H Matsui

Research and development on vanadium alloys for fusion applications

The current status of research and development on unirradiated and irradiated V-Cr-Ti alloys intended for fusion reactor structural applications is reviewed, with particular emphasis on the flow and fracture behavior of neutron-irradiated vanadium alloys. Recent progress on fabrication, joining, oxidation behavior, and the development of insulator coatings is also summarized. Fabrication of large (>500 kg) heats of V-4Cr-4Ti with properties similar to previous small laboratory heats has now been demonstrated. Impressive advances in the joining of thick sections of vanadium alloys using GTA and electron beam welds have been achieved in the past two years, although further improvements are still needed.

Effect of temperature change on microstructural evolution of vanadium alloys under neutron irradiation in JMTR

Abstract Recently, it has been pointed out that the effect of varying temperature on microstructural evolution in materials under neutron irradiation can be very complicated. In this paper, evolution of microstructure in vanadium alloys in each step of temperature changes is closely examined in order to analyze the effects after several temperature cycles. Neutron irradiation was performed in JMTR with an upward temperature change. TEM observation, micro-Vickers hardness test and positron annihilation lifetime spectroscopy have been carried out on vanadium alloys. During the irradiation at 220°C, a lot of small defects have nucleated. They grew up after the upward temperature step to 420°C. On the other hand, microstructure remained essentially the same in the irradiation at 340/530°C temperature combination. Vanadium alloys containing titanium show different tendencies; unlike other alloys, a number of radiation induced precipitates were found by TEM in these alloys.

Microstructural evolution in vanadium irradiated during ion irradiation at constant and varying temperature

Abstract To understand the influence of stepwise change of irradiation temperature on microstructural evolution in pure vanadium, ion irradiations were performed at 473/873, 673/873 and 873/473 K. The defect cluster density was strongly affected by the pre-irradiation temperatures. Suppression of interstitial type loop formation was prominent by pre-irradiation at lower temperatures. These results were explained by the appearance of vacancy-rich conditions at the beginning of the higher temperature irradiation due to the reclustering of the vacancies formed in the lower temperatures.

Swelling behavior of V–Fe binary and V–Fe–Ti ternary alloys

Abstract V–Fe binary alloys with different Fe concentrations, i.e., V–1, 3 and 5 at.% Fe, and V–5% Fe alloy added with 1, 3 and 5 at.% of Ti were irradiated in EBR-II at 380–615°C to about 11 dpa. TEM observation was performed after irradiation. A systematic increase in cavity size was observed with increasing iron concentration in the binary alloys, especially at 510°C and 615°C irradiation. On the other hand, the density of cavities decreased with increasing iron concentration and irradiation temperature. Maximum swelling in V–Fe system occurred between 500°C and 600°C and the amount of swelling was up to 30% at a damage level of 11 dpa. The alloy containing only 1% Fe already showed substantial swelling. The effect of titanium addition to the swelling was very remarkable. One atomic percent of titanium addition to V–5 at.% Fe significantly suppressed cavity formation, and 3 at.% of titanium addition entirely suppressed swelling. There seems to be a threshold titanium concentration for suppression of swelling in V–5 at.% Fe. Radiation-induced precipitation of titanium oxide may be one reason why titanium additions suppress the swelling in vanadium alloys. Homogeneous titanium oxide precipitates were not observed so that the titanium in solution is more likely to be playing an important role for suppression of swelling.

Irradiation creep of vanadium-base alloys

A study of irradiation creep in vanadium-base alloys is underway with experiments in the Advanced Test Reactor (ATR) and the High Flux Isotope Reactor (HFIR) in the US. Test specimens are thin-wall sealed tubes with internal pressure loading. The results from the initial ATR irradiation at low temperature (200-300 C) to a neutron damage level of 4.7 dpa show creep rates ranging from {approx}0 to 1.2 x 10{sup {minus}5}/dpa/MPa for a 500-kg heat of V-4Cr-4Ti alloy. These rates were generally lower than reported from a previous experiment in BR-10. Because both the attained neutron damage levels and the creep strains were low in the present study, however, these creep rates should be regarded as only preliminary. Substantially more testing is required before a data base on irradiation creep of vanadium alloys can be developed and used with confidence.

Blanket design using FLiBe in helical-type fusion reactor FFHR

Abstract The blanket design for a force-free helical reactor (FFHR) is presented, which is a demo-relevant heliotron-type D-T fusion reactor based on the first all-superconducting-coils device, LHD (large helical device) under construction in NIFS at present. For the goal of a self-ignited reactor of 3 GW thermal output, the design parameters at the first stage for concept definition of FFHR have been investigated. The main feature of FFHR is a force-free-like configuration of helical coils, which makes it possible to simplify the coil supporting structure and to use a high magnetic field instead of high plasma beta. The other feature is the selection of molten-salt FLiBe as a self-cooling tritium breeder for mainly safety reasons owing to the low tritium inventory, low reactivity with air and water, low pressure operation, and low MHD resistance compatible with a high magnetic field. In particular, as common issues in fusion reactors, the FLiBe blanket system in FFHR is expressed in detail by showing engineering possibilities to overcome key issues on tritium permeation, material corrosion, heat transfer, operation pressure, etc. The basic design for maintenance and repair of the blanket is also discussed.

