IFF
Scientific Report 2000/2001

An essential part of the activities of the group is still concerned with radiation damage effects in candidate structural fusion materials. This work is a contribution of the IFF to the Nuclear Fusion Project in Jülich which is financed to 25 and 45%, respectively, by EURATOM. The research topics are part of the European Fusion Programme. In addition to these activities, the group is increasingly involved in materials investigations for the targets of the planned European high-power spallation neutron source ESS (see below).

Scientific goal of the work are investigations of irradiation-induced changes of properties of technical- and model-material. Because an intensive source of fusion neutron is still missing, their action is simulated by light ion bombardment at a cyclotron. The emphasis of the efforts is an improvement of the basic understanding of processes underlying radiation damage effects. A close interaction with theoreticians is therefore mandatory and the close long-standing and fruitful collaboration with H. Trinkaus (Institut Theorie II) was continued.

In the field of metallic structural fusion materials the work concentrated on the combined influence of atomic displacements, hydrogen and helium on the mechanical properties and the hydrogen permeation of martensitic steels, particularly their low activation versions. These investigation will be extended to steel-ceramics composites in order to test the effectiveness of ceramic layers as diffusion-barriers for hydrogen isotopes.

The influence of (n, a )-produced helium in beryllium (a favourite plasma-facing candidate material) on its tensile properties and microstructure has been studied for a wide range of temperatures and He-concentrations. This work is close to comletition.

The emphasis of this research is on the most highly loaded components of the European Spallation Source (ESS), i.e. target and moderators. The integrity of these parts will be decisive for the duration of uninterrupted operation periods of the entire facility. The extraordinary loads on the mercury target are on the container proper and its secondary enclosure, in particular the respective proton beam windows. These loads have mainly two causes. First, the stress waves, which are due to the shock-like energy deposition into the target and its multiple shells. Stress waves within the container walls are generated by the direct heating of the beam window as well as by the pressure waves due to the pulsed heating of the mercury. The second concern is radiation damage and foreign atom production (mainly hydrogen and helium) induced by the high energetic protons and neutrons.

Radiation damage and foreign atom production are investigated with proton accelerators. Life time estimates of components are made by analyzing long term irradiated targets and proton beam windows of already operating medium power spallation sources (LANSCE, Los Alamos and ISIS, Rutherford Appleton Lab). The stress wave problem is studied with experiments on pulsed high power proton accelerators.

The mechanical tests and micro-structural investigations of samples cut from LANSCE components of existing spallation sources are nearly finished. The results show a remarkable strengthening and embrittlement with all three investigated materials classes (austenitic and martensitic steels as well as nickel-based alloys). The residual ductility observed with specimens subject to the highest available dose of 10 dpa (corresponding to about 2 months of operation of ESS) are, however, sufficient for being employed as structural materials of ESS targets.

An unexpected result was found in the investigation of the "spent" ISIS target: Pure tantalum remained a ductility of more than 10% after irradiation up to the maximum available dose of 13 dpa, making Ta an attractive candidate material for target structural components. Microstructural studies and simulation experiments (in collaboration with ORNL) are under way to uncover the reason for this behaviour.

A new tensile machine allowing tests in the temperature range from RT to 350°C was developed and installed in the FZJ Hot Cells. It will be used for testing he specimens from the STIP irradiation program at SINQ (PSI) which will arrive in Jülich in early 2001.

Cold moderators have gained increasing importance in the past. Quality and quantity of neutrons produced with a pulsed source can be particularly improved, if cold moderators can deliver and sustain short pulses over a broad energy range. The ideal slowing down medium for that purpose is methane because of its high proton density and many low lying rotational vibration modes. Unfortunately, in the radiation field of a target, highly active radicals are formed in methane, in particular CH_3- und H⁺. In liquid methane (100-K-moderator) this gives rise to the formation of higher alkane homologues, which is eventually clogging the piping. In its solid state (20-K-moderator), in addition to radiolysis, crystal defects like interstitials are generated. The stored energy together with recombination of radicals can lead to spontaneous energy release (Wigner effect), which in turn may destroy the moderator vessels.

Within the present R&D phase several paths for developing radiation resistant or at least better manageable cold moderators are being followed. One way is the production of small methane pebbles (2 to 3 mm diameter), which as a bed are cooled by flowing liquid hydrogen. A second possibility is the inclusion of methane in porous substances (e.g. zeolites) or clathrates (e.g. from water ice), both again as small pebbles. Radiation damage and Wigner effect will thus be restricted to small particles. A timely and regular exchange of the pebble beds would prevent the destruction of the vessel and sustain the neutronic quality of the moderator. A third way would be the utilization of different hydrocarbons (with many freely rotating methyl groups), which do not exhibit the unfavorable radiolysis behavior of methane.

Irradiation behavior of moderator media are being performed at reactors (CARE in England and IBR-2 in Russia). The neutronic properties (intensities and pulse shapes) of the different variants will be studied in a to scale mock-up of the ESS target-moderator-reflector module. This test facility has been installed at the cooler synchrotron (COSY) of the Institut für Kernphysik of Forschungszentrum Jülich and first successful experiments have been performed. The experiments are performed under the auspices of the international collaboration JESSICA (Jülich Experimental Spallation target Set-up In COSY Area).

H. Ullmaier and H. Conrad