Neutron Capture in Hydrogen Fusion Reactor

[trainman2001]

A recent issue of Aviation Week described a novel design for a fusion reactor from the Lockheed Skunk Works. It was smaller, simpler and lighter than the extant systems now being built and they're optimistic about it's test run in five years. I wrote a letter to the editor touting this departure and related it to other major breakthroughs in the last 100 years that all came from industries other than those traditionally tasked with them. This prompted an editorial in the magazine from the Holland brothers, two men who've spent long careers in the conventional nuclear power industry, who basically said that commercial fusion isn't ever going to happen because of two factors: the creation of large masses of highly radioactive waste in the form of the steel vacuum vessels, and the destructive flaking of the inner lining. The cause of the former was the highly energetic neutrons that penetrate deeply into the steel structure, and the latter from the alpha particles changing the crystal structure of the lining leading to failure. I don't deny that these two effects present onerous engineering challenges.

That being said, I wrote another letter to the editor yesterday saying that neither of these men has the vision necessary to solve the problem and there will be solutions generated, but probably not from the nuclear fission power industry.

My question (took me a while to get to this point) is this. What other materials or systems could capture energetic neutrons and convert that energy directly to electric current without going through the traditional steam/turbine capture scheme? Would there, could there be some form of solid state material that could use that energy to change the energy state of electrons, or transmute materials in some way to absorb the energy and transform it to useful form. Could the steel alloy be doped with an element or isotope that when irradiated would become stable? Or... are these guys right and neutron/alpha capture always will lead to destruction and more nuclear waaste? I am not a physicist, but I am also not without hope.

 

[mfb]

Neutrons will always damage solid materials - they can scatter at nuclei and displace them from their original position.
Liquids don't have this type of damage, but they are not solid... and you still get nuclear reactions that can induce some radioactivity (there is no material that has absolutely no way unstable isotopes can get produced).

As neutrons are not charged, there is no direct way to use them for electricity production. They will always release a large part of their energy as thermal energy. There are reactions where neutrons can lead to the emission of charged particles, but using those directly for electricity production is quite unrealistic. D-T-fusion also needs neutrons to breed new tritium.

Could the steel alloy be doped with an element or isotope that when irradiated would become stable?

It is easy to allow 90% or even 95% of the atoms to capture some neutron to become another stable (or very short-living) isotope. The remaining few percent are the problem, and steel alloys always have various different elements, some less problematic, some more.
You cannot avoid radioactive waste - but you can try to keep its amount, radioactivity and lifetime as small as possible.

 

[Astronuc]

That being said, I wrote another letter to the editor yesterday saying that neither of these men has the vision necessary to solve the problem and there will be solutions generated, but probably not from the nuclear fission power industry.

That's a bold statement. The statement by the Holland brothers reflects the challenges (potential roadblocks) to commercial nuclear fusion, and are well known. I suspect they are familiar with nuclear fusion technology as well as conventional fission technology.

My question (took me a while to get to this point) is this. What other materials or systems could capture energetic neutrons and convert that energy directly to electric current without going through the traditional steam/turbine capture scheme? Would there, could there be some form of solid state material that could use that energy to change the energy state of electrons, or transmute materials in some way to absorb the energy and transform it to useful form. Could the steel alloy be doped with an element or isotope that when irradiated would become stable? Or... are these guys right and neutron/alpha capture always will lead to destruction and more nuclear waaste? I am not a physicist, but I am also not without hope.

Fast neutrons (up to 14.1 MeV) will exit the fusion plasma. They will react with the first wall causing radiation damage as mfb mentioned, but at high temperature, annealing will resolve some of the damage. On the other hand, the alloying elements will become activated/transmuted (n,γ) to radionuclides, which will decay into higher Z elements. In addition, a number of nuclides will experience spallation reations (n, α), which is one source of He-4 in the structural materials. Other sources of He are neutral He-4 and He-3, and energetic tritons (t) that leak through the magnetic confinement. The tritons form metal hydrides and the T decays to He-3, and He-3 can absorb neutrons to become He-4.

