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 experimental results 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 theoretical models and calculations. The aim for 2008 was to determine the structure and the physical properties of EFDA and DEMOKRITOS FeCr alloys.

Work performed in 2008 (in co-operation within Materials and Modelling group):

Fe-Cr alloys with concentrations of 5, 10, 15 and 20 at. % Cr have been successfully fabricated with good homogeneity by means of the arc-melting technique. Detailed magnetic measurements, in particular, magnetic hysteresis loops measurements and magnetisation vs. temperature measurements, have been performed on samples of 5, 10, and 20 at. % Cr concentration in the temperature range 300 to 1100 K. The dependence of the saturation magnetisation, remanence magnetisation and coercive field on Cr concentration and temperature has been studied. In addition, X-ray diffraction measurements were performed on all samples and the dependence of the lattice constant on Cr concentration was studied.

Model Fe-Cr alloys, specially fabricated, were received from EFDA. The physical properties of these were determined, as for the FeCr alloys fabricated by Demokritos, through the performance of

i) X-ray diffraction measurements (Annex 41);

ii) Detailed magnetic measurements in the temperature range 300 to 1100 K on a-Fe and Fe-5%Cr (Annex 42);

iii) Mössbauer measurements at room temperature (Annex 42);

iv) Electrical measurements (Annex 43).

After discussions within the Materials Group and on the suggestion of the Chair of the Materials Topical Group it was decided that fast neutrons irradiation would not give any effect and all the effort should be concentrated on proton and deuteron irradiations (see Annex 43). Special samples have been fabricated for proton and deuteron irradiation and electrical measurements have been performed to characterise them prior to irradiation.

(The above results were presented at the meetings of the Materials Group organised by EFDA in June and December 2008.)

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. Initially the aim is to optimise the deposition parameters of SiC coatings using rf magnetron sputtering technique and evaluate the quality of the coatings on the steel substrates. Further investigations will assess the interface reactions between SiC and ODS-Eurofer after heat treatments under high vacuum in the intended operating temperature range of a fusion reactor.

Work performed in 2008:

In order to control and optimise the conditions of sputtering deposition of silicon carbide on steels, SiC layers were initially deposited on silicon Si(100) substrates which were further examined by Fourier Transform Infrared (FTIR) spectroscopy measurements to verify the growth of SiC. X-ray reflectivity (XRR) measurements were carried out in order to define the deposition parameters, to calibrate the deposition rate and finally to determine the mass density profile of the deposited coating. X-ray diffraction (XRD) measurements were also performed and verified the amorphicity of the SiC coatings in agreement with the FTIR measurements. The optimisation parameters for the deposition concern mainly the deposition temperature, the argon pressure and the rf power.

Further, thin disks of ODS-Eurofer were cut from a rod supplied by EFDA and were polished to mirror-like quality surface. These were used as substrates to deposit SiC coatings of 200 and 1000 nm thickness. The SiC coatings were found to be stable and well-adhering to the steel substrates. In Annex 44 are discussed the results of the work performed. In progress is the investigation of the reactions occurring at the Eurofer/SiC interface after heat treatments of these structures under vacuum.

4.1.4. Development of irradiation processes

Background and objectives: Within the Materials Modelling Group there is a considerable effort in understanding the radiation damage induced in FeCr alloys and correlating the experimental results with modelling. The general objectives of this activity is to provide experimental data for the resistivity recovery after irradiation with protons and deuterons and to further assist in the development of theoretical models and calculations. The aim for 2008 was to make a feasibility study of proton and deuteron irradiations at DEMOKRITOS TANDEM accelerator and to optimise the experimental set-up for the resistivity measurements. A second objective was to estimate the neutron field characteristics at the irradiation positions of the Greek Research Reactor facility. This study provided important data for the retrospective evaluation of the material irradiation experiments performed at GRR-1 and for the assessment of the neutron field properties of the proposed Low Enrichment Uranium (LEU) fuelled core.

Work performed in year 2008:

