Low Activation Properties Research Articles

Activation analysis for the reference low-activation vanadium alloy NIFS-HEAT-2

• Activation analysis was performed for a NIFS-HEAT2 (V-4Cr-4Ti) sample. • The sample was irradiated in Japan Material Testing Reactor (JMTR) ~15 years before. • Detected impurities were 0.41 wppm Co and 0.71 wppm Nb. • Ag impurity concentration was estimated to be <0.034 wppm. • Results are supporting the superior low activation properties of NIFS-HEAT-2. Activation analysis of impurities was performed on a test specimen of the reference low-activation vanadium alloy NIFS-HEAT-2 (V-4Cr-4Ti) which has been irradiated in Japan Materials Testing Reactor (JMTR) ~16 years before. Since the radioactive nuclides from V, Cr and Ti are produced with short half-lives, intense peaks of 60 Co (T 1/2 = 5.27 years) were dominant in the measured gamma-ray energy spectra and peaks of 94 Nb (2.03 × 10 4 years) were also observed. The concentrations were evaluated to be 0.41 wppm and 0.71 wppm respectively from comparison with results of radioactivity calculation using the FISPACT-2005 code. In the analysis, detection of 108m Ag (418 years), which would have a considerable influence on the material recycling after the usage in a fusion reactor, was intense motivation. Although the peaks of 108m Ag has not been detected, the concentration of Ag was estimated to be <0.034 wppm from analysis of detection limit in gamma-ray peak count. Gamma-ray peaks originated from other impurity elements were not observed in the spectra, and the present analysis are supporting the superior low activation properties of NIFS-HEAT-2.

W0.5TaTiVCr-based composite reinforced with W-mesh for fusion plasma-facing applications

We present research into tungsten (W) alloy-based composites reinforced with a W-mesh. Due to low activation and higher strength properties, W0.5TaTiVCr was used as a matrix material. Layers of W-mesh (Wmesh) were embedded in W0.5TaTiVCr for improving ductility and toughness. We employed elemental powder mixing and spark plasma sintering (SPS) at 1600 °C for sample preparation, which is a simpler method as compared to chemical vapor infiltration and hot isostatic pressing. The microstructural analysis shows a W-mesh that is well-bonded with the W0.5TaTiVCr matrix, which exhibits multiple phases and a BCC structure. The room temperature compressive fracture strain of W0.5TaTiVCr/Wmesh composites show an improvement from ~3.5% to ~15.8% due to increase in Wmesh concentration from 10 wt% to 50 wt%, whereas the compressive yield strength changes from ~1900 MPa to ~1700 MPa (at room temperature) and ~1200 MPa to ~950 MPa (at 1200 °C). The W0.5TaTiVCr matrix alone shows ~7.7 MPa m1/2 fracture strain, and the addition of 10 wt%Wmesh in W0.5TaTiVCr results in more than a two-fold increase in fracture toughness (up to ~20 MPa m1/2), which suggests a potential use of this material in fusion reactors.

Microstructure and mechanical properties of low activation Fe–Ti–Cr–V–W multi-principal element alloys

Single-phase multi-principal element alloys can have low activation and rapid induced radioactive decay properties that make them ideal structural materials for the first wall/cladding structure of nuclear fusion reactors. The present work proposes the design of multi-principal element alloys consisting of low activation elements Fe, Ti, Cr, V, and W with both equiatomic and optimized compositions. The optimal composition of the multiple components is based on the formation of a single-phase solid solution with a minimal Gibbs free energy. The microstructures and mechanical properties of the alloys are investigated. The equiatomic alloy is observed to be composed of dendritic crystals consisting of a body-centered cubic (bcc) solid solution phase and a complex intermetallic compound, while the optimized alloy is composed of cellular dendritic crystals consisting of a single bcc solid solution phase in its as-cast state. Homogenization treatment is observed to reduce the fracture strength of the equiatomic alloy due to grain coarsening, although it has little effect on the hardness and other mechanical properties. However, homogenization increased the hardness and compressive yield strength of the optimized alloy to different degrees. The optimized alloy has better plastic toughness than the equiatomic alloy, which increases its workability and radiation resistance.

