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• W2079873646 abstract "Tritium diffusion coefficients have been measured in ceramic materials of potential interest as insulator or wall materials in controlled thermonuclear reactors. Tritium was recoil injected into samples to about 2×1014 atoms/cm2 through thermal‐neutron transmutation of a surface blanket of 6Li‐enriched Li2CO3 in the NCSU Pulstar research reactor. Diffusion coefficients were determined from measurement of the tritium release rate during postirradiation isothermal annealing. Released tritium was detected either with an internal‐flow proportional counter using P‐10 sweep gas or with an ion chamber using a 90% helium–10% hydrogen gas mixture. The total amount of tritium in a sample was determined by heating to successively higher temperatures until no further tritium was released.Tritium release fractions were linear with t1/2 for short times as predicted by classical solutions for recoil‐injected specimens.1 Diffusion coefficients were calculated from the release curves employing calculated values for the tritium recoil ranges. Arrhenius plots (ln D vs 1/T) were straight lines over the range of D’s measured (usually five orders of magnitude with an upper limit on D of 10−9 cm2/s due to system response time). Least‐squares, best‐fit values of D0 and Q for the expression D=D0 exp(−Q/RT) are tabulated in Table I along with the temperature ranges for the materials studied.Some data were taken with a liquid‐nitrogen cold trap between the sample tube and the ion chamber to determine if part of the tritium release occurred as water vapor, but no indication of this was found.Diffusion coefficients were much smaller than those generally observed for metals at comparable temperatures.2 The high observed activation energies (30 to 65 kcal/mole) suggest the possibility of chemical bonding effects. The pronounced difference in measured D’s [five orders of magnitude at 500 °C for pure alumina compared to Lucalox (MgO doped Al2O3) and four orders of magnitude at 650 °C for pure SiC compared to Al‐doped SiC] also suggests that impurities may be important in controlling tritium diffusion rates. Similar effects have been observed for self‐diffusion3 and for impurity diffusion4,5 in these materials.Diffusion coefficients obtained for single‐crystal and sintered samples of BeO are very near those obtained by Scott and Wassell6 for single crystals, but are three to four orders of magnitude larger than their results for BeO powder. Anomalous release rates were obtained for Lucalox at temperatures below 450 °C. The initial release rates were characteristic of low diffusion coefficients with a high activation energy (63 kcal/mole), but values of D consistent with higher temperature results and the D’s in Table I were obtained when t1/2 exceeded about 40 s1/2. No explanation is offered for this behavior.Pyrolytic carbon exhibited a tritium release pattern which could be interpreted in terms of classical diffusion, but which also appeared to involve tritium desorption from surface‐connected pores. Tritium release rates increased when hydrogen was added to the sweep gas. Tritium profile measurements showed that the released tritium came from surface layers with very little movement of tritium at greater depths. Exposing specimens to tritium gas gave tritium buildup in just the top 25 μm; this tritium exhibited the same release characteristics as recoil‐injected specimens during postexposure heating.Table II lists compositions for some of the materials studied, as measured by the supplier or by electron‐microprobe analysis. A comparison of the data in Table I and II suggests that specimens containing small amounts of heterovalent impurities gave higher diffusion coefficients and somewhat lower activation energies than the purer materials.The low tritium‐diffusion coefficients observed for all materials supports their possible use as tritium barriers in fusion systems." @default.
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• W2079873646 title "Abstract: Tritium diffusion in ceramic materials for thermonuclear reactors" @default.
• W2079873646 doi "https://doi.org/10.1116/1.568929" @default.
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