By David Warmflash in Discover Magazine
Nuclear power has long been a
contentious topic. It generates huge amounts of electricity with zero carbon
emissions, and thus is held up as a solution to global energy woes. But it also
entails several risks, including weapons development, meltdown, and the hazards
of disposing of its waste products.
But those risks and benefits all
pertain to a very specific kind of nuclear energy: nuclear fission of uranium
or plutonium isotopes. There’s another kind of nuclear energy that’s been
waiting in the wings for decades – and it may just demand a recalibration of
our thoughts on nuclear power.
Nuclear fission using thorium is
easily within our reach, and, compared with conventional nuclear energy, the
risks are considerably lower.
Thorium’s Story
Ideas for using thorium have been
around since the 1960s, and by 1973 there were proposals for serious, concerted
research in the US. But that program fizzled to a halt only a few years later.
Why? The answer is nuclear weapons. The 1960s and ’70s were the height of the
Cold War and weaponization was the driving force for all nuclear research. Any
nuclear research that did not support the US nuclear arsenal was simply not
given priority.
Conventional nuclear power using a
fuel cycle involving uranium-235 and/or plutonium-239 was seen as killing two
birds with one stone: reducing America’s dependence on foreign oil, and
creating the fuel needed for nuclear bombs. Thorium power, on the other hand,
didn’t have military potential. And by decreasing the need for conventional
nuclear power, a potentially successful thorium program would have actually
been seen as threatening to U.S. interests in the Cold War environment.
Today, however, the situation is
very different. Rather than wanting to make weapons, many global leaders are
worried about proliferating nuclear technology. And that has led several
nations to take a closer look at thorium power generation.
How Thorium Reactors Work
The isotope of thorium that’s being
studied for power is called Th-232. Like uranium, Th-232 comes from rocks in
the ground.
A thorium reactor would work like
this: Th-232 is placed in a reactor, where it is bombarded with a beam of
neutrons. In accepting a neutron from the beam, Th-232 becomes Th-233, but this
heavier isotope doesn’t last very long. The Th-233 decays to protactinium-233,
which further decays into U-233. The U-233 remains in the reactor and, similar
to current nuclear power plants, the fission of the uranium generates intense
heat that can be converted to electricity.
To keep the process going, the U-233
must be created continuously by keeping the neutron-generating accelerator
turned on. By contrast the neutrons that trigger U-235 fission in a
conventional reactor are generated from the fuel itself. The process continues
in a chain reaction and can be controlled or stopped only by inserting rods of
neutron-absorbing material into the reactor core. But these control rods aren’t
foolproof: their operation can be affected during a reactor malfunction. This
is the reason that a conventional fission reactor has the potential to start
heating out of control and cause an accident. A thorium fuel cycle, by
contrast, can be immediately shut down by turning off the supply of neutrons.
Shutting down the fuel cycle means preventing the breeding of Th-232 into
U-233. This doesn’t stop the heating in the reactor immediately, but it stops
it from getting worse.
The increased safety of thorium
power does not end there. Unlike the U-235 and plutonium fuel cycles, the
thorium reactors can be designed to operate in a liquid state. While a
conventional reactor heading to meltdown has no way to jettison the fuel to
stop the fission reactions, a thorium reactor design called LFTR features a plug at the bottom of the reactor that will
melt if the temperature of the reacting fuel climbs too high. If that happens
the hot liquid would all drain out and the reaction would stop.
Powered Up
Thorium power has other attractions,
too. Its production of nuclear waste would be orders of magnitude lower than
conventional nuclear power, though experts disagree about exactly how much:
Chinese researchers claim it’s three orders of magnitude (a
thousandth the amount of waste or less), while U.S. researchers say a hundredth the amount of waste.
Thorium would be easier to obtain
than uranium. While uranium mines are enclosed underground and thus very
dangerous for the miners, thorium is taken from open pits, and is estimated to
be roughly three times as abundant as uranium in the Earth’s crust.
But perhaps the most salient benefit
of thorium power, in our geopolitically dicey world, is that the fuel is much
harder to turn into a bomb. Thorium itself isn’t fissile. The thorium fuel
cycle does produce fissile material, U-233, which theoretically could be used in a bomb. But
thorium would not be a very practical route to making a weapon, especially with
LFTR technology. Not only would the proliferator have to steal the fissile
U-233 as hot liquid from inside the reactor; they’d also be exposed to an
extremely dangerous isotope, U-232, unless they had a robot to carry out the
task.
Future Fuel
China has announced that its researchers will produce a
fully functional thorium reactor within the next 10 years. India, with one of
the largest thorium reserves on the planet but not much uranium, is also
charging ahead. Indian researchers are planning to have a prototype thorium
reactor operational early next year, though the reactor’s output will
be only about a quarter of the output of a typical new nuclear
plant in the west. Norway is currently in the midst of a four-year test of using thorium fuel rods in existing
nuclear reactors.
Other nations with active thorium
research programs include the United Kingdom, Canada, Germany, Japan, and
Israel.
There are some drawbacks to thorium
fuel cycles, but they are highly technical. For instance, thorium reactors have
been criticized as potentially having more neutron
leak compared with conventional reactors. More neutron leak means more
shielding and other protection is needed for workers at the power plant. And as
in most types of alternative energy, thorium power faces a lack of funding for
research and of financial incentives for power companies to switch over.
In recent decades, stories about
safe, green nuclear power in popular media have tended to focus on the quest
for nuclear fusion. Certainly, we can expect, and should hope, for continued
progress toward that type of power. But while that happens, the investments by
China, India, and other countries suggest that thorium is en route to
contribute to the grid in the near term – and to dramatically improve the
world’s energy sustainability in the process.
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