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How big a problem is neutron activation?
Posted: Thu Dec 01, 2011 2:14 pm
So, I know that one reason they would like to try to get p-11B fusion, if possible, instead of d-d or d-t is that the hydrogen fusion reactions release neutron. I guess the reason that's a problem is that the neutron flux will "activate" some of the atoms in the structural elements of the reactor, and turn them radioactive?
What I don't have a good feel for, is just how bad a problem is neutron activation? Are there materials that can be used in construction which will reduce the activation problem? Just how radioactive do the activated materials become? How long do they tend to stay radioactive?
I have the impression, though I might be wrong, that even with the neutron activation problem, a deuterium or tritium fusion reactor would still be much less of a potential radiation hazard than a fission reactor, and you would produce much, much less radioactive waste?
From an economics perspective, I think I heard once that neutron activation might cause a short lifetime for some parts of the reactor, meaning they might have to be replaced every 5 or 10 years or something? Even if that's the case, is that really that much of a problem? I get the impression that a lot of people believe that if polywell's work and actually have a decent net power ratio, they should be so much cheaper than anything else that you could afford to replace those parts ever 5 or 10 years, and still be an order of magnitude (or 3) cheaper than every other energy source?
Posted: Thu Dec 01, 2011 3:00 pm
The are two issues with neutrons from a longer term operating standpoint.
1.) Activation - The issue here is that the longer you run the unit, the more things become sources. This is dependant on the materials you use and there proximity to neutron flux. it will produce growing significant maintenance and disposal problems over the life of the unit.
2.) Embrittlement - another issue here. Metal lattices do not not to get shot up over time by nuetrons. It alters the structure making them more brittle. This in turn over time reduces the designed integrity of the exposed components. This is the primary driver for most core and system components established lifetimes of safe use. Again this is controlled to some degree by materials used and proxitmities to neutron flux.
Both of these issues are unique to the desgin of the unit. Given varied materials and structures, as well as flux management, it is not so simple to calculate effects in a simple and realistic manner. They are considered in the design process, then once the unit is constructed and operated, tells are watched for to see how things are going. Most of the time the desing constraints and restraints have been over done, and thus it ifs found that things are better than predicted. However, early designs of plants were the learning curves, and some of these got really hot from activation issues, as well as were much limited operationally later in life due to embrittlement concerns.
Posted: Thu Dec 01, 2011 3:26 pm
Embrittlement tends to be less of an issue with vacuum chambers as it is with a pressure vessel. Its kind of like concrete. Pretty good in compression, not so much in tension.
Oh, and catastrophic failure of the vacuum chamber is no where near the issue that catastrophic failure of a typical nuclear pressure vessel would be.
Posted: Thu Dec 01, 2011 4:00 pm
The embrittlement issue it would seem, for vacuum vessels, would be more for external support components, given Kite's well founded observation.
Posted: Thu Dec 01, 2011 8:53 pm
Problems of neutron bombardment is multifactorial.
Induced radiation can be significant. The major difference between fission and fusion reactor radioactive waste products is longevity. You need to safely store fission plant fuel rods, etc for hundreds of thousand of years. For fusion neutron induced radioactive products, the storage time may be as little as a hundred years. Selection of appropriate structural material to minimize the intensity and/ or half life of the secondary radiation will would be a major engineering consideration (just as in the Tokamak). Most of the neutron induced radiation will decay within a few days. I think I read that a moderate Polywell D-D reactor could be approached and serviced within a couple of days after shutdown (with protective clothing and limited time exposures).
Neutrons deposit their KE as heat in the absorbing material. This places limits of the minimum size that can handle this heat load (~1-2 MW per square meter) and the associated cooling requirements. Direct conversion of P-B11 or D-He3 reactors may allow for more compact units, and possibly much cheaper units as the steam plant is a major cost component of power plants, especially considering the expected conversion efficiencies.
Neutrons are also a potential enemy of superconductors. Current superconductors are damaged by neutrons and this would possibly greatly shorten their service lifetimes. Much more resistant superconductors and/ or 'aneutronic' fuels like D-He3 or P-B11 would help to solve this problem.
Aside from neutrons ( which is perhaps ~ 1/2 the dose (you need to consider not only the flux, but also the much more energetic neutrons from the D-T reaction) with a D-D Polywell vs a D-T Tokamak at the same power output levels; there is the issue of tritium production. In a Tokamak extreme efforts are required to generate and process tritium fuel. In a D-D reactor, the tritium is a waste product which could have several possible fates, some of which could be profitable- such as Beta batteries, or recycling into the D-D reactor to boost the total fusion output. The He3 could also be used for this. Any reactor that can burn D-D (excludes the Tokamak which is limited to a starting fuel of D-T) can also utilize D-T or possibly D-He3 reactions. Bussard accounted for this in a power optimized D-D reactor. This may be especially useful if the Q of the D-D burning is marginal, the resultant 'free' tritium burns much easier and could push the system to profitable levels.
Finally, the neutrons are not necessarily all bad. Aside from the heat generated directly, Bussard also mentioned that Boron 10 in the wall of the reactor could efficiently capture neutrons and produce lithium and an alpha particle with the resultant generation of additional energy.
Other neutron absorbing reactions could possibly do this also (both endothermic and exothermic reactions may occur) but on longer time frames than the B10 reaction.
Unlike fission reactors, this secondary radiation/ heat production is probably of a smaller magnitude, and decays much more rapidly, so that melt downs due to coolant failure is impossible, or at least extremely more limited and manageable.