December 5, 2012
Thorium is being touted as an ideal fuel
for a new generation of nuclear power plants, but in a piece in this
week's Nature, researchers suggest it may not be as benign as
The element thorium, which many regard as a potential nuclear
"wonder-fuel," could be a greater proliferation threat than
previously thought, scientists have warned.
Writing in a Comment piece in the new issue of the journal, Nature,
nuclear energy specialists from four British universities suggest
that, although thorium has been promoted as a superior fuel for
future nuclear energy generation, it should not be regarded as
inherently proliferation resistant.
The piece highlights ways in which small
quantities of uranium-233, a material useable in nuclear weapons,
could be produced covertly from thorium, by chemically separating
protactinium-233, during its formation.
The chemical processes that are needed for protactinium separation
could possibly be undertaken using standard lab equipment,
potentially allowing it to happen in secret, and beyond the
oversight of organizations such as the International Atomic
Energy Agency (IAEA), the paper says.
The authors note that, from previous experiments to separate
protactinium-233, it is feasible that just 1.6 tonnes of thorium
metal would be enough to produce 8kg of uranium-233 which is the
minimum amount required for a nuclear weapon.
Using the process identified in their
paper, they add that this could be done "in less than a year."
"Thorium certainly has benefits, but
we think that the public debate regarding its
proliferation-resistance so far has been too one-sided," Dr
Steve Ashley, from the Department of Engineering at the
University of Cambridge and the paper's lead author, said.
"Small-scale chemical reprocessing of irradiated thorium can
create an isotope of uranium - uranium-233 - that could be used
in nuclear weapons. If nothing else, this raises a serious
Thorium is widely seen as an alternative
nuclear fuel source to uranium.
It is thought to be three to four times
more naturally abundant, with substantial deposits spread around the
world. Some countries, including the United States and the United
Kingdom, are exploring its potential use as fuel in civil nuclear
Alongside its abundance, one of thorium's most attractive features
is its apparent resistance to nuclear proliferation, compared with
uranium. This is because thorium-232, the most commonly found type
of thorium, cannot sustain nuclear fission itself.
Instead, it has
to be broken down through several stages of radioactive decay. This
is achieved by bombarding it with neutrons, so that it eventually
decays into uranium-233, which can undergo fission.
As a by-product, the process also produces the highly radiotoxic
uranium-232. Because of this, producing uranium-233 from
thorium requires very careful handling, remote techniques and
heavily-shielded containment chambers. That implies the use of
facilities large enough to be monitored.
The paper suggests that this obstacle to developing uranium-233 from
thorium could, in theory, be circumvented.
The researchers point out
that thorium's decay is a four-stage process:
thorium-232 breaks down into thorium-233.
After 22 minutes, this
decays into protactinium-233.
And after 27 days, it is this
substance which decays into uranium-233, capable of undergoing
Ashley and colleagues note from previously existing literature that
protactinium-233 can be chemically separated from irradiated
Once this has happened, the protactinium
will decay into pure uranium-233 on its own, with little radiotoxic
"The problem is that the neutron
irradiation of thorium-232 could take place in a small
facility," Ashley said. "It could happen in a research reactor,
of which there are about 500 worldwide, which may make it
difficult to monitor."
The researchers note that from an early
small-scale experiment to separate protactinium-233, approximately
200g of thorium metal could produce 1g of protactinium-233 (and
therefore the same amount of uranium-233) if exposed to neutrons at
the levels typically found in power reactors for a month.
This means that 1.6 tonnes of thorium
metal would be needed to produce 8kg of uranium-233. They also point
out that protactinium separation already happens, as part of other
Given the need for access to a research or power reactor to
irradiate thorium, the paper argues that the most likely security
threat is from potential willful proliferator states. As a result,
the authors strongly recommend that appropriate monitoring of
thorium-related nuclear technologies should be performed by
organizations like the IAEA.
The report also calls for steps to be
taken to control the short-term irradiation of thorium-based
materials with neutrons, and for in-plant reprocessing of
thorium-based fuels to be avoided.
"The most important thing is to
recognize that thorium is not a route to a nuclear future free
from proliferation risks, as some people seem to believe,"
"The emergence of thorium
technologies will bring problems as well as benefits. We need
more debate on the associated risks, if we want a safer nuclear