4.1 Development of material science and advanced materials for DEMO

4.1.1. Understanding of the radiation damage in Fe-Cr steels

Background and Objectives: Within the Materials Modelling Group there is a considerable effort in understanding the structure, phase transformation and magnetic properties of FeCr alloys before and after irradiation from electrons, ions and neutrons and correlating these findings with modelling. The general objectives of this activity are to provide experimental data for the testing of the different theoretical predictions and to further assist in the development of the modelling activities. Within 2009 the aim was to determine the magnetic properties and structural properties of EFDA FeCr model alloys continuing the work that started in 2008 using X-ray diffraction, magnetisation, Small-Angle Neutron Scattering and Mössbauer measurements and in addition to perform the first trial electrical resistivity measurements on proton irradiated samples.

Work performed in 2009:

The results of the following tasks were presented in the meetings of the Materials Group organised by EFDA.

1. X-ray diffraction measurements were performed on all FeCr alloys received from EFDA and the thermal expansion was studied as a function of Cr concentration and temperature. The detailed work and results are described in Annex 37. Small-Angle neutron scattering experiments were performed on all FeCr alloys in order to establish whether the as-received EFDA FeCr alloys have any inhomogeneities or short range order or density fluctuations. The detailed work and results are described in Annex 38.
2. Detailed magnetic measurements, i.e. magnetic hysteresis loops measurements and magnetic-gravimetric measurements, have been performed on samples of 5wt% and 15wt% Cr concentration in the temperature range 300 to 1100 K. The dependence of the saturation magnetisation, remanence magnetisation, coercive field and Curie temperature on Cr concentration and temperature has been studied. The detailed work and results are described in Annex 39.
   Mössbauer measurements were carried out at 78 K on all FeCr alloys and the Fe local environment was assessed. The detailed work and results are described in Annex 40.
3. Preliminary designs and feasibility studies for the irradiation of iron-chromium alloys with protons and deuterons started in 2008. During 2009 we proceeded with the actual installation of the irradiation facility at the TANDEM accelerator, the coupling of the cryostat to the beam line and the realisation of the in-situ electrical resistivity measurement system. This allowed us to carry out the first successful irradiation experiments with 5 MeV protons. These experiments are described in detail in Annex 38. It has been shown that the sample is kept at cryogenic temperatures (~40 K) during the irradiation. Traces of the electrical resistivity of the sample could be successfully recorded in real-time, during and after the irradiation, providing valuable information on the amount of radiation damage induced by the proton beam. First trial electrical resistivity measurements were carried on proton irradiated samples. The detailed work and results are described in Annex 41.

4.1.3. Development of radiation resistant coatings

Background and objectives: As ODS-Eurofer is the candidate structural material for the blanket construction of a fusion reactor it would be challenging to be integrated with SiC, which has a variety of attractive properties such as low activation, high thermal conductivity coupled with low thermal expansion and high strength which give this material exceptional thermal shock resistant qualities at high operating temperatures. Our group has already experience in the fabrication and characterisation of SiC coatings. The objective of this activity is mainly to evaluate and optimise the functionalities of SiC coatings on ODS-Eurofer substrates. The aim for 2009 was to assess the interface reactions between SiC and ODS-Eurofer after heat treatments in the temperature range 600 to 900ºC.

Work performed in 2009:

1. Preparation of Eurofer (ODS) disks to deposit amorphous SiC films of various thicknesses up to 1000 nm by RF sputtering deposition. The conditions of deposition were previously optimised.
2. Thermal annealing of the films at various temperatures up to 900^oC under vacuum and investigation of interface reactions by X-ray diffraction and grazing-incidence diffraction measurements. The various phases formed at the ODS-Eurofer/SiC interface were identified. The detailed work and results are described in Annex 42.

4.1.4. Development of irradiation processes (Optimisation of the neutron field characteristics of the new GRR1 LEU core for fusion material irradiation studies)

Background and objectives: Evaluation of the neutron field characteristics at the irradiation positions of the Greek Research Reactor new Low Enrichment Uranium fuelled core is important in order to design fusion material irradiation campaigns, validate activation calculations and plan sample handling.

Work performed in year 2009:

