Transmutation of long-lived fission products (LLFPs) utilizing electron Bremsstrahlung and compact subcritical assembly

• An innovative design concept for transmuting long-lived fission products—Tc-99, I-129, Cs-135, Zr-93, Se-79, and Pd-107—using electron Bremsstrahlung and a subcritical fuel assembly. • The spherical geometry and low fuel-to-moderator ratio enable significantly lower power generation while enhancing fission product transmutation efficiency, • MCNP6.2 simulations show that a system with a multiplication factor of 0.95 and an electron beam of 500 MeV, 20 mA can achieve transmutation half-lives of under 10 years, with Tc-99 and I-129 exhibiting the shortest half-lives of less than 5 years. • Transmutation half-lives can be further reduced by increasing the multiplication factor beyond 0.95, provided that system safety is maintained at higher values. • Unlike conventional reactors and accelerator-driven systems (ADS), which serve dual purposes of transmutation and power generation, the proposed system is free from constraints such as power peaking, thermal-hydraulic limits, and reactivity control . A novel approach is proposed for neutron transmutation utilizing an electron accelerator coupled with a compact subcritical system. The transmutation converter features a spherical target surrounded by a subcritical assembly composed of a moderator and low-enriched uranium in shell plates. The transmutation of six fission products , Tc-99, I-129, Cs-135, Zr-93, Se-79, and Pd-107, was investigated using electron beams with energies of 300 MeV, 30 mA and 500 MeV, 20 mA. The MCNP6.2 analyses indicate that with an electron beam of 500 MeV, 20 mA, the transmutation half-lives are all under 10 years, with 4.19 years and 5.45 years for Tc-99 and I-129, respectively, except for Zr-93, which has the longest half-life of 35 years. These transmutation half-lives represent systems with multiplication factors of approximately 0.95, and the transmutation half-life can be further reduced by raising the multiplication factor above 0.95 provided that the system’s safety at this multiplication factor is ensured.