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Highlights • The packing structures of mono-sized cylinder and U-shaped pebble beds were further analyzed and discussed respectively. • The effects of cylinder to pebble diameter ratio on the packing factor were investigated and compared with empirical model. • The effects of U-shaped container dimension and pebble size on the packing structures of Li 4 SiO 4 pebble bed were analyzed. Abstract The packing structures of tritium breeder and neutron multiplier pebble beds are important to estimate the thermal mechanical properties and the flow characteristics of purge gas in solid tritium breeding blanket. In this study, the pebble packing structures were numerical studied based on DEM simulation. The packing structures of cylinder pebble beds were further analyzed. The results show that the aspect ratio has a great influence on packing structure of pebble bed. Increasing the aspect ratio, a higher average packing factor can be obtained. Further, the U-shaped Li 4 SiO 4 pebble beds were investigated. The results show that when the width of the U-shaped channel, the height of the bed and the diameter of the pebbles are given, the dimensions of the U-shaped container have little effect on the pebble bed packing structures. In addition, the influences of pebble size on packing structures of U-shaped pebble bed were analyzed and discussed. By decreasing the pebble size, the average packing factor can be increased, the wall effect region of the U-shaped pebble bed can be also reduced relatively and a more uniform pebble bed can be obtained. [ABSTRACT FROM AUTHOR]

In solid tritium breeder blanket, the tritium breeder and neutron multiplier are used in formed of pebbles. To reveal the inner packing structures of the pebble bed, in this paper, the discrete element method (DEM) was applied to simulate the pebble bed packing structures by pouring pebbles from the top of container. The current numerical results obviously show that with the increase of aspect ratios the packing factor can be significantly increased in both cylinder pebble bed and cubic pebble bed. Namely, the larger aspect ratios in cylinder and cubic pebble bed, the smaller the proportion of the wall affected region, the greater the average packing factor of the cylinder and cubic pebble bed. Furthermore, the pebble packing structures in a U-shaped container were also simulated and analyzed. In the bend pebble bed, the average packing factor of 0.6278 was obtained. Close to the convex and concave container wall, the wall effects of pebble distribution and packing factor were observed. With the increase of the distance to the container wall, the layered arrangements disappear gradually and the oscillating characteristics of local packing factor are also damped little by little. The oscillations of local packing factors are limited in about 5 diameters. A transition region is formed between the layered packing in the near wall region and the uniformly random packing in the inner bulk region. In addition, the contact force distribution and coordination number distribution were also detailed analyzed. [ABSTRACT FROM AUTHOR]

• Effects of bed width on the packing structures of pebble bed were investigated by DEM simulation. • Effects of pebble properties (friction coefficient between pebbles, pebble size distribution and pebble density) on the packing fraction of pebble bed were simulated and analyzed. • Packing structures of both the tritium breeder (Li 4 SiO 4 , Li 2 TiO 3) and the neutron multiplier (Be, Be 12 Ti) pebble beds were simulated and analyzed by DEM. In solid tritium breeder blanket, the packing structures of the tritium breeder pebble bed and the neutron multiplier pebble bed are very important to analyze the tritium breeder ratio and the heat and mass transfer process in the pebble bed. In this study, the numerous simulations were conducted to investigated the effects of bed dimension, friction coefficient between pebbles, pebble size distribution and pebble density on the packing structures of the ceramic tritium breeder (Li 4 SiO 4 , Li 2 TiO 3) pebble beds and the neutron multiplier (Be, Be 12 Ti) pebble bed by discrete element method (DEM), respectively. The simulation results reveal that the pebble bed dimension, friction coefficient and pebble size distribution have a significant impact on the pebble bed packing fraction. As the width of the pebble bed container increases, the average packing fraction of the pebble bed gradually increases, meanwhile the influence of volume fraction of the wall effect gradually decreases. In addition, a lower friction coefficient between pebbles (smoother surface of the tritium breeder and the neutron multiplier pebbles) and wider pebble size distribution will result in a higher packing fraction of the pebble bed. However, the material density of the ceramic tritium breeder (Li 4 SiO 4 , Li 2 TiO 3) pebbles and the neutron multiplier (Be, Be 12 Ti) pebble seems to have no obvious effect on the packing fraction of the wider pebble bed for the solid tritium breeder blanket. [ABSTRACT FROM AUTHOR]

ITER is an unique opportunity to test tritium breeding blanket mock-ups in integrated Tokamak operating conditions. Helium-cooled ceramic breeder (HCCB) test blanket module will be the primary option of the Chinese ITER TBM program. China plans to test its own test blanket modules (TBM) during ITER different operation phase (H-H,D-D, D-T). A preliminary design of HCCB TBM have been completed in 2013. In order to reduce the effects of magnetic field ripple, the HCCB TBM design has been updated with reduced RAFM mass recently. Related R&D on key components fabrication, welding technology, materials development and mock-up test of TBM modules and components have being implemented. In this paper, the current status on design and related R&D, as well as the development of auxiliary system (TES, CPS,HCS, etc.,) of Chinese HCCB TBS program were introduced. [ABSTRACT FROM AUTHOR]