ARC reactor materials: Activation analysis and optimization

Abstract Nowadays, Fusion Energy is one of the most important sources under study. During the last years, different designs of fusion reactors were considered. At the MIT, an innovative design was created: ARC, the Affordable Robust Compact reactor. It takes advantage of the innovative aspects of recent progress in fusion technology, such as high temperature superconductors, that permit to decrease the dimensions of the machine, reaching at the same time high magnetic fields. Our main goal is the low-activation analysis of possible structural materials for the vacuum vessel, which is designed as a single-piece placed between the first-wall and the tank that contains the breeding blanket. Due to its position, the vacuum vessel is subject to high neutron flux, which can activate it and cause the reduction of the component lifetime and decommissioning problems. The activation analysis was done also for the liquid breeder FLiBe, compared with Lithium-Lead. Codes used for the low-activation analysis were MCNP and FISPACT-II. The first one is based on a neutronics model and for each component a certain neutron flux is evaluated. For FISPACT-II, the main input is the composition of the analyzed material, the neutron flux and the irradiation time. Results from FISPACT-II are the time behavior of specific activity, contact dose rate. To assess suitable structural materials for the vacuum vessel, low-activation properties were considered. Vanadium alloys turn out to be one of the best alternatives to the present material, Inconel-718. Finally, isotopic tailoring and elemental substitution methods were applied. Here, the composition of each alloy is analyzed and critical isotopes or elements are eliminated or reduced. After the modifications, new simulations are done, and those leading to significant improvements in the final results are highlighted.

Electron beam welding of oxide dispersion strengthened 9 Cr martensitic steel-an experimental and theoretical perspective

Abstract Advanced nuclear energy systems present significant improvements in economics, safety, reliability and sustainability over currently operating reactor technologies. Proper choices of cladding and structural material with a. high neutron damage resistance b. good mechanical properties at high operating temperatures and c. low activation properties are essential for the safe and reliable operation of the system. The 9-12% Cr transformable steels with lower carbon (∼0.1%) contents and addition of Mo, W, V, Nb present better high temperature strength than austenitic steels. The use of 9%Cr steel is considered as an optimum solution taking into consideration the present day requirements like environment, efficiency, elasticity, experience and economy that dominate considerations of modern power station design, layout, fabrication and operation. The pursuit of improved efficiency/power plant performance resulted in the design of system operating at higher temperature and pressure, which requires materials with appropriate high-temperature mechanical properties and metallurgical stability at operating temperatures. Oxide Dispersion Strengthened (ODS) 9% Cr steels have the potential to advance the high temperature performance of associated components in advanced nuclear energy systems with marginal oxidation resistance. The research focuses on Electron beam welding (EBW) of Hot Isostatically Pressed (HIP) ODS 9 Cr martensitic steel in as-received condition to arrive at the optimized weld process parameters. The weldments were characterized using Optical Microscopy for depth of penetration and width of weldments. The experimental results were theoretically verified using Minitab software

Peptide with endothelial cell affinity and antiplatelet adhesion property to improve hemocompatibility of blood‐contacting biomaterials

AbstractClinical applications of various blood‐contacting biomaterials have been limited by the complications associated with hemolysis and coagulation on material surfaces. We have been developing anticoagulant‐free polymeric mechanical heart valves with an expanded polytetrafluoroethylene (ePTFE) sewing cuff and have found that the insufficient hemocompatibility of ePTFE is one of the biggest problems to be overcome for their durability. In this work, we present a set of novel peptide sequences that bind selectively to endothelial cells (ECs) and have low platelet activation properties. Five 7mer‐oligopeptides selected from a phage display library were immobilized onto the ePTFE surface by a simple manufacturing protocol and evaluated in vitro. All sequences displayed a better EC‐binding property than the REDV. Two of the peptides displayed extremely low platelet adhesion and activation. In particular, HGGVRLY displayed the highest EC affinity and maximally reduced platelet adhesion. HGGVRLY is a promising peptide for modifying various types of blood‐contacting biomaterials.

