B.A. Loomis

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.

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.

Alloying and impurity effects in vanadium-base alloys☆

Abstract Vanadium-base alloys offer a number of significant advantages over other alloy systems for fusion reactor structural applications. However, the properties and performance of vanadium are strongly influenced by the choice of alloying additions and the presence of impurity elements, and these effects are reviewed in this paper. In particular, contraints imposed by neutron-activation considerations upon possible alloying additions and permissible levels of impurities are discussed, and typical impurity levels resulting from reduction and refining operations are summarized. The effects of alloying additions and impurities upon mechanical properties are also reviewed, with particular attention given to recent observations on the intergranular segregation of sulfur and other impurities. Effects on corrosion/compatibility and fabrication properties are also summarized. Finally, consideration is given to future directions in alloy optimization, in view of recent results on the apparent neutron-irradiation and helium embrittlement of certain V-Cr-Ti alloys.

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.

Evaluation of low-activation vanadium alloys for use as structural material in fusion reactors

Abstract The V-7.2Cr-14.5Ti, V-9.2Cr-4.9Ti, V-9.9Cr-9.2Ti, V-13.5Cr-5.2Ti, V-4.1Cr-4.3Ti, Vanstar-7, V-4.6Ti, V-17.7Ti, V-10.0Ti, V-3.1Ti-(0.5-1.0)Si, and V-2.5Ti-1.0Si alloys were evaluated for use as structural material in fusion reactors. The alloys were evaluated on the basis of their unirradiated and neutron-irradiated yield strength; swelling resistance; ductile-brittle transition temperature; resistance to hydrogen, helium, and irradiation embrittlement; and compatibility with a lithium reactor coolant. On the basis of these evaluations, alloys of V-(0–7)Cr-(5–10)Ti-(0–1)Si are recommended for more intensive evaluation for use as structural material in fusion reactors.

Flux pinning by radiation damage in oxygen-doped niobium

Abstract Niobium foils, doped with 10, 250, 480 or 904 wt. p.p.m. of oxygen, were irradiated with fast neutrons (E>0·1 MeV) to a total fluence of 9×1019 cm−2 at ∼50°C. The radiation damage as characterized by transmission electron microscopy consists of dislocation loops and ‘black spot’ damage (probably small, unresolved dislocation loops). The density of dislocation loops increases by about a factor of 20 with increasing oxygen content whereas the root mean square dislocation loop diameter decreases so as to maintain the total area of dislocation loop approximately constant. Despite the great increase in dislocation loop density ρ with oxygen content, the pinning force density F p at low reduced magnetic induction decreases by more than a factor of 20 from the 10 to the 904 p.p.m. oxygen sample. These results show that the decrease in loop size through its effect on the elementary pinning force f p of the loop far outweighs the increase in loop density. Tests of summation models are made using calculate...

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.

Properties of V4Cr4Ti for application as fusion reactor structural components

Abstract Vanadium-based alloys are promising candidate materials for application in fusion reactor first-wall and blanket structures because they offer several important advantages, i.e. inherently low irradiation-induced activity, good mechanical properties, good compatibility with lithium, high thermal conductivity and good resistance to irradiation- induced swelling and damage. As part of a program to screen candidate alloys and develop an optimized vandanium-base alloy, extensive investigations of various Vue5f8Ti, Vue5f8Crue5f8Ti and Vue5f8Tiue5f8Si alloys have been conducted after irradiation in lithium in fission reactors. From these investigations, Vue5f84wt.%Crue5f84wt.%Ti was indentified as the most promising alloy. The alloy exhibited attractive mechanical and physical properties that are prerequisites for first-wall and blanket structures, i.e. high tensile strength, high ductility, good creep properties, high impact energy, low ductile—brittle transition temperature before and after irradiation, excellent resistance to irradiation-induced swelling and microstructural instability and good resistance to corrosion in lithium. In particular, the alloy is virtually immune to irradiation-induced embrittlement, a remarkable property compared with other candidate materials being investigated in the fusion—reactor—materials community. The effects of helium, charged dynamically in simulation of realistic fusion reactor conditions, on tensile, ductile—brittle transition and swelling properties were insignificant. The thermal creep behavior of the alloy was significantly superior to that of austenitic and ferritic/martensitic steels.

High swelling rates observed in neutron-irradiated V-Cr and V-Si binary alloys

Additions of 5 to 14 wt% chromium to vanadium lead to very large swelling rates during neutron irradiation of the binary alloys, with swelling increasing strongly at higher irradiation temperatures. Addition of 2 wt% silicon to vanadium also leads to very large swelling rates but swelling decreases with increasing irradiation temperature. Addition of 1 wt% zirconium does not yield high swelling rates, however.

Length and electrical resistivity changes of neutron irradiated uranium

Abstract The length change of uranium single crystals in the [100], [010], and [001] crystallographic directions was determined during thermal neutron irradiation at temperatures between 5°K and 550°K. The electrical resistivity induced in uranium single crystals during neutron irradiation at temperatures between 4·5K and 20°K was also determined. The recovery of the irradiation-induced electrical resistivity and length changes was correlated with the temperature dependence of the irradiation-induced length changes. These results are consistent with the hypothesis that elongation in the [010] direction and contraction in the [100] direction during thermal neutron irradiation of uranium single crystals are due to the anisotropic condensation of vacancy and interstitial defects in planar clusters. The substantial decrease in magnitude of the length changes on increasing the irradiation temperature from 20°K to 140°K was attributed to the annihilation of interstitials at vacancy defects because of interstiti...