(i) Development of irradiation facilities using protons, 14 MeV neutrons and other ions of the “Demokritos” TANDEM accelerator: We have evaluated the feasibility of neutron and proton irradiation of samples at low temperatures for performing in situ electrical measurements, using “Demokritos” TANDEM accelerator (Annex 45). The parameters that were taken into account concern the sample heating, the homogenous production of defects and how to avoid ions trapping in the sample. In addition, we have developed an apparatus for in situ electrical measurements during the irradiation of samples using the “Demokritos” TANDEM accelerator (Annex 42 and Annex 45). A closed-cycle refrigerator has been modified in order to be coupled with the irradiation beam line of the accelerator and the necessary equipment has been selected. Also, a study has been made for the optimum energy selection so that the beam’s ions will pass through the sample but on the other hand to have a considerable number of defects produced in the sample. Various calculations were made for beam energies ranging from 4.0 up to 9.5 MeV, using the computer code SRIM in order to find the appropriate energy where the number of the produced defects, according to the Quince- formula, will reach a maximum.
(ii) Design of fast neutron irradiation facilities using the GRR-1 reactor: A series of numerical simulations was performed using the Monte Carlo code MCNP (version 5). The core and irradiation devices were geometrically modelled in detail, while the respective isotopic inventories were calculated using the WIMS-ANL code. Since the irradiation experiments were performed during the GRR-1 transition phase from High Enrichment Uranium (HEU) to Low Enrichment Uranium (LEU) fuel, simulations included both the actual HEU/LEU core configuration used in the sample irradiation studies and a proposed future core composed fully of LEU fuel assemblies. The neutron field characteristics at the GRR-1 irradiation devices for the two core configurations are shown in Annex 46. The results of the study showed an increase in total neutron flux after conversion to LEU fuel. At the fast neutron irradiation assembly, D4, there was an increase in thermal, epithermal and fast neutron flux of 7.8%, 23.3% and 23.8%, respectively at a set reactor power level. Moreover, the epithermal to thermal and fast to thermal flux ratios were increased by a factor of 1.14 and 1.16, respectively. In addition, a preliminary calculation of heating rate at EUROFER-97 sample irradiated at assembly D-4 was performed.

4.2 Materials Modelling

4.2.1 Car-Parinello modelling of proton-wall interaction

Background and Objectives: The use of ab-initio methods and in particular of ab-initio molecular dynamics can play an important role in fusion related research since the results do not depend on empirical force fields as in the case of classical molecular dynamics. Only a few works, however, take advantage of its potential for modelling the plasma wall interactions. Consequently, in 2008 the objective was to focus apply ab-initio (Car-Parinello) molecular dynamics to a known system (proton + graphite) in order to compare the results with the more abundant literature coming from classical molecular dynamics. The goal was to validate the model and, in the future (starting in 2009), use it in the case of beryllium, which is considered more critical due to the lack of available empirical force fields.

Work performed in year 2008 (partially in collaboration with the Institute of Ion Physics, Innsbruck University):

(i) Our ab-initio (Car-Parinello) molecular dynamics calculations were used to confirm that the C–C covalent bonds cannot be easily broken by direct bombardment of low energy (< 100 eV) hydrogen atoms and that the mechanism of erosion must be found elsewhere. This conclusion is consistent with classical MD data available in the literature. Additional details are given in Annex 47.
(ii) In addition, we showed that CH2 groups are not stable in the system generated by hydrogen collision on graphite. As a consequence, they cannot be considered the precursors of graphite corrosion as elsewhere suggested (Koga and Tanaka, J. Kor. Phys. Soc. 49 1 2006).
(iii) Despite the fact that the CH2 groups were found to be unstable, other structures like C2H3 can exist on the graphite. We were able to show that, in this case, the C–C bond is replaced by a C–H–C bridge. The presence of this group does not break up the graphitic net, but it can weaken, to a certain degree, the overall structure.
(iv) Potential Energy Surfaces of the C2H3 configuration were calculated.

(This work, not included in the initial work programme, was performed under the PWI Task Force, contract PWI-08-TA-05/CY/PS/0)

4.4 Development of HT superconductors for DEMO

4.4.1. Development of high and low Tc superconductor hybrid metamaterials

Background and Objectives: Superconducting materials play an important role in producing high magnetic fields, an essential element for plasma confinement. Superconducting coils are mostly fabricated from classical superconductors like Nb3Sn and NbTi. After the discovery of high-Tc superconductors in 1986, with Tc ranging from 40 to130 K, numerous scientists have tried to manufacture coils able to carry high current densities in-high magnetic fields, operating above 4.2 K. In addition, attempts have been made to manufacture hybrid coils consisting of high Tc and classical superconductors, targeting to increase the produced magnetic field operating at 4.2 K. In this spirit, we propose the manufacturing of hybrid prototypes constituted from the Bi2223 superconductor and MgB2 in order to study the possibility of hybrid specimens, to transport high critical current. The particular period we have been dealing with the preparation and the magnetic characterisation of the Bi2223 and Mg(B1-xCx)2 superconductors.

