The University of Missouri–Columbia’s research reactor (MURR) and the energy technologies company Babcock & Wilcox (B&W) are independently working toward producing molybdenum-99 in the US within several years. If such plans proceed, nuclear medicine and nuclear nonproliferation both stand to gain.
Molybdenum-99 decays into technetium-99m (m is for metastable), the most widely used radioisotope in medical diagnostic imaging; roughly 30 million procedures worldwide use 99mTc annually, according to a report from an international workshop on 99 Mo held last December in Sydney, Australia. Most 99 Mo comes from fission of uranium-235, and the main production facilities—in Canada, Belgium, the Netherlands, and South Africa—use highly enriched uranium, for which transportation, storage, and waste pose proliferation hazards. HEU is defined as having more than 20% fissile 235U—and for 99 Mo production more than 90% enrichment is the norm.
Only a small fraction of the HEU is consumed in the fission reaction, which leaves a lot of weapons-grade waste. In fact, waste from isotope production is more enriched than spent HEU fuel. And, says Pablo Adelfang, who coordinates research reactor activities and is responsible for HEU minimization projects at the International Atomic Energy Agency (IAEA), because of the short irradiation time, the burn-up of the target is extremely low, making it not only more dangerous in terms of proliferation than typical spent fuel, but also easier to handle and thus to steal.
The broader push toward low-enriched uranium took a blow from the Energy Policy Act in 2005, when Congress struck down a requirement that countries importing US HEU for isotope production be working toward converting their reactors to use LEU targets. At the same time, Congress requested that the National Academy of Sciences do a study on the technical and economic feasibility of procuring medical isotopes from LEU; the study includes consideration of savings due to reduced security for LEU waste. The NAS findings are expected to be released in October.
US 99 Mo plans
“Our program is to minimize civilian use of HEU,” says Parrish Staples, manager of the National Nuclear Security Administration’s program to convert reactors from HEU to LEU fuel. “The only HEU the US is currently exporting is for production of 99 Mo in foreign production facilities.” The US exports about 25 kg of HEU each year, or about half the total used for making 99 Mo, he says. If the US stops exporting HEU, he adds, according to the IAEA definition, “a weapon’s worth of material would be removed [from circulation each year].”
Also paving the way for US production of 99 Mo is the 99mTc market. The isotope is used to diagnose such illnesses as heart disease, cancer, and bone, liver, and kidney malfunction. The US accounts for about half the world’s 99mTc use, and the world market is projected to grow by 7–10% a year for the next decade or so.
Moreover, the disruption late last year of 99 Mo production in Canada threw the nuclear medicine community into a panic. With a half-life of 66 hours, 99 Mo can’t be stockpiled. The Canadian reactor was down for maintenance, and its startup was delayed because of safety violations. Such was the upset in the nuclear medicine community that the Canadian government stepped in and ordered the reactor to start up despite some remaining safety concerns—and demoted the head of Canada’s nuclear regulatory agency, Linda Keen.
“At some point there will be an incident somewhere in the world that will cause the US to close its borders to radioactive materials for a day, a week, two or three weeks, whatever,” says MURR director Ralph Butler. “And when you think that there are tens of thousands of patients per day [in the US] utilizing this diagnostic tool [radioactive isotopes], that’s a huge impact.” The US does not have a 99 Mo source, he adds. “There is a national need, and it’s an opportunity we [at MURR] can meet.”
With MURR, Butler aims to supply half of the US 99 Mo demand. Production would follow standard protocol: A uranium target is placed in the neutron field of the reactor, the incident neutrons induce fission, and after some hours the target is removed and the 99 Mo is separated out chemically. Unlike most current facilities, the target at MURR would be LEU, although ironically the 10-MW Missouri reactor uses HEU fuel. “We have the right reactor. We run steady state, and we have considerable FDA [US Food and Drug Administration] experience,” says Butler. Last year MURR made 42 different isotopes for research and commercial applications; from 1969 to 1984, it made 99 Mo.
