4.2 Materials Modelling

4.2.1. Radiation damage studies on Fe and Fe-Cr alloys by electrical resistivity measurements

Background and Objectives: Within the Fusion Materials Topical Group there is considerable effort in understanding and modelling radiation effects in Fe and Fe-Cr alloys, which constitute the base of EUROFER low-activation structural steels. Experimental validation of the developed modelling tools is of primary importance for the reliable prediction of radiation damage at fusion relevant dose levels. The general objective of this activity is to provide relevant experimental data to assist the modelling activities and test theoretical predictions. For this purpose, a low temperature ion irradiation device has been under development, offering in-situ electrical resistivity measurements and fast annealing capabilities. Within 2010 the fast temperature control loop has been completed and the first resistivity recovery experiments have been conducted on proton irradiated Fe-Cr alloys.

Work performed in 2010 (within Materials Topical Group (TG-M), collaboration with CIEMAT):

1. Fast heating and cooling necessary for the electrical resistivity recovery measurements after low-temperature irradiation requires very tight control of the sample temperature. For this purpose a versatile computerised temperature control system was developed and integrated in the low temperature irradiation device. The control algorithms are implemented on a personal computer (PC) for maximum flexibility. Apart from the standard types of temperature transducers as e.g. thermocouples, the system is capable of utilising the electrical resistivity of the sample itself as a temperature probe. This allows for very accurate and instantaneous temperature sensing. The temperature control system has been successfully tuned and tested in the real working conditions of the irradiation environment.
2. Two distinct methods of sample heating have been tested: resistive and radiative. Radiative heating presents the advantage that the heating power can be selectively supplied (focused) only on the sample keeping the surrounding sample holder at cryogenic temperature. However it was found that the resulting temperature profile on the specimen is highly in-homogeneous, leading to undesirable uncertainties in the annealing temperature. To remedy this problem, resistive heater foils were implemented in close contact with the sample. The system is designed so that only the sample and the heaters are at high temperature during annealing. Due to the small thermal masses, very fast heating and cooling rates have been achieved. The electrical leads which supply the heating current are at the same time responsible for cooling. The rate of heat supply and loss through these leads were carefully balanced and optimised so as to achieve heating/cooling rates of ~1000 deg/min with final annealing temperatures up to 600 K. The temperature of the surrounding sample holder remains at cryogenic levels. Typical performance of the system is described in Annex 37.
3. For the electrical resistivity measurements under irradiation specimens in the form of thin foils with a thickness of 50 – 100 µm are required. Several methods were tested to achieve this: mechanical lapping, grinding and cold rolling. The later method gives excellent thickness homogeneity and is therefore preferred. After rolling to the required thickness and cutting the specially formed resistivity specimens out of the thin foils, the samples are chemically polished so that possible surface contamination is removed. Cold-working induced by the rolling process is relieved by annealing at 800 - 850ºC for 24 hours under vacuum. Under these preparation conditions, the electrical resistivity measured on different specimens of the same alloy shows differences off only 1 – 2 % from the mean value, indicating that the procedure is accurate and repeatable.
4. As the main technical issues related to the resistivity recovery measurements have been resolved, the first irradiation campaigns were started within 2010. The recovery of radiation damage produced by 5 MeV protons in a Fe-10%Cr alloy has been successfully recorded by means of electrical resistivity measurements after successive annealing steps. Four recovery stages were clearly identified. These results will be compared to recent theoretical results after a complete systematic experimental data-set has been acquired.

The above results are reported in Annex 37 and were presented at the meetings of the Materials Topical Group (MATREMEV) organised by EFDA.

4.2.2. Experimental validation of phase stability by XRD, SANS and Mössbauer measurements

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 2010 the aim was to complete the determination of 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.

Work performed in 2010 (within Materials and Modelling group (TG-M), IPPLM-Poland):