Work performed in year 2008:

Bi2-xPbxSr2Ca2CuO10+δ has been prepared by standard solid state reaction by heating stoichiometric amount of Bi2O3, PbO2, SrCO3, CaCO3 and CuO. After several heatings at 800°C and grindings, the specimens were heated at 970oC for a month. Bi2-xPbxSr2CaCu2O8 (2212 Tc = 80 K) was initially formed (and some minor impurity phases) but as the reaction temperature raised at 870oC, the transformation 2212 → 2223 takes place with slow kinetic. This is the reason for the one month annealing time at 870°C. The partial substitution of Bi for the Pb (x = 0.3) is needed in order to improve the slow 2212 → 2223 kinetics. At this stage of our study we also have estimated the magnetic phase diagram (vortex matter properties) of MGBC compound in single crystal form, in order to define the limit of concentrations being appropriate for the hybrid superconductors. 2223 compound is a high anisotropic superconductor with anisotropic parameter roughly. Since for the 2223 superconductor a single crystal is very difficult to be prepared we will use polycrystalline samples. In this case we measure the most advantage situation e.g. . Our measurements show that contrary to low Tc superconductors the diamagnetic onset does not change appreciably as the external magnetic field increases, due to the fluctuation effects. Only below the so-called irreversibility temperature (see Annex 48) the sample is capable to support high critical current (a parameter useful for practical applications). For the magnetic characterisation of the MGBC superconductor we used ac and dc magnetic measurements. Measurements under several magnetic fields show that for this superconductor, although having a significant critical temperature (Tc = 35 K), the diamagnetic onset moves to lower temperatures as the external magnetic field increases. Measurements in several angles of the c-axis of the crystal, with magnetic field, reveal that MGBC is also an anisotropic superconductor with more disadvantageous the situation θ = 0. The crystal with high carbon content (x = 0.1) displays a lower critical temperature in respect to the x = 0.04 crystal, but a lower anisotropy and higher Hc2-line for θ = 0. In conclusion, based on these results in order the hybrid superconductor to be useful for magnetic fields B > 1 T and temperature interval 5-20 K, the sample needs to have a carbon content x = 0.1.

4.6 Techniques for waste recycling

4.6.1 Waste assessment and management procedure determination

Background and Objectives: Determination of activity levels at components of a future fusion power plant is important in order to plan work activities and decrease collective radiation doses received by the plant personnel according to the ALARA principle. Moreover, accurate radiological characterisation of the plant materials allows development of waste management and material recycling strategies. Therefore, a measurement campaign will be required in order to determine radiation dose levels, identify radioisotopes and monitor radionuclide concentrations in activated materials (i.e. structural components, first wall etc.) of a fusion plant. A mobile, remote controlled, High-Purity Ge (HPGe) detector based gamma spectrometry device can be used for such measurements. Scope of this work is the conceptual design of a gamma spectrometer system capable to perform in situ scanning of activated components of a fusion power plant.

Work performed in year 2008: A series of numerical simulations and verification measurements was performed in order to develop a computational model of the proposed spectrometer.

(i) Monte Carlo code MCNP was applied to model a coaxial HPGe detector. The model was applied to predict the detector full energy peak efficiency for point sources in the photon energy range of 122 to 1332 keV. The geometric parameters defining the detector active volume were optimised by comparison against experimental measurements. The effects of germanium crystal dead-layer thickness and counting geometry on full energy peak efficiency were studied. The developed calibration technique enabled efficiency characterisation of the HPGe detector using minimum experimental data. The results of this study are summarised in Annex 49.
(ii) A comparison of HPGe detector full energy peak efficiency prediction using Monte Carlo codes MCNP and PENELOPE was performed. The results of the calculations were compared amongst each other and against measurements performed using standard sources in the photon energy range of 80 keV to 1332 keV. Very good agreement was obtained between the results of the two codes and the experimental data. Therefore both codes can be applied in modelling HPGe detector full energy peak efficiency successfully. The results of this comparison study are shown in Annex 50.
(iii) European Activation System (EASY) code package was used to evaluate induced activity, gamma-ray emission spectra and gamma dose rate levels from EUROFER-97 considered as structural first wall material of a fusion power plant. A 2.5 years reactor operation period, at a first wall power of 1 MW⋅m-2, was assumed. Calculations were performed for cooling times ranging from 1 hour to 100 years after reactor shut-down. This study provided the gamma ray source data required for the design of the gamma spectrometer (gamma ray spectrum and activity levels). The results of this evaluation are shown in Annex 51.
(iv) A preliminary design of a gamma spectrometer system for in situ assay of activated components of a fusion power plant was developed. The system incorporated a HPGe detector, gamma ray shielding and an adjustable collimator. Monte Carlo code MCNP was used to predict the system response for assaying a slab composed of EUROFER-97. Gamma ray full energy peak efficiency was predicted for the extended photon energy range from 50 keV to 5 MeV. The response of different detector segments was investigated and their relative contribution to the measurement was analysed. The results of the calculations are shown in Annex 52. This work is in progress.