Monolith and Target Systems

The heavy metal target absorbs the proton beam from the accelerator and transforms into fast neutrons. The moderator-reflector surrounding the target transforms the fast neutrons emitted by the target into slow neutrons, making them useful for research.

The tungsten target and moderator-reflector are surrounded by a radiation shielding system of 7000 tons of steel in order to contain the extreme level of highly penetrating gamma and fast neutron radiations created in the target and its vicinity. The beam extraction system provides intense slow neutron beams through beam tubes going across the target shielding. At the surface of the shielding, the neutrons are delivered to be used at the neutron scattering instruments. The proton beam window separates the high vacuum in the accelerator from the inert Helium gas. All of these systems sit inside large vessel. Together they form the target monolith, a large cylinder 12 m in diameter and 10 m high. 

The Wheel

A rotating tungsten wheel is the baseline option for the target, which distributes the irradiation over a large volume of target material. Metallic liquid lead-bismuth eutectic is being retained as an comparative target for licensing purposes. Both of these options offer comparable neutron-yield performance, and satisfy the eSS safety goals in terms of waste release and disposal under normal operation and in case of accidents. After periods of operation the target emits significant amount of ‘after-heat’, gradually decaying in time. both technologies are new for spallation sources, none of the established target designs being adequate for the higher power level of ESS.

The moderators

The ESS target station will contain two liquid-hydrogen moderators with a volume of about 2.5 l each, partially surrounded by water pre-moderators of comparable volume. The moderators are placed inside an inner reflector of about 1 m3 of beryllium. These components will be kept at their desired operational temperature by dedicated cooling systems. These systems will not emit significant after-heat.

Beam extraction

The beam-extraction system will consist of more than 40 beam tubes arranged in four sectors with a 60 to 65° horizontal angular spread. Each beam tube will be equipped with a beam shutter within the target monolith to assure that the residual radiation escaping through the closed beam line when the target station is not in operation is reduced to safe working levels at the wall of the target monolith. This monolith will be surrounded by a combination of integrated and individual radiation shielding for each beam line, which guarantees safe working access to the areas outside of these shielding structures all the time, including full power operation.

 

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