Warm tungsten walls reduce tritium retention in fusion reactors
Neutron damage to the tungsten wall material of future fusion reactors such as ITER can trap the precious fusion fuel tritium. Research by PhD student Rianne 't Hoen at FOM Institute DIFFER now indicates that at the expected high operating temperature of the walls, this additional tritium retention can be reduced by up to 80%. Self-healing and enhanced mobility cause the atoms to quickly vacate trapping sites in the metal. Rianne 't Hoen published her results in the journal Nuclear Fusion.
Trapping hydrogen isotopes
The metal tungsten is the prime candidate for the exhaust of the international fusion project ITER, which will start its experiments in 2019. In the hot interior of ITER, the hydrogen isotopes deuterium and tritium will fuse together to form the harmless gas helium and a free neutron, releasing a great amount of energy. ITER is designed to be the first fusion reactor that will produce more fusion power than it consumes itself.
Tungsten's good thermal properties and low erosion rate make it favourable over other possible wall materials. One concern is how neutrons impacting from the plasma will influence these properties. For instance, resulting vacant sites in tungsten's atomic lattice can trap hydrogen isotopes such as the fusion fuel tritium. The total amount of tritium in the entire reactor at any one moment is limited to 700 g for reasons of efficiency and safety. Therefore, keeping tritium retention in the reactor wall as low as possible is highly important to the success of ITER.
Simulating neutron damage
To investigate what fundamental processes influence the trapping of tritium, Rianne 't Hoen exposed samples of tungsten to the intense plasma in DIFFERs experiment Pilot-PSI, one of few in the world which can produce plasma's with the temperature and density that will occur at the ITER exhaust. By first bombarding her tungsten targets with tungsten ions, she created extra trap sites in the metal lattice to simulate the damage that neutrons from the fusion reaction will inflict on wall materials. A series of diagnostics and a tritium migration model revealed how defects in tungsten and tritium retention in those defects behave.
Self-healing effect
Testing retention at both below 227 ºC as in earlier experiments and at the higher 527-927ºC which ITERs exhaust will operate at, Rianne found that the hydrogen isotope density in the tungsten is lower when the wall is hotter. Compared to the lower temperature experiments, the tungsten retained up to 80% less hydrogen isotopes. The exact reason is not yet known, but the processes involved are: "The higher the temperature, the more the atoms in the metal move, so that defects in the atomic lattice can heal themselves", explains 't Hoen. This effect will evict hydrogen isotopes in the defects. "Also, the hydrogen itself is more mobile at higher temperatures. That means that less hydrogen will be trapped in the wall."
Dance your PhD
In October 2012, the research now published in Nuclear Fusion featured in the international contest Dance your PhD. This contest challenges young researchers to present their research in the form of a dance performance, showing instead of telling what their work is about. Rianne 't Hoen won the contest's popular choice award with her dance The Great Escape about tritium retention in tungsten.
Reference
M.H.J. 't Hoen, M. Mayer, A.W. Kleyn, H. Schut and P.A. Zeijlmans van Emmichoven, Reduced deuterium retention in self-damaged tungsten exposed to high-flux plasmas at high surface temperatures, Nuclear Fusion (2013).