Thorium: The Future Of Nuclear Power – Part I

Introduction

A steadily rising population combined with improved living standards among the emerging economies is fuelling an increasing demand for energy. Global electricity demand is forecast to rise 76 per cent by 2030, the majority of which still comes from burning fossil fuels. Not only are coal, oil and natural gas finite sources of energy, there are also geopolitical, environmental and financial incentives for developing alternatives.

Perhaps one of the best available alternatives is thorium. When burned in a new type of safer and cleaner nuclear reactor, thorium, which is a naturally occurring radioactive metal, can provide vast amounts of power with very few drawbacks.

A History of Thorium

The element thorium (Th) was discovered in 1828 by a Swedish chemist Jons Jakob Berzelius who named the silvery white metal after Thor, the Norse god of thunder. It was another sixty years however before Marie Curie discovered that thorium was radioactive.

The process of nuclear fission was first discovered in 1938 by a chemist named Otto Hahn in Germany. Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller fragments releasing tremendous amounts of energy. Following its discovery it was quickly realised that a fission chain reaction could be utilised in the making of a bomb and so on the eve of World War II, fearing that the Nazi’s would develop the weapon first, the United States launched the Manhattan Project.

Scientists working on the project began by looking at uranium as the fission fuel for their new bomb. Uranium has two isotopes, uranium-235 and uranium-238. Uranium-235 is naturally fissile and thus provided one avenue for the project to explore. They also found that the more common uranium-238 could be bombarded with neutrons to create a new element, plutonium, which was also fissile and also provided the potential for an explosive device.

They then looked at thorium and found that when thorium-232 absorbs a neutron it becomes uranium-233 which is also fissile and could be used in a bomb. Using thorium-232 to make a weapon had severe drawbacks however and therefore attention was focused on uranium and plutonium. The result was the Hiroshima bomb which was a uranium-235 bomb and the Nagasaki bomb which was a plutonium bomb.

The move towards generating electrical power from nuclear really began in the 1950’s at which time there were two schools of thought. There were those in favour of the current technology of light water reactors (LWRs) that produce plutonium, and those who wanted to go in the direction of thorium. Since uranium and plutonium were better understood they were considered a safer bet and therefore these technologies received the bulk of the research effort in the atomic power programme.

Despite this decision a group of scientists at the Oak Ridge National Laboratory (ORNL) led by Alvin Weinberg continued their research into thorium, developing and testing the first molten salt reactor (MSR). The reactor ran successfully from 1965 to 1969 proving the validity and benefits of thorium as a nuclear fuel and becoming the blueprint for the Liquid-Fluoride Thorium Reactor or LFTR (pronounced “lifter”).

The Advantages of Thorium & the LTFR

It’s possible to use thorium in today’s solid fuel water cooled reactors, however it is the unique combination of the thorium cycle and the Liquid-Fluoride Thorium Reactor (LFTR) that provide the big advantages.

The LFTR is a type of molten salt reactor (MSR), so-called because it uses uranium and thorium dissolved in fluoride salts. These fluoride salts are impervious to radiation damage and because they are chemically extremely stable they can be heated to very high temperatures without needing to be pressurised. As the fluoride solution heats up its expansion reduces the amount of fuel in the reactor’s core, causing the reaction rate to slow down and as the salt cools the reaction rate increases. This feedback mechanism operates without human intervention making the LFTR much safer than uranium reactors and preventing a runaway reaction or reactor melt down.

The LFTR also features a special safety device called a ‘freeze plug’ that sits at the base of the reactor vessel. The plug is made from a piece of frozen fluoride salt that is kept frozen by a stream of cool gas. If the liquid fluoride gets too hot or the reactor loses power the plug melts and the fluoride solution automatically drains out into a tank that is passively cooled. This is in contrast to LWRs which require a constant source of power in order to keep the cooling water circulating and prevent a meltdown.

Unlike today’s light water reactors that operate at very high pressure, LFTR operates at atmospheric pressure so there is no need for large pressure containment buildings, allowing the reactor facility to be much smaller whilst still maintaining the same power output. The reduced size and complexity of the LTFRs design leads to a significant reduction in cost and even though a full-scale LFTR has never been built it’s expected that the full lifecycle cost of a thorium reactor would be at least 30% to 50% less than equivalent uranium-based facility.

Conventional reactors also have to be shutdown to be refuelled whereas LTFR can be fuelled continuously. The compact nature of the LTFR also means that a fully operational modular unit could be transported on the back of a lorry to remote sites or disaster zones. Due to their high operating temperature they also lend themselves to applications such as hydrogen production, desalination and coal to liquid production.

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