Sandia Labs turns to tritium in quest for fusion power

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Two fast neutron radiation-effects cassettes aim toward the center of the containment system for tritium within the Z vacuum chamber at Sandia National Laboratories(Credit: SNL)

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Sandia National Laboratories is raising the stakes in its quest to develop fusion power by introducing tritium into its experiments with its Z machine. Tritium, the heaviest of the three hydrogen isotopes, promises to boost energy output by a factor of 500 as the scientists continue to seek ways to produce a self-sustaining fusion reaction that generates more energy than it consumes. However, the isotope is difficult and potentially dangerous to work with.

The Z Machine has been used by Sandia for decades to create laboratory fusion reactions to verify computer models to make sure that US nuclear warheads remain safe and reliable without resorting to underground test explosions. It's essentially a giant shock machine made up of a ring of giant condensers that feed electricity into a central vacuum chamber via cables as thick as a Volkswagen Beetle.

Inside the chamber, the energy is released in a single shot that Sandia says is 1,000 times greater than that of a lightning bolt and 20,000 times faster. This is focused on a target made of hundreds of hairlike tungsten wires, which instantly flash into a plasma that implodes to generate two million joules of X-Ray energy and heats the chamber to 1.8 million degrees Celsius (3.2 million degrees F).

Up until now, this energy was focused on samples of deuterium, which is a hydrogen atom with a neutron in addition to its proton. Tritium is hydrogen with two neutrons. Sandia wants to introduce a 50/50 mix of deuterium and tritium into the Z Machine, which will theoretically generate 500 times more energy and 80 times more neutrons than deuterium alone.

Sandia National Laboratories principal investigator Dean Rovang checks out the Z Machine’s tritium gas transfer system(Credit: SNL/Randy Montoya)

So far, the Sandia team has only done dry runs this year because of the hazards that handling tritium present. Tritium is unstable and highly radioactive with a half life of 12 years. It also must be handled very carefully because it's still a form of hydrogen and can easily slip into areas where it shouldn't, like the millions of gallons of water and oil used to cool the Z Machine, but Sandia is confident that its experience in working with plutonium will provide the answer.

"There was a high level of integration on facility containment and radiation protection, to do it right," says Brent Jones, facility integration lead. "The Sandia-California gas transfer group, with decades of experience dealing with tritium, developed a method of housing, delivering and containing the material. They built a device that could load a small but defined quantity of tritium; the neutron generator people filled the target with tritium; and the plutonium confinement folks contributed their shot expertise."

The laboratory hopes to one day use unconfined tritium inside the Z Machine, but that will require extensive testing on how the isotope will interact with the components and working out methods to purge the tritium from the target chamber after each shot so it doesn't adhere to the walls.

Sandia says unconfined experiments are necessary because keeping the gas in a chamber doesn't provide accurate measurements of fusion outputs. The team hopes to gradually increase the amount of tritium used in the experiments until they reach the 50/50 mark in about three years.

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