Review of Tritium Retention in Beryllium Sandia National Laboratories Rion Causey Sandia National Laboratories Livermore, CA IAEA Meeting Vienna September 25-27, 2006 Materials & Engineering Sciences Center Atoms to Continuum

Beryllium Identified in oxide form in 1798 by Nicholas L. Vauquelin Isolated in pure form by Wohler and Bussy in Became readily available to industry in 1957 Has the highest melting point of all the light elements (1287 o C) Has excellent thermal conductivity (~50% of that of copper) Resists oxidation at normal temperatures Is also named Glucinium (due to it having a sweet taste!) Is a low Z material Sandia National Laboratories

Beryllium Beryllium was used extensively in the JET fusion reactor. Very little was known at that time about the behavior of tritium in beryllium. The JET beryllium was noted to pump the hydrogen isotope plasma gas for several seconds into the reactor pulse, and to dump the gas back out after the pulse was over. This behavior was not understood. When the ITER design project began, tritium inventory calculations using existing parameters for diffusivity and solubility predicted kilograms of tritium. Sandia National Laboratories

Experiments performed for ITER all showed hydrogen isotope retention in beryllium to be low R.A. Anderl et al., J. Nucl. Mater. 273 (1999) 1-16 Experiments were performed at various fluences. Agreement between results suggested a saturation effect. (data adjusted to 100 eV) Sandia National Laboratories

Work at the Russian Academy of Sciences showed hydrogen isotope bombardment of beryllium to open surface connected porosity R.A. Anderl et al., J. Nucl. Mater. 273 (1999) 1-16 Chernikov et al., Proceedings 2nd IEA International Workshop on Beryllium Technology for Fusion (1995) Sandia National Laboratories

Surface Connected Porosity in Beryllium The open porosity in beryllium is generated due to the inability of the hydrogen isotopes to remain in solution in the metal (low solubility). We now believe the solubility of hydrogen in beryllium to be VERY low. We also now believe that the long term pumping of hydrogen isotopes reported by JET to simply be due to molecular buildup in the porosity (verified by Alimov and others). The open porosity should make it virtually impossible to generate large tritium inventories in beryllium generated by plasma exposure. Sandia National Laboratories

Experiments in TPE demonstrate the difficulty of generating high inventories Sandia National Laboratories At 250 o C, increasing the plasma exposure time by two orders of magnitude increased the tritium retention by only a factor of 2.4. The 40 hour exposure represents over 100 one thousand second shots in ITER.

Removal of Tritium from Beryllium After gas exposure at elevated temperatures After long term neutron exposure Sharapov et al., J. Nucl. Mater (1996) 870. heating rate 10 K/s Andreev et al., J. Nucl. Mater (1996) 880. Once tritium is in beryllium, it is not easily removed. Do these plots say something about the location of the tritium in the Be?

Permeation - Many measurements, are any right? If you accept the idea that tritium retention in the ITER beryllium will be small, is there still a problem? Codeposition of beryllium with both tritium and oxygen can only be a problem if oxygen is readily available in ITER. I leave it to the Jeff Brooks of the world to determine the answer here. What about permeation? Will beryllium joined to copper permeate much tritium into the coolant? Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Permeation experiment by Al’tovskiy et al* These authors used both tubes and disks for their measurements. Tubes were powdered hot extruded beryllium, 98.4 wt.% pure, 1.3 wt.% O, 56 µm grain size. Disks were hot-pressed beryllium, 99.2 wt.% pure, 0.3 wt.% Fe and 0.2 wt.% O, 30, 56, and 600 µm grains. Temperatures from o C were used with pressures < 1 atm Permeation (Tubes) = 5.46x exp(-10.6 kcal/RT) mol/(m-s-Pa 1/2 ) Permeation (membranes) = 1.9x10 -6 exp(-23.0 kcal/RT) mol/(m-s-Pa 1/2 ) *R.M. Al’tovskiy et al., Russ. Metall. 3 (1981) 51 Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Permeation experiment by Kizu and Miyazaki* Powder metallurgy membranes with thicknesses of 0.2 and 0.4 mm were used Primary impurity was oxygen at 0.9%. Density was 99% theoretical. Temperatures from 735 to 1000 K were used with pressures from 10 to 1000 Pa Permeation = 1.0x10 -6 exp(-17.5 kcal/RT) mol/(m-s-Pa 1/2 ) *K. Kizu and K. Miyazaki, Fusion Technol. 28 (1995) Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Permeation experiment by Tazhibaeva et al* FP-56 beryllium with a purity level of 98% was used. The primary impurity was oxygen with a level of 1.6 wt.% Sample thickness = 0.2 mm Temperatures from 673 to 873 K and pressures from 1 to 1000 Pa were used Permeation = 1.3x10 -9 exp(-15.2 kcal/RT) mol/(m-s-Pa 1/2 ) *I.L. Tazhibaeva et al., 18th Symposium on Fusion Technology, Aug. 1994, Karlsruhe, Germany; p vol.1 Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Diffusivity and solubility experiment by Jones and Gibson* Beryllium was vacuum cast metal. Purity was 98.5 vol% with 1.5 vol% oxygen Sample thickness was 0.79 mm Temperatures from o C Permeation = SxD = 5.9x exp(-4.4 kcal/RT) mol/(m-s-Pa 1/2 ) *P.M.S. Jones and R. Gibson, J. Nucl. Mater. 21 (1967) 353 Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Permeation Results Are Not At All Consistent Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum Results using the permeation technique differ by three orders of magnitude. Using the product of solubility times diffusivity adds one more order of magnitude difference.

