Permeation Barriers for Fusion Reactor Blankets

1. Materials & Engineering Sciences Center Atoms to Continuum Permeation Barriers for Fusion Reactor Blankets Rion Causey Departments 8758 Hydrogen and Metallurgical Science Sandia National Laboratories Livermore, CA 94550 Idaho Falls, Idaho May 11, 2007

2. Materials & Engineering Sciences Center Atoms to Continuum Steady state permeation of hydrogen through materials depends on three different parameters Diffusivity – rate of transport through the material Solubility – concentration of hydrogen in the material (depends on pressure) Dissociation rate of hydrogen into the material (surface limited ingress) Diffusivity and solubility are fundamental properties of the bulk material and cannot be changed

3. Materials & Engineering Sciences Center Atoms to Continuum Lets look at diffusivities for several materials From a diffusivity point of view, aluminum and copper do not look good as a barrier but, lets not make any choices before we look at the solubilities as well.

4. Materials & Engineering Sciences Center Atoms to Continuum And, lets look at solubilities for several materials Tungsten, gold, alumina, and aluminum look good. SiC looks very bad. That is why you must really look at the product of diffusivity times solubility. SiC is still as good as it gets. Beryllium looks bad, but that is only because people don’t know how to measure solubility.

5. Materials & Engineering Sciences Center Atoms to Continuum One common barrier materials is missing (aluminide) What is aluminide? It is usually designated as MAlX where M signifies a metal and X is typically between 1 and 3. It is not uncommon for M to be stainless steel. Various techniques are available getting the aluminum onto and into the metal, but usually high temperatures are used to drive the aluminum in. The result is a film several microns thick that is an alloy of metal with aluminum. It is my personal opinion that any barrier capability that aluminide has is due to a very thin layer of Al2O3 on the outer surface.

6. Materials & Engineering Sciences Center Atoms to Continuum Lets look at some results for aluminide as a barrier According to Hollenberg et al.*, an aluminide coating can give you a Permeation Reduction Factor (PRF) as great as 10,000. This is the rate of permeation through a bare metal to that of a coated metal. There are many other references stating an ex-reactor PRF of 10,000 or greater. Hollenberg goes on to show European and US in-core values for the PRF to drop down to < 80 and 150. As far as the classified program that I work on, I can say that the PRF is in the double digits. * G.W. Hollenberg et al., Fus. Eng. and Design 28 (1995) 190.

7. Materials & Engineering Sciences Center Atoms to Continuum What is going on with this aluminide? First of all, why was the data for diffusivity and solubility in this material not shown? Because no one has ever measured them. Why? Perhaps only because it is difficult. I think if they did measure the properties, they would determine a diffusivity and solubility that would give you a permeation coefficient (SxD) that would provide a PRF much less than 10,000. I would guess the value to be in the double digit range. Why do I say this? I think the low permeability for an aluminide coating comes from a very low dissociation coefficient.

8. Materials & Engineering Sciences Center Atoms to Continuum Dissociation coefficient?? For hydrogen to enter a material (other than glass), it must dissociate into two hydrogen atoms. Unless the temperature is sufficient to break the bond by itself, there must be an active site on the surface to participate in the dissociation process. A well annealed Al2O3 layer has no such sites. It is my personal opinion that the radiation in reactors and fusion reactor blankets helps overcome the dissociation barrier either by breaking the hydrogen-hydrogen bond or by producing a defect on the surface layer that provides an active site.

9. Materials & Engineering Sciences Center Atoms to Continuum Where should we look for barriers that perform well in a radiation field? • It is not sufficient to simply perform permeation measurements in the lab. That has gotten us nowhere. • Measure the diffusivity and solubility, and look at the product. Diffusivity and solubility are fundamental properties that are not strongly affected by radiation.

10. Materials & Engineering Sciences Center Atoms to Continuum What materials look very promising? Carbon – Theoretically dense carbon films work very well. The hydrogen solubility is effectively zero. When films such as pyrolytic carbon are used, there is high temperature migration along grain boundaries, but temperatures as high as 1000 C are required for the migration. Silicon carbide – Same as carbon above. Silicon carbide has been shown to be an excellent barrier even in a radiation field (New Production Reactor). Beryllium – I contend the solubility of hydrogen in beryllium is also zero. Solubility measurements such as those by Swansiger have been fooled by hydrogen retention in the oxide. Permeation measurements need to be performed to examine the contribution of grain boundary diffusion.

11. Materials & Engineering Sciences Center Atoms to Continuum Conclusions and Recommendations • Don’t rely on ex-reactor permeation testing. Measure the diffusivity and the solubility. • Look for ways to bond known barriers (C and SiC) to surfaces. • Lets give beryllium (and other HCP materials?) another look. • If barriers have not been developed, don’t put a blanket module on ITER. Leaky blankets will kill fusion energy.