 10 MWth reactor could reduce its inventory by 68 percent. Policy recommendations for integrating transmutation considerations into Nigerian nuclear waste management regulation are developed. Keywords: nuclear transmutation, radioactive waste, research reactor, Nigerian Atomic Energy Commission, neutron activation