1. The development of a detailed GRR-1 core model using the MCNP code and the exact core geometry and fuel composition was continued in 2009. The developed model was validated against experimental data. In particular, the results of the calculations were compared against experimentally determined control rod reactivity worths and neutron flux measurements performed in several core irradiation positions. The ratio of the calculated to measured integral reactivity (% Δk/k) of each of the five control rods was found 0.972 ± 0.098, 1.084 ± 0.110, 1.157 ± 0.117, 0.874 ± 0.089 and 1.097 ± 0.112.
2. The neutron field characteristics of the proposed Low Enrichment Uranium core were analysed at the irradiation traps used for fusion material activation studies (D4, A7, Z7, Z9, Be) The calculated to measured thermal neutron flux ratio for the five irradiation positions tested was found to be 1.217 ± 0.058, 1.082 ± 0.055, 1.117 ± 0.057, 0.979 ± 0.049 and 1.120 ± 0.067. Therefore, good agreement between MCNP calculated and experimental values was observed.
3. The predicted flux characteristics at the irradiation traps of the proposed reference LEU core were compared with the flux characteristics of the last operating mixed HEU and LEU core and a significant advantage of 15 % in increase in fast (E > 0.8 MeV) neutron flux was observed for the proposed LEU core.
4. The reference LEU core model was used for prediction of the nuclear parameters (induced activity, heating, gamma dose rate) in specimens of steel, silicon carbide, tungsten, beryllium and Li₄SiO₄ irradiated. These specimens were considered irradiated at the in-core fast neutron irradiation position (D4) of the proposed LEU reference core. The radionuclide inventory was estimated using FISPACT code assuming several irradiation and decay schemes. The calculations showed that control of Cobalt trace element impurity is of great importance for reducing gamma dose rate of structural components.
5. The potential for material irradiation studies utilising the new fully Low Enrichment Uranium core of the GRR-1 facility was demonstrated. The neutron field characteristics at several irradiation positions were studied. Sample related nuclear parameters that will assist in the design of irradiation campaigns and sample handling post-irradiation were determined. A summary of the results of the above work activities is provided in Annex 43.

4.4 Development of HT superconductors for DEMO

4.4.1. Development of high and low T_c superconductor hybrid metamaterials (Metamaterial fabrication consisting from layers of high and low Tc superconductors)

Background and Objectives: We have proposed to fabricate and study the magnetic and transport properties of hybrid materials consisting of high Tc Bi2-xPbxSr2Ca2CuO10+x (2223) and the intermediate critical temperature MgB2-xCx (MGBC) superconductors.

Work performed in year 2009:

1. We optimised the heat process in preparing single phase samples consisting of Bi_1.7Pb_0.3Sr₂Ca₂CuO₁₀ (2223) and MgB₂ superconductors, with Tc=109 K and 38 K, respectively. The quality of the sample and the evolution of the heat treatment process were monitored using x-ray diffraction data and the Rietveld method. Bulk magnetisation measurements have been performed in a SQUID magnetometer. The hybrid samples were prepared by mixing fine powders (few microns) of 2223 and MgB₂ superconductors compressed in a matrix 2cm in diameters, in several pressing forces. From the compressed pastilles we cut rectangular in shape specimens for the magnetic measurements.
2. The critical current was indirectly estimated from hysteresis magnetisation loops using the Bean model for rectangular specimens. Below 38 K (the critical temperature of MgB2) the hysteresis magnetisation loops are nearly symmetric in respect to the magnetic field axis, indicating that a volume critical current can flow between the grains of the 2223 and MgB₂ superconductors. As temperature increases the width of the loops decreases and, practically, above 39 K only 2223 grains are superconducting and the sample cannot support volume critical currents. Hybrid samples rich in MgB₂ can operate as bulk superconductors in the temperature interval 4.2-15 K with a reduced slope of the critical current J_c(H) curve, in comparison with the pure MgB₂. Such hybrid materials could operate more efficiently in comparison with pure MgB₂ or 2223 superconductors in the temperature interval 5<T<15 K. The detailed work and results are described in Annex 44.

4.6 Techniques for waste recycling

4.6.1 Waste assessment and management procedure determination (Optimisation of the design of a gamma spectrometry system to be used for remote evaluation of induced activity at components of a future fusion device for Waste Management purposes.)

Background and Objectives: Evaluation of activity levels at components of a future fusion plant is important in order to plan work activities and decrease collective doses received by personnel according to the ALARA principle. Moreover, accurate radiological characterisation of the plant materials allows development of waste management and material recycling strategies.

Work performed in year 2009:

1. The first effort for defining the approach to predict induced activity in components of the future fusion plant for preliminary estimation of radiological exposures were taken. Preliminary calculations were carried out. However, the results are not conclusive yet. This work will continue in 2010.
2. A HPGe detector based gamma spectrometry device capable to perform in-situ scanning of activated components was designed. In 2009 the computational modelling of the spectrometer was continued in order to derive the detector response for a range of gamma emitting sources and measurement geometries. Calculations were performed to predict full energy peak efficiency over the photon energy range between 50 keV and 5 MeV for surface and slab source configurations. Different collimator apertures were considered and their effect on the detector response was shown. The contribution of different HPGe crystal regions in the detector efficiency was evaluated showing the enhanced response of a large detector to high energy photons, due to energy deposited in the side and back segments of the crystal. This observation justifies the use of a large volume HPGe detector for collimated gamma spectrometry in applications requiring measurements in an extended photon energy range up to several MeV such as evaluation of induced activity in activated components of a fusion device. The proposed methodology minimises the amount of experimental work required for the spectrometer characterisation and gives valuable insight into different HPGe crystal regions contributing in the detector response. The results of this work are summarised in Annex 45.
3. A semi-empirical non-destructive technique to evaluate the activity of gamma-ray emitters (key radionuclides) in contaminated pipes was developed. The efficiency of the detector for the complex pipe and detector configuration was evaluated by Monte Carlo calculations. Gamma ray detector full energy peak efficiency was predicted assuming homogeneous activity distribution over the internal surface of the pipe. However, the effect of activity inhomogeneity on the accuracy of the technique was also examined. The results of this study are presented in Annex 46.