Body-centered cubic (bcc)-structured V-Cr-Ti alloys (particularly composition ranges of around V-4Cr-4Ti and V-15Cr-5Ti) have low neutron cross-sections and the isotopes that do form with neutron capture have short half lives (51V has a half-life of <4 min). As noted in Chapter 4.12, Vanadium for Nuclear Systems, these characteris­tics, along with reasonable operating temperatures (limited by radiation hardening and helium bubble formation to «575-775 K129), make V-Cr-Ti an attractive material for first walls and blankets, but the tritium retention characteristics of vanadium alloys leave much to be desired. Vanadium has a large solubility for hydrogen and a very large diffu­sion coefficient for hydrogen. These two traits make the permeability of hydrogen in vanadium compara­ble to that of titanium and palladium.1 Vanadium absorbs hydrogen exothermically. Additions of chro­mium tend to increase this energy, while titanium additions tend to decrease it and, to first order, alloys with roughly equal and small amounts of chro­mium and titanium (such as V-4Cr-4Ti) are assumed to react similarly to hydrogen isotopes as pure vana — dium.129 Vanadium alloys form hydrides below «450 K,129 which is below the typical operating temperatures.

The diffusivity of hydrogen in vanadium is «10- m2 s-1 in the range of operating temperatures, a larger value than in most metals.129 There has been extensive experimental measurement of several V-Cr-Ti alloys using different hydrogen isotopes. These are summar­ized in Figure 14. Schaumann et at}31 and Cantelli eta/.132 independently measured the diffusivity of both protium and deuterium in pure vanadium after charg­ing them with gas at «775 Kusing the Gorsky anelas — tic relaxation effect.131 Both groups found that the prefactor did not depend strongly on the isotope, whereas the activation energy did. This is contrary to the common naive expectation, where the activation energies would be identical and the prefactors would differ by a factor of the square root of the mass. The two groups also each reported a deviation from expo­nential behavior at lower temperatures, which could be attributed to some combination of surface effects, trapping, and a V-H phase transition at «200 K.1 However, deviation from Arrhenius behavior is com­mon in bcc metals133 and has been reported in a number of studies of vanadium. The transition tem­perature from exponential behavior varies widely with the technique used to measure diffusivity and the group that measures it,131,132,134-139 and has been as high as 813 K when measuring uncharged specimens using the absorption technique.134 This supports the notion that much of the deviation reported in the literature may be due to surface recombination limita­tion at lower temperatures. Compounding the recom­bination limitation of the vanadium base metal is the fact that surface oxides (particularly TiO) form and limit recombination more.140,141 At lower tempera­tures, there was an increased deviation in the measured Dh/Dd from /2, which has also been observed by others100,138 and in the diffusivity of titanium.142 The heavier hydrogen isotopes do not diffuse much slower than predicted until temperatures below ~-373 K, thus the physics associated with deviations from the predic­tions of classical rate theory cannot be exploited for the use in fusion applications.

The electrochemical pulse experiments of Boes and Zuchner143 derived an activation energy that is twice as large using the electrochemical pulse method, which was also supported by absorption experiments by Eguchi and Morozumi.134 Both tech­niques are influenced by the surface, while Gorsky effect measurements and electrical resistivity mea­surements are only influenced by the bulk.1 The

absorption experiments also indicated that the hydro­gen diffusion coefficient in vanadium alloys is decreased by additions of chromium (as well as iron and niobium), but that it could be increased by large titanium additions, which is thought to be due to electronic contributions.134 Most other experiments have found that moderate amounts of titanium decrease the hydrogen diffusion coefficient much more than chromium is able to.133,139 Ti’s strong ability to trap hydrogen isotopes may explain the discrepancy in these measurements. Increasing tita­nium content decreases the Dh/Dd ratio, while increasing chromium content increases the ratio.139

While the solubility of hydrogen in vanadium is lower than that in either zirconium or titanium, it is still very large, being greater than the value in palladium and much greater than the value in the other structural metals considered here (Figure 15). The reported hydrogen lattice solubilities in vana­dium alloys are in reasonable agreement, regardless of composition.1 Alloying additions do, however, change trapping in the alloy.

Titanium has a higher heat of solution for hydrogen than vanadium and titanium138 additions

increase the lattice parameter of vanadium.133 How­ever, titanium is a much stronger trap than other elements that increase the lattice parameter as much or more (including niobium, molybdenum, and zirconium).133 Pine and Cotts139 assert that tita­nium solute atoms trap not only hydrogen isotopes at nearest-neighbor interstitial sites, but also hydrogen substitutionally. They demonstrated that the binding energy varied from 3 kJ mol-1 in V-3Ti to 9.84 kJ mol-1 in V—8Ti. The trapping energy for D is larger than that for hydrogen for both alloys. However, it should also be noted that there is considerable short — range ordering in V—Ti alloys with more than ^4 at. % Ti.133 This ordering means that trapping will not obey an Oriani-type behavior, in which trapping would be linearly dependent on the number of solute atoms, because the solid solution is not random. The elements chromium, iron, and copper all reduce the lattice parameter of vanadium and the diffusivity change in alloys containing these elements is also much lower than that in alloys with Ti.133 In fits to the apparent diffusivity in tritium diffusion experi­ments in ternary V-Cr-Ti alloys, Hashizume eta/.135

show that, in addition to single titanium atoms, the most likely secondary trap is not chromium. Instead, the secondary trap has much higher energy and a lower concentration when compared to the monomer titanium trap. They also speculated that this was due to solute dimers and larger clusters. Interstitial oxygen, carbon, and nitrogen are also com­mon in vanadium alloys. One or more hydrogen atoms bind with single carbon or nitrogen atoms readily, and oxygen atoms tend to trap at least two hydrogen atoms each.144

Other defects, such as dislocations, may still be effective traps at 773 K.145 As with other materials, vanadium can be damaged by radiation, and this will likely be the dominant trap in fusion reactors.146,147

The recombination coefficient for hydrogen is over five orders of magnitude slower in vanadium than in nitrogen in the range of operating tempera­tures, and is relatively insensitive to the surface concentration of sulfur.129 Because of this and the high diffusivity of tritium, release is recombination limited in vanadium alloys. Deuterium ion-driven permeation experiments148 of V-15Cr-5Ti have

estimated the recombination-rate coefficient to be

2.4 X 10-29 m4 s-1 (although this measurement is three orders of magnitude lower than measurements on more dilute alloys149 and two orders of magnitude higher than measured in pure vanadium1 0). It should be noted that most measured recombination rates are lower bounds due to surface oxides. In environ­ments in which this native oxide layer may be dam­aged (such as by radiation in a fusion reactor), the actual recombination rate may be higher.147

V-Cr-Ti alloys have hydrogen permeabilities that are at least two orders of magnitude more than nearly any other blanket material and form detrimental hydrides.129’130’151-156 The ongoing stud­ies of permeation barriers may allow mitigation of this significant disadvantage so that V’s positive traits in a high-energy neutron environment can still be utilized.