Study of ceramic pebble beds in Post Irradiation Examination of the Pebble Bed Assemblies irradiation experiment

The irradiation experiment Pebble Bed Assemblies (PBA) consists of four mock-up representations (test elements) of the EU Helium Cooled Pebble Bed (HCPB) concept. The four test elements contain a ceramic breeder pebble bed sandwiched between two beryllium pebble beds and are regarded as one of the first DEMO representative HCPB blanket irradiation tests, with respect to temperatures and power densities. The design value of the PBA were to irradiate pebble beds at a power density of 20–26 W/cc in the ceramic breeder, to a maximum temperature of 800 °C.Two test elements contain lithium orthosilicate pebbles (Li₄SiO₄; FZK/KIT) and were irradiated with target temperatures of 600 and 800 °C, respectively. The other test elements have lithium metatitanate (Li₂TiO₃; CEA) with different grain sizes and were both irradiated with a target temperature of 800 °C. The PBA have been irradiated for 294 Full Power Days (12 cycles) in the High Flux Reactor (HFR) in Petten to a total neutron dose of 2–3 dpa in Eurofer, and an estimated (total) lithium burnup of 2–3% in the ceramic pebbles.This work presents results of Post Irradiation Examinations (PIE) on the four HCPB test elements. Using e.g. SEM, the evolution of compressed pebble beds and pebble interactions like swelling, creep, sintering, etc., under irradiation and thermal loads are studied for the candidate pebble materials Li₂TiO₃ and Li₄SiO₄. (Chemical) interactions between ceramic pebbles and Eurofer (e.g. chrome diffusion) are observed. Looking at different sections of the pebble beds, correlations between temperatures and thermal–mechanical behaviour are clearly observed.     

Thermal conductivity of SiC after heavy ions irradiation

In this study, we performed irradiation experiments on α-SiC samples, with heavy ions at room temperature (74 MeV Kr, fluence of 5 × 10¹⁴ ions cm^?2). This energy results in an irradiated layer of about 9.6 μm for SiC. TEM and Raman analyses reveal a graded damaged material. In the electronic interactions domain SiC is weakly damaged whereas it becomes fully amorphous in the nuclear interactions domain. According to the structural examinations, the irradiated SiC is considered as a multilayered material. Thermal conductivity in both electronic and nuclear interactions domains is measured as a function of temperature and annealing temperature. It appears that such an approach is reliable to estimate thermal conductivity of ceramics under neutron irradiation.     

Development of the IFMIF Tritium Release Test Module in the EVEDA phase

This paper presents the engineering design of the IFMIF (International Fusion Materials Irradiation Facility) Tritium Release Test Module (TRTM). The objectives of the TRTM are: (i) in-situ measurements of the tritium released from lithium ceramics and beryllium pebble beds during irradiation, (ii) studying the chemical compatibility between lithium ceramics and structural materials under irradiation, and (iii) performing post irradiation examinations for the irradiated materials. The TRTM has eight rigs which are arranged in two rows (2 × 4) perpendicular to the beam axis and enclosed by a structural container. Each rig includes one capsule that contains lithium ceramic or beryllium pebbles for irradiation. Neutrons reflectors are implemented at different locations to reflect the scattered neutrons back to the active region aiming to improve the tritium production. The TRTM is required to provide irradiation temperature range of 400–900 °C for the capsules filled with lithium ceramics and 300–700 °C for the ones packed with beryllium. The engineering design of the TRTM components such as container, rigs, capsules, pebble beds, neutrons reflectors, and purge gas and coolant tubes are presented. In addition a test matrix for the irradiation campaign is proposed.     

