Thorium nuclear fuel has risks

Thorium fuel has risks

Simple chemical pathways open up proliferation possibilities for the proposed nuclear ‘wonder fuel’, warn Stephen F. Ashley and colleagues.

Thorium is being touted as a potential wonder fuel. Proponents believe that this element could be used in a new generation of nuclear-power plants to produce relatively safe, low-carbon energy with more resistance against potential nuclear-weapons proliferation than uranium. Although thorium offers some benefits, we contend that the public debate is too one-sided: small-scale chemical reprocessing of irradiated thorium can create an isotope of uranium that could be used in nuclear weapons, raising proliferation concerns.

Naturally-occurring thorium is made up almost entirely of thorium-232, an isotope that is unable to sustain nuclear fission. When bombarded with neutrons, thorium is converted through a series of decays into uranium-233, which is fissile and long-lived — its half-life is 160,000 years. A side product is uranium-232, which decays into other isotopes that give off intense γ-radiation that is difficult to shield against. Spent thorium fuel is typically difficult to handle and thus resistant to proliferation.

We are concerned, however, that other processes, which might be conducted in smaller facilities, could be used to convert 232Th into 233U while minimizing contamination by 232U, thus posing a proliferation threat. Notably, the chemical separation of an intermediate isotope — protactinium-233 — that decays into 233U is a cause for concern.

Thorium is not a route to a nuclear future that is free from proliferation risks. Policies should be strengthened around thorium’s use in declared nuclear activities, and greater vigilance is needed to protect against surreptitious activities involving this element.

Protactinium pathway

The decay path of thorium is well understood. If bombarded with neutrons, isotopically pure 232Th forms 233Th, which has a half-life of 22 minutes and β-decays to 233Pa. That isotope has a half-life of 27 days and β-decays to 233U, which can undergo fission. The International Atomic Energy Agency (IAEA) considers 8 kilograms of 233U to be enough to construct a nuclear weapon1. Thus, 233U poses proliferation risks.

Although 233U is not used today in commercial reactors, the United States accumulated two tonnes of it during the cold war. Plans to dispose of much of it by burial are controversial and pose security and safety risks, according to a 2012 report2.

The chemical reprocessing needed to separate 233U from spent nuclear fuel requires major infrastructure, such as large reprocessing plants, which are difficult to hide. With thorium fuel, the presence of highly radiotoxic 232U means that the spent fuel must be handled using remote techniques in heavily-shielded containment chambers

After irradiating thorium with neutrons for around one month, chemical separation of 233Pa could yield minimal 232U contamination, making the 233U-rich product easier to handle. If pure 233Pa can be extracted, then it merely needs to be left to decay to produce pure 233U. The problem is that neutron irradiation of 232Th could take place in a small facility, such as a research reactor, of which around 500 exist worldwide. The 232Th need not be part of a nuclear-fuel assembly nor be involved in energy generation.

It has been demonstrated that around 200 g of thorium metal could produce 1 g of 233Pa — and hence 1 g of 233U — if exposed to neutrons at levels typically found in power reactors and some research reactors for a month, followed by protactinium separation3. Thus, only 1.6 tonnes of thorium metal would be required to produce the 8 kg of 233U required for a weapon. This amount of 233U could feasibly be obtained by this process in less than a year.

The separation of protactinium from thorium is not new. We highlight two well-known chemical processes — acid-media techniques3,4 and liquid bismuth reductive extraction5,6,7 (see ‘Ways to obtain pure protactinium’) — that are causes for concern, although there may be others. Both methods use standard nuclear-lab equipment and hot cells — containment chambers in which highly radioactive materials can be manipulated safely. Such apparatus is not necessarily subject to IAEA safeguards………..

Given the need for access to a research or power reactor to irradiate thorium, the most likely security threat stems not from terrorist organizations but from wilful proliferating nation states. We have three main concerns:

First, nuclear-energy technologies that involve irradiation of thorium fuels for short periods could be used covertly to accumulate quantities of 233U by parallel or batch means, perhaps without raising IAEA proliferation flags.

Second, the infrastructure required to undertake the chemical partitioning of protactinium could be acquired and established surreptitiously in a small laboratory.

Third, state proliferators could seek to use thorium to acquire 233U for weapons production. These three points should be included in debates on the proliferation attributes of thorium……….

Thorium is not as benign as has been suggested and we call for greater debate on its associated risks….

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