|By Michele Boyd and Arjun Makhijani
Thorium "fuel" has been proposed as an alternative
to uranium fuel in nuclear reactors. There are not "thorium reactors",
but rather proposals to use thorium as a "fuel" in different types of reactors,
including existing light-water reactors and various fast breeder reactor
Thorium, which refers to thorium-232, is a
radioactive metal that is about three times more abundant than uranium
in the natural environment. Some of the largest reserves are found in Idaho
in the U.S. Large known deposits are in Australia, India, and Norway. The
primary U.S. company advocating for thorium fuel is Thorium Power (www.thoriumpower.com).
Unlike the claims made or implied by thorium proponents, however, thorium
doesn't solve the proliferation, waste, safety, or cost problems of nuclear
power, and it still faces major technical hurdles for commercialization.
Not a Proliferation Solution
Thorium is not actually a "fuel" because it
is not fissile and therefore cannot be used to start or sustain a nuclear
chain reaction. A fissile material, such as uranium-235 (U-235) or plutonium-239
(which is made in reactors from uranium-238), is required to kick-start
the reaction. The enriched uranium fuel or plutonium fuel also maintains
the chain reaction until enough of the thorium target material has been
converted into fissile uranium-233 (U-233) to take over much or most of
The use of enriched uranium or plutonium in
thorium fuel has proliferation implications. Although U-235 is found in
nature, it is only 0.7% of natural uranium, so the proportion of U-235
must be industrially increased to make “enriched uranium” for use in reactors.
Highly enriched uranium and separated plutonium are nuclear weapons materials.
In addition, U-233 is as effective as plutonium-239
for making nuclear bombs. In most proposed thorium fuel cycles, reprocessing
is required to separate out the U-233 for use in fresh fuel. This means
that, like uranium fuel with reprocessing, bomb-making material is separated
out, making it vulnerable to theft or diversion. Some proposed thorium
fuel cycles even require 20% enriched uranium in order to get the chain
reaction started in existing reactors using thorium fuel. It takes 90%
enrichment to make weapons-usable uranium, but very little work is needed
to move from 20% enrichment to 90% enrichment.
It has been claimed that thorium fuel cycles
with reprocessing would be much less of a proliferation risk because the
thorium can be mixed with uranium-238. In this case, fissile uranium-233
is also mixed with non-fissile uranium-238. The claim is that if the U-238
content is high enough, the mixture cannot be used to make bombs without
a complex uranium enrichment plant. This is misleading. More uranium-238
does dilute the uranium-233, but it also results in the production of more
plutonium-239 as the reactor operates. So the proliferation problem remains
– either bombusable uranium-233 or bomb-usable plutonium is created and
can be separated out. Even if the mixture of U-238 and U-233 contains so
much U-238 that it cannot be used for making weapons, the U-233 proportion
can be increased by enrichment – the same process used to enrich natural
uranium in U-235. The enrichment of U-233 is easier than the enrichment
of U-235 because U-233 is much lighter than U-235 relative to U-238 (five
atomic weight units lighter compared to three).
There is just no way to avoid proliferation
problems associated with thorium fuel cycles that involve reprocessing.
Thorium fuel cycles without reprocessing would offer the same temptation
to reprocess as today's once-through uranium fuel cycles.
Not a Waste Solution
Proponents claim that thorium fuel significantly
reduces the volume, weight and long-term radiotoxicity of spent fuel. Using
thorium in a nuclear reactor creates radioactive waste that proponents
claim would only have to be isolated from the environment for 500 years,
as opposed to the irradiated uranium-only fuel that remains dangerous for
hundreds of thousands of years. This claim is wrong. The fission of thorium
creates long-lived fission products like technetium-99 (halflife over 200,000
years). While the mix of fission products is somewhat different than with
uranium fuel, the same range of fission products is created. With or without
reprocessing, these fission products have to be disposed of in a geologic
If the spent fuel is not reprocessed, thorium-232
is very-long lived (half-life: 14 billion years) and its decay products
will build up over time in the spent fuel. This will make the spent fuel
quite radiotoxic, in addition to all the fission products in it. It should
also be noted that inhalation of a unit of radioactivity of thorium-232
or thorium-228 (which is also present as a decay product of thorium-232)
produces a far higher dose, especially to certain organs, than the inhalation
of uranium containing the same amount of radioactivity.
Finally, the use of thorium also creates waste
at the front end of the fuel cycle. The radioactivity associated with these
is expected to be considerably less than that associated with a comparable
amount of uranium milling. However, mine wastes will pose long-term hazards,
as in the case of uranium mining. There are also often hazardous non-radioactive
metals in both thorium and uranium mill tailings.
Ongoing Technical Problems
Research and development of thorium fuel has been undertaken in Germany,
India, Japan, Russia, the UK and the U.S. for more than half a century.
India is often cited as the country that has successfully developed thorium
fuel. In fact, India has been trying to develop a thorium breeder fuel
cycle for decades but has not yet done so commercially.
One reason reprocessing thorium fuel cycles
haven't been successful is that uranium-232 (U-232) is created along with
uranium-233. U-232, which has a half-life of about 70 years, is extremely
radioactive and is therefore very dangerous in small quantities: a small
particle in the lung would exceed legal radiation standards for the general
public. Therefore, fabricating fuel with U-233 is extremely expensive and
Not an Economic Solution
Thorium may be abundant, but it does not mean
that it is economical. Compared to uranium, the thorium fuel cycle will
be costly. In a once-through mode, both thorium target rod production and
uranium enrichment (or plutonium separation) are needed. In addition, as
noted, inhalation of thorium-232 produces a higher dose than the same amount
of uranium-238 (either by radioactivity or by weight). In a breeder configuration,
reprocessing is required, which is costly. Reprocessing is made even more
costly, because the highly radioactive U-232 created in the reactor from
thorium makes worker protection more difficult and expensive for a given
level of annual dose.
Fact sheet completed in January 2009