The Heart of Modern Nuclear Medicine: Mo-99 Medical Isotope Production in the United States
More than 55,000 Americans rely on it every day for the purpose of diagnosing and staging a multitude of diseases, including cancer and heart disease. Molybdenum-99 (99Mo) is the parent isotope of technetium-99m (99mTc), a light-emitting element that is helping to save lives. 99mTc is used in over 80% of nuclear diagnostic procedures worldwide, and thus lies at the heart of modern nuclear medicine. The market for this product is estimated at over one billion dollars.
For the past few decades, the majority of the US supply of 99Mo has come from the NRU reactor in Canada. That reactor was shutdown for 17 months in 2009/2010 to repair corrosion of the vessel, which had led to heavy water leaking from the base of the reactor. This was an unanticipated event that caused major shortages of 99Mo around the world and highlighted the US’ vulnerability. At the same time, the National Nuclear Security Administration (NNSA) had become concerned about the fact most of the 99Mo produced in the world utilizes processes reliant on highly enriched uranium (HEU), which raises concerns regarding proliferation of nuclear weapons. With the NRU reactor scheduled to shut down permanently in 2016, NNSA has encouraged US-based production technologies that eschew HEU, and has offered partial financial backing to companies with promising designs.
Because Molybdenum has a relatively high fission yield, the most efficient production method is to simply irradiate HEU and separate out the Molybdenum that is produced as a fission product. This same process can be utilized with low enriched uranium (LEU), but the efficiency of the process is reduced. There are several alternative routes to 99Mo production. Irradiating 98Mo targets with neutrons produces 99Mo through the neutron capture process. Bombarding 100Mo targets with high energy gamma rays produces 99Mo through neutron ejection. This latter process can only be accomplished by utilizing a high-energy electron accelerator.
NNSA entered into cost sharing arrangements with four companies, beginning as early as 2007, to spur the development of domestic 99Mo produced without HEU. Each company is eligible for up to $25M in federal funding, provided it is matched by private funds. One of those companies was GE-Hitachi. Their concept utilized a 98Mo target that was proposed to be irradiated in a commercial nuclear reactor, producing 99Mo via neutron capture. However, GE-Hitachi abandoned the project early in 2012, after determining the financial projections did not support the remaining cost to complete the development program.
One early candidate technology developed by B&W utilizes a homogeneous solution reactor. The B&W Medical Isotope Production System (MIPS) is envisioned to utilize multiple 240 kilowatt reactor modules, in which uranium solution (uranyl nitrate, uranyl sulfate, and/or uranyl fluoride) forms the reactor core. Control rods are used for reactivity control. After irradiation, the solution is transferred to hot cells for separation of the molybdenum. After reconditioning, the remaining uranium solution is returned to the reactor.
Although B&W entered a cooperative agreement with NNSA in 2007, it is unclear whether the company continues to pursue the MIPS reactor. B&W reduced its funding of the project early in 2012, and there is little mention of it on the company’s website. It appears B&W is instead focusing on its small modular power reactor concept, the mPower.
SHINE Medical Technologies has developed an advanced accelerator technology for the production of 99Mo as a fission product. The technology irradiates a low-enriched uranium target with neutrons, but the device is essentially a subcritical assembly – not a nuclear reactor. SHINE is an acronym for Subcritical Hybrid Intense Neutron Emitter. The SHINE production process has been validated by research at Los Alamos National Laboratory. SHINE has selected a site in Janesville, Wisconsin, and intends to have their facility operational by 2016. When full production capacity is reached, the SHINE facility is designed to supply 50% of the US demand for 99Mo.
Northstar Medical Radioisotopes, LLC, has developed separation and generator technologies for 99Mo /99mTc. The company was working with GE-Hitachi and also with the University of Missouri as suppliers of 99Mo (both from reactor sources). Both of those arrangements failed as both potential suppliers elected not to continue their efforts. Northstar now proposes a production method that does not involve reactors or uranium. Details of the technology are scarce, but it appears to utilize a linear accelerator in the 50 MeV range to bombard a metal target with high energy electrons, thus producing gamma rays via bremsstrahlung. These gamma rays then irradiate a 100Mo target, ejecting a neutron via gamma-neutron interaction to produce 99Mo. Northstar has developed and tested this technology at Argonne National Laboratory. This process yields a lower specific activity than fission-based processes, and requires recycle of the 100Mo. Northstar has announced a site in Beloit, Wisconsin that is within 20 miles of the SHINE site in Janesville. Northstar expects to break ground on the facility in 2013, and to have the facility operational by 2014.
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