European Spallation Neutron Source Research Articles

Microstructural changes in the welding of titanium-stabilized steels

Abstract At the Central Institute for Engineering, electronics, and Analytics (ZEA-1 by its acronym in German) of the Jülich Research Center GmbH, a cryostat was developed for the European Spallation Neutron Source (ESS) and manufactured from a titanium-stabilized austenitic stainless steel alloy (EN 1.4571). After tungsten inert gas welding, the welds exhibited a black, patchy discoloration. After materialographic preparation, the phenomena were examined using microscopic methods to assess their impact on the strength of the weld and thus on the usability of the cryostat. Not only could the impact on the cryostat operation be assessed, but the formation mechanism could be elucidated and suitable measures could be outlined to avoid the black, patchy discoloration.

HEIMDAL: A novel materials science neutron powder diffractometer at ESS

Developing new materials is the most challenging task for the future demands due to limitations in energy, resources but also environmental damage. Improvements in material performances are reached for example by the incorporation of advanced ceramics and polymers into heterogeneous systems. Their performances usually depend on the interplay between properties defined by the atomic, nano/mesoscopic and microscopic structure. In-situ and in-operandi investigations will be in the focus of such investigations. The instrument HEIMDAL proposed for the European spallation neutron source ESS will offer here perfect prospects, as the instrumental resolution of this powder diffractometer can widely be adapted taking either full advantage of the broad pulse of ESS (2.86ms) offering highest intensity, or using a fraction of the pulse for highest resolution. A thermal and a cold guides pointing to the same virtual source extend the spatial window of the instrumental from an atomic scale (0.3Å-1≤Q≤50Å-1) to a nano/meso scale, 0.002Å-1≤Q≤0.1Å-1 by adding a narrow-band SANS instrument behind. Our chopper system allows switching the different operation modes electronically. Traditionally such structural information has been collected in separated experiments such as powder diffraction (PD), wide angle diffraction scale, small angle diffraction and direct space imaging techniques (sub-micronic to millimeter scale), whereas HEIMDAL can offer these options in its final stage at the same time at therefore for absolutely identical experimental conditions. The top-loading geometry foreseen not only accepts auxiliary from the ESS pool (cryogenics, pressure cells, magnets) but also allows implementing bulky brought-in user equipment. It can be pretested off-line at the instrument, but already fully connected to the HEIMDAL electronics. Figure 1: a) Operation modes of the HEIMDAL b) Layout of the instrument HEIMDAL

Homogeneous bubble nucleation limit of mercury under the normal working conditions of the planned European spallation neutron source

In spallation neutron sources, liquid mercury is the subject of big thermal and pressure shocks, upon adsorbing the proton beam. These changes can cause unstable bubbles in the liquid, which can damage the structural material. While there are methods to deal with the pressure shock, the local temperature shock cannot be avoided. In our paper we calculated the work of the critical cluster formation (i.e. for mercury micro-bubbles) together with the rate of their formation (nucleation rate). It is shown that the homogeneous nucleation rates are very low even after adsorbing several proton pulses, therefore the probability of temperature induced homogeneous bubble nucleation is negligible.

Materials related R&D work for the ESS target stations

Current planning for the European Spallation Neutron Source Project (ESS) provides for two target stations of 5 MW beam power each: a long pulse station (2 ms, 16 2/3 Hz) and a short pulse station (1.4 μs, 50 Hz). In both target stations liquid mercury is the reference target material. Materials related R&D work concentrates on: • The effects of the high pulsed power input into the short pulse target (stress pulses, pitting, high cycle fatigue) and their mitigation (with emphasis on the introduction of bubbles of non-condensable gas in the liquid metal). • The effects of radiation on the mechanical properties of structural target components (hardening and embrittlement). • Investigations of novel cryogenic moderator materials (ice and solid organic compounds) under fast neutron irradiation. Most of these efforts are carried out together with other projects within international collaborations such as ASTE, JESSICA, LiSoR, STIP, URAM and others.

Nuclear Simulation and Radiation Physics Investigations of the Target Station of the European Spallation Neutron Source

The European Spallation Neutron Source (ESS) delivers high-intensity pulsed particle beams with 5-MW average beam power at 1.3-GeV incident proton energy. This causes sophisticated demands on material and geometry choices and a very careful optimization of the whole target system. Therefore, complex and detailed particle transport models and computer code systems have been developed and used to study the nuclear assessment of the ESS target system. The purpose here is to describe the methods of calculation mainly based on the Monte Carlo code to show the performance of the ESS target station. The interesting results of the simulations of the mercury target system are as follows: time-dependent neutron flux densities, energy deposition and heating, radioactivity and afterheat, materials damage by radiation, and high-energy source shielding. The results are discussed in great detail. The validity of codes and models, further requirements to improve the methods of calculation, and the status of running and planned experiments are given also.

Radiation Shielding and Protection of the European Spallation Neutron Source (ESS)

The shielding and the radiation protection of high intensity spallation neutron sources as ESS has special features and importance compared to usual accelerator shielding and protection considerations. For the shielding we are interested in very thick shields, so small uncertainties e.g. in cross sections lead to large errors; we are interested in detailed predictions, such as accurate particle spectra over large energy ranges, and the geometries of interest are complex, e.g. shields with multiple beam holes. Containment of radioactivity for the high power target station is also a very important issue and has highest hazard. The different methods, which are used for designing the ESS shielding, are discussed. Comparison with performed measurements is also given.

The project “European Spallation Neutron Source (ESS)”: status of R&D programme

At present the ESS-Project is in its R&D phase (1997–2001) comprising activities in the areas of accelerator (linac and rings), target station (target, moderators, reflector) and instruments. The work is carried out by 14 laboratories and universities in the European Union and Switzerland and co-ordinated by the ESS R&D Council. Intense and fruitful collaborations with partners in the United States, Japan and Russia have been established. In this article, examples of typical results of R&D work on the ESS linac and target will be given.

Design considerations for the linac system of the ESS

The proposed European spallation neutron source (ESS) will be the leading neutron scattering research facility based on pulsed high intensity/high energy proton beams. According to the present reference design, a linear accelerator produces 1.2 ms long H − beam pulses of 1.334 GeV with a repetition rate of 50 Hz. The beam is accumulated in two rings. The stored beams are extracted in one turn resulting in a pulse of 1.3 μs duration which is delivered to one of the two liquid mercury target stations. Beams from two H − ion sources are pre-accelerated by radio frequency quadrupoles (RFQs), then combined to one beam of about 120 mA. After acceleration in a drift tube linac (DTL) to 70 MeV a coupled cavity linac (CCL) accelerates the beam to the final energy. The CCL can be realized in normal conducting or in superconducting (SC) version which is expected to result in a significant cost reduction for investment and operation.