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99Mo is an essential medical isotope that comprises of at least 70% of radioactive procedures globally. Currently an essential component of 99Mo manufacturing is the uranium target from which 99Mo is produced by fission. As the world moves towards low enriched uranium (LEU) targets due to non-proliferation concerns it is becoming of interest to find methods to increase the efficiency of the LEU targets in order to reduce the ever increasing nuclear waste levels of which a long term solution for disposal or treatment has yet to be satisfactorily found. Advantages and disadvantages of various target designs are investigated and discussed along current disposal and reprocessing methods. The idea of a reusable target is introduced as a way forward in reducing the nuclear waste burden for future generations. • Fission based manufacturing of 99Mo is moving towards Low Enriched Uranium methods. • Low Enriched Uranium leads to increased radioactive waste volumes. • The majority of uranium targets are single use and highly inefficient. • Reusable targets have the potential to lower the nuclear waste burden. [ABSTRACT FROM AUTHOR]

A new target material combination was modelled to replace the existing uranium-aluminium design used for 99Mo manufacture to increase the sustainability of the production process. Previous efforts to develop a more sustainable uranium target for 99Mo production, resulted in the levels of 239Pu in the target after irradiation being elevated due to the increase in 238U present. MCNP6.2 was used to model 4 different cylindrical targets based on 4–7 days irradiation to further understand this effect. To reduce the resultant 239Pu levels, ratios of 0–99% of Ce were used as a replacement for 238U. The results show that the addition of 140Ce and the removal of 238U reduced the 239Pu levels in the target significantly thus increasing the sustainability of the target and giving a slight increase to the 99Mo output of the targets. [ABSTRACT FROM AUTHOR]

The incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. [ABSTRACT FROM AUTHOR]

Abstract: The structure of one of the oxides proposed to be present in the grey phase of irradiated mixed oxide fuel, the double perovskite Ba2SrUO6 has been investigated from room temperature to 1300K using synchrotron X-ray powder diffraction methods. The divalent strontium and hexavalent uranium are found to be fully ordered in the double-perovskite arrangement of alternating octahedra sharing corner oxygen atoms. At room temperature Ba2SrUO6 adopts a monoclinic structure in space group P21/n. Heating to above 900K induces a first order transition to a rhombohedral structure, and further heating to above 1200K results in a continuous transition to a cubic structure. The sequence of structures is associated with the progressive loss of cooperative tilting of the corner sharing SrO6 and UO6 octahedra. [Copyright &y& Elsevier]

Current fission-based methods of 99Mo production require single use uranium targets which leads to spent uranium waste. This waste could be reduced if a target is developed that does not require dissolution so that it can be reused for multiple production runs. MCNP6.2 was used to model reusable targets of 20% and 1% enrichment for activity produced, target efficiency and burnup. The 1% enriched target was found to be much more efficient but had a lower activity produced compared to the 20% enriched target. The ideal target design for 99Mo production that optimises efficiency and reusability and reduces the self-shielding effect of UO 2 was found to be a target that is made from 1% enriched UO 2 with density as high as allowable for sufficient yields, efficient 99Mo extraction and having an irradiation time of 5 days, with the target able to be re-irradiated and re-processed 2–4 times. • Current fission based manufacturing of 99Mo using uranium targets is based on target destruction. • Target destruction leads to increased radioactive waste volumes. • MCNP6 modelling of uranium targets show that higher density targets have more self-shielding effects and are less efficient. • Low density uranium targets with 5 days irradiation maximise 99Mo output and minimising radioactive waste. [ABSTRACT FROM AUTHOR]