Thermo-mechanical analyses and assessment with respect to the design codes and standards of the HCPB-TBM Breeder Unit

The Helium Cooled Pebble Bed-Test Blanket Module (HCPB-TBM) is one of the two breeding blanket concepts currently under development in Europe. Key component of the HCPB-TBM, the Breeder Unit (BU), has entered the detailed engineering design phase. After establishing a base design, thermal and thermo-mechanical analyses have been performed under typical ITER operational conditions: the results are presented and discussed in this paper.A full scaled finite element model of the base design of the HCPB-TBM BU has been built to run thermal analyses of the beryllium and Li₄SO₄ pebble beds and thermo-mechanical analyses of the BU structure, both in steady state and in a typical transient regime during a pulse of the ITER D–T phase. The temperatures reached in the Li₄SO₄ and beryllium pebble beds in the BU base design are 930.8 °C and 712.9 °C, respectively, which are above the recommended values of 920 °C for Li₄SO₄ and 650 °C for the beryllium pebbles. The maximum temperature in the structural steel is 548.4 °C, which remains under the design limit defined for the TBM studies (550 °C). In order to decrease the temperature in the hot spots identified in the pebble beds, a reduction of the Li₄SO₄ and beryllium bed volumes has been adopted. As for the structural material, the thermo-mechanical analyses have been assessed with respect the RCC-MR design code (completed for irradiation damages with ITER SDC-IC). The results reveal some problematic points in the base design, concentrated in the coolant inlet and outlet pipes and in the connection region of the BU cooling plates with the BU backplate. Submodeling technique has been used to improve the design in these regions. An increase in the thickness of the coolant inlet and outlet pipes and a redesign of the BU backplate have led to a fulfillment of the codes and standards. The design modifications of pebble bed region and structural material have been implemented in the final design of the BU that is presently used as reference for the design and test of a BU mockup in KIT.     

Micro-Raman studies of swift heavy ion irradiation induced structural and conformational changes in polyaniline nanofibers

Polyaniline (PAni) nanofibers doped with camphor sulfonic acid have been irradiated with 90 MeV O⁷⁺ ions at different fluences (3 × 10¹⁰?1 × 10¹² ions/cm²) using a 15UD Pelletron accelerator under ultra-high vacuum. XRD studies reveal a decrease in the domain length and an increase in the strain upon SHI irradiation. The increase in d-spacing corresponding to the (1 0 0) reflection of PAni nanofibers with increasing irradiation fluence has been attributed to the increase in the tilt angle of the chains with respect to the (a, b) basal plane of PAni. Decrease in the integral intensity upon SHI irradiation indicates amorphization of the material. Micro-Raman (μR) studies confirm amorphization of the PAni nanofibers and also show that the PAni nanofibers get de-doped upon SHI irradiation. μR spectroscopy also reveals a benzenoid to quinoid transition in the PAni chain upon SHI irradiation. TEM results show that the size of PAni nanofibers decreases with the increase in irradiation fluence, which has been attributed to the fragmentation of PAni nanofibers in the core of amorphized tracks caused by SHI irradiation.     

Swift heavy ion induced modifications of optical and microstructural properties of silver–fullerene C60 nanocomposite

In this paper, we study the optical and microstructural properties of silver–fullerene C₆₀ nanocomposite and their modifications induced by swift heavy ion irradiation. Silver nanoparticles embedded in fullerene C₆₀ matrix were synthesized by co-deposition of silver and fullerene C₆₀ by thermal evaporation. The nanocomposite thin films were irradiated by 120 MeV Ag ions at different fluences ranging from 1 × 10¹² to 3 × 10¹³ ions/cm². Optical absorption studies revealed that the surface plasmon resonance of Ag nanoparticles showed a blue shift of ～49 nm with increasing ion fluence up to 3 × 10¹³ ions/cm². Transmission electron microscopy and Rutherford backscattering spectroscopy were used to quantify particle size and metal atomic fraction in the nanocomposite film. Growth of Ag nanoparticles was observed with increasing ion fluence. Raman spectroscopy was used to understand the effect of heavy ion irradiation on fullerene matrix. The blue shift in plasmonic wavelength is explained by the transformation of fullerene C₆₀ matrix into amorphous carbon.     