Toughening of a low-activation tungsten alloy using tungsten short fibers and particles reinforcement for fusion plasma-facing applications

Novel low-activation tungsten (W) alloy-based composites are investigated here. A W0.5TaTiVCr matrix (M) was selected due to its promising strength (more than two times that of pure W) and low activation properties. W short fibers (Wsf) or W particles (Wp) were embedded in W0.5TaTiVCr in an effort to improve the fracture strain. Unlike W-fiber-reinforced-W-matrix-composites, near-isotropic composites with nearly full densities were developed via the simple fabrication method (as compared to hot isostatic pressing and chemical vapor infiltration) of elemental powder mixing followed by spark plasma sintering (SPS) at 1600 °C. Microstructural characterization reveals randomly dispersed and well-bonded W reinforcements with the matrix, which shows the body-centered cubic (BCC) crystal structure and multiple phases. The fracture strain of W0.5TaTiVCr under compressive stress shows a two-fold improvement from ~4.3% to ~9.3% due to the addition of 10 wt.%Wp while maintaining a high yield strength of ~1900 MPa. An addition of 50 wt.%Wp results in ~16% fracture strain. The addition of 10 wt.%Wsf results in four times higher fracture toughness (from ~7.7 to ~29.6 MPa · m1/2). The mechanisms determining the improvement in the fracture strain and toughness due to the addition of the Wsf and Wp are discussed based on the fracture surface analysis.

Phase evolution progress and properties of W-Si composites prepared by spark plasma sintering

Due to the low neutron activation and favourable self-passivating properties, Si is introduced to tungsten matrix by spark plasma sintering (SPS) technique. The phase evolution progress and mechanical properties of the composites are investigated. The results show that Si prefers to react with residual oxygen in the powders to form SiOy (y = 1, 1.5, 2) at low Si addition (≤3 at%). In this case, the formation of W5Si3 is suppressed. But when further increasing the Si content up to 32 at%, W5Si3 phase starts to form and its content increases with the elevating ratio of silicon suboxide to SiOy. Combined the results of XPS and tensile testing, it is concluded that silicon oxide is beneficial to the strength of W but not beneficial to the elongation. However, W5Si3 shows positive effects on both strength and elongation. W-32Si composite achieves the best properties with tensile strength 353 MPa and elongation 4.58%. Also, the thermal conductivities were measured, which reveal a growing trend with testing temperature in W-Si composites in contrast with pure W.

The Interactions between Blood and Polymeric Nanoparticles Depend on the Nature and Structure of the Hydrogel Covering the Surface

Polymeric surfaces in contact with blood in vivo are foreign bodies and are quickly isolated from blood by the non-specific defense systems. Nanoparticles (NP) used as drug carriers are normally quickly taken up by phagocytes and sequestered in liver and spleen to which they can deliver drugs. Long-circulating and/or low complement activating core-shell NPs can be obtained from PEO/PEG amphiphilic copolymers forming brush or loops on the surface. Core-shell NPs can also be obtained from polysaccharidic graft or block amphiphilic copolymers. Complement activation by the NPs and protein adsorption both depend on the structure, nature and molecular weight of the polysaccharide chains composing the shell. NPs showing low complement activation can be obtained if the polysaccharide on the surface is long and in a brush configuration. Fragile molecules such as hemoglobin or siRNA can be loaded and protected by appropriate brush shells without modifying the low complement activation properties.

Mannose-binding lectin complement pathway plays a key role in complement activation by Paracoccidioides brasiliensis

Paracoccidioides brasiliensis (Pb) is a dimorphic fungal pathogen that causes paracoccidioidomycosis, the most severe deep mycosis from South America. Although cell mediated immunity is considered the most efficient protective mechanism against Pb infection, mechanisms of innate immunity are poorly defined. Herein, we investigated the interaction of the complement system with high and low virulence isolates of Pb. We demonstrated that Pb18, a high virulence Pb isolate, when incubated with normal human serum (NHS) induces consumption of hemolytic complement and, when immobilized, promotes binding of C4b, C3b and C5b-C9. Both, low virulence (Pb265) and high virulence (Pb18) isolates consumed C4, C3 and mannose-binding lectin (MBL) of MBL-sufficient, but not of MBL-deficient serum as revealed by deposition of residual C4, C3 and MBL on immune complexes and mannan. However, higher complement components consumption was observed with Pb265, as compared with Pb18. The suggested relationship between low virulence and significant complement activation properties of Pb isolates, was confirmed by the demonstration that virulence attenuation of Pb 18 results in acquisition of the ability to activate complement. Conversely, reactivation of attenuated Pb18, results in loss of the ability to activate complement. Our results demonstrate for the first time that Pb yeasts activate the complement system by the lectin pathway, and there is an inverse correlation between complement activating ability and Pb virulence. These differences could exert an influence on innate immunity and severity of the disease developed by infected hosts.