Mechanical behavior and microstructural evolution of vanadium alloys irradiated in ATR-A1

Abstract An irradiation experiment has been done in the ATR-A1 to investigate irradiation behavior of vanadium alloys in the low temperature regime from 200°C to 300°C with damage levels of 3 to 4 dpa. In creep measurements, creep tubes of V–3Fe–4Ti–0.1Si with inner pressures up to 165 MPa did not rupture during irradiation. The effective strain rate of creep was below 0.2% dpa−1 and it showed the same tendency as V–4Cr–4Ti alloys. In Charpy impact tests, all specimens of V–4Cr–4Ti–0.1Si and V–3Fe–4Ti–0.1Si showed brittle behavior at room temperature and the DBTT increased to 60–150°C. The fracture surface showed cleavage. Tensile tests conducted both at room temperature and at the irradiation temperature showed significant irradiation hardening and brittle responses. TEM showed that high densities of tiny defect clusters were formed in V–Cr–Ti and V–Fe–Ti alloys. Precipitates could not be seen in specimens irradiated below 300°C, however, fine defect clusters are considered to be the origin of brittle behavior in V–Cr–Ti alloys irradiated at low temperatures.

Effects of temperature change on the microstructural evolution of vanadium alloys under ion irradiation

Abstract The evolution of microstructure in vanadium alloys after upward and downward temperature changes has been closely examined. Vanadium alloys have been irradiated in the high fluence irradiation facility at the University of Tokyo (HIT). Irradiations have been performed either at constant temperature of 500°C or in a stepwise temperature sequence of either 350/500°C, 400/500°C, 450/500°C, 500/350°C or 350/500/350°C up to 0.5 or 0.75 dpa. After 350/500°C and 400/500°C temperature change irradiations, small dislocation loops have been observed. The density of these dislocation loops decreased with the pre-irradiation temperature. After 450/500°C irradiation, the microstructure was coarse, indicating that the initial temperature (450°C) was high enough to be in the regime, where the growth of defects mainly occurs. In the case of downward temperature change, microstructures coarser than those of higher temperature irradiations were observed. This apparent anomaly may be understood in terms of the rate theory.

Study of point defect behaviors in vanadium and its alloys by using HVEM

Abstract Microstructural evolution and point defect behavior in vanadium and V– x Fe ( x =0.1, 0.2, 0.3, 3, 5 at.%) have been examined by using high voltage electron microscopy. During irradiation, interstitial-type dislocation loops are formed and grow in all materials. In V– x Fe, measured saturated loop number density is much higher than that in pure vanadium, indicating iron atoms in the matrix strongly interact and trap self-interstitial atoms (SIAs). The shapes of loops formed in V– x Fe are complicated, i.e., loops grown to >100 nm show stacking fault-like shapes. Those complicated shapes become more significant with increasing iron concentration. This means iron atoms segregate to loops through the strong interaction with SIAs. The apparent migration energies of 0.21 eV and 0.81 eV have been determined from the temperature dependence of loop number density for pure vanadium and V– x Fe, respectively. Various observed phenomena are discussed in terms of the obtained binding energy.

EFFECTS OF VARYING TEMPERATURE IRRADIATION ON THE NEUTRON IRRADIATION HARDENING OF REDUCED-ACTIVATION 9CR-2W MARTENSITIC STEELS

Abstract In order to clarify the effects of varying temperature during irradiation on the irradiation hardening of 9Cr–2W steels, tensile tests and positron annihilation lifetime measurements were carried out following the varying temperature irradiation (220/420°C and 340/530°C) utilizing a so called multi-section and multi-division controlled irradiation capsule in JMTR. After the irradiation at 220°C to 0.053 dpa, the steels show irradiation hardening as much as 110 MPa. The hardening was almost completely diminished immediately after the elevation of the irradiation temperature to 420°C. Subsequent irradiation at 420°C up to 0.14 dpa did not cause any hardening. The results of positron annihilation lifetime measurements indicate that microvoids are formed by the irradiation at 220°C but disappear upon elevating the temperature to 420°C and are then formed again by the subsequent irradiation at 420°C up to a total dose of 0.14 dpa. This behavior may be interpreted in terms of decomposition of interstitial loops or migration of small interstitial loops during temperature elevation. There is no good correlation between irradiation hardening and formation of microvoids in neutron-irradiated reduced-activation martensitic steels.