M. Bencomo, “Parametric Study of molybdenum-99 Production using a Sub-critical Low Enriched Uranium Assembly Design Proposed by Niowave, Inc.”, M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2016).
The radioisotope, technetium-99m (Tc-99 with 6-hour half-life), is used in over 80% of diagnostic medical imaging and is the daughter product from the radioactive decay of the isotope, molybdenum-99 (Mo-99 with 66-hour half-life). Mo-99 is a fission product, with a fission yield of 6.1%, and therefore can be produced by nuclear reactors or by other methods. While Mo-99 has been produced using highly enriched uranium (HEU), there is an international interest to produce this isotope using low enriched uranium (LEU) due to the nuclear proliferation concerns of HEU. Niowave Inc. is a facility that has plans to produce Mo-99 in the United States. The production of Mo-99 in the U.S. ensures its seamless availability to benefit the people who need Tc-99 based medical diagnostics in the country. Mo-99 production was studied for an electron beam and sub-critical LEU assembly design proposed by Niowave Inc. by applying Monte Carlo radiation transport and coupled isotope generation-depletion calculations. In addition, the production of Xe-135, I-131, Pu-239, Ru-105 and Rh-105 were also investigated. The photoneutron source emanating from electron-photon- neutron production scheme of Niowave was studied by varying neutron moderators in the sub-critical system and LEU enrichment to predict optimal production of Mo-99 and other radioisotopes products of interest. The neutron moderators that were considered for this study are light water, heavy water and beryllium. Mo-99 production rate was studied, the predicted value for this study is ~9 kCi per week with a U-235 enrichment of 10% and light water as the neutron moderator. This amount of Mo-99 production could meet 15% of the US demand from one production facility. Studies found that water is the best neutron moderator for the current design to maximize the production of Mo-99. The light-water-moderated system achieves highest criticality level as well as a highest thermal neutron flux and power, when compared to the other two candidates. Heavy water is a better neutron moderator than beryllium for the current design, however, it is not as good as water. Both, heavy water and beryllium can achieve a similar performance as that of light water moderator only if the U-235 enrichment is higher than 19.9%, however the facility is limited to only using LEU fuel. Even at 19.9% enriched fuel, heavy water and beryllium do not achieve the neutron flux, power or Mo-99 production levels when water is used moderator in the system. In conclusion, the studies conducted found that water is the best moderator candidate for this design, maximizing the production of Mo-99.