To produce 99 Mo on a large scale, Butler adds, MURR needs a new processing facility. He estimates the facility would cost upward of $35 million and says he is “seeking funding from public and private donors. Then we have to do a detailed design of the building and submit it to the NRC [Nuclear Regulatory Commission]. Our goal is to be in production by 2012.”
B&W is pursuing a different reactor type to produce 99 Mo. “We have a patented technology to use an aqueous homogeneous reactor,” says Evans Reynolds, program manager for the company’s medical isotope production system. In an AHR, also known as a solution reactor, uranium salt dissolved in water and acid serves as both fuel and target. A solution reactor is attractive, says Reynolds, who is based at B&W’s facility in Lynchburg, Virginia, “because the reaction cannot go out of control. In the liquid environment, gas bubbles form, resulting in a large negative power coefficient of reactivity, and it is thus self-regulating. It’s kind of a fail-safe nuclear reaction.” Moreover, he adds, solution reactors are low cost—he estimates less than $70 million for 200 kW—use less uranium, and have simplified fuel handling, processing, and purification. Solution reactors have been around for a long time, but because of the acid, corrosion has been a problem.
The B&W reactor would be modular, with a basic 200-kW unit capable of producing perhaps 20% of US demand for 99 Mo, Reynolds says. “One of these machines is about the size of a big trash can. They are fairly simple—a drum of liquid, cooling, control rods, and gas management and support systems.”
Every 120 hours, Reynolds continues, when the 99 Mo has built up enough to reach equilibrium, the reaction will be stopped and the 99 Mo will be harvested. “The trick is that we have a solution with uranium, and instead of throwing it away, we just use it again.”
“We believe that this system offers enough commercial advantage, in capital and operating cost, that it should offer a return on investment for someone to build one, rather than converting an existing system from HEU to LEU,” says Reynolds. “The pharmaceutical end of the business buys the targets and puts them in the reactor, and changing from highly enriched uranium to low-enriched uranium may require a new facility, which I suspect is why they are balking at doing it.”
“Since the process of separating the moly from our nitric acid solution is similar to the process used in the existing technology, there is no question that it will work,” says Reynolds. “The question is, How efficient will it be? That’s where we are. We are beginning to look at the separation and purification efficiencies to optimize commercial viability.” Full operation, he adds, “would require a license from the NRC and FDA approval to use this as a pharmaceutical product.”
Writing on the wall
It’s hard to say what the global impact on nonproliferation would be if the US starts producing 99 Mo, says Adelfang. “The trend in the business is that everybody is more open to discuss conversion than they were a year or two or three ago. It’s not just technical issues. It’s a political and financial issue. But one can be sure it would have a psychological impact, and that would be strong.” Adds Alan Kuperman, a professor of public affairs at the University of Texas at Austin and a senior policy analyst for the Nuclear Control Institute, “It would finally put a stake through the heart of the myth that while you can produce isotopes from LEU, you can’t do it on a large scale.” If the US starts making 99 Mo, he adds, “the message [to current producers] would be that your only shot to save your market share is to convert to LEU.”
A major producer of isotopes, the high-flux reactor in Petten, the Netherlands, converted to LEU fuel a couple of years ago, and at the workshop in Australia the reactor’s manager announced that a planned successor will only allow LEU targets. “I think the Dutch announcement reflects an understanding of the way the world is moving,” says Kuperman. “The way to lock it in and make sure it really happens is for the US to move ahead with funding domestic production with LEU. That would get everybody to see the writing on the wall.”
In this total body bone scan using 99mTc linked to methylene diphosphonic acid, the black spots indicate metastases from prostate cancer. The MDP binds to bone and accumulates in areas of cancer.
In this total body bone scan using 99mTc linked to methylene diphosphonic acid, the black spots indicate metastases from prostate cancer. The MDP binds to bone and accumulates in areas of cancer.
