Introduction built at the Oak Ridge National Laboratory,

Introduction

 

Nuclear
power has long been one of the solutions in our quest to reduce our reliance on
fossil fuels, as well as meeting the world’s increasing energy demands. In
nearly 500 nuclear reactors all around the world, the fission of uranium or plutonium
is taking place, allowing us to generate vast amounts of energy with little to
no carbon emissions. However nuclear power has its disadvantages,
which include highly radioactive waste products as well as the risk of nuclear
meltdown. 12 The idea of using Thorium as a fuel in nuclear
reactors has been around since the very beginning, and may just be the long-term
solution that we need. There are many experts that believe that a thorium reactor
would offer considerable advantages over its uranium counterpart, and “can mean a 1000+ year solution or a quality low-carbon bridge to
truly sustainable energy sources solving a huge portion of mankind’s negative
environmental impact.”34

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Background and History

 

Research
into thorium based nuclear power was carried out in the 1960s and 1970s in the
United States. In 1964, a molten salt reactor (MRSE) was built at the Oak Ridge
National Laboratory, which used U-233 as its fuel (produced by bombarding
thorium with neutrons). 4 The reactor was in operation very
successfully for several years, from 1965 to 1969 with minimal problems, and
demonstrated that a thorium based nuclear reactor was possible. 5 However
in the 1970s, the US government abandoned the program for various reasons. It
was argued that uranium based reactors were already established and proven. The
1970s were also the height of the cold war, meaning nuclear weapons were a
major factor. Unlike uranium, the thorium fuel cycle produces almost no weapons
grade plutonium, leading the US to prioritising uranium reactors as nuclear
power was heavily linked with nuclear weapons research at the time.

 

However,
thorium research has been making a bit of a comeback in recent times. In India,
a prototype thorium reactor is being built at the Madras Atomic Power Station
in Kalpakkam with an estimated output of 500MWe. In addition, a next generation
molten salt reactor (SALIENT) is under construction by a research team in the Netherlands.
67

 

The Thorium Fuel Cycle

 

The
isotope of thorium used in the thorium fuel cycle is thorium-232. However, the
thorium does not itself undergo fission. Instead, it is placed in a reactor and
bombarded with neutrons. The thorium-232 absorbs a neutron to become thorium-233,
which then decays into protactinium-233 and then finally uranium-233. The
uranium then undergoes fission to produce energy, similarly to conventional
nuclear power plants. 89 This series of reactions are
shown in figures 1 and 2 below:

In
order to keep the reaction going, there must be a continuous supply of neutrons
so that the thorium-232 is able to decay into uranium-233. This removes the
need for control rods in the reactor (which can melt at high temperatures,
sending the reactor into meltdown). Instead, the reaction can be stopped by
simply cutting off the supply of neutrons. This in turn prevents the production
of uranium-233, stopping the fission reaction. 2

 

There
are many types of thorium based nuclear reactors, but the one we will be looking
at in this article is the Liquid fluoride thorium reactor (LFTR). This is a
type of molten salt reactor, meaning the nuclear fuel is mixed with a molten
carrier salt (in this case, a fluoride based salt) to form a fluid. The use of
a fluid fuel has many benefits ranging from efficiency to safety as well as
some drawbacks, all of which will be discussed in the next section. 10 11
A diagram of a typical LFTR is shown in figure 3.

·       Fuel
abundance. It is estimated that
thorium ores (mostly monazite) are three to four times more abundant in the
earth’s crust than uranium ores. It is estimated that the world thorium reserves
(that we can feasibly extract) number around 2.8 million tons and that there is
enough thorium in these sources to meet global energy demands for tens of
thousands of years. We already mine thousands of tons of thorium as a
by-product of rare earth metals mining, but it is usually discarded as waste
due to the lack of current demand. 1 12

 

·       No fuel
enrichment. Nearly all natural
thorium can be used as fuel, meaning there is no expensive fuel enrichment
needed. In comparison, only 0.7% of naturally occurring uranium is the fissile
uranium-235 isotope used in conventional reactors and therefore requires
processing before being used in a reactor. 4

 

·       Difficult to
make nuclear weapons. Another
advantage of the thorium fuel cycle is that it produces very little to no
weapons grade nuclear materials such as plutonium-239. Although uranium-233
(which is produced) could theoretically be used in a bomb, it is very difficult
as the uranium-232 tends to “poison” the uranium-233 and it’s very difficult to
separate them. 8

 

