A team of scientists at the US government’s Los Alamos National Laboratory is working to turn nuclear waste into tritium, an isotope of hydrogen that could one day fuel nuclear fusion power plants. The work was presented by Terence Tarnowsky, a physicist from Los Alamos, at the autumn meeting of the American Chemical Society in Washington DC.
A nuclear fusion power plant would take deuterium and tritium, two isotopes of hydrogen, and fuse them creating helium and releasing large amounts of energy while producing very little radioactive waste. While deuterium is readily extracted from seawater, tritium is vanishingly rare on Earth.
‘The initial fusion power plants are going to be based on the deuterium-tritium (D–T) fusion process,’ Tarnowsky explains. ‘The easiest path to fusion power is through D–T fusion, because it’s the lowest temperature that’s reachable here on Earth since we don’t have a big ball of gravity like the sun.’ During D–T fusion a deuterium nucleus fuses with a tritium nucleus to produce helium-4, a high-energy neutron and significant amounts of energy.
There is no large-scale commercial tritium production in the US because dedicated production facilities were shut down in 1988. ‘There was no push to do commercial tritium because there wasn’t really a market for it outside of the weapons conference,’ Tarnowsky notes.
He estimates the total tritium inventory on the planet is about 25kg and that figure has a large margin of error. Right now, the major commercial source of tritium in the western world is in Canada. The Canada Deuterium Uranium reactor (Candu), is Canada’s heavy water reactor that de-tritiates nuclear wastewater.
If fusion power plants are one day going to power the world then they’re going to need a lot of tritium, Tarnowsky states. ‘As they start up, these reactors are going to require tens of kilograms of tritium, and this is a difficult proposition when tritium doesn’t really exist naturally on Earth but is only formed in the upper atmosphere by cosmic rays and then drifts down to the ocean,’ he explains.
Computer simulations
Tarnowsky is currently in the process of conducting computer simulations of reactors that use particle accelerators to produce tritium using nuclear waste. His next step will be to refine these simulations to more precisely evaluate the efficiency and safety of the proposed reactor’s design.
These putative accelerator-driven reactors would use spent nuclear fuel dissolved in a molten lithium salt that would help to cool the fuel. A gigaelectronvolt accelerator would then be used to fire a proton beam at the salt to create spallation neutrons, which would initiative fission in the spent fuel, which would act as a neutron multiplier reducing the energy the process needs. Neutrons released by this reaction could then be harnessed to turn lithium into tritium and helium-4.
Tarnowsky estimates that his theoretical system could create about 2kg of tritium per year. ‘It’s scalable, so you can go up in yield or down in yield … if you have enough accelerators to drive that,’ Tarnowsky states. This is on par with the total yearly output from all reactors in Canada. However, with a 2GW fusion reactor calculated to require 112kg of tritium a year, many of tritium-producing reactors would be required.
‘If it turns out that this is feasible, we’d like to take the project to the wider world, because … it will really need buy-in from somebody on the federal level – that’s where the interface is going to happen,’ he states.
David Krofcheck, a nuclear physicist at the University of Auckland in New Zealand, is supportive of such proposals. ‘Harvesting of tritium would be a major side benefit from this process for planned D–T fusion reactors, particularly since Canada’s Candu reactors, North America’s major tritium supplier, will approach their 50-year lifetime in the next decade,’ he states.
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