Part 2 of 2 Parts (Please read Part 1 first)
Ultimately, physicists will use the knowledge acquired from JET’s decommissioning to improve how they incorporate recycling into the design of the Spherical Tokamak for Energy Production (STEP). It is a prototype commercial fusion reactor being planned in Britain. The information will also shape future regulation, according to Buckingham.
JET and ITER are both ‘tokamak’ design fusion reactors, which confine gas in their doughnut-shaped cavities. JET uses powerful magnets to compress a plasma of hydrogen isotopes, ten times hotter than the Sun, until the nuclei fuse. The last time the fusion community decommissioned a comparable device was in 1997. The Tokamak Fusion Test Reactor at Princeton Plasma Physics Laboratory in New Jersey was shut down. Many parts, such as the equipment for injecting hot beams of gas into the reactor, were reused. The site itself was repurposes. The tokamak had to be filled with concrete, cut up and buried.
JET scientists hope that the decommissioning will leave little overall waste. Buckingham says that the main challenge is to understand where the tritium is and to remove it from materials, including from metal tiles that line the inside of the tokamak. JET engineers will utilize a refurbished robotic system to remove sample tiles for analysis. They will use remotely operated lasers to measure how much tritium is in samples that remain inside the experimental equipment. Like all hydrogen isotopes tritium is a gas that “penetrates all materials. and we need to know exactly how deep the tritium has penetrated”, says Buckingham.
Studies at JET this year will remove and study sixty wall tiles. They are the first of more than 4,000 components in the facility. Buckingham adds that “We can use this information to move from lab-scale research to industrial-scale processes, to detritiate the many tons of tiles and components which will be removed from JET over the next few years.”
In order to extract the tritium from metal parts, engineers will heat the components in a furnace before capturing the released isotope in water. Tritium can be removed from the water and turned back into fuel. The leftover materials become low-level waste, the same classification given to radioactive waste made by universities and hospitals. Variations on this process are being tested for other materials such as resins and plastics.
JET researchers are exploring how to dispose of low-level waste. They also need to deal with the much smaller amount of intermediate-level radioactive waste in which nuclear decay occurs more frequently. Options for those low and intermediate level wastes remaining include re-treating the waste, removing it to special disposal sites or storing it until it decays to lower levels of radioactivity. Some unaffected parts of JET that are not radioactive, such as diagnostic and test equipment, have already been repurposed in fusion experiments in France, Italy and Canada.
In its final experiments last December, JET was deliberately damaged. Scientists researched inverting the shape of the confined plasma in a way that might more readily confine heat. They also deliberately damaged the facility by sending a high-energy beam of ‘runaway’ electrons careering into the reactor’s inner wall. This beam is produced when plasma is disrupted.
Joelle Mailloux leads the scientific program at JET. He said, “Analysis of the damage, after the machine is opened up, will provide useful data to test the detailed predictions.”