Weapons proliferation risks of thorium nuclear reactors

Thorium: Proliferation warnings on nuclear ‘wonder-fuel’ Phys Org,  Thorium is being touted as an ideal fuel for a new generation of nuclear power plants, but in a piece in this week’s Nature, researchers suggest it may not be as benign as portrayed.

The element thorium, which many regard as a potential nuclear “wonder-fuel”, could be a greater proliferation threat than previously thought, scientists have warned.

Writing in a Comment piece in the new issue of the journal, Nature, nuclear energy specialists from four British universities suggest that, although thorium has been promoted as a superior fuel for future nuclear energy generation, it should not be regarded as inherently proliferation resistant. The piece highlights ways in which small quantities of -233, a material useable in , could be produced covertly from thorium, by chemically separating another isotope, protactinium-233, during its formation.

The chemical processes that are needed for protactinium separation could possibly be undertaken using standard lab equipment, potentially allowing it to happen in secret, and beyond the oversight of organisations such as the  (IAEA), the paper says.

The authors note that, from previous experiments to separate protactinium-233, it is feasible that just 1.6 tonnes of thorium metal would be enough to produce 8kg of uranium-233 which is the minimum amount required for a nuclear weapon. Using the process identified in their paper, they add that this could be done “in less than a year.”

……….”Small-scale chemical reprocessing of irradiated thorium can create an isotope of uranium – uranium-233 – that could be used in nuclear weapons. If nothing else, this raises a serious proliferation concern……..

Alongside its abundance, one of thorium’s most attractive features is its apparent resistance to nuclear proliferation, compared with uranium. This is because thorium-232, the most commonly found type of thorium, cannot sustain nuclear fission itself. Instead, it has to be broken down through several stages of radioactive decay. This is achieved by bombarding it with neutrons, so that it eventually decays into uranium-233, which can undergo fission.

As a by-product, the process also produces the highly radiotoxic isotope uranium-232. Because of this, producing uranium-233 from thorium requires very careful handling, remote techniques and heavily-shielded containment chambers. That implies the use of facilities large enough to be monitored.

The paper suggests that this obstacle to developing uranium-233 from thorium could, in theory, be circumvented. The researchers point out that thorium’s decay is a four-stage process: isotopically pure thorium-232 breaks down into thorium-233. After 22 minutes, this decays into protactinium-233. And after 27 days, it is this substance which decays into uranium-233, capable of undergoing nuclear fission……..

“The problem is that the neutron irradiation of thorium-232 could take place in a small facility,” Ashley said. “It could happen in a research reactor, of which there are about 500 worldwide, which may make it difficult to monitor.”…….

“The most important thing is to recognise that thorium is not a route to a nuclear future free from proliferation risks, as some people seem to believe,” …….

The researchers are: Dr Stephen F. Ashley and Dr. Geoffrey T. Parks from the University of Cambridge; Professor William J. Nuttall from The Open University; Professor Colin Boxall from Lancaster University; Professor Robin W. Grimes from Imperial College London.  http://phys.org/news/2012-12-thorium-proliferation-nuclear-wonder-fuel.html#jCp

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