Local Mixing Model Research Articles

Tritium inventory and permeation in TFTR

The control of tritium in TFTR has both important safety and environmental implications. Current modelling techniques allow realistic predictions of the tritium inventory in and permeation through in-torus components. The source of the tritium was computed using a three-dimensional description of the neutral particle flux on the TFTR vacuum vessel wall from the neutral transport code DEGAS, under plasma conditions modelled with the one-dimensional transport code BALDUR. The movable limiter (graphite) was modelled using an extension of the Local Mixing Model for hydrogen retention and isotope exchange. In particular, the calculations consider the inventory and recycling for the movable limiters (which are expected to experience transient temperatures up to 2000°C) as well as the bumper limiters and protective plates which will remain somewhat cooler. Transient effects in the bulk graphite limiter and in the stainless steel wall were modelled using the DIFFUSE code with materials parameters taken from the most recent literature. It appears that for the expected material properties, specifically the hydrogen recombination constant measured in a clean TFTR environment, and operating scenario, there will be essentially no tritium permeation through the stainless steel walls or bellows and less than 0.1 kCi of tritium inventory in the stainless steel first wall. The limiters may contain as much as 5 kCi of tritium.

Retention and recycling of plasma edge hydrogen isotopes in C and TiC

The retention, release, and isotopic exchange of hydrogen, deuterium, and tritium incident on room temperature C and TiC with Maxwellian velocity and isotropic angular distributions have been calculated using the Local Mixing Model. The calculations are based on the hydrogen saturation concentration, range distributions and isotopic exchange behavior, all of which have been verified experimentally by monoenergetic implants. Particle fluences (10 16 to 10 20/cm 2) and characteristic Maxwellian energies (50 eV to 3200 eV) were chosen in the range of interest for tokamak plasma edge conditions. The results of these calculations are used to estimate tritium inventory buildup and to predict tritium changeout rates by isotopic exchange in tokamaks and other fusion devices. Tritium inventories in the TiC components (which comprise ≳ 20% of the TFTR first-wall surface) are anticipated to remain at reasonable levels in TFTR, and isotopic exchange is shown to be a feasible method to reduce these tritium inventories.

Extended local mixing model for hydrogen retention and isotope exchange

The previously reported local mixing model (LMM) for hydrogen retention and isotope exchange is extended to include thermal release of the hydrogen from its trap sites. In addition, a new mechanism for beam-enhanced thermal transport (BETT) is described which accounts for a major portion of the thermal release at elevated temperatures. BETT involves the beam-induced promotion of trapped hydrogen from stronger traps to weaker traps and arises naturally from the basic concepts of the LMM. Comparison of the predictions of the model for hydrogen retention and isotope exchange at elevated temperatures in graphite and TiC show good agreement between experiment and theory.

Trapping and release of implanted D/H ions in fused silica

Elastic recoil detection techniques have been used to measure hydrogen isotope profiles in fused silica implanted with 20 keV H and/or D ions. Saturation is obtained for both H and D implantations at concentrations of 2.1 × 10 21 ions/cm 3, in good agreement with results of gas loading experiments. Infrared measurements indicate that most of the implanted H bonds in the form of OH. Theoretical analyses using the local mixing model (LMM) of H-trapping provide a good fit to the saturation behavior and subsequent H movement toward the surface. Isochronal annealing shows that both H and D are released in a single stage centered about 550°C. The best theoretical fit is for a 1.9 eV H-trap activation energy.

A trap activation model for hydrogen retention and isotope exchange in some refractory marterials

Our recently, developed Local Mixing Model (LMM) has been successful in describing and predicting the properties of hydrogen retention and isotope exchange for a variety of refractory materials. For some materials, however, the detailed predictions of the LMM are not observed. A Trap Activation Model (TAM) is proposed here to account for the observed departures from the LMM. Comparison of experimental room temperature saturation depth profiles for H + → Si with the predictions of TAM suggests that the hydrogen traps are multiple-vacancy complexes in this system. The observed profiles result from a beam-induced competition between trap creation/ annihilation and H-trapping/detrapping.

A local mixing model for deuterium replacement in solids

Abstract A new model for hydrogen isotope exchange by ion implantation has been developed. The basic difference between our approach and previous work is that we include the depth distribution of the implanted species. The outstanding feature of this local mixing model is that the only adjustable parameter is the saturation hydrogen concentration which is specific to the target material and dependent only on temperature. The model is shown to give excellent agreement both with new data on H/D exchange in the low Z, coating materials VB2, Tie, TiB2, and B reported here and with previously reported data on stainless steel. The saturation hydrogen concentrations used to fit these data were 0.15, 0.25, 0.15, 0.45, and 1.00 times atomic density respectively. This model should be useful in predicting the recycling behavior of hydrogen isotopes in tokamak limiter and wall materials.

Hydrogen in Fusion First Wall Surfaces

The concentration at which deuterium implanted into the materials B, C, Si, TiC, TiB2, B4C, and VB2 saturates has been measured using nuclear microanalysis. The local mixing model for H saturation and isotopic replacement agree well with these data and yield H saturation concentrations close to .5 H/host atom ratio for the elemental samples and lower concentrations for the compounds. The local mixing model has also been used to model tritium buildup and isotope exchange in proposed first-wall materials for magnetic confinement fusion reactors. Two satatistical models are introduced to discuss the buildup to saturation.

Saturation, Exchange and Retention of Implanted D/H Ions in Fused Silica

ABSTRACTElastic recoil detection techniques have been used to measure hydrogen isotope profiles in fused silica implanted with 20 keV H and/or D ions. Saturation is obtained for both H and D implantations at a concentration of 2.2 × 1021 ions/cm3. H and D exchange are observed for saturated samples subsequently implanted with a different H isotope. A previously developed local mixing model of H-trapping has been extended to include retrapping and beam-induced detrapping for application to these studies. This theoretical analysis provides a good fit to the H/D saturation and isotope exchange behavior. Isochronal annealing shows that both H and D are released in a single stage centered about 550°C.

Depth resolved measurements of hydrogen isotope exchange in carbon

High resolution measurements of the depth profiles of hydrogen and deuterium implanted in carbon were made using SIMS. Nuclear reaction analysis was used to calibrate the concentration scales for the deuterium. The depth profiles of low fluence H and D implants show the D extends to slightly greater depths than the H at 1.5 keV. The retention of deuterium at high fluences was in good agreement with predictions of a saturation model. Depth profiles were also measured on samples implanted with 10 1 8D/cm 2 at 1.5 keV followed by hydrogen implants of 2 × 10 1 7H/cm 2 and 10 1 8H/cm 2 at the same energy. A comparison between these measured profiles and profiles calculated using the local mixing model shows that the model predicts the depth dependence of the H, D exchange reasonably accurately.