Controlled nuclear fusion is widely accepted as one of the best long-term hopes for meeting the world’s energy needs. After 70 years of research, the construction of ITER and the proliferation of smaller experiments, some of them commercial enterprises, mark a significant step towards the realization of these hopes. However, the viability of reactor wall materials, such as tungsten, under the extreme conditions necessary for fusion remains an unresolved problem.
Fusion energy production relies on the reaction of the hydrogen isotopes, deuterium and tritium, which form a plasma at temperature of 100 million degrees C and combine to release highly energetic neutrons. The plasma and neutrons can interact with the reactor vessel wall, which may cause erosion of the surface and may also cause tritium (which is radioactive) to become trapped in the wall material, making it unavailable for fusion. The damage caused by these interactions is perhaps the most serious impediment to the realization of fusion energy production.
Tritium retention impacts safety as well as cost. In ITER and in any successor facility there will be strict limits on the amount of tritium that may be trapped in the wall material. Therefore, ITER will use tungsten, which has a high melting point and low propensity for absorbing hydrogen fuel, as the wall material for the regions of highest heat load.
Like all solid materials, tungsten would get damaged over time as a result of interactions with the high-energy neutrons produced in fusion reactions. The IAEA has conducted a coordinated research project (CRP) Plasma-Wall Interaction for Irradiated Tungsten and Tungsten Alloys in Fusion Devices, that has helped reduce the uncertainties associated with tungsten damage in fusion device components by providing scientists with valuable data to use in simulating its behaviour and properties for future reactor designs.
Nineteen institutions (national laboratories and universities) from ten countries participated in the CRP, which resulted in reduced uncertainties surrounding some of the key issues relating to tungsten in fusion environments and recommendation of best practices for the experimental technique of thermal desorption spectroscopy in determining the amount of hydrogen retained in a sample of damaged material. The main findings of the CRP were reported in a Special Issue of the IAEA’s journal series Atomic and Plasma-Material Interaction Data for Fusion (APID) and associated data is available on the web pages of the project.
One of the challenges that motivated this CRP was the very high neutron fluence – the number of particles incident on the wall surface per unit area over a defined period of time – expected for a commercially-viable power reactor. For tungsten the predicted level of neutron irradiation will cause many dislocations and change the composition of the pure metal to an alloy of (primarily) tungsten, rhenium and osmium. Pure crystalline tungsten has an extremely low affinity for tritium, but this beneficial property will degrade over time.
These issues are critically important for fusion energy development beyond ITER, which will need to sustain fusion for more than a few minutes. Investigations into properties of irradiated fusion materials are hampered, however, by the unavailability of an adequate neutron source for materials investigations (at least until the International Fusion Material Irradiation Facility, IFMIF, is completed) and by the great difficulty of relevant computational studies. Therefore, the material properties, resistance to sputtering and ablation, and the behaviour of trapped tritium in tungsten-based materials after neutron irradiation remain poorly known.
This CRP brought together experimentalists from fusion research institutes and from laboratory plasma-material interaction experiments with theorists involved in molecular dynamics and quantum simulations of plasma interaction with nuclear-modified tungsten. The CRP facilitated fusion plasma and fusion materials modelling by enhancing the knowledge base on properties of tungsten as a plasma-facing material in a fusion nuclear environment. In doing so, it is supporting planning and design efforts towards a Fusion Power Plant.
Specific Research Objectives
- To create an inventory of knowledge about effects of neutron irradiation and charged particle surrogate irradiation on the microstructure and surface properties of tungsten-based plasma-facing materials.
- To perform coordinated experiments and computations (based on quantum theory and molecular dynamics) to improve the knowledge base on effects of irradiation upon tungsten microstructure, and thereby the implications for the retention and transport of tritium.
- To synthesize new information, extrapolate to relevant fusion neutron fluence, and provide best expert estimates and uncertainties on data relating to tungsten-based materials in a fusion reactor environment.
Impact
For further information related to this CRP, please see the CRP page.