Effect of Li3+ irradiation on the transport properties of La0.7Pb0.3MnO3 type CMR material

We report the low temperature (below the metal–insulator transition temperature T_im) resistivity and magnetoresistance (MR) behavior of 50 MeV Li³⁺ beam irradiated La_0.7Pb_0.3MnO₃ for three different fluences. Ion beam irradiation causes a decrease of T_im leading to the increase of insulating regime. Resistivity data of the unirradiated as well as irradiated samples fitted well with an equation of the form ρ = ρ₀ + ρ_2.5T^2.5 which indicates predominant contribution from the electron–magnon interaction (second term). The temperature dependent MR data of samples irradiated with different ion fluences follow the simple relation [MR = a + b/(T + C)] showing appreciable effect of radiation on the parameters a, b and C. The physical significance of the radiation effect on these parameters is not yet very clear.     

Low temperature formation of luminescent Si nanocrystals with combined process of excimer UV-light irradiation and RTA

Si ion implantation was widely used to synthesize specimens of SiO₂ containing supersaturated Si and subsequent high temperature annealing induces the formation of embedded luminescent Si nanocrystals. In this work, the potentialities of excimer UV-light (172 nm, 7.2 eV) irradiation and rapid thermal annealing (RTA) to achieve low temperature (below 1000 °C) formation of luminescent Si nanocrystals in SiO₂ have been investigated. The Si ions were introduced at acceleration energy of 180 keV to fluences of 7.5 × 10¹⁶ and 1.5 × 10¹⁷ ions/cm². The implanted samples were subsequently irradiated with an excimer-UV lamp for 2 h. After the process, the samples were rapidly thermal annealed at 1050 °C for 5 min before furnace annealing (FA) at 900 °C. Photoluminescence spectra were measured at various stages at the process. Effective visible photoluminescence is found to be observed even after FA at 900 °C, only for specimens treated with excimer-UV lamp and RTA, prior to a low temperature FA process. Based on our experimental results, we discuss the mechanism for the initial formation process of the luminescent Si nanocrystals in SiO₂, together with the effects with excimer lamp irradiation and RTA process on the luminescence.     

Comparison of neutron and gamma irradiation effects on KU1 fused silica monitored by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) studies have been carried out on KU1 fused silica irradiated with neutrons at fluences 10²¹ and 10²² n/m², and gamma-ray doses up to 12 MGy. The effects of post-irradiation thermal annealing treatments, up to 850 °C, have also been investigated. Paramagnetic oxygen-related defects (POR and NBOHC) and E′-type defects have been identified and their concentration has been measured as a function of neutron fluence, gamma dose and post-irradiation annealing temperature. It is found that neutrons at the highest fluence generate a much higher concentration of defects (mainly E′ and POR, both at concentrations about 5 × 10¹⁸ spins/cm³) than gamma irradiations at the highest dose (mainly E′ at a concentration about 4 × 10¹⁷ spins/cm³). Moreover, for gamma-irradiated samples a lower treatment temperature (about 400 °C) is required to annihilate most of the observed defects than for neutron-irradiated ones (about 600 °C).     

Soft X-ray fluorescence measurements of irradiated polymer films

Fluorescent soft X-ray carbon Kα emission spectra (XES) have been used to characterize the bonding of carbon atoms in polyimide (PI) and polycarbosilane (PCS) films. The PI films have been irradiated with 40 keV nitrogen or argon ions, at fluences ranging from 1 × 10¹⁴ to 1 × 10¹⁶ cm⁻². The PCS films have been irradiated with 5 × 10¹⁵ carbon ions cm⁻² of 500 keV and/or annealed at 1000°C. We find that the fine structure of the carbon XES of the PI films changes with implanted ion fluence above 1 × 10¹⁴ cm⁻² which we believe is due to the degradation of the PI into amorphous C:N:O. The width of the forbidden band as determined from the high-energy cut-off of the C Kα X-ray excitation decreases with the ion fluence. The bonding configuration of free carbon precipitates embedded in amorphous SiC which are formed in PCS after irradiation with C ions or combined treatments (irradiation and subsequent annealing) is close to either to that in diamond-like films or in silicidated graphite, respectively.     

