Any US site that produces molybdenum-99, a parent to the
radioisotope used in 70–80% of medical imaging, would use low-enriched uranium, which
could aid nonproliferation by prodding suppliers in other countries to do the same.
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 99Mo held last December in Sydney, Australia. Most
99Mo 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 99Mo 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 99Mo 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 99Mo in foreign production facilities." The US exports about 25 kg
of HEU each year, or about half the total used for making 99Mo, 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 99Mo 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 99Mo production in Canada threw the nuclear medicine community into
a panic. With a half-life of 66 hours, 99Mo 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 99Mo 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 99Mo 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 99Mo 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 99Mo.
To produce 99Mo
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 99Mo. "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 99Mo,
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 99Mo has built up enough to reach equilibrium, the reaction will
be stopped and the 99Mo 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 99Mo, 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 99Mo,
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."
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