• Microstructural characterizations and deuterium permeation behaviors for RAFM steel CLF-1 corroded by Li 4 SiO 4 were investaged. • Three layered corrosion products including outer Li 4 SiO 4 layer, middle layer LiFeO 2 and inner layer LiCrO 2 were found. • A cracking and reoxidation process was proposed to explain the decreasing deuterium permeability of CLF-1 during corrosion by Li 4 SiO 4. In a solid tritium breeding blanket, Li 4 SiO 4 is in contact with reduced activation ferritic/martensitic (RAFM) steel at high temperatures for years and forms a corrosion layer. The effects of the corrosion layer on the hydrogen isotope permeability of a RAFM steel CLF-1 were thus investigated. X-ray diffraction, secondary ion mass spectroscopy and spherical aberration corrected transmission electron microscopy demonstrated the formation of a three-layered corrosion product, comprising a middle layer of LiFeO 2 and an inner layer of LiCrO 2. Li 4 SiO 4 powder with the (104) crystal plane orientation was adsorbed on the outermost surface of the CLF-1, forming a nanoscale layer with a thickness of ∼150 nm. Deuterium permeation tests show that the deuterium permeability of the corroded samples was two orders of magnitude higher than that of the blank substrate that was pre-heated for 30 days, but one order of magnitude lower than that of the blank substrate that was not preheated. The CLF-1 corroded for 10 days exhibited the highest deuterium permeation flux. There was a decrease in the deuterium permeability of the steel that corroded for 15 and 30 days; thus, there was a decrease in the permeability with an increase in the corrosion period. By the weighting method and TEM analysis, a cracking and reoxidation process was proposed to explain the decreasing deuterium permeability of CLF-1 during corrosion by Li 4 SiO 4. Our study provides a promising approach for preventing tritium permeation. [ABSTRACT FROM AUTHOR]

• α-Al 2 O 3 and α-Al 2 O 3 /Cr 2 O 3 tritium permeation barriers can be prepared by the metal organic decomposition (MOD) method. • The nanohardness and corrossion resistance of α-Al 2 O 3 MOD coatings are improved by a Cr 2 O 3 buffer. • An Al 2 O 3 MOD single layer tends to perform lower deuterium permeability at higher temperature. Four kinds of well-crystallized tritium permeation barrier coatings, including γ-Al 2 O 3 , α-Al 2 O 3 , γ-Al 2 O 3 /Cr 2 O 3 and α-Al 2 O 3 /Cr 2 O 3 , were prepared by the metal organic decomposition (MOD) method using a dipping technique. In this study, we concentrated on the preparation and properties of α-Al 2 O 3 and α-Al 2 O 3 /Cr 2 O 3 double-layer coatings. The results showed that the α-Al 2 O 3 coating exhibited a pure phase structure, which was entirely and directly transformed from the γ-Al 2 O 3 at an annealing temperature of 1000 °C, and no obvious substrate oxides were formed. An unannealed Cr 2 O 3 MOD layer with a thickness of 0.8 μm featured higher crystallization and was taken as a buffer layer. However, the template effect of Cr 2 O 3 was not obvious because the formation temperature of the α-Al 2 O 3 did not decrease. The corrosion resistance (1.068 × 10−7 A/cm2) and nanohardness (18.49 GPa) of the α-Al 2 O 3 /Cr 2 O 3 double-layer coating improved compared to those of γ-Al 2 O 3 , α-Al 2 O 3 , and γ-Al 2 O 3 /Cr 2 O 3 coatings. In contrast, the deuterium permeation test showed that the α-Al 2 O 3 MOD coating had the lowest deuterium ion current at temperatures of 400 ˜ 600 °C of the films considered in this study, demonstrating better resistance to deuterium permeation. Relevant mechanisms and explanations are discussed in detail. [ABSTRACT FROM AUTHOR]

Chinese Fusion Engineering Testing Reactor (CFETR) is aimed to obtain the technologies to fill the gaps between ITER and DEMO. The helium cooled ceramic breeder (HCCB) blanket is one of the candidates for CFETR. Ceramics Li 4 SiO 4 , beryllium and helium of 8 MPa were selected as tritium breeding material, neutron multiplier and coolant, respectively. CLF steel, one of reduced activation ferritic/martensitic (RAFM) steels, was applied as structural materials of blanket. Neutron and gamma transport was simulated with MCNP code and FENDL/MC-2.1 neutron cross section data. Activation calculation was performed with FISPACT-2007 code and EAF-2007 activation file. In this study, neutronics characteristic of HCCB blanket such as tritium breeding ratio (TBR) and shielding were assessed. The arrangement of Be, Li 4 SiO 4 and cooling plate in blanket module was optimized to improve TBR. The sensitive study for First Wall on TBR was also performed. The radioactivity inventories of HCCB blanket were estimated. The dominant radionuclides in structural materials, Li 4 SiO 4 and Beryllium were discussed, respectively. The damage of plasma facing components such as First Wall and divertor due to DT neutron irradiation was analyzed in terms of DPA value and gaseous product. [ABSTRACT FROM AUTHOR]

