The ESS AB has taken a decision that the baseline design for the target is a rotating wheel of solid tungsten. This will now be the base for the efforts of the ESS Design Update work during the Pre-Construction Phase, which will result in an ESS Technical Design Report, to be completed in early 2013.

ESS has considered, in a broad collaboration with laboratories in Europe and experts globally, all realistically thinkable options for the target station design concept, including the target material. The ESS Steering Committee - comprised of representatives from the 17 partner countries - and the ESS AB Board – the legally responsible governing body - have now taken a decision on the baseline design concept for the target station.

During the last year, ESS has mainly investigated two different options for the target concept and the target material: 1) a rotating wheel of solid tungsten, cooled with water or helium gas, and 2) a liquid metal target made of a mixture of lead and bismuth. Earlier options have also included liquid mercury and liquid pure lead as target materials.

The different design options have mainly been studied with respect to potential environmental safety, conventional safety, technological risks, scientific performance and waste management. The chosen concept has the best properties regarding potential environmental impact, safety and scientific performance.

It is even better, since it has the potential to produce around 10 % more neutrons than the earlier baseline design from 2002/2003.

This is now the ESS baseline design, and it will be the base for the remaining ESS design work during the Pre-Construction Phase. The decision is underpinned by practical experience and solid analysis, and it is therefore very unlikely that it will change.

Yes, ESS will not use mercury as target material. The former ESS technical design, decided on almost ten years ago, was based on a liquid mercury target station. The current ESS technical design, planned to be ready in the beginning of 2013, will be based on the new ESS baseline with a tungsten target.

The design options that ESS has studied have been derived from the different spallation target designs currently in operation at ISIS in Oxford, SNS in Tennessee, USA, JSNS in Tokai, Japan, LANL in New Mexico, USA, and PSI in Villigen, Switzerland, and also from the planned second target station at SNS. All of these target designs deliver high performance, good scientific possibilities in combination with high safety. Tungsten is today the most common target material for spallation sources, used f.ex at ISIS and LANL.

ESS needs a target station to produce the neutrons that scientists use to study materials.

Historically, neutrons for science have been produced in small research reactors located on research institutes and laboratories. For scientific, environmental and safety reasons, scientists today prefer neutron sources built on particle accelerators and spallation technology. This method produces more useful neutrons, and thus better research results. It also produces much less radioactive waste and has not the same risks as fission reactors.

Spallation resembles bowling: accelerating protons knock out neutrons. More precisely, the neutrons that scientists need to study different materials can be obtained by letting protons knock out these from a material rich in neutrons, e.g. a heavy metal. Neutrons are also “boiled off” through excitation. Spallation is one of the many sub-atomic processes involving atomic nuclei taking place around us. In nature, spallation can happen for example in the atmosphere, where oxygen, nitrogen and coal can be subject to spallation.

Tungsten is a hard, rare metal under standard conditions when uncombined. Tungsten is found naturally on Earth only in chemical compounds. It is considered a very robust metal, and has the highest melting point of all pure metals. Tungsten was discovered in 1783 by the Swedish chemist Carl Wilhelm Scheele. The mineral scheelite as alreday known, and was called ”tung sten”, i.e ”heavy stone”, because of its high density.

Tungsten is used in many electrical appliances. Tungsten alloys and compounds have numerous applications, most notably in incandescent light bulb filaments, X-ray tubes, (as both the filament and target), superalloys, and catalysts. Tungsten carbide is used for drills, cutters and other tools within metalworking, woodworking, mining, petroleum and construction industries.

The rotation ensures that when the proton beam hits one part of the target, the rest of the target can be cooled. Cooling is needed to remove the excess heat resulting from the spallation process.

Will radioactivity be produced in the target station?

Yes, the spallation process will result in ionising radiation from radioactive isotopes deriving from the target material. Therefore the target material will be surrounded by shielding to stop radiation from reaching the surrounding environment.

No matter how ESS will be built, we will fulfill all safety requirements with a good margin. The chosen target concept has been selected on the basis of several criteria, including safety. The chosen concept is found to have the highest possible inherent safety, which will make it relatively easy to fulfil the safety requirements.

No, not even right outside of the target station. A worker at ESS will be exposed to around the same amounts of ionising radiation as an airplane pilot.

A person living in Sweden is, during normal conditions, exposed to a so-called background radiation of 3-4 mSv per year. This is generated both from natural sources in the earth and the cosmos, as well as from housing (building materials) and health care (X-rays). ESS has set the goal of keeping the exposure to the public and the environment below 0,05 mSv, i.e around 1 % of the background radiation. This is half of the dose rate limit of 0,1 mSv set by the Swedish authorities for any facility (industries, hospitals, nuclear power plants etc) working with ionising radiation. In conclusion, from a radiation point of view, ESS will have a marginal effect on the surrounding environment.