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The International Atomic Energy Agency has identified the radioactive material thorium as a fuel to power existing and new electrical generation plants, but a new study shows that along with this plentiful “wonder fuel” comes a dangerous weapons proliferation risk.
The processing of just 3,200 pounds of raw thorium metal would yield 18 pounds of uranium-233, an amount the agency considers to be enough to construct a nuclear weapon, evidence shows. And a nuclear reactor using thorium for a fuel source would generate that amount of uranium in less than a year.
Further, the processing of the fuel could be done with simple laboratory equipment, negating the need for sophisticated centrifuges that could be easily hidden from IAEA inspectors.
Thorium is a naturally occurring radioactive element that is found naturally as thorium-232 and has a half-life of about 14 billion years. Thorium is estimated to be about three to four times more abundant than uranium and is being touted as a replacement for uranium in nuclear reactors.
Several countries, including the United States, the United Kingdom, Germany and India, have experimented with the material as a substitute nuclear fuel in reactors, and some research reactors have been constructed. India, which has approximately 30 percent of the world’s thorium reserves, is especially interested in constructing a thorium-fueled reactor.
According to the IAEA, there is an increased interest among the member states for the use of thorium in nuclear power generation. The international agency states that the major incentives for use of thorium include its wide availability relative to uranium, the ease of handling, a higher matter-to-energy conversion than uranium and the inherent proliferation resistance characteristics of thorium-based fuels.
Some member states believe that near-term deployment of thorium fuels in proven reactor types is not only feasible but also attractive for contributing to meeting expanding energy needs. Several options are also currently under consideration and even under active development for deployment for a longer term.
But in a recent report, a team of nuclear energy specialists led by Steve Ashley of the Department of Engineering at the University of Cambridge, the paper’s lead author, suggests that, contrary IAEA statements, simple chemical processes hold some inherent dangers in fissile material falling into the wrong hands.
Ashley and his colleagues, from the University of Cambridge, The Open University, Lancaster University and Imperial College, believe that not enough research has been done on the question of proliferation. They show that small-scale chemical reprocessing of irradiated thorium can be transformed into an isotope of uranium that could be used in nuclear weapons.
“Thorium certainly has benefits, but we think that the public debate regarding its proliferation-resistance so far has been too one-sided,” Ashley said.
“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.”
One of the new type of nuclear reactors being researched is a thorium-based molten salt reactor. Molten salt reactors were first developed in the 1960s and use liquid nuclear fuels rather than solid fuel rods. Proponents of these reactors claim they are more efficient and safer than existing plants.
Thorium, in its natural occurring state, cannot provide the radiation needed to fuel a nuclear reaction. To make it suitable for power generation or weaponry the thorium needs to be bombarded with neutrons, which turns the thorium, through several steps, into uranium-233, which is fissile and can be used in nuclear weapons. The spent thorium fuel, however, is difficult to handle, which makes it hard to convert into a usable weapon.
Ashley’s team believes, however, that there are other processes that could be used to convert naturally occurring thorium into uranium-233 while minimizing contamination by other uranium isotopes, a process that would pose a proliferation threat.
Normally, the chemical processing needed to separate U-233 from spent nuclear fuel requires large reprocessing plants, which are difficult to hide.
However, if thorium is bombarded with neutrons for about one month and the resulting isotope (Protactinium-233) allowed to decay naturally, it will produce pure U-233. The proliferation problem stems from the fact that neutron irradiation of thorium could be done in a small research reactor, of which 500 already exist worldwide. The thorium would not have to be inventoried in a nuclear generation plant and, therefore, would be extremely hard to trace by inspectors.
The conversion of thorium into U-233 can be done using well-known chemical processes that use standard laboratory equipment and are not subject to IAEA safeguards.
It’s not that the process does not have its challenges. The conversion process generates a fair amount of heat, which would make purifying large amounts of thorium difficult though not insurmountable. The purification process could be dispersed to several locations processing the material in parallel.
Given the need for access to a research or power reactor to process the thorium, a terrorist organization acquiring the U-233 is not the most likely threat. The greater threat comes from a nation-state. Because Iran’s nuclear program is under such close scrutiny, using thorium as a precursor for fissile material may be an attractive option.
The Cambridge team’s report expressed three main concerns.
First, the processes used to prepare thorium for use in nuclear-energy technologies could be used for short periods of time to produce U-233 without raising the suspicion of IAEA inspectors.
Second, the equipment required to complete the processing the thorium into U-233 could be acquired and operated secretly in a small laboratory.
Third, a nation-state could use thorium to acquire U-233 for weapons production.
“The most important thing is to recognize that thorium is not a route to a nuclear future free from proliferation risks, as some people seem to believe,” Ashley added. “The emergence of thorium technologies will bring problems as well as benefits. We need more debate on the associated risks, if we want a safer nuclear future.”