Superconducting Properties and Microstructure of ${\rm V}_{3}{\rm Ga}$ Multifilamentary Wires Through a PIT Process Using High Ga Content Compounds

We have reconsidered V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ga compound wire for future fusion reactor application. 14 MeV fast neutrons will be formed during the deuterium-tritium fusion reaction, consequently future fusion reactors will be constructed using low activation materials wherever possible. V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ga compound has a short radioactive decay time and it will be one of the candidate superconductors with both low activation and high field properties so called ldquolow activation superconducting materialsrdquo. The present critical current density, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , is insufficient for large fusion magnet. We investigated a new PIT process, using high Ga content compound, in order to improve superconducting properties. The upper critical field, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> , of the samples using high Ga content Cu-Ga compounds, was increased with increasing Ga content, and measured 22.5 T when was used 50 at%Ga compound powder. Moreover, the effect of Mg addition into the powder filament was also investigated. The small amount of Mg addition within 1 at%, was improved <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> to about 23.0 T. The Ga content dependence and the Mg addition effect on the superconducting properties in the new PIT process are reported in this paper.

Vanadium Alloys for Fusion Blanket Applications

Vanadium alloys are attractive candidate structural materials for breeding blanket of fusion reactors because of their low activation properties and high temperature strength. Studies on various blanket designs are being carried out using vanadium alloys as structural materials and liquid lithium or molten salt Flibe as breeding and coolant materials. Recently significant progress in fabrication technology has been made for vanadium alloys through a program of producing large ingots of high purity V-4Cr-4Ti (NIFS-HEATs). Fundamental understandings on the effects of interstitial impurities (C, N, O) on mechanical properties, radiation effects on microstructure and mechanical properties, corrosion and compatibility in various environments were enhanced. New promising candidates were identified for the MHD insulator coating. Among the remaining critical issues are the effects of transmutant helium on mechanical properties and development of long life MHD coating. Development of vanadium alloys will he carried out in coordination with IFMIF and ITER-TBM schedules as essential tools for verification of their performance in fusion blanket environments.

Structural characterisation of SiC/SiC f composites exposed to chemical interaction with Be at high temperature

Fibre-reinforced SiC composites are candidate materials for application in nuclear fusion technology as structural materials due to their low activation and high performance properties. We studied the chemical compatibility between SiC/SiC f composites and Be under isothermal conditions at 800 °C. Two types of SiC/SiC f composites were used, one of them with a surface CVD coating of SiC powder. Results show that, after the chemical compatibility tests, the surface of uncoated samples is heavily oxidised with C surface depletion and Be in-diffusion. For the coated samples the chemical reactions are not so severe. Experiments indicate the presence of Si and Be 2C as reaction products in the surface of both samples.

High temperature chemical compatibility between SiC composites and Be pebbles

SiC composite reinforced fibres are still considered to be an important material to be used in nuclear fusion reactors due to their high temperature and low neutron activation properties. Two different kinds of SiC/SiC f composite were manufactured, one of them presenting an extra SiC coating obtained by chemical vapour deposition technique. Several samples of both materials were placed inside a Be pebble bed and the whole set-up annealed at 800 °C for 550 h in a reducing atmosphere, simulating fusion reactor conditions. Surface chemical reactions were investigated with nuclear microprobe analyses techniques and complemented with SEM analysis. For the uncoated samples, surface oxidation is accompanied by a strong C depletion and a Be diffusion. Two different behaviours were found for the coated samples. One of those samples showed extended regions where surface was left almost unaltered. The general behaviour, however, was an increase in the number and extension of the cracks already observed at the surface of the coated virgin samples.

Mechanical properties of two chromium alloys in as-received and heat-treated conditions

The main attractions of chromium alloys for fusion applications are their high corrosion resistance and low activation properties. For the improvement of mechanical strength properties, these materials use oxide dispersion strengthening (ODS). The main drawbacks are low impact fracture toughness up to 1150 K, coupled with limitations in welding, shaping and other manufacturing processes. This paper describes the work performed in 1999 by SCK-CEN for the European Fusion Programme, in order to obtain exploratory results for the assessment of the possible use of such materials in a reactor concept. Tensile, instrumented impact and static three-point bend tests have been carried out on two fine-structured alloys, denominated DUCROPUR (99.97% pure Cr) and DUCROLLOY, an oxide dispersion strengthened alloy (Cr–5%Fe–1%Y 2O 3), in as-HIPped and heat-treated conditions. Some tensile tests have also been carried out on specimens irradiated at 300 °C to approximately 0.5 dpa.

