Researchers discover the cause of irradiation-induced instability in materials surfaces.
"Our discovery overturns a long-held paradigm about what causes surfaces to erupt into patterns under energetic particle bombardment. The blasting away of individual atoms from energetic particle impacts has long been thought to determine whether a surface is stable or unstable," says Aziz.
"The effect of atoms blasted away turns out to be so small that it is essentially irrelevant. The lion's share of the responsibility of what makes a surface stable or unstable under irradiation comes from the cumulative effect of the much more numerous atoms that are just knocked to a different place but not blasted away."
Spalling/ sputtering may not be the dominate method of material degradation, but it is extreamly important to the conditions within the vacuum vessel. Any one doing vacuum deposited coatings work knows this. And depsoiting thin metal coatings on structures can lead to insulation loss,etc.
And changing the structural properties due to applied force is obvious to anyone working with metals, plastics etc- things like case hardening, crack propagation, tempering, hydrogen embritlement, etc.
If this has any significance, it would be in modifying materials to resist this structural change rather than considering only the erosion of surfaces. This seems to be a consideration that has been generally know for centuries.
The research does not objected that, nor enters into the issue of surface erosion.
What they are stating is that the mechanical degradation of the material is not depending mainly from the atoms removed from the surface but on the change of the crystalline structure of the metal due to relocation of the atoms that undergo the impacts.
"The effect of atoms blasted away turns out to be so small that it is essentially irrelevant. The lion's share of the responsibility of what makes a surface stable or unstable under irradiation comes from the cumulative effect of the much more numerous atoms that are just knocked to a different place but not blasted away."
The team found that the cumulative effect of these displacements can be either ultra-smoothening, which may be useful for the surface treatment of surgical tools, or topographic pattern-forming instabilities, which can degrade materials. The outcome depends on the type of material, energetic particle, and irradiation conditions.
Giorgio wrote:What they are stating is that the mechanical degradation of the material is not depending mainly from the atoms removed from the surface but on the change of the crystalline structure of the metal due to relocation of the atoms that undergo the impacts.
This is already very well understood. Not sure what is new here.... do an internet search on something like "dpa neutrons" or "dpa damage neutrons" [dpa = displacements per atom]
Different materials degrade at different dpa rates. Mostly well-mapped out by now, but still worth searching for the best materials (this being a big area of the ITER research).
Giorgio wrote:What they are stating is that the mechanical degradation of the material is not depending mainly from the atoms removed from the surface but on the change of the crystalline structure of the metal due to relocation of the atoms that undergo the impacts.
This is already very well understood. Not sure what is new here.... do an internet search on something like "dpa neutrons" or "dpa damage neutrons" [dpa = displacements per atom]
Different materials degrade at different dpa rates. Mostly well-mapped out by now, but still worth searching for the best materials (this being a big area of the ITER research).
My knowledge was that DPA was used as a measurement unit of surface degradation, and it was the surface degradation that was linked to the mechanical degradation of the material subjected to irradiation.
Giorgio wrote:My knowledge was that DPA was used as a measurement unit of surface degradation, and it was the surface degradation that was linked to the mechanical degradation of the material subjected to irradiation.
I will give a look into it.
Neutron-induced dpa is very much throughout the material. Neutrons penetrate matter easily - no ionsiation nor Colulomb scattering, y'see. They only get slowed by bouncing off of atomic nucleii and direct kinetic collision losses.
By shifting atoms out of the crystal structures of the metals by whacking into them, you get an assortment of effects. Materials also embrittle not only because of this destruction to the material structure being compromised but also the neutrons remain within the metal, decay to protons and so then you have hydrogen embrittlement deep within the material. Dealing with this is a Big Issue.
For ITER the materials have been specified to a limit of less than 3 dpa which is the conservative value for SS survivability I believe. This limits the total number of neutrons that it can emit in its lifetime, as it will be for any fusion reactor.
B.3. Means to Enhance Radiation Resistance
13. Given the origins and effects of radiation damage as outlined above, a major focus of current
research is on finding ways to stabilize the displaced atoms, vacancies, and lattice distortions . This can
be done, for example, by creating features in the material such as grain boundaries or other vacancy
sinks to capture and hold migrating radiation defects. Engineering and nucleonic considerations
largely determine the major elemental constituents and phase composition of core structural materials,
but there remains scope for the adjustment of the micro- and nanostructures of the material to improve
radiation resistance. These options include items such as grain size and orientation, trace elements,
introduced sinks, and dispersed strengthening precipitates.
What I understood from this new research is that to Enhance the radiation resistance we should switch the focus from recapturing/stabilize the dislocated cascade atoms to preventing them to relocate at all in the lattice structure, because relocation (and not dislocation) is what makes the surface less stable under irradiation.
chrismb wrote:For ITER the materials have been specified to a limit of less than 3 dpa which is the conservative value for SS survivability I believe. This limits the total number of neutrons that it can emit in its lifetime, as it will be for any fusion reactor.
I knew the following amounts:
Iter 2-4 DPA
Demo 50-80 DPA
Power 100-150 DPA
It will be fun to see what type of material they will come out with for the Power reactor. That is IF they will ever reach that stage...
Would glasses- noncrytalized metals have an advantage?
And why would a Operational Tokamak need so much more neutron tolorance than typical fission reactor? It might be ~ 10-30X more powerful, but that equivalent neutron flux is spread over a much larger surface area than in a PWR.
Is it because most of the neutrons are trapped in the fuel rods of the fission reactor (most neutrons are consumed in propagating the fission reactions)?
Is it due to the energy of the D-T fusion neutron?
How would this compare to the D-D fusion neutron effects in a more power dense Polywell?
And finally, if stainless steel has a DPA of ~ 2, what is the upper limit of any currently available material?
D Tibbets wrote:Would glasses- noncrytalized metals have an advantage?
Good question.
D Tibbets wrote:And why would a Operational Tokamak need so much more neutron tolorance than typical fission reactor? It might be ~ 10-30X more powerful, but that equivalent neutron flux is spread over a much larger surface area than in a PWR.
Average Neutron energy in fission is around 2 MeV.
In D-T Fusion Neutrons will have an energy level of 14,1 MeV. Hence the need for a bigger tolerance to Neutron.
Also, all the atomic elements that can become highly activated under those energy levels cannot be used in the first wall (even as alloying materials). This means that the First Wall must be free from elements like Nb Mo Cu Al Ni Co Si. I believe they will have some hard time to find a good alloy.
D Tibbets wrote:How would this compare to the D-D fusion neutron effects in a more power dense Polywell?
I have no idea, I never thought about a D-D polywell.
D Tibbets wrote:And finally, if stainless steel has a DPA of ~ 2, what is the upper limit of any currently available material?
I have seen a report of DPA limits in Tungsten of up to 80 DPA. If you are interested I can go back to dig into my links and see if I can find it. I do not remember what the testing conditions was right now.
The other major consideration for materials is activation. If you pick the wrong stuff, it may last longer from a structural standpoint, but it gets way hotter from an activation standpoint.