Fusor Forums

I was going to post this as a reply to the "Farnsworth on the Fusion Powered Future" thread, but I figured the discussion could warrant a new thread of its own. As part of a course I recently took on Materials Science in Nuclear Engineering, I did some research into the materials requirements for fusion reactors, should they become a viable technology.

The thermal, structural and radiation stresses in any sort of fusion system we can envision are so intense that there is no one "magic material" that can satisfy all three criteria. Let's assume for a moment that breakeven is possible and repeatable in the laboratory. The engineering hurdles that must be overcome to make a viable fusion power plant are enormous, just from a materials standpoint.

In particular, D-D and D-T neutron bombardment of a 304/316 stainless reactor vessel will all but turn it to powder in short order, activation issues notwithstanding. Over a 30 year period, every atom in the steel could be expected to be displaced 200-700 times! Oxide-dispersion steels show a lot of promise, but they too will still succumb to radiation effects. The down time required to repair the structure would result in a lot of lost dollars for the electric utility.

Neutron absorbers can (and must) be placed inside the chamber to mitigate the radiation effects to the vessel and its exterior, as well as to extract the fusion energy. This is the so-called Lithium blanket that often gets mentioned in the literature. The lithium can be molten, in which case it must be surrounded by a suitable plumbing system, once again presenting the issue of radiation damage.

Couple the radiation damage issues with the more conventional problems of thermal and mechanical stresses, high vacuum, plasma exposure, corrosion and the like, and you have quite a technical challenge on your hands. To even make continuous operation of a fusion reactor possible (i.e. many years), major breakthroughs in materials science will be needed (just another 50 years, right?).

I can post some of the papers we found if people are interested. They focus on ITER and magnetic fusion though, which is clearly not the focus of the fusor forum. That said, the physics of radiation damage is the same whether you're talking about magnetic fusion, fusors, ICF or what have you.

Material science is one of the minor areas that every fusor builder will have to dip into as part of the fusor technology, though certainly not to the level you just mentioned.

This area is often overlooked as folks attempt fusors built out of old steel tanks or aluminum balls, etc. It is nice to use what is at hand, but warranted success is usually found along long proven, tradiational lines with traditional materials found to be the shortest path to functionality, if not the cheapest path to be followed.

As noted, a lot of material science will be needed to support the 24-7 power effort should fusion power ever develop in the future.

What is cool about this effort, at the amateur level, is the number of different sciences and technologies one is forced to dip into to complete the effort. Limited material science is one of those areas.

Richard Hull

Brian,

Post those papers if you don't mind.

As with the 'another 50 years joke', not only do we need a gifted set of engineers to design a viable reactor and make break even but also a group of politically aware and gregarious scientists that can get the required money together. Fusion, especially ITER, is intense on so many levels, but the rewards from it would be great.

Awesome comments though. Where are you studying Nuclear Engineering at?

-Brian

While we often think of stainless steel as having a negligible neutron cross section, it is actually rather significant when you look at the high fluxes found in fission and fusion reactors. Among other things, it has a nasty property of expanding its volume significantly upon irradiation with neutrons as defects form. This is one of the many reasons why Zircalloy is chosen over SS as cladding in fuel rods in fission reactors.

A lot of the new materials being researched are cool in that their crystal and grain structure is arranged so that they suppress or "gobble up" radiation defects before they have a chance to propagate through the material. On an industrial scale, the cost would be reasonable, but there needs to be an industry first! Conveniently enough, the needs of the fission industry are almost identical, especially as we move to extending reactor lifetimes and increasing fuel burnup levels.

Not all the papers could fit due to file size limits, so I'll post the bigger paper in the files section. It was an interdepartmental course in which I worked with a materials science student on the final review paper, which I've also attached here as a word document. I just graduated from RPI with my BS in Nuclear Engineering, but I'll be staying on for PhD work.

Brian McDermott wrote:
> In particular, D-D and D-T neutron bombardment of a 304/316 stainless reactor vessel will all but turn it to powder in short order, activation issues notwithstanding. Over a 30 year period, every atom in the steel could be expected to be displaced 200-700 times!
Ah, but bear in mind that these kinds of figures (and the papers uploaded here) are true for tokamak sized kit at 10s of MW. As you make a device smaller, so the surface area to volume ratio increases so that the neutron flux per unit area would be reduced for a given specific power [to volume] output. In other words, if a reactor could be made small enough then it would not only be easier to manage the waste [maybe you could just throw the whole thing away in one go, no 'dismantling' necessary] but that the materials may not have to be so exotic.

At the moment as ITER is, a single disruption will dump so much energy onto the divertors that they can only take about a dozen such disruptions in the whole lifetime of ITER. The smaller JET has had many such disruptions but its divertors last a while because of this volume:surface area factor.

Stainless is usually presumed to be OK for 3dpa, so if you made a device 100th the size of ITER [that produces one millionth (500W) of the power] then you could run it for that 30 year life and not exceed 3dpa, by those figures.

Brian,

Thanks for posting these papers, interesting reading. As I suspected, the materials used in a commercial 24/7 rector is an issue.

Assuming that we don't discover a new amazing material tomorrow, the solution must be to have a really simple reactor design, where the core can simply be lifted out and replaced after a certain number of hours. Somehow, I don't think a "tokamak" fits that description.

Steven

There have long been many weighty books written solely on the subject of neutron damage to materials for use in reactors. I have a few older ones with lots of nasty images of what we normally consider strong stuff exposed to reactor level neutron fluxes for periods as short as 6 months that are just decimated and degraded to virtual non-functionality. Early power reactors benefited from this destructive testing just after WWII.

No one and nothing handles multi-megawatt energies without extensive materials training be it flowing water in a dam or neutrons in a reactor. It is a another world from the normal stream of conciousness that few but the engineers involved understand.

Fission reactors are not something one pulls apart to diddle with and make engineering changes or updates within the core. In many cases, once fired up, the door is shut for good. Thus, it has got to be right from the get go.

Richard Hull

With aneutronic fusion such as boron-hydrogen reactions many of the structural problems in design are resolved.

Steven,

The core of my reactor, BSFusion, can be replaced in seconds, not hours. And there are no material activation problems, none!