A brake for neutrons

The world's most modern neutron source, the European Spallation Source (ESS), is being built with the help of the Forschungszentrum in Lund ( southern Sweden). A high-intensity neutron beam can be generated in two ways: by nuclear fission or by spallation. The special feature of the latter is that the neutron beam has a higher energy density and therefore a higher intensity, but in particular that it is pulsed, which makes time-resolved measurements possible. The ESS is a joint European project involving 17 countries and will be the world's most powerful neutron source with an average beam power of 5 MW.

A very special heart for neutrons

In addition to a proton accelerator and the target, the heart of a spallation source is the so-called cold moderator. A proton accelerator accelerates proton salvos to almost the speed of light and directs them onto the atomic nuclei of a target. Although the impact itself only releases a few neutrons directly, it excites the atomic nuclei so that 20 to 30 usable neutrons are released per nucleus. Because the free neutrons are much too fast and energetic for experiments, they have to be "slowed down". Reducing the speed of the neutrons coming from the surface from around 20,000 kilometres per second to less than 0.5 kilometres per second in the moderator presented the team at the Central Institute of Engineering, Electronics and Analytics (ZEA), Engineering and Technology (ZEA1) at the research centre with numerous technical challenges. The result is a para-hydrogen-filled moderator made of a high-strength aluminium alloy, the so-called cold heart. This pressurised container has to be operated at 250°C and 10 bar pressure while simultaneously dissipating around 4000 W of radiant heat. A team led by Yannick Beßler is responsible for the design, further development of the next generation, production and acceptance tests of the cold moderator for the ESS.

In addition to extensive fluid mechanics simulations, carried out using the Jülich supercomputer, and complex structural mechanics simulations, all official requirements for the operation of a hydrogen-carrying component in the nuclear sector were met and the manufacturing process was successfully completed. Manufacturing also presented the teams at the institute with further challenges. The aluminium alloy used was generally considered to be almost impossible to weld. Thanks to extensive parameter studies and preliminary tests on the electron beam welding machine and during TIG welding, the experts at ZEA1 were nevertheless able to fulfil the high requirements for machining, surfaces and testing technology with the highest quality as usual.

The realisation of such a challenging project is, as almost always, based on close, cross-departmental collaboration between the different disciplines at ZEA1. From project management, design, simulation, joining technologies and additive processes, to precision machining and assembly, surface treatment, testing and acceptance, the manufacturing of the hearts is a team effort that everyone is proud of. The two hearts have been completed, tested and approved and will now be integrated into the Moderator & Reflector System, which was also built at ZEA1, before travelling to Lund.