Safety aspects of joints and coatings for plasma facing components with composite structures

Silicon carbide fibres silicon carbide matrix (SiC/SiC) composites are among the most promising materials for high temperature applications, but they cannot be joined by traditional ways because they do not melt and the typical mechanical joints (rivets, bolts, etc.) are ineffective with these new materials: this is a problem for the preparation of complex structures, such as those of plasma-facing components (PFCs) for fusion reactors. Furthermore, these composites show a poor resistance towards the oxidation, they are porous and permeable to fluids. PFCs made of composite materials and in which joints and coatings are made with advanced materials pose some new problems regarding their safety. In particular, the following aspects have been addressed in this paper: (a) tritium inventory; (b) activated dust in plasma chamber; (c) accident-relevant low-activation properties; and (d) long-term relevant low-activation properties. As a result of all the above mentioned aspects, some safety-relevant guidelines for the choice of joining and coating materials for PFCs are finally discussed.

Mechanical Characterization of Two Low-Activation Chromium Alloys in As-Received and Heat-Treated Conditions

In recent years, within the fusion long-term programmes, attention has been devoted to the characterization of Chromium (Cr) alloys, in view of their elevated corrosion resistance, low activation properties and high-temperature mechanical strength.As part of the European Fusion Programme, an activity has been launched in 1999 with the aim of exploring the potential of Cr alloys as structural materials in fusion reactors, for example, as first wall or blanket materials. Recent investigations have focused attention on two commercially available materials: high-purity 99.7% Cr (DUCROPUR) and Cr alloyed with 5% Fe and 1%Y203 (DUCROLLOY), both of which have shown excellent low activation characteristics.The mechanical properties of these two alloys, in both as-received and heat-treated conditions, have been characterized at SCK•CEN by means of tensile, instrumented impact and static three-point bend tests, using standard and sub-size specimens. Tensile tests have also been carried out on samples irradiated at 300 °C in the BR2 reactor in Mol up to an accumulated dose of about 0.5 dpa.

Failure mechanisms in continuous-fiber ceramic composites in fusion energy environments

Silicon carbide composites are attractive for structural applications in fusion energy systems because of their low activation and afterheat properties, excellent high-temperature properties, corrosion resistance, and low density. These composites are relatively new materials with a limited database; however, there is sufficient understanding of their performance to identify key issues in their application. To date, dimensional changes of the constituents, microstructural evolution, radiation-enhanced creep, and slow crack growth have been identified as potential lifetime limiting mechanisms. Experimental evidence of these mechanisms, the factors that control them, and their implications on component lifetime will be discussed.

SiC–SiC f CMC manufacturing by hybrid CVI–PIP techniques: process optimisation

SiC–SiC f ceramic matrix composites (CMC) are candidate structural material for fusion power reactor applications because of their favourable thermo-mechanical and low-activation properties. Among their different manufacturing techniques, present, the most employed ones are chemical vapour infiltration (CVI) and polymer infiltration and pyrolysis (PIP). These two techniques are based on the common principle of filling the porosity among the fibres with SiC resulting from precursor decomposition. CVI process deposits high purity crystalline SiC with good properties onto fibres whereas PIP leaves lower characteristic amorphous SiC with traces of oxygen between fibres. PIP, on the other hand, seems to be much more industrially effective than CVI. In the attempt to maximise the properties and reduce costs, some work has been done on the so called ‘hybrid techniques’ in which CVI and PIP are both employed. The work performed by ENEA and FN S.p.A. consists of a series of combined CVI–PIP process cycles and the subsequent product characterisation.

Low-activation characteristics of V-alloys and SiC composites

Two novel low-activation materials for a fusion reactor first wall and blanket structures are examined: a vanadium-based alloy (V–4Cr–4Ti) and a SiC/SiC composite. The comparison concerns low-activation properties, and stresses the importance of data on the impurity content. In particular, new data about impurity levels in vanadium alloys show that long-term activation of such materials may be strongly reduced if certain impurities are kept under control.