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Materials & Engineering Sciences Center. Atoms to Continuum. Separation of Molecular Tritium from Atomic Tritium in Plasma Exposed Tungsten Rion Causey and Robert Kolasinski Sandia National Laboratories Livermore, CA 94550
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Materials & Engineering Sciences Center Atoms to Continuum Separation of Molecular Tritium from Atomic Tritium in Plasma Exposed Tungsten Rion Causey and Robert Kolasinski Sandia National LaboratoriesLivermore, CA 94550 9th International Workshop on Hydrogen Isotopes in Fusion Reactor Materials Salamanca, Spain June 2-3, 2008
Materials & Engineering Sciences Center Atoms to Continuum Unpublished Work from 1991 High purity tungsten foils were exposed in the old Tritium Plasma Experiment Flux = 7.5x1020 T/m2-s Exposure Time = 7 hrs at 250 C, 2 hrs at 400 C Energy = 100 eV Fluence = 1.9x1025 T/m2 (250 C) and 5.4x1024 T/m2 (400 C) Samples Annealed and Unannealed (1000 C 1 hr) The atomic tritium content of the samples was determined by chemical dissolution and liquid scintillation counting of the neutralized acid solution. Atomic tritium is released as the acid dissolves the tungsten with the T+ joining the high concentration of H+ already in the solution. The molecular tritium content of the samples was determined by capturing all gases from above the acid solution and sweeping them along with helium through an ionization chamber. Tritium preexisting at T2 would not be retained in the acid solution.
Materials & Engineering Sciences Center Atoms to Continuum Experimental Results Tritium Retention at 250 C Atomic (T/m3) Molecular (T/m3) Unannealed 4.7x1022 3.2x1022 Annealed 1.3x1022 4.8x1022 Tritium Retention at 400 C Unannealed 1.8x1022 7.3x1021 Annealed 4.2x1021 5.5x1021
Materials & Engineering Sciences Center Atoms to Continuum Experimental Results • Annealing reduces the atomic concentration for both temperatures • Annealing somewhat increases the molecular concentration at 250 C • Annealing has no effect on the molecular concentration at 400 C • After annealing, at 250 C, molecular dominates the atomic retention • At 400 C, atomic dominates molecular before annealing, but they are similar after annealing
Materials & Engineering Sciences Center Atoms to Continuum Speculation on Molecular Component?? Based on results shown in my PSI paper, I contend that the molecular component is simply that residing in voids underneath blister caps. One key reason for believing this is based on modeling by Don Cowgill. His bubble coalescence model kept predicting very near surface bubble agglomeration that prevented further ingress. Therefore, no bubbles! The blisters are not thought to be due coalescence of bubbles or voids. They are due to stresses caused by the trapped hydrogen in the near surface. The molecules arrive after the blisters form. Past modeling has over estimated atomic trap sites because the modeling assumed all retention was atomic. TPE experiments will be performed in the coming year to determine both the atomic and molecular retention as a function of temperature. Because the results will depend on flux, fluence, energy, and atomic trapping, conditions as close to those for ITER as possible should be chosen.
Materials & Engineering Sciences Center Atoms to Continuum More Speculation?? The lowest possible inventory for tritium in tungsten occurs for tungsten containing no traps at all. There would be no atomic or molecular retention (no trapping => no stress => no blisters => no retained molecules). We have recommended annealing tungsten to minimize inventory. Annealing even at 1000 C for one hour significantly reduces the atomic trapping. Based simply on inventory, is this correct? The higher the concentration of atomic traps, the sooner blisters form. For high concentrations of traps, blisters are likely to be small with a very high density. Blisters should effectively shield the area beneath them from ingress. Almost all of the retention would be molecular and limited by fracture of the blister cap. What are the plasma impurity implications of this? Concern about high inventories of tritium in neutron damaged tungsten are not founded. Blisters will prevent filling of the traps.
Materials & Engineering Sciences Center Atoms to Continuum Conclusions • Both atomic and molecular hydrogen isotopes exist in plasma exposed tungsten • The ratio of atomic to molecular varies with temperature • It is contended that the molecular component exists in blisters • Blisters will play a very important role in plasma impurities and tritium inventory (both good and bad)
