Cheap electricity, no plutonium, no meltdowns nor long-lived radioactive waste. These are the promises of the thorium molten salt reactor that TU Delft is working on. A crucial experiment in which the salt is irradiated was started at the NRG High Flux Reactor in Petten last month.
Continuous neutron bombardments at 700 degrees Celsius in corrosive salt - the inside of a thorium reactor is about as dangerous as it gets. Can the nickel piping network in a reactor sustain such hardship? Delft researchers are about to find out.
If so, the thorium reactor is one step closer to reality. And with that, we can look forward to cheap nuclear electricity without the risk of serious accidents, no long-lived radioactive waste and no plutonium. Dr Jan Leen Kloosterman of the Reactor Institute Delft is used to summing up all the advantages of the thorium molten salt reactor compared to uranium reactors. He is leader of a European thorium research project that was initiated in 2015. He is also willing to admit that a huge amount of research is still needed to prove the feasibility of such a reactor.
The thorium molten salt reactor, or MSR, that the Delft researchers are working on in collaboration with partners in Germany, France, Italy and Switzerland – among others - is a type of reactor in which the fuel (thorium) is dissolved in molten lithium fluoride salt. This type of reactor is said to be inherently safe because the salt also serves as a coolant. If a leak occurs, the fuel flows out of the reactor along with the coolant and the reactions inside the reactor cease. The salt solution clots and all the radioactive material is trapped in the salt. That’s the theory.
To validate their models and to prove the feasibility of this technology, the researchers started an experiment last month in the NRG High Flux Reactor in Petten. For two years the researchers will irradiate a piece of fluoride salt measuring a few cubic centimetres. “Since we use a high flux beam, we can simulate twenty years of exposure in a real - still to be built – thorium reactor in two years’ time,” says Kloosterman.
The behaviour of fluoride salt is another major point of concern
Too little is known about the lifespan of the materials used in the reactor. Besides the continual bombardment by neutrons the material is also subjected to very high temperatures. The salt is 700 degrees Celsius, which is half the melting temperature for nickel.
The behaviour of fluoride salt is another major point of concern within the European project. The initiator of this line of research is Dr. Rudy Konings, professor of nuclear fuel cycle chemistry (Applied Sciences), who also works for the Institute for Transuranium Elements in Karlsruhe. Konings hopes to discover whether the theory that radioactive material (including caesium and iodine) becomes trapped in the salt in the event of a leak is really true.
To start the reactor, a bit of uranium (U-235) is initially incorporated in order to make nuclear fuel, as thorium is not fissile. In principle, the reaction that follows - in which U-233 is produced and consumed - continues indefinitely. That is what the models predict. Will this really happen? That is also something the researchers hope to find out with their experiment.
“The building of a prototype thorium molten salt reactor should be feasible twenty years from now,” says Kloosterman. “But only if we can upscale this experiment. We want to perform an experiment with larger quantities of salt that is irradiated while flowing through a loop. There are only two places in the world where this can be done; in Petten and in the US.”
China is investing much in this technology. The country started a research programme several years ago involving hundreds of researchers that dwarfs the Delft project in size. “But as far as I know, they haven’t started irradiating salt yet. They are following our steps with much interest.”