H.M. Chung

Status of vanadium alloys for fusion reactors

Abstract Advantages of vanadium alloys for fusion reactor structural applications are: low induced activation, excellent thermal stress factor, high strength at elevated temperatures, and superior ductility at low temperatures. Resistance to irradiation damage is also very impressive, i.e. very small DBTT shift by irradiation and low swelling. Research and development of vanadium alloys have made a remarkable progress in recent years partly supported by ITER-related activities. Composition range centered about V4Cr4Ti is the target of many of the studies conducted in the US, Japan and RF. Most of the studies have demonstrated the superior performance of this alloy family, while some alloys containing large amount of impurities have shown relatively poor properties. In addition to the baseline mechanical properties, neutron irradiation effects, e.g., swelling and radiation embrittlement are covered in this paper. Of particular importance is the study of the effects of dynamically-charged helium on mechanical properties and swelling. Recent developments of insulator coatings and welding are also covered. The importance of mechanistic studies of the damage behavior is emphasized for efficient alloy development and prediction of materials life in service.

Development and testing of vanadium alloys for fusion applications

V base alloys have advantages for fusion reactor first-wall and blanket structure. To screen candidate alloys and optimize a V-base alloy, physical and mechanical properties of V-Ti, V-Cr-Ti, and V-Ti- Si alloys were studied before and after irradiation in Li environment in fast fission reactors. V-4Cr-4Ti containing 500-1000 wppM Si and <1000 wppM O+N+C was investigated as the most promising alloy, and more testing is being done. Major results of the work are presented in this paper. The reference V-4Cr-4Ti had the most attractive combination of the mechanical and physical properties that are prerequisite for first-wall and blanket structures: good thermal creep, good tensile strength/ductility, high impact energy, excellent resistance to swelling, and very low ductile-brittle transition temperature before and after irradiation. The alloy was highly resistant to irradiation-induced embrittlement in Li at 420-600 C, and the effects of dynamically charged He on swelling and mechanical properties were insignificant. However, several important issues remain unresolved: welding, low-temperature irradiation, He effect at high dose and high He concentration, irradiation creep, and irradiation performance in air or He. Initial results of investigation of some of these issues are also given.

Development of vanadium-base alloys for fusion first-wall—blanket applications

Abstract Vanadium alloys have been identified as a leading candidate material for fusion first-wall—blanket applications. Certain vanadium alloys exhibit favorable safety and environmental characteristics, good fabricability, high temperature and heat load capability, good compatibility with liquid metals and resistance to irradiation damage effects. The current focus is on vanadium alloys with (3–5)% Cr and (3–5)% Ti, with a V4Cr4Ti alloy as the leading candidate. The available database indicates that the VCrTi alloys provide the following advantages: 1. (1)|These alloys are readily formable and weldable; however, atmospheric contamination must be avoided during welding and high temperature processing. 2. (2)|The relatively high thermal conductivity and low thermal expansion provide for a high heat load capability. 3. (3)|These alloys exhibit good high temperature tensile and creep properties which permit high temperature operation. 4. (4)|VCrTi alloys exhibit low long-term activation and low radioactive decay heat which provide safety and environmental advantages. 5. (5)|These alloys are characteristically resistant to liquid metal corrosion. 6. (6)|Alloys with a few per cent titanium are highly resistant to irradiation-induced swelling, which provides for the possibility of long lifetime. 7. (7)|Results obtained to date indicate that the V4Cr4Ti alloy is highly resistant to irradiation-induced degradation of the mechanical properties. Substantial progress has been made in the development of these alloys for fusion application. Larger heats of the V4Cr4Ti alloy have been prepared in the USA and Russia. The baseline property database has been expanded. Recent results indicate that these alloys are resistant to irradiation damage. The irradiation-induced swelling is low, the uniform elongation of alloys irradiated at 400–600 °C remains above 8% and the ductile—brittle transition temperature of the V4Cr4Ti alloy after irradiation remains well below room temperature. Preliminary results indicate that the crack growth rates of certain alloys are not highly sensitive to irradiation. Results from the dynamic helium charging experiment (DHCE) which simulates fusion relevant helium/dpa ratios are similar to results from neutron-irradiated material.

