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99 Mo, the parent of the widely used medical isotope 99m Tc, is currently produced by irradiation of enriched uranium in nuclear reactors. The supply of this isotope is encumbered by the aging of these reactors and concerns about international transportation and nuclear proliferation. Methods: We report results for the production of 99 Mo from the accelerator-driven subcritical fission of an aqueous solution containing low enriched uranium. The predominately fast neutrons generated by impinging high-energy electrons onto a tantalum convertor are moderated to thermal energies to increase fission processes. The separation, recovery, and purification of 99 Mo were demonstrated using a recycled uranyl sulfate solution. Conclusion: The 99 Mo yield and purity were found to be unaffected by reuse of the previously irradiated and processed uranyl sulfate solution. Results from a 51.8-GBq 99 Mo production run are presented. Thedaught er of 99 Mo (half-life, 66 h), 99m Tc (half-life, 6 h), is used in more than 45 million diagnostic nuclear medicine procedures annually worldwide, with approximately 16.7 million procedures performed in the United States alone (1). Despite being the largest single user of 99m Tc, the United States currently imports 100% of its supply (2). Current supply chains of 99 Mo rely on aging nuclear reactors, such as the High Flux Reactor in The Netherlands and the National Research Universal reactor in Canada (1). The major U.S. supplier, located in Canada, will cease normal production in late 2016 but will maintain the ability to produce 99 Mo for another 2 y, if a severe shortage occurs (3). The High Flux and National Research Universal reactors have exceeded their initial design lifetimes of 40 y, having been in operation for 55 and 59 y, respectively. In 2009-2010, both reactors were shut down for extended periods of time, causing a severe 99 Mo shortage (4,5). The 99 Mo shortage forced clinicians to ration imaging procedures, which delayed critical diagnostic tests or resulted in older, less effective techniques that in many cases increased the radiation dose to the patient (6). The U.S. 99 Mo market is also fragile because it relies solely on international air transportation, which has been halted in the past due to inclement weather, natural phenomena, flight delays, and terrorist threats (6).The predominant global 99 Mo production route is irradiation of highly enriched uranium (HEU, $20% 235 U) solid targets in nuclear reactors fueled by uranium (2). Other potential 99 Mo production paths include (n,g) 98 Mo and (g,n) 100 Mo; however, both routes require enriched molybdenum material and produce low-specificactivity 99 Mo, which cannot be loaded directly on a commercial 99m Tc generator. The U.S. National Nuclear Security Administration implements the long-standing U.S. policy to minimize and eliminate HEU in civilian applications by working to convert research reactors and medical isotope production facilities to low enriched uranium (LEU, ,20% 235 U) worldwide (7). In 2009, the Global...

Molybdenum-99 is the parent of Technetium-99m, which is used in nearly 80% of all nuclear medicine procedures. The medical community has been plagued by Mo-99 shortages due to aging reactors, such as the NRU (National Research Universal) reactor in Canada. There are currently no US producers of Mo-99, and NRU is scheduled for shutdown in 2016, which means that another Mo-99 shortage is imminent unless a potential domestic Mo-99 producer fills the void. Argonne National Laboratory is assisting two potential domestic suppliers of Mo-99 by examining the effects of a uranyl nitrate versus a uranyl sulfate target solution configuration on Mo-99 production. Uranyl nitrate solutions are easier to prepare and do not generate detectable amounts of peroxide upon irradiation, but a high radiation field can lead to a large increase in pH, which can lead to the precipitation of fission products and uranyl hydroxides. Uranyl sulfate solutions are more difficult to prepare, and enough peroxide is generated during irradiation to cause precipitation of uranyl peroxide, but this can be prevented by adding a catalyst to the solution. A titania sorbent can be used to recover Mo-99 from a highly concentrated uranyl nitrate or uranyl sulfate solution; however, different approaches must be taken to prevent precipitation during Mo-99 production.

SHINE Medical Technologies plans to use fissioning of a low enriched uranium (LEU) solution as uranyl sulfate for molybdenum-99 production. One of the major concerns for SHINE is peroxide formation from radiolysis, which can lead to precipitation of uranyl peroxide. Bench-top experiments where peroxide was added directly to a uranyl sulfate solution were performed to determine the concentration where precipitation occurs as a function of temperature and concentration of ferrous or ferric ion to aid in peroxide destruction. Based on the experimental results and relevant literature, a thermodynamic/kinetic model for the precipitation of uranyl peroxide for a given set of conditions was developed and tested. The conditions that must be specified in the model are temperature, uranyl sulfate concentration, ferrous-or ferric-ion concentration, and the H 2 O 2 production rate. Additionally, experiments were performed using the Van de Graaff accelerator as a radiation source for radiolytically induced peroxide formation. Uranyl peroxide precipitated at 12°C when a uranyl sulfate solution was exposed to a radiation dose of 7900 Mrad in the pulse mode, but precipitation did not occur in the direct current (DC) mode at much higher powers. Because precipitation could not be achieved in the DC mode for this set of experiments, the effects of temperature and power on uranyl peroxide formation could not be determined. Future experiments will monitor current continuously, measure gas flow rates continuously, and use a uranyl sulfate solution that contains little to no iron impurity.

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