Eventually He-bubbles can form and this leads to blistering of the metal surface of the first wall.

One can try to stop neutrons with light elements, e.g., hydrogen, or capture them in Li, such that Li-6 can form T and He, so the T is available as fuel. Ideally though, one would like to have an aneutronic reaction, but that has its own challenges.

mfb covered the other matters quite well.

 

[yaakov]

Xenon 131 I believe poisons fission reactions...via neutron capture? Maybe a boundry layer of this gas may inhibit damage to tokomak inner wall metals...

 

[Astronuc]

Xenon 131 I believe poisons fission reactions...via neutron capture? Maybe a boundry layer of this gas may inhibit damage to tokomak inner wall metals...

Xe-135 is a reactor poison due to a high thermal neutron cross-section. It's not much of a poison for fast neutrons, and it won't work in a fast flux or in a vacuum. Xe would be strong poison in plasma with a high number of electrons and strong brehmsstrahlung.

 

[DEvens]

Fusion research is exploring a variety of notions. As yet there certainly has not been a complete solution demonstrated to the issue of neutrons damaging the wall. This is related to the issue of getting the energy out and converting it to a useful form. Both Tokamaks and laser fusion, even if they could operate efficiently, have issues with getting the energy into the form of electricity. At this time most fusion research labs are not happy with their suggestions for how to do that. But they are working at the plasma side of the problem. It does not matter what you will do with the energy if you do not have the energy.

The usual thoughts are things like Lead blankets and similar ideas. You put Lithium in the Lead to breed more Tritium.

There are at least two approaches that are coming "from left field" so to speak.

One is to use Boron instead of D-T. There is a Boron reaction that produces energy but no neutron. This has its own set of big challenges. Using the higher Z makes achieving plasma very much harder. It is not clear that this approach will work, but if it did it would have a lot of attractive benefits.

Another interesting approach is to use liquid Lead as the wall. The idea is to spin molten lead to open a vortex, and operate inside that vortex. The lead will then accept the high energy neutrons, and slow down most of them. The Lead has no structure to break down, and does not produce much in the way of radioactive isotopes. The folks at http://www.generalfusion.com/ have a small prototype for working on this design. Again, there are some major engineering challenges between where they are and making it work.

 

[yaakov]

Thanx...maybe a way to protect interior surface of tokomak. Some type of disposable boundry layer of gas or film...similar to TIG welding process with argon etc. Shalom

 

[yaakov]

What is the new ITER using?

 

[Astronuc]

Thanx...maybe a way to protect interior surface of tokomak. Some type of disposable boundry layer of gas or film...similar to TIG welding process with argon etc. Shalom

No gas! The neutral gas atoms would find their way into the plasma. Liquid Li would be more effective, but even a moderate layer would not provide much protection to a fast neutron flux. The first wall must interact with the fast neutrons without much spallation or sputtering.

 

[yaakov]

Beryllium xenon hydride...semi solid ...with lithium and graphite as a composite multilayer protective surface. I am sure ITER developers looked into any every possible solution. True xenon gas would migrate into neutron flux..unless confined in solid as micro scopic bubbles within a solid .

 

[mheslep]

You cannot avoid radioactive waste ...

Well not with current technology (p-B11)

 

[mheslep]

... the steel vacuum vessels, ...the steel structure, ... the steel alloy

A steel wall is not necessarily required for a fusion reactor, either magnetic confinement or implosion. One reason to use steel in fission reactors is, for example in PWRs, to sustain the high pressure (~150 atm).

Also see the decades old, "The Trouble with Fusion" essay that covers some of these issues.

 

[mheslep]

BTW, George Miley et al have released a paper in 2009 where they find that aneutronic proton-Boron 11 fusion has a significantly better chance given some of the laser implosion developments coming out of NIF, in particular fast-ignition techniques. Previously the p-B11 has been consider far more difficult than DT fusion. Miley et al contend that the reach is not so far with fast ignition.