Reference vanadium alloy V4Cr4Ti for fusion application

Vanadium alloys exhibit important advantages as a candidate structural material for fusion first-wall/blanket applications. These advantages include high temperature and high wall load capability, favorable safety and environmental features, resistance to irradiation damage, and alloys of interest are readily fabricable. A substantial data base has been developed on laboratory-scale heats of V-Ti, V-Cr-Ti and V-Ti-Si alloys before and after irradiation. Investigations in recent years have focused primarily on compositions of V-(0--15)Cr-(0--20)Ti (0--1)Si. Results from these investigations have provided a basis for identifying a V-4Cr-4Ti alloy as the US reference vanadium alloy for further development. Major results obtained on one production-scale heat and three laboratory heats with compositions of V-(4--5)Cr-(4--5)Ti are presented in this paper. Properties measured were input properties, tensile properties, creep, and radiation effects.

Pseudobinary zircaloy-oxygen phase diagram☆

The zirconium-rich portion of the pseudobinary zircaloy-4/oxygen phase diagram was investigated by resistometric measurements and by metallographic analysis of equilibrated and quenched specimens. Accurate results for the phaseboundary locations were obtained from metallographic analysis of specimens in which oxidation and homogenization were limited to the α-phase region, and the specimens were rapidly quenched from equilibration temperatures in the β- and (αa + (β)-phase regions. The a- and (3-phase boundaries can be represented by the realtions In Cα = −2.28 + 0.535 In (T — 1083) and In Cβ = 5.02 − 8 220/T (forT > 1280 KorCβ > 0.2), where Cα and Cβ are the oxygen concentrations in weight percent and T is temperature in K.

Progress in vanadium alloy development for fusion applications

Abstract Vanadium alloys have been identified as a leading candidate low-activation structural material for fusion first-wall blanket applications. Candidate vanadium alloys exhibit favorable safety and environmental characteristics, good fabricability, high temperature and heat load capability, good compatibility with liquid metals and resistance to irradiation damage. The focus of the vanadium alloy development program has been on the vanadium-chromium-titanium (0–15%Cr, 1–20%Ti) alloy system. Investigations include effects of minor alloy elements such as Si, Al and Y and substitution of iron for chromium in the ternary alloy. A V-4Cr-4Ti alloy is currently regarded as the reference alloy. Significant progress has been made in the development of vanadium alloys for fusion applications. Two production-scale heats (500 and 1200 kg) of the V-4Cr-4Ti alloys have been produced with controlled levels of impurities. The baseline properties of the 500 kg heat are similar to those of the previous laboratory-scale heats. Additional data have been obtained on baseline tensile and fracture properties. Results obtained on several heats with minor variations in composition indicate high uniform and total elongation of these alloys at temperature from RT to 700°C. Results obtained to date indicate that the V-Cr-Ti alloys are resistant to swelling and embrittlement after exposure to relatively high neutron fluences at temperatures of 400–600°C. The properties are not significantly different when modest amounts of helium are generated during neutron irradiation by the dynamic helium charging experiment method. However, recent results have indicated that these alloys are susceptible to irradiation embrittlement at lower temperatures. Additional irradiation experiments are in progress to investigate these effects at temperatures of 200–400°C. This paper presents an update on the experimental results on candidate low activation vanadium alloys.