SHI induced re-crystallization of Ge implanted SiO2 films

Ge nanocrystals embedded in SiO₂ matrix have been synthesized by swift heavy ion irradiation of Ge implanted SiO₂ films. In the present study, 400 keV Ge⁺ ions were implanted into SiO₂ films at dose of 3 × 10¹⁶ ions/cm² at room temperature. The as-implanted samples were irradiated with 150 MeV Ag¹²⁺ ions with various fluences. Similarly 400 keV Ge⁺ ions implanted into Silicon substrate at higher fluence at 573 K have been irradiated with 100 MeV Au⁸⁺ ions at room temperature (RT). These samples were subsequently characterized by XRD and Raman to understand the re-crystallization behavior. The XRD results confirm the presence of Ge crystallites in the irradiated samples. Rutherford backscattering spectrometry (RBS) was used to quantify the concentration of Ge in the SiO₂ matrix. Variation in the nanocrystal size as a function of ion fluence is presented. The basic mechanism of ion beam induced re-crystallization has been discussed.     

Development and experimental study of beryllium window for ITER radial X-ray camera

Radial X-ray camera (RXC) is a diagnostic device planned to be installed in the ITER Equatorial Port #12. Beryllium window will be installed between the inner and outer camera of RXC, which severs as the transmission photocathode substrate and also the vacuum isolation component. In this paper the design and manufacture process of two types of beryllium windows were introduced. Although 50 μm thickness of beryllium foil is the best choice, the 80 μm one with X-ray threshold of 1.34 keV was selected for safety consideration. Using the intermediate layer (low purity of beryllium) between the beryllium foil and the stainless steel base flange is an effective strategy to limit the welding thermal deformation and thermal stress of the thin foil caused by bonding between different materials. By using ANSYS software, the feasibility of the aperture design was analyzed and validated. Metal sealing ring was applied in the mechanical clamped beryllium window for its good stability under high temperature and neutron radiation. Although both of the hollow metal sealing ring with 0.03 mm silver coating and the pure silver sealing ring can satisfy the sealing requirement, the later one was chosen to produce the final product. Two hours 240 °C high temperature baking test, two hours 3.3 Hz vibration test and fatigue test were performed on the two types of beryllium windows. Based on the tests results, the two types of beryllium windows could stand the high temperature baking during the wall conditioning phase of ITER tokamak and the vibration during transportation without causing large leakage. Both of the two types of beryllium windows could bear impact load (0.1 MPa pressure difference) for many times without failure.     

Study of modifications in Lexan polycarbonate induced by swift O6+ ion irradiation

Swift Heavy Ion (SHI) irradiation of the polymeric materials modifies their physico-chemical properties. Lexan polycarbonate films were irradiated with 95 MeV oxygen ions to the fluences of 10¹⁰, 10¹¹, 10¹², 10¹³ and 2 × 10¹³ ions/cm². Characterization of optical, chemical, electrical and structural modifications were carried out by UV–Vis spectroscopy, FTIR spectroscopy, Dielectric measurements and X-ray Diffraction. A shift in the optical absorption edge towards the red end of the spectrum was observed with the increase in ion fluence. The optical band gap (E_g), calculated from the absorption edge of the UV–Vis spectra of these films in 200–800 nm region varied from 4.12 eV to 2.34 eV for virgin and irradiated samples. The cluster size varied in a range of 69–215 carbon atoms per cluster. In FTIR spectra, appreciable modification in terms of breaking of the cleavaged C–O bond of carbonate and formation of phenolic O–H bond was observed on irradiation. A rapidly decreasing trend in dielectric constant is observed at lower frequencies. The dielectric constant increases with fluence. It is observed that the loss factor increases moderately with fluence and it may be due to scissoring of polymer chains, resulting in an increase in free radicals. A sharp increase in A.C. conductivity in pristine as well as in irradiated samples is observed with frequency and is attributed to scissoring of polymer chains. XRD analyses show significant change in crystallinity with fluence. A decrease of ～9.02% in crystallite size of irradiated sample at the fluence of 2 × 10¹³ ions/cm² is observed.     

