Credit: Justin Randall

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AN ALTERNATIVE DESIGN does away with the requirements for uranium or plutonium altogether, and relies on thorium as its primary fuel source. This design, which was originally dubbed an Energy Amplifier but has more recently been named an Accelerator Driven System (ADS), was proposed by Italian Nobel physics laureate Carlos Rubbia, a former director of one of the world's leading nuclear physics labs, CERN, the European Organisation for Nuclear Research.

An ADS reactor is sub-critical, which means it needs help to get the thorium to react. To do this, a particle accelerator fires protons at a lead target. When struck by high-energy protons the lead, called a spallation target, releases neutrons that collide with nuclei in the thorium fuel, which begins the fuel cycle that ends in the fission of U-233.

A nuclear reactor that requires a particle beam to keep it running might seem a bit strange. But on the contrary, this is one of the ADS design's most attractive features. If the particle beam is switched off, it is impossible for the fuel to enter a chain reaction and cause a meltdown. Instead, the rate of fission will immediately begin to slow and the fuel will eventually cool down and die out. According to Sydney's Hashemi-Nezhad, a sub-critical reactor such as this has clear safety benefits over uranium reactors. "It has zero chance of a Chernobyl-type accident," he said.

Another major advantage of this design is that it only requires thorium as fuel.

Hashemi-Nezhad also says thorium is a highly abundant resource "550 times more abundant in nature than uranium-235".

It's also an element in which Australia is well blessed - we have the largest known thorium reserves in the world. Thorium mining is also less complex than uranium mining; and the ore doesn't even require enrichment before use in an ADS reactor.

In a non-proliferation sense, there are also good reasons to prefer a sub-critical thorium reactor, as it is impossible to make weapons-grade materials from thorium.

Even traces of unburnt U-233 in thorium reactor waste products are more difficult to convert into a usable nuclear weapon than U-235 or Pu-239. Imagine the West offering thorium-fuelled ADS reactors to countries such as Iran or North Korea: this would satisfy their demands for cheap nuclear power, but entirely avert the risk of the civil nuclear program leading to the development of nuclear weapons.

The other key advantage of the ADS design is that it can be used to dispose of dangerous weapons-grade material and commercial reactor by-products in a similar way to mixed thorium fuel.

While the ADS design has promise, it presents challenges. First, there's the design itself: while lab tests have proven the concept of using a particle beam to start the thorium fuel cycle, the physics of scaling it up to the size of a commercial reactor are unproven and could be more complex. Then there's the way the particle beam interacts with the spallation target and the fuel in order to operate efficiently. Also, while there are plenty of existing conventional nuclear reactors that can be fairly inexpensively converted to mixed thorium fuel, an ADS reactor would have to be designed, built and paid for from scratch.

Retrofitting old reactors is not an option.

Does this make a large-scale ADS reactor viable? CERN thinks so. It recently released a detailed report covering the financial viability of the ADS design for power generation, and found it to be at least three times cheaper than coal and 4.8 times cheaper than natural gas. Any nuclear reactor will have a high establishment cost, but CERN stresses that a long-life reactor will be highly competitive compared to fossil and renewable energy fuels.

Hashemi-Nezhad has been working on the ADS reactor concept with colleagues in Germany, Russia, India and Eastern Europe, and is enthusiastic about it. "The future of nuclear reactors is in ADS because it operates in a sub-critical condition. Only under this condition it is possible to transmute waste isotopes while gaining energy and producing fuel at low cost. And it's safe," he said.

He also thinks Australia could play a leading role in the development and promotion of thorium-fuelled reactors. "It is up to the Australian government to make an investment in this research. Huge thorium resources in Australia can provide green energy at low cost for several centuries." An enticing prospect, to say the least.

CAN ATOMIC POWER be green? Physics suggests it can. And our consumption of energy is accelerating at the same time the climate is being affected by power generation.

Unless we start seriously exploring energy alternatives to burning fossil fuels, erratic and destructive weather conditions could be with us for generations to come. Renewable energy such as wind and solar have bright futures, and will play a large role in any future energy program - but they can never hope to satisfy baseload requirements of a city.

Hydroelectric power is an option - but most of the economical sites have been exploited, and biodiversity suffers when valleys are flooded to create dams. So, unless some groundbreaking discovery in nuclear fusion is made, making it not only possible but efficient and economical - then nuclear fission will remain on the agenda for promising baseload energy alternatives.

Despite its drawbacks, conventional uranium-fuelled nuclear power is a realistic option that is likely to be continued worldwide.

But it is thorium reactors that present a real quantum leap forward. Humble thorium could potentially alleviate three of the most pressing issues facing modern civilisation in the 21st century: the hunger for energy, the spectre of climate change and the need to eliminate nuclear weapons.