·       Efficiency. A LFTR is also far more efficient than a conventional
reactor. The use of a liquid fuel instead of a solid one means that the reaction
is more thermally efficient, as it can operate at higher temperatures. The molten
salt also acts as a coolant, removing the need for a specialised coolant and is
far more effective than the water used in uranium reactors, as the molten salts
have a much higher specific heat capacity. It is also more fuel efficient, as
it uses up almost all of its fuel, leaving almost no waste. This increased
efficiency means it is estimated that one ton of raw thorium can produce as
much energy as 300 tons of uranium or four million tons of coal. 1

 

·       Less Waste. The efficiency as discussed above means that far less
thorium is needed to produce the same amount of energy as uranium. This means
that there will be much less fission waste products. It is estimated that there
are two to three orders of magnitude less waste from LFTRs compared with
current reactors. The waste itself is also less radioactive, with the
radioactivity dropping down to background levels after a few hundred years, as
opposed to tens of thousands of years. 1

 

·       Safety. There are also many important safety considerations. Since
LFTRs are designed to work at low pressures (either at or even below
atmospheric pressure), any leaks or failures do not lead to a massive increase
in volume, preventing the reactor from exploding. There is also a negative
temperature coefficient of reactivity. This means that increasing the
temperature lowers the reactivity of the fuel. This is due to thermal
expansion. The fuel expands as its temperature increases, reducing the amount
of fuel in the active region of the reaction (as the fuel density will
decrease) and therefore lowering the reactivity, keeping the reaction stable. 13

 

·       Meltdown
Proof. As shown in figure 3, LFTRs
have a freeze plug at the bottom of the reactor. If the temperature inside the
reactor exceeds a certain limit, the freeze plug will melt, causing the fuel to
drain into emergency dump tanks for safe storage and stopping the reaction. As
expected, this is a feature exclusive to molten salt reactors, due to the fuel
being a fluid.  

 

·       Online
refuelling. The liquid fuel also
allows for online refuelling, as extra fuel can just be pumped into the reactor
while it is running. Unlike solid fuel reactors, there is no need to shut down
the reactor to refuel it, making it more economical. 14

 

·       Mining. Thorium is also easier to mine than uranium. Most
thorium ores can be extracted from open pit mines, which are much easier and
safer than most uranium mines, which are usually underground and therefore
require ventilation and can potentially have dangerous levels of radioactive
radon gas. Due to the efficiency of the LFTRs and the fact that nearly all
natural thorium can be used in a reactor, the volumes of thorium required are
likely to be much less than the volumes of uranium required today. This means
that less mining needs to take place to meet demand, making thorium as a fuel
source less environmentally damaging. 15

 

Disadvantages:

 

·       Much more
development is needed. Since thorium
research was largely abandoned in the 1970s and the last fully operational LFTR
was shut down in 1969, a lot more development and research will be needed
before we will be able to build a feasible and efficient thorium based reactor.
This will require a great of time and investment, with some believing that the
perceived advantages are not clear and therefore not worth the investment. 14

 

·       U-232 is
extremely dangerous in the short term. The
thorium fuel cycle produces uranium-232 (during irradiation for use in
reactors) which is extremely radioactive in the short term, giving off strong
gamma rays (2-2.6 MeV) which are very difficult to stop. 8 14

 

·       Little operational
experience. Whereas the uranium cycle
has been in use for over 50 years and is tried and proven, the thorium fuel
cycle is largely untested and unproven due to the limited thorium experiments
conducted throughout the years. This means we have very limited operational
experience with thorium based reactors. 8

 

 

Conclusion

 

In
conclusion, thorium based nuclear power is a much safer, more efficient and
sustainable version of the uranium fuel cycle that is in use today. It has the
potential to provide us with a safe and low carbon solution to the worlds
increasing demand for energy for hundreds of years, until we are able to
develop truly renewable energy sources such as fusion. I believe that many of
the technical difficulties associated with thorium power could be overcome with
sufficient research and development. However as with most alternative energy
sources, thorium research faces a lack of funding and incentive from power
companies to make the jump. A promising development in this respect is that the
Generation IV International Forum seems to have taken thorium based power into
consideration. The Generation IV International Forum is an international group
made up of fourteen countries that is dedicated to the research and development
of the next generation of nuclear reactors. In 2002, six reactor designs were
selected to represent the next generation of nuclear energy. These designs were
chosen for their safety, sustainability, cost effectiveness and resistance to
weapons proliferation, and the molten salt reactor was selected as one of the
six Generation IV reactor designs. This is a very promising development in the
future of thorium power. 16