• CLF-1 RAFM steels were irradiated by using 6 MeV Au2+ at room temperature. • The hardness of the irradiated CLF-1 was increased with increasing irradiation damage levels. • The diffusion depth of the Li+ is much larger than the thickness of the corrosion layer. • The thickness of the corrosion layer increased with increasing irradiation damage levels. • The Li 2 Fe 3 O 4 appeared on the sample corresponding to the highest irradiation damage level. The corrosion behavior of the CLF-1 steel irradiated by 6 MeV Au-ions with different fluences was studied by exposing it to a solid tritium breeder (Li 4 SiO 4) in a 0.12 MPa He + 1% H 2 atmosphere at 550 °C. The results showed that irradiation-induced hardening appeared in the irradiated CLF-1. The double corrosion layer appeared on the irradiated samples, and the thickness of the corrosion layer increased with the irradiation damage level. A newly formed corrosion product of Li 2 Fe 3 O 4 appeared on the sample surface corresponding to the highest irradiation damage level. The underlying mechanism for the changed mechanical properties and corrosion products is discussed. [ABSTRACT FROM AUTHOR]

• A double layered corrosion products including outer LiFeO 2 and inner layer LiCrO 2 were found. • A localized LiCrO 2 (003) / LiFeO 2 (003) interface was observed. • The deuterium permeability of the corrosion layer formed in ambient air atmosphere was evaluated. In a solid tritium breeding blanket, tritium breeder Li2TiO3 is in contact with reduced activation ferritic martensitic steel at high temperatures for years and accordingly form a corrosion layer on the blanket structural steel. The effects of the corrosion layer on the deuterium permeability of RAFM steel CLF-1 were thus investigated. X-ray diffraction, secondary ion mass spectroscopy and high-resolution transmission electron microscopy proved the formation of a double layered corrosion products with a thickness of ∼200nm. A localized LiCrO2 (003)/LiFeO2 (003) interface between the outer layer LiFeO2 and the inner layer LiCrO2 was observed. With the increase of corrosion periods (120∼360 h), there is a first deuterium permeability increase due to cracking and an amorphization process occurred at the interface between the corrosion layer and the substrate. The CLF-1 steel corroded for 720 hours performed a lowest deuterium permeation flux as the amorphous oxides have been entirely crystallized. A localized corrosion mechanism coexisting with uniform corrosion was proposed to explain the changing deuterium permeability of the CLF-1 steel corroded by Li2TiO3 in 1 atm ambient air atmosphere. Our study provides a novel thought on reducing the tritium permeability of the RAFM steel. [ABSTRACT FROM AUTHOR]

• Effect of thermal cycles on structure and deuterium permeation of Al 2 O 3 coating prepared by MOD method. • No visible peeling-off was observed on the sample surface after thermally shocked for 150 cycles. • The deuterium permeation flux of the Al 2 O 3 coating increased with increasing the number of thermal cycles. • The disappearance of the (400) and (441) diffraction peaks of the γ-Al 2 O 3 are observed after deuterium permeation tests. The effect of thermal cycles on the microstructure and deuterium permeation of the Al 2 O 3 coatings prepared by the MOD method was investigated. An as-deposited Al 2 O 3 coating showed a uniform, smooth and dense surface morphology and was composed of γ-Al 2 O 3 and α-Al 2 O 3 phases. After 550 °C thermal cycles, all the Al 2 O 3 coatings exhibited good phase stability. The Al 2 O 3 coating remained intact and dense after 100 thermal cycles, while obvious cracks appeared on the coating surface after 150 thermal cycles. Furthermore, a deuterium permeation test of the as-deposited and thermally cycled Al 2 O 3 coatings was carried out. The deuterium permeation flux of the Al 2 O 3 coating increased with increasing the number of thermal cycles. The Al 2 O 3 coatings with fewer than 100 thermal cycles remained dense and had a smooth surface morphology. However, several cracks appeared on the surface of the coating after 100 thermal cycles. In particular, delamination of the coating occurred after 150 thermal cycles. Disappearance of the (400) and (441) diffraction peaks of the γ-Al 2 O 3 was observed after the deuterium permeation tests. The microscopic mechanism of the influence of thermal cycles on the microstructure and deuterium permeation of the Al 2 O 3 coatings was analyzed. [ABSTRACT FROM AUTHOR]

Reduced activation ferritic/martensitic steel (RAFM) is recognized as the primary candidate structural material for fusion power reactors blankets. China Low-activation Ferrite steel (CLF-1) is a type of RAFM steel developed by Southwestern Institute of Physics (SWIP) since 2003. The paper reviewed recent research progress of CLF-1 steel including the industrial 5-ton scale ingot fabrication technologies research, material properties evaluation and joints technologies optimization. And also, the main issues of technique research and certification strategy for ITER use were discussed. [ABSTRACT FROM AUTHOR]