1. The experimental set-up of measuring X-ray diffraction patterns down to liquid nitrogen temperature has been completed and successfully tested. X-ray diffraction patterns have been measured for iron samples in the temperature range 93 to 300 K. Data analysis provided the lattice constant variation as a function of temperature.
2. Samples of FeCr alloys were thermally aged in the temperatures 400, 500, 600 and 700^oC for 2 and 9 days after being solid solution treated at 800^oC for 4 h. Two neutron beam time applications were submitted and approved after review from external committees, at Laboratoire Leon-Brillouin, CEA, Saclay and at JCNS at FRMII reactor in Garching, in order to perform Small-Angle Neutron Scattering (SANS) experiments on the thermally aged FeCr samples. In total 10 beam days were allocated and the experiments were carried out. Initial analysis of the SANS data was performed for the determination of the agglomeration phenomena and it is going to be continued in 2011 (see Annex 38).
3. Sample thinning methodologies (chemical polishing and grinding and cold rolling) have been tested in order to fabricate thin foils from FeCr alloys for Mössbauer measurements. Both were methodologies proved to be successful. Mössbauer spectroscopy (MS) measurements have been performed on (a) thermally aged samples and (b) proton irradiated samples.
4. An invitation to Prof Dubiel (IPPLM-Poland) resulted in a visit at the end of 2010 which was very productive. Data on several samples was subsequently analysed using the specialised software developed for such alloys at Prof Dubiel’s laboratory. These analyses indicated the importance and influence of sample preparation conditions and parameters in determining local strain and order/disorder in the samples. The detailed results are presented in Annex 39.

The above results were presented in the meetings of the Materials Topical Group (MATREMEV) organised by EFDA.

4.2.3. Oxidation protection layers for EUROFER

Background and objectives: The ferromagnetic 9Cr steel EUROFER 97 is considered as a structural material for the first wall of future fusion reactors. The operation temperature for EUROFER 97 is limited to 550^oC due to loss of its creep strength at high temperatures. This disadvantage has been overcome by the development of oxide dispersion strengthened (ODS) variants of this steel. The EUROFER 97 steel strengthened with Y₂O₃ particles (0.3–1wt%) allows an increase in the operation temperature by about 100^oC. In this work SiC based coatings deposited on ODS-Eurofer are developed and the structure and chemical reactions with the ODS-Eurofer substrate are investigated using various techniques such as X-ray diffraction at grazing incidence (GIXRD), X-ray reflectivity (XRR), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) with EDX analysis and Rutherford Back Scattering (RBS).

Work performed in 2010 (within Materials Topical Group (TG-M), KIT):

1. Thick SiC coatings were deposited on ODS-Eurofer using the RF magneton sputtering technique and assessment of their quality was performed.
2. SEM measurements have been performed on the surfaces and the cross section of as deposited and annealed SiC coatings on ODS-Eurofer in order to investigate surface morphology and the chemical composition at the interface. The oxidation tests planned to be performed in the second part of 2010 were not completed because the responsible researcher for this work left NCSR “D”.
3. Investigation of the interface of SiC/ODS-Eurofer using RBS measurements.
4. Investigation of the surface morphology at each heat treatment stage by AFM measurements.

The above results are presented in Annex 40.

4.4 Development of HT superconductors for DEMO

4.4.1. Fabrication of hybrid contacts

Background and Objectives: The aim of this activity is to prepare high quality samples of the superconductors MgB₂, (T_c=39 K), Bi_2-xPb_xSr₂Ca₂Cu₃O₁₀ (T_c=110 K) and YBa₂Cu₃O₇ (T_c=93 K) in polycrystalline form, in order to fabricate hybrid samples consisting of two superconductors (MgB₂/ Bi_2-xPb_xSr₂Ca₂Cu₃O₁₀ and MgB₂/ YBa₂Cu₃O₇) and to study the superconducting properties, mainly critical current as a function of magnetic field and temperature.

Work performed in year 2010:

We have prepared and magnetically characterised hybrid specimens consisting of high T_c and intermediated T_c, MgB₂ superconductor. The hybrid specimens were fabricated by compressing powders of the two superconductors in a cylindrical matrix using a hydraulic press. The orientation of the grains is estimated using x-ray diffraction data by modelling the preferred orientation with the March model. The characterisation of the samples and the contact between the two superconductors is being made by employing dc magnetisation and ac susceptibility measurements.