Experimental study and analysis of the purge gas pressure drop across the pebble beds for the fusion HCPB blanket

The lithium ceramic and beryllium pebble beds of the breeder units (BU), in the fusion breeding blanket, are purged by helium to extract the bred tritium. Therefore, the objective of this study is to support the design of the BU purge gas system by studying the effect of pebbles diameter, packing factor, pebble bed length, and flow inlet pressure on the purge gas pressure drop. The pebble bed was formed by packing glass pebbles in a rectangular container (56 mm × 206 mm × 396 mm) and was integrated into a gas loop to be purged by helium at BU-relevant pressures (1.1–3.8 bar). To determine the pressure drop across the pebble bed, the static pressure was measured at four locations along the pebble bed as well as at the inlet and outlet locations. The results show: (i) the pressure drop significantly increases with decreasing the pebbles diameter and slightly increases with increasing the packing factor, (ii) for a constant inlet flow velocity, the pressure drop is directly proportional to the pebble bed length and inlet pressure, and (iii) predictions of Ergun's equation agree well with the experimental values of the pressure drop.     

Temperature dependence of surface morphology and deuterium retention in polycrystalline ITER-grade tungsten exposed to low-energy,high-flux D plasma

Surface topography and deuterium retention in polycrystalline ITER-grade tungsten have been examined after exposure to a low-energy (38 eV/D), high-flux (10²² D/m² s) deuterium plasma with ion fluences of 10²⁶ and 10²⁷ D/m² at various temperatures. The methods used were scanning electron microscopy equipped with focused ion beam, thermal desorption spectroscopy, and the D(³He,p) ⁴He nuclear reaction at ³He energies varied from 0.69 to 4.0 MeV. During exposure to the D plasma at temperatures in the range from 320 to 815 K, small blisters of size in the range from 0.2 to 5 μm, depending on the exposure temperature and ion fluence, are formed on the W surface. At an ion fluence of 10²⁷ D/m², the deuterium retention increases with the exposure temperature, reaching its maximum value of about 10²² D/m² at 500 K, and then decreases below 10¹⁹ D/m² at 800 K.     

Helium-implanted CLAM steel and evolutionary behavior of defects investigated by positron-annihilation spectroscopy

China Low Activation Martensitic (CLAM) steel has been chosen as the primary candidate structural material for the first wall/blanket for fusion reactor. The excessive helium irradiation induced damage of CLAM steel at high temperatures and the evolution of defects were investigated in this paper. The samples were homogeneously implanted with 1e + 17 ions/cm² and 100 keV of helium at room temperature, 473, 673, and 873 K. Irradiation induced damage of CLAM steel and the annealing behavior of defects were probed by slow positron beam Doppler broadening technique. Helium implantation produced a large number of vacancy-type defects which bound with helium and formed helium–vacancy complexes. Target atoms’ displacement capacity was strengthened with rising irradiation temperatures, so the S parameter increased with increasing irradiation temperatures, and helium–vacancy complexes were main defects after helium implantation at damage layers. Helium bubbles would be unstable and the desorption of helium bubbles would promote the density of defects above 673 K. By analyzing the curves of S–W and annealing tests of irradiated specimen, it suggested that there werenot only one type of defect in damage layers. Though helium–vacancy complexes were primary defects after helium implanted, introducing excessive helium might also generated other point defects or dislocation loops in the material.     

Ion beam shaping of Au nanoparticles prepared by micellar technique

Irradiation-induced burrowing and ion-induced shaping effects of Au nanoparticles are investigated. Hexagonally arranged Au nanoparticles prepared by micellar technique with diameter ～10 nm and inter-particle distance of about 80 nm were sequentially irradiated with 200 keV Ar⁺ ions to fluences of 5×10¹⁵ ions/cm². Irradiation with Argon ions causes sinking of the Au nanoparticles into the subjacent SiO₂ layer due to capillary driving forces related to specific wetting conditions while the spherical shape is conserved. Subsequent irradiation with 54 MeV Ag⁸⁺ swift heavy ions of these spherical Au nanoparticles confined within a silica matrix shapes them into prolate nanorods and nanowires whose principal axes are aligned along the beam direction. Above a threshold fluence two deformation regimes have been observed. For relatively low fluences Au nanoparticles elongate into nanorods depending on their volume. For high fluences, the formation of nanowires is observed provided that the inter-particle distance is short enough to allow for an efficient mass transport through the silica matrix.     

