IFF

 IKP

 PES

 ZCH

 ZEL

 ZAT

In charge: Dr. G. Bauer, PES, g.bauer@fz-juelich.de

Aims and Objectives

The European Spallation Source Project (ESS) was jointly supported by 17 institutions from 11 countries with the goal to design, build and operate the world's most powerful pulsed neutron source for application in science and research. In two target stations specifically matched to the needs of neutron scattering experiments the neutrons were to be produced in mercury targets bombarded with 5 MW of beam power each from a 1.34 GeV linear accelerator. The highest scientific potential is offered by the combination of a long pulse target station with 2 ms pulse duration at 16 2/3 Hz repetition rate and a short pulse target operating at 50 Hz and a pulse duration of a few microseconds. Following a recommendation by the German delegation, the European Strategy Forum for Research Infrastructures (ESFRI) issued a statement in January 2003, which rated a new European neutron source as a long term need only. As a consequence the tasks of the Jülich Project group were reduced to the orderly completion and documentation of ongoing work by the end of 2003. The Project was terminated per Dec. 31, 2003.

Significant Results in 2003

At FZJ the main emphasis of the work in 2003 was on the technical layout of the target block complete with shielding, moderator and reflector unit, proton beam window and of the target proper with its mercury loop and all handling equipment and procedures. Innovative solutions to many problems allowed to devise a system with higher overall availability and better operating conditions than anticipated for the facilities currently under realisation in the USA and in Japan. Due to greatly reduced funding and to staff reductions over the course of the year it was not possible, though, to achieve the original goal of generating a Project Baseline that would allow a dependable cost estimate by industrial enterprises.

In an attempt to solve the problem of cavitation erosion on the liquid metal target container caused by the pulsed operation at very high power input systems were developed that would generate gas bubbles of suitable size and concentration and would allow to simulate and analyse the pressure pulse in a mercury loop. The promising techniques will now be taken over and developed further by the corresponding US project.

Important progress was also made in qualifying the martensitic steel as a target structural material, which was originally selected due to its superior thermo-mechanical properties relative to austenitic steels:

• Surface hardening as a protective measure against cavitation erosion yielded HV-values up to 700 over a depth of a few hundred microns if done by laser treatment, while nitriding in a pulsed plasma resulted in up to 1200 HV in a depth of some 50 microns;
• It could be demonstrated that intermittent thermal annealing during the irradiation process can largely reverse the radiation induced embrittlement while most of the hardening is retained;
• Furthermore, it was shown that the drastic deterioration of the fracture toughness caused by the transmutation product Helium at high concentrations does not occur up to 0.25 at%, which corresponds to a full power operation of the ESS-target during at least three months;
• Electron beam welding and Laser welding were shown to be viable techniques for joining also complex parts of the difficult-to-weld material even under realistic working conditions.

Neutronic calculations for a novel moderator system proposed by the Instrument Group showed that, while the significantly increased moderators produced lower flux densities than the original ones, an overall intensity gain is still possible in cases where the whole moderator surface can be taken advantage of by the instrument.

Investigating the moderation properties of molecular solid moderators at the experimental setup JESSICA it was confirmed that this class of materials has the potential of superior performance relative to the commonly used liquid hydrogen moderators. While no detailed evaluation of a technical concept for this type of moderators was possible within the reduced scope, some principal features such as the preference for horizontal insertion of the moderators were accounted for in the layout of the target station.

Detailed accounts on the work done and the results obtained may be found in the contributions made to the ICANS-XVI meeting (http://www.fz-juelich.de/ess/icans-xvi/) and in Chapter 4 of the ESS-Update Report (http://www.neutron-eu.net/en/index.php?cat=122).