The advent of ultra-high power lasers allows laser power levels that are about 1000 times the power of all the power stations in the USA. This opens the way to new approaches for inertial confinement fusions (ICF) that in turn can drastically reduce the laser input energy needed to achieve practical ICF power. The specific approach discussed here involves inducing a fusion burn wave by laser-driven impact of a relatively large block of plasma on the outside of a solid density fusion target. This new method is specifically selected to enable the extremely attractive, but demanding, neutron-free proton–B-11 fusion that potentially can lead to the long sought goal of an ultra “clean” fusion power plant

Paper seems to be behind pay wall now. Intro here: http://nextbigfuture.com/2011/07/fusion-energy-without-radioactivity.html. Paper has been cited a couple dozen times now.

The possibility of direct conversion of alphas to electric power also could improve efficiency by several multiples of that of a heat engine.

 

[e.bar.goum]

What is the new ITER using?

I recently was at a talk by Prof. Steven Cowley (from ITER), and he highlighted that materials issues are a really key challenge. We really don't know what 14 MeV neutrons will do.

He mentioned that in a full scale fusion reactor, each and every atom in the wall will be displaced from it's equilbrium position 20 times each year. That's a ginormous materials challenge! Apparently, we just don't know how the steel will hold up.

It's interesting. It seems that we're starting to get a pretty good handle on fusion instabilities, and the next thing is to figure out the materials science side of things.

 

[Astronuc]

I recently was at a talk by Prof. Steven Cowley (from ITER), and he highlighted that materials issues are a really key challenge. We really don't know what 14 MeV neutrons will do.

He mentioned that in a full scale fusion reactor, each and every atom in the wall will be displaced from it's equilbrium position 20 times each year. That's a ginormous materials challenge! Apparently, we just don't know how the steel will hold up.

It's interesting. It seems that we're starting to get a pretty good handle on fusion instabilities, and the next thing is to figure out the materials science side of things.

There's been ongoing materials development programs in US, Europe and Asia for fast and fusion reactors. Basically, as Cowley indicated, lots of atomic displacements (dpa) and activation (hence need for low activation, high strength materials), and for fusion systems, permeation of d & t in the structural systems.

 

[e.bar.goum]

I doubt if carbon steel is even a candidate for first wall material.

Indeed:

Because of its unique physical properties, beryllium has been chosen as the element to cover the first wall. The rest of the blanket shield will be made of high-strength copper and stainless steel.

https://www.iter.org/mach/blanket

And yet, the steel is of concern, apparently. I guess you don't get 100% stopping of 14.1 MeV n in the Be layer.

ETA: The idea is to eventually have Li there for tritium breeding. Which brings up another problem: For each tritium you use, you only get one neutron. The efficiency for t production isn't going to be good enough, surely?

 

[Hologram0110]

ETA: The idea is to eventually have Li there for tritium breeding. Which brings up another problem: For each tritium you use, you only get one neutron. The efficiency for t production isn't going to be good enough, surely?

This is a common concern. The usual answer is that neutrons are also produced from neutron multiplication reactions like (n,2n) or (n,3n) and some other reactions like (gamma,n). When considering these reactions it is possible to breed sufficient tritium.

Other options are synergies with fission reactors. For example, fast neutrons from a fusion reactor can induce fission in depleted uranium creating significantly more neutrons (depending on how close it is to critical). Since the fission component doesn't need to be critical it is possible to get much higher burn up and greatly reduce waste. Tritium is also produced in heavy water reactors as a by product (although in much smaller amounts).

 

[mheslep]

Tritium production and the associated neutron accounting have been discussed in an interesting and spirited thread previously on PF. In particular, see here: https://www.physicsforums.com/threads/iter-useless.422576/page-2

 

[yaakov]

Awesome...I am just an engineer not a physicist...I had worked for DOE at BNL on long island on AGS and RHIC targets and witnessed degradation of various metals from high radiation events.I am very excited anticipating ITER s start up...I feel it is one of the most important projects in history of mankind to succeed...Shalom