Design of a large irradiation channel at MNSR facility in Ghana

In the design of new slant tube for large sample irradiation in the Ghana Research Reactor-1 facility, Monte Carlo N-Particle Code version 5 (MCNP-5) was employed to simulate the neutron flux profile of the new design. The results show that the neutron flux peaks at different points, at an average thermal neutron flux of (1.1406 ± 0.0046) × 10¹¹, (1.1849 ± 0.0047) × 10¹¹ and (1.0580 ± 0.0044) × 10¹¹ n cm^?2 s^?1 around the reactor vessel. The first two peaks happened to coincide with pneumatic transfer pipes in the pool, but the third peak happened to be in line with the slant tube position. It was observed that as the diameter of the tube varies from 3.90 cm to 23.40 cm, the average thermal neutron flux decreased exponentially from (1.1849 ± 0.0047)10¹¹ n cm^?2 s^?1 to (3.3241 ± 0.0100) × 10¹⁰ n cm^?2 s^?1. The average thermal neutron flux decreases exponentially along the diameter of the designed slant tube from (1.0366 ± 0.0042) × 10¹¹ n cm^?2 s^?1 to (9.7396 ± 0.0136) × 10⁹ n cm^?2 s^?1. From the results, it is evident that a slant tube of diameter 15.00 cm can be installed at the original slant tube position for large sample irradiation.     

Characterization and qualification of advanced insulators for fusion magnets

Intensive research over the past decades demonstrated that the mechanical material performance of epoxy based glass fiber reinforced plastics, which are normally used by industry as insulating materials in magnet technology, degrades dramatically upon irradiation to fast neutron fluences above 1 × 10²² m^?2 (E > 0.1 MeV). which have to be expected in large fusion devices like ITER. This triggered an insulation development program based on cyanate ester (CE) and blends of CE and epoxies, which are not affected up to twice this fluence level, and therefore appropriate for large fusion magnets like the ITER TF coils. Together with several suppliers resin mixtures with very low viscosity over many hours were developed, which renders them suitable for the impregnation of very large volumes. This paper reports on a qualification program carried out during the past few years to characterize suitable materials, i.e. various boron-free R-glass fiber reinforcements interleaved with polyimide foils embedded in CE/epoxy blends containing 40% of CE, a repair resin, a conductor insulation, and various polyimide/glass fiber bonded tapes. The mechanical properties were assessed at 77 K in tension and in the interlaminar shear mode under static and dynamic load conditions prior to and after reactor irradiation at ~340 K to neutron fluences of up to 2 × 10²² m^?2 (E > 0.1 MeV). i.e. twice the ITER design fluence. The results confirmed that a sustainable solution has become available for this critical magnet component of ITER.     

Deuterium and helium retentions of V–4Cr–4Ti alloy used as first wall of breeding blanket in a fusion reactor

The deuterium and helium retention properties of V–4Cr–4Ti alloy were investigated by thermal desorption spectroscopy (TDS). Ion energies of deuterium and helium were taken at 1.7 and 5 keV, respectively. The retained amount of deuterium in the sample irradiated at 380 K increased with the ion fluence and was not saturated to fluence of up to 1 × 10²³ D/m². For the irradiation at 773 K, 0.1% of implanted deuterium was retained at the highest fluence. For the helium ion irradiation at room temperature, three groups of desorption peaks appeared at around 500, 850, and 1200 K in the TDS spectrum. In the lower fluence region (<1 × 10²¹ He/m²), the retained helium desorbed mainly at around 1200 K. With increasing fluence, the amount desorbed at 500 K increased. Total amount of retained helium in the samples saturated at fluence up to 5 × 10²¹ He/m² and saturation level was 2.7 × 10²¹ He/m².