Published online 25 May 2010 | Nature 465, 408-409
(2010) | doi:10.1038/465408a
Declan Butler
Nuclear Non-Proliferation
Treaty talks grapple with legacy of highly enriched fuel.
Sources: HEU data: Nuclear threat initiative/Monterey Inst. Intl studies;
reactor data: O. Reistad & S. Hustveit Nonprolif. Rev. 15, 266–287
(2008)
As the month-long conference reviewing the
Nuclear
Non-Proliferation Treaty comes to an end this week in New York, efforts
to minimize the world's nuclear arsenal are centre stage.
But many countries at the meeting, held every
5 years, are calling for action on an underappreciated but pressing risk:
getting rid of the legacy of hundreds of research reactors, mainly civilian,
that use weapons-grade highly enriched uranium (HEU).
The total quantity of HEU in research reactors
is small compared with military stocks, but still amounts to a few hundred
tonnes - more than enough to pose a threat, as a nuclear bomb can be made
with just a few dozen kilograms. Security can often be lower at research
reactors, which are typically operated by universities and civilian labs,
raising fears that nuclear material could fall into the hands of terrorists.
The Vienna-based International Atomic Energy
Agency (IAEA) has been helping countries convert their reactors to use
low-enriched uranium (LEU), but cannot force them to do so. Nations including
the United States want a renewed worldwide effort to speed up this process;
in April, President Barack Obama hosted an international nuclear security
summit in Washington DC, which agreed in principle to the ambitious goal
of locking down unsecured civilian nuclear material within 4 years.
Yet progress in converting HEU reactors to
LEU has been slow (see map). Despite the renewed momentum for change, many
of the resolutions supporting reactor conversion at this month's conference
contained key caveats, such as "where this is technically and economically
feasible".
Nature discussed the problem and possible
solutions with Pablo Adelfang, head of the IAEA Research Reactor
Section.
How important are HEU reactors for research?
Very. Research reactors are the cornerstone
of nuclear science and a stepping stone towards nuclear power. They are
unique tools for testing materials and fuel, and for training scientists
and engineers. Other civilian HEU reactors, which share many of the design
characteristics of HEU research reactors, produce vital medical radioisotopes.
How secure are HEU stocks against theft?
Security standards have improved widely. Some
of the HEU is so highly irradiated that it would quickly kill anyone trying
to steal it. But there is a dispute over what levels of irradiation make
HEU self-protecting. Some material that has been cooling in a pool is less
lethal, and one could imagine terrorists taking the risk of stealing it.
Why are reactors difficult to convert?
HEU reactors used for research and to produce medical radioisotopes
have uranium enrichment levels of the order of 90% fissile uranium-235
atoms and just 10% uranium-238. [Unenriched uranium is largely non-fissile,
containing about 99% uranium-238.] The problem is that the volume of the
core of these existing HEU reactors, and their component fuel elements,
are fixed by their initial design. You cannot increase the size of the
core, which would be one way to achieve almost the same number of uranium-235
atoms in the core using LEU (typically about 20% enriched). In any case,
that would amount to designing a new reactor.
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So the only option for conversion is to design
LEU fuels with higher densities of uranium. The challenge has been to increase
fuel density while ensuring that fuels can be easily fabricated and will
perform properly under irradiation to accommodate released fission gases,
for example. Fortunately, almost all HEU research reactors were designed
as relatively low-power, low-fuel-density reactors. That means the high-density
LEU fuels already available are adequate for converting them. But that
leaves 20 or so higher power, higher performance HEU reactors, which would
require LEU fuel of much higher density to match the uranium-235 loading
of their HEU cores.
How are researchers trying to create higher
density LEU fuels?
The highest possible density for a uranium
fuel is pure uranium metal, but it is unstable under reactor conditions.
So it is alloyed with other elements, typically molybdenum at 7-8%. The
density of a uranium–molybdenum alloy is very close to that of pure uranium.
But in tests in 2003, the first promising
next-generation LEU fuel - in which the uranium-molybdenum powder was dispersed
in a heat-conducting aluminium matrix - failed badly. The fuel reacted
with the matrix, became amorphous, and was unable to retain fission gases,
which gathered together in big bubbles, causing breakaway swelling of the
fuel and pillowing of the fuel plates. One solution was to add pure silicon
to either the fuel or the aluminium. Another, the main route being pursued,
was to use aluminium as a cladding rather than a matrix, with a thin layer
of zirconium separating it from a sheet of uranium. Final development and
qualification of this latter fuel should take just a couple of years. It
would allow conversion of all but about ten HEU reactors.
Is cost an obstacle to conversion?
Converting a reactor can cost from US$1 million
to $10 million, depending on its type. The main expenses are purchasing
new fuels and making changes to the reactor's safety and operating systems.
These costs are met by most countries, although the Global Threat Reduction
Initiative, a programme launched in 2004 by the US energy department to
improve nuclear security worldwide, provides technical and funding support
to lower-income countries.
The IAEA cannot demand conversion, but should
countries wish to convert, we will provide technical support. There is
some reluctance from scientists because they fear that conversion might
reduce reactor performance. To reassure them, the IAEA does a detailed
reactor assessment, including calculating any changes in performance.
What's happening with medical reactors?
Less progress has been made in converting
the targets used for medical radioisotope production to LEU, although South
Africa is converting one. Other countries, including Argentina and Australia,
are already using LEU for this purpose. I think that the companies who
produce radioisotopes are inclined to convert, but they are concerned about
costs and the effect on the price of their products. A 2009 report by the
US National Academies, however, found that there was no technical obstacle
to converting, and that it would cause at most a 10% increase in the cost
of medical imaging.
What should happen next?
We need to move on from debating whether it
is economically or technically viable to convert HEU reactors and targets,
and push ahead with doing it. The non-proliferation stakes are too high
to do otherwise. |