Kieran P. Dolan is a second-year doctoral student at MIT’s Department of Nuclear Science and Engineering. He is working in the MIT Nuclear Reactor Laboratory on MIT Reactor-based experiments with respect to designing fluoride-salt-cooled high-temperature nuclear reactors known as FHRs. He has said “I've been interested in advanced reactors for a long time, so it's been really nice to stay with this project and learn from people working here on-site."
The U.S. Department of Energy (DOE) has funded an Integrated Research Project (IRP) Grant for MIT, the University of Wisconsin at Madison, and the University of California at Berkeley to carry out a multi-year collaboration on a series of studies on FHRs. There is high interest in the nuclear industry with respect to the FHR concept because molten salt transfers heat very efficiently. This enables advanced reactors employing molten salt to run at higher temperatures than current operational power reactors. FHRs also have some unique safety features that are not found in conventional reactors. Dolan says, “Molten salt reactors offer an approach to nuclear energy that is both economically viable and safe.”
The MITR reactor is simulating the likely operating environment of a working advanced reactor including high temperatures in the experimental capsules. Dolan’s FHR tests utilizes billiard-ball-sized capsules that are composites of fuel particles. These balls of fuel are suspended in molten salt that is constantly circulating. The molten salt is a special blend of lithium fluoride and beryllium fluoride which is called “flibre”. The circulating flibre absorbs and distributes the heat produced by fission reactions in the capsules.
There is a big technical challenge in using molten salt coolants in FHRs. Dolan explains, “The salt reacts with the neutrons released during fission, and produces tritium, Tritium is one of hydrogen’s isotopes, which are notorious for permeating metal. The worry is that tritium might escape as a gas through an FHR's heat exchanger or other metal components." Tritium can be a danger to human health and the environment if it escapes from the reactor.
One possible solution to this tritium problem is the use of graphite which can trap fission products. It can absorb tritium before it can escape. Dolan says, “While people have determined that graphite can absorb a significant quantity of hydrogen, no one knows with certainty where the tritium is going to end up in the reactor.” He is focusing his doctoral research on experiments with the MITR to figure out just how effective graphite can be in absorbing tritium. He says, “We want to predict where the tritium goes and find the best solution for containing it and extracting it safely, so we can achieve optimal performance in flibe-based reactors.” Dolan has been analyzing samples from three MITR experiments in which various types of specialized graphite samples are irradiated in the presence of molten salt. He says, “Our measurements so far indicate a significant amount of tritium retention by graphite. We're in the right ballpark.”