In depth: Decreased proliferation danger
You may find extended discussions on the question whether or not thorium-MSR’s could be either decreasing or increasing proliferation of weapon-grade nuclear materials.
Understanding of this important topic is not really helped by discussing separate technical properties of materials and/or processes. In practice, any nuclear device, molten salt reactors included, will need very specific design properties, and be subject to specific regulatory demands, including adequate safety procedures, required by regulators. Proliferation resistance will have to be assured. In the case of molten salt reactors, designers will have a new set of properties that will have to be adequately employed in order to meet the safety demands. The technology of molten salt reactors offers a wide range of options to meet these demands.
The bottom line of this argument is that molten salt reactors as such will not make it easier to obtain the knowledge, materials or devices to make a nuclear weapon. At present, every autonomous state that is determined to produce nuclear weapons and has the money to pursue this goal, has easier ways to achieve it than through molten salt reactors. That being said, it is useful to point out several mechanisms that are relevant to designers of molten salt reactors who set out to minimize proliferation risks.
One important mechanism is that, unless in the design stage very specific facilities are included and implemented, 233U that is produced from 232Th is always accompanied by 232U. This 232U acts as a proliferation prophylactic: while chemically identical and essentially inseparable from 233U, 232U produces strong and penetrating gamma radiation. This radiation is easily detectable and highly destructive to ordnance components, circuitry, and especially human beings. In normal operation, this is no problem due to adequate shielding provided by design. But the presence of 232U – at levels specified by the design – makes the uranium, unless highly specialized skills and equipment are used, difficult and very dangerous to handle outside the reactor core.
Another reason has to do with the number of free neutrons that are produced in the thorium-MSR core. When 233U absorbs a neutron, fission occurs and on average just over 2 new free neutrons are released. One is used to drive a subsequent fission by being absorbed by another 233U atom and sustain the chain reaction, and one is used to convert 232Th to 233U. A well designed thorium-MSR therefore creates just enough neutrons to generate just enough fuel, but no more. If meaningful quantities of 233U would be diverted, power generation would wind down – an event that would be difficult to go unnoticed in combination with remote detection.
On the other hand, thorium-MSR’s can help lowering proliferation risks, as they can be used to deal with the plutonium from decommissioned nuclear weaponry. Plutonium can be used as start-up-fuel for thorium-MSR’s or it can be added to the salt mixture where it is used for energy and broken down into rest-products that are unsuited for use in nuclear weapons (Juhasz, et al., 2009, p. 4), (Hargraves & Moir, 2010, p. 312), (Hart, 2011, pp. 9,10).
The Protactinium route: proliferation is possible, but not increased
There has been some controversy on the internet about the so called ‘protactinium route’. In this route protactinium, an element that forms after thorium has caught a neutron, is chemically separated before it decays into 233U – the fissile isotope of the thorium cycle. The production of very pure protactinium allows for the production of very pure 233U, which could pose a proliferation risk.
It is clear that regulatory authorities would only be satisfied with a design that excludes such possibilities. This means that such concerns should and will be an integral part of the design process. This also means that proliferation risks can only be assessed by evaluating final MSR designs. MSR designers have a broad range of properties at their disposal to ensure that protective mechanisms add up to a system that is highly proliferation resistant. The ‘protactinium route’ drew attention because it seemed to offer a way to circumvent one possible protective mechanism: 233U is usually accompanied by 232U, which produces strong gamma radiation, that acts as a protection against unauthorized handling. However, many more mechanisms are available, like remote inventory screening, reactivity control, the placement of the reprocessing unit in radioactive environment and many more.
The above route for instance would require complete control over the reactor processes for extended periods of time. For states with this possibility, there are other, easier ways to produce weapons grade nuclear materials. And here again: any future reactor will be subject to adequate safety regimes, including IAEA inspections (Hargraves & Moir, 2010), (Ashley, 2012).