1. We continue our effort in order to further optimise the preparation method of Bi_2-xPb_xSr₂Ca₂Cu₃O₁₀ superconductor. We study a large number of hybrid samples consisting of MgB₂/ Bi_2-xPb_xSr₂Ca₂Cu₃O₁₀ by varying the volume percentage of two superconductors and the pressure used to prepare the hybrid specimens. The orientation of the grains in the hybrid samples is studied using the Rietveld method and by employing the empirical March model. Strong preferred orientation has been observed in the grains of Bi_2-xPb_xSr₂Ca₂Cu₃O₁₀ superconductor. The superconducting properties of the hybrid samples were studied by dc magnetisation measurements as function of the external magnetic field at several temperatures. Hybrid specimens were fabricated by compressing and thoroughly mixing, appropriate quantities (volume percentage 90/10, 81/19, 50/50, 19/81 and 10/90, where the first number corresponds to MgB₂ and the second to the YBa₂Cu₃O₇) of fine powders of the MgB₂ and YBa₂Cu₃O₇ superconductors, in a cylindrical matrix, using a hydraulic press. Rietveld refinement of the x-ray diffraction of patterns of the compressed hybrid samples revealed that a strong preferred orientation of the high T_c superconductor grains exists. The critical current density of the hybrid specimens is estimated using the Bean model, supposing a “roof” type profile of the magnetic induction inside samples. By using ac-susceptibility measurements as a function of the dc-magnetic field and the amplitude of the ac magnetic field we tried to assess the ability of the contact between MgB₂ and high T_c-superconductors to support superconducting current. The detailed work and results are described in Annex 41.

4.6 Techniques for waste recycling

4.6.1 Gamma ray spectrometry calibration method for non-destructive evaluation of induced radioactivity at components of a future fusion device

Background and Objectives: Evaluation of induced activity at structural components of a future fusion device is important in order to plan work activities, estimate occupational collective doses, apply ALARA principle, and implement waste management and material recycling strategies. Objective of the present activity is to develop a gamma spectrometer system enabling to perform evaluation of activity at components of a fusion device remotely. The spectrometer incorporates a shielded coaxial High Purity Germanium semiconductor detector and an adjustable collimator. In 2009 a model of the system was developed and used to predict detector Full Energy Peak (FEP) efficiency for a set of collimator apertures and source configurations representing a uniformly activated first wall of a fusion device. Moreover, the contribution of different crystal sections in detection efficiency was evaluated.

Work performed in year 2010:

In 2010, the study focused on the optimisation of the detector-collimator configuration for analysis of bulk samples or components with non-homogeneous activity distribution.

1. The detector model was further improved by implementing a better representation of the crystal inactive layer. The full energy peak efficiency of the detector was semi-empirically evaluated using the efficiency transfer method as a function of photon energy. The evaluation was based on calculations performed using Monte Carlo code MCNP and taking into consideration experimental full energy peak efficiency results obtained from a multi-nuclide volume reference source. In addition, X-ray images of the detector and mount configuration were taken in order to verify detector nominal data provided by the manufacturer and reveal fine differences between nominal and actual geometrical characteristics of the detector especially at the crystal edge region.
2. Based on the results of the above studies, a detailed model of the detector, shielding, collimator aperture and bulk sample was developed. The model was used to study the effect of a set of parallel-hole collimator apertures for analysis of bulk samples on the detector response. The effect of collimator thickness, aperture diameter, source to detector, detector to collimator distance was examined and a final configuration was proposed.
3. The model was then used in order to demonstrate the feasibility of detecting an active source within a bulk matrix material. A 3-D gamma ray emission scan of the sample was produced and an inhomogeneity in activity distribution was successfully predicted.

An extended summary of the work and results is provided in Annex 42.

4.6.2. Development and experimental evaluation of the in situ gamma spectrometry system under selected measurement geometries simulating configurations to be encountered in a fusion plant

Background and Objectives: A collimated High Purity Germanium (HPGe) semiconductor detector based gamma spectrometry device was designed to be used for remote evaluation of induced activity at components of a future fusion device (section 4.6.1). The system incorporates a HPGe detector, gamma ray shielding and an adjustable collimator. Aim of the present activity is the experimental evaluation of the system under selected counting configurations.

Work performed in year 2010:

1. A mock-up of the spectrometer was developed. The system incorporated an 85% efficiency HPGe detector, gamma ray shielding and a set of parallel-hole type collimator apertures. Full energy peak efficiency was experimentally determined for different source to detector distances and collimator apertures in the photon energy range from 60 keV to 1836 keV.
2. Two calibrated multi-nuclide gamma standards of extended geometry were procured and tested. The sources represented a foil surface source and a cylindrical source homogeneously incorporated in resin matrix.
3. The MCNP model of the in situ gamma spectrometer was verified against the measurements for the extended radioactive source configurations. A very good agreement between calculated and experimental results within 5% was obtained for all photon energies.

This work demonstrated that the proposed spectrometry system has the capability to evaluate induced activity in materials for different gamma sources and measurement geometries. In addition, the measurements enabled validation of the detector model. Therefore, the model can be used for computational efficiency characterisation of the spectrometer for a wide range of source sizes, shapes and matrices that may be encountered in radioactivity assay applications in a future fusion device. The work and results are provided in Annex 43.