Posted October 25th 2011
Materials science, along with plasma research, plays an essential role in the development of fusion power plants. The strength of the reactor wall limits the power of the plasma and thus the possible physics regimes. Frequently replacing damaged wall elements is not a viable economical option. New, advanced materials must be developed in order to implement the plasma power levels required by fusion energy. EFDA supports the process of materials research for ITER and tackles many long-term issues, such as, design studies for power plants.
The challenge
The core of a fusion plasma is hotter than 100 million degrees Celsius. It exhausts extremely large heat and particle fluxes. At some places on the vessel wall, for instance, at the divertor, temperatures can reach well in excess of 1000 degrees Celsius. Under certain plasma conditions, bursts with even greater power can also occur. Neutrons with unprecedented energy levels damage the crystal structure of the wall material and activate it. Moreover, fusion power plants will be burning tritium-deuterium plasmas, and some of the radioactive tritium may be deposited into the wall. Dust and debris from the wall surface build up in the plasma. Their atoms absorb energy and thus reduce plasma performance. This effect is worst for elements with high atomic numbers. Most of today’s fusion experiments use steel walls lined with replaceable carbon tiles. The steel wall of a power plant will be water cooled and lined with thick, water cooled, steel blankets protected by removable first wall panels. Even then, standard steels may be weakened if they absorb excessive amounts of heat and fast neutrons.
The options
The protective wall panels must withstand the plasma, but wall debris may not affect the plasma: Carbon is an extremely heat resistant material, the light atoms do not harm plasma performance, but it nevertheless provides an ideal nesting place for tritium. Tungsten is a metal with a very high melting point, and shows a low affinity for tritium but as a result of its high atomic number its debris may not be permitted to enter the plasma. Beryllium is a very light metal which does not absorb tritium and it is sufficiently heat resistant to be used for those parts of the wall that do not come into contact with plasma. Its dust, however, is highly toxic and requires careful handling. ASDEX Upgrade is the first tokamak that operates successfully with a tungsten wall. With its newly installed ITER-Like-Wall, JET will be the only experiment to gather expertise with a wall made of beryllium and tungsten prior to ITER.
New materials
Alongside looking into existing options, new materials are also being developed. Silicon-carbides, special alloys and liquid metal wall concepts are potential solutions to the plasma facing component problem. Low activation and highly heat resistant steels are being developed for the vessel structure. All materials must be capable of production in sufficient quantities and be tested under fusion reactor conditions. Creating these conditions is a challenge: Intensive investigations are underway regarding the viability of an intense high energy neutron source, referred to as International Fusion Materials Facility, which will be built in parallel with ITER.