Lets look at a 1st order approximation of the permeation through the ITER beryllium Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum Be Tiles CuCrZr (heat extraction) 316L- (N) 80 mm The beryllium thickness is 10 mm. The copper alloy substrate is 22 mm thick. The coolant tubes are 8 mm in diameter. For (very) 1st order approximation, assume that the coolant tubes subtend 50% of the area at a depth of 10 mm into the copper. Details from: G. Federici et al., Physica Scripta T91 (2001) 76.

Continue Permeation Area of 1st Wall = 690 m 2 DT particle flux = 1x10 20 /m 2 Temperature = 483 K (assume it to be uniform) Total exposure time = 12,000 pulses x 400 s = 4.8x10 6 s Assume tritium coming out of solution into open porosity generates an equilibrium pressure of 100 Pa Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Continue Permeation Use permeation coefficient of Kizu and Miyazaki (highest) Permeation coefficient = 1.0x10 -6 exp(-17.5 kcal/RT) mol/(m-s- Pa 1/2 ) Divide by diffusivity [D = 1.3x10 -7 exp(-14.1 kcal/RT) m 2 -s] Solubility then = 1.2x10 -2 exp(-3.4 kcal/RT) atom fraction/atm 1/2 The atom fraction unit is needed by the DIFFUSE code. Note that dividing the permeation coefficient by the diffusivity (both given by Kizu and Miyazaki) results in an unreasonably high solubility. Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Continue Permeation Assuming the total exposure time to be 4.8x10 6 seconds, and not considering diffusion to occur during the time between shots, the calculated permeation is 0 !!!. This is due to the failure to break through the 1 cm of beryllium followed by 1 cm of copper. If we assume the 100 Pa DT pressure to be there steady-state for the entire 10 years, the calculated permeation for this long exposure time is 3x10 14 T/cm 2. When multiplied by the area, the total permeation is only calculated to be 170 curies !!! Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Continue Permeation On an earlier slide, I mentioned that dividing the Kizu permeability by the diffusivity gives an unreasonably high solubility. At a few hundred degrees, the solubility was calculated to be over 1000 appm. If the solubility was this high, the porous structure in the implant zone would never occur. I ran one last DIFFUSE code run with the Kizu diffusivity multiplied by 100 and the solubility divided by 100 (permeability would remain the same). In this case, with the pressure held at 100 Pa for 10 years, the integrated tritium permeation is 10 grams. This number should not be taken seriously. Remember, I used the Kizu permeability which is about 2 orders of magnitude larger than the next closest values. I then had to leave the pressure on continuously. JET has shown that the tritium oozes back out of the beryllium porous layer in about 10 minutes after the run is over. Then, I had to increase the measured diffusivity by 100 to get early breakthrough. Not realistic!! Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Permeation Conclusion Only by assuming unreasonable conditions is it possible to calculate problematic tritium permeation through the ITER beryllium first wall. Even if there was a fast diffusion path (grain boundary diffusion), the copper substrate would mitigate the low concentration of rapidly moving tritium with its own low solubility. There should be no need for a tritium cleanup system for the first wall cooling system! Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum

Conclusions Tritium retention in beryllium is low Tritium permeation through beryllium is low Tritium is bred in the beryllium itself, but it is trapped and difficult to get out If available oxygen is low, codeposition of beryllium with tritium is relatively low Beryllium must be a good fusion reactor material (at least until neutron flux becomes high in DEMO and beyond Sandia National Laboratories Materials & Engineering Sciences Center Atoms to Continuum