IFF
Scientific Report 1999/2000

The main 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.

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. For the spallation case there is the additional possibility to investigate "spent" targets of operating medium power sources (ISIS, LANSCE) in the Hot Cells of FZJ.

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) will be 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 has been studied for a wide range of temperatures and He-concentrations (see following article). Transmission-electron-microscopy (TEM) is under way to uncover the reasons for the observed severe embrittlement.

During the work on irradiation-induced dimensional changes in ceramics, unusual morphologies of helium bubbles have been found. Systematic experiments and TEM investigations together with the development of a theoretical model led to a detailed understanding of the development of such bubble structures.

Concluding electron irradiations confirmed the suspicion that the alledged Radiation Induced Electrical Degradation (RIED) effect is an artefact. It could be clearly shown that the drastic increases in conductivity of Al₂O₃ found by some authors were due to unsuitable techniques of measuring high insulation resistances.

H. Ullmaier

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.

Experiments on the stress wave problem have been performed at the Alternating Gradient Synchrotron (AGS) at Brookhaven with the mercury target of the international ASTE (AGS Spallation Target Experiment) collaboration. A recently developed laser interferometric technique for measuring pressure waves in liquid metal targets provided reproducible results.

The comparison of experiments with results from numerical calculations (employing finite elements) of the expected stress and pressure waves within the ASTE target subject to high power proton pulses is satisfactory only for the time interval between power input and arrival of the pressure wave front at the target container wall. For a quantitative description of the pressure wave at later times a more sophisticated modeling of the pressure sensor is needed and being developed.

Numerical calculations show that the time dependence of the pressure wave within a mercury target for a given total energy deposition strongly depends of the proton beam profile. This way may offer a means of mitigating the impact of pressure waves on the target structure.

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 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.

Spallation nuclides have been identified applying capillary electrophoresis combined with ICP mass spectroscopy to separate the rare earth elements. All 14 lanthanides from an 800 MeV irradiated tantalum target from ISIS have been analyzed. The results were in good agreement with theoretically predicted distributions as well as with data from HPLC-ICP mass spectroscopy.

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 is under construction at the cooler synchrotron (COSY) of the Institut für Kernphysik of Forschungszentrum Jülich. The experiments will be performed under the auspices of the international collaboration JESSICA (Jülich Experimental Spallation target Set-up In COSY Area).

IKP, ZAT, ZEL, ZFK-HZ

BNL (Upton, USA) , KEK (Tsukuba, Japan), JAERI (Tokai, Japan), LANL (Los Alamos, USA), ORNL (Oak Ridge, USA), PSI (Villigen, CH), RAL (Chilton, UK).

Dr. J. Chen Mechanical tests and TEM on irradiated spallation materials 23.60.0

Dr. H. Conrad (Institute for Scattering Methods) European Spallation Source: Target and Moderators 23.60.0

Dr. P. Jung Radiation damage and hydrogen effects in metals and ceramics, thermal desorption spectroscopy 23.80.5

Dr. W. Kesternich Radiation effects in metals and insulators, TEM 23.80.5

Dr. G. Küppers (ZCH) Chemical analysis of spallation products 62.50.0

Dr. H. Tietze-Jaensch Co-ordinator of international ESS-experiment "JESSICA" Instruments for pulsed neutron sources 23.60.0

Prof. H. Ullmaier (Head of Project "European spallation Source ESS" at FZJ) Mechanical properties of irradiated metals, 23.60.0 23.80.5

J. Deutz (until end of 99) Preparation of ceramic TEM specimens 23.80.5

A. Fournier Secretary, Project Assistant ESS 23.80.5 23.60.0

H. Klein Instrumentation and data processing, TEM, irradiation experiments 23.80.5 23.60.0

W. Schmitz SEM, irradiation experiments, specimen preparation 23.80.5

J. Abasolo-Hernandez (IPN Mexico) Radiation-induced segregation in metals and ceramics 23.80.5

C. Byloos (Institute for Scattering Methods) Shock waves in ESS target 23.60.0

F. Carsughi (Univ. of Ancona, Italy) Investigations of spent spallation target components 23.60.0

A. Garcia-Borquez (IPN Mexico) Radiation-induced segregation in metals and ceramics 23.80.5

C. Liu (NPCI, Chengdu, China) Hydrogen embrittlement of ferritic/ martensitic steels 23.80.5

A. Ryazanov (Kurchatov Inst., Moskau) Theory of radiation damage in metals 23.60.0

E. Shabalin (JINR, Dubna) Radiation damage in solid methane and ice 23.60.0