Irradiation-assisted stress corrosion cracking of austenitic stainless steels: recent progress and new approaches

Irradiation-assisted stress corrosion cracking (IASCC) of several types of BWR field components fabricated from solution-annealed austenitic stainless steels (SSs), including a core internal weld, were investigated by means of slow-strain-rate test (SSRT), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and field-emission-gun advanced analytical electron microscopy (FEG-AAEM). Based on the results of the tests and analyses, separate effects of neutron fluence, tensile properties, alloying elements and major impurities identified in the American Society for Testing and Materials (ASTM) specifications, minor impurities, water chemistry, and fabrication-related variables were determined. The results indicate strongly that minor impurities not specified by the ASTM-specifications play important roles, probably through a complex synergism with grain-boundary Cr depletion. These impurities, typically associated with steelmaking and component fabrication processes, are very low or negligible in solubility in steels and are the same impurities that have been known to promote intergranular SCC significantly when they are present in water as ions or soluble compounds. It seems obvious that IASCC is a complex integral problem which involves many variables that are influenced strongly by not only irradiation conditions, water chemistry, and stress but also iron and steelmaking processes, fabrication of the component, and joining and welding. Therefore, for high-stress components in particular, it would be difficult to mitigate IASCC problems at high fluence based on the consideration of water chemistry alone, and other considerations based on material composition and fabrication procedure would be necessary as well.

Effects of neutron irradiation and hydrogen on ductile-brittle transition temperatures of V-Cr-Ti alloys

The effects of neutron irradiation and hydrogen on the ductile- brittle transition temperatures (DBTTs) of unalloyed vanadium and V-Cr-Ti alloys were determined from Charpy-impact tests on 1/3 ASTM standard size specimens and from impact tests on 3-mm diameter discs. The tests were conducted on specimens containing <30 appm hydrogen and 600-1200 appm hydrogen and on specimens after neutron irradiation to 28-46 dpa at 420, 520, and 600C. The DBTTs were minimum (< {minus}220{degree}C) for V-(105)Ti alloys under for V-4-Cr-4Ti alloy with <30 appm hydrogen. The effect of 600-1200 appm hydrogen in the specimens was to raise the DBTTs by 100--150{degree}C. The DBTTs were minimum (< {minus}220{degree}C) for V-(1-5)Ti alloys and V-4-Cr-4Ti alloys after neutron irradiation.

Effect of irradiation damage and helium on swelling and structure of vanadium-base alloys☆

Abstract Swelling behavior and microstructural evolution of V-Ti, V-Cr-Ti, and V-Ti-Si alloys were investigated after irradiation at 420–600°C up to 114 dpa. The alloys exhibited swelling maxima between 30 and 80 dpa and swelling decreased on irradiation to higher dpa. This is in contrast to the monotonically increasing swelling of binary alloys that contain Fe, Ni, Cr, Mo, and Si. Precipitation of dense Ti5Si3 promotes good resistance to swelling of the Ti-containing alloys, and it was concluded that Ti of > 3 wt% and 400–1000 wppm Si are necessary to effectively suppress swelling. Swelling was minimal in V-4Cr-4Ti, identified as the most promising alloy based on good mechanical properties and superior resistance to irradiation embrittlement. V-20Ti doped with B exhibited somewhat higher swelling because of He generation. Lithium atoms, generated from transmutation of 10B, formed γ-LiV2O5 precipitates and did not seem to produce undesirable effects on mechanical properties.

Microstructural examination of irradiated V–(4–5%)Cr–(4–5%)Ti

Microstructural examination results are reported for two heats of V-(4-5%) Cr-(4-5%)Ti irradiated in the EBR-II X530 experiment to 4.5 dpa at {approximately}400 C to provide an understanding of the microstructural evolution that may be associated with degradation of mechanical properties. Fine precipitates were observed in high density intermixed with small defect clusters for all conditions examined following the irradiation. The irradiation-induced precipitation does not appear to be affected by preirradiation heat treatment at 950-1125 C. There was no evidence for a significant density of large (diameter >10 nm) dislocation loops or network dislocations. Analytical investigations successfully demonstrated that the precipitates were enriched in titanium, depleted in vanadium and contained no nitrogen. These results are discussed in terms of future alloy development options.