Questions about fusion safety, waste

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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KitemanSA
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Questions about fusion safety, waste

Postby KitemanSA » Wed Mar 23, 2011 12:27 pm


happyjack27
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Postby happyjack27 » Wed Mar 23, 2011 1:50 pm


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Postby jsbiff » Wed Mar 23, 2011 3:05 pm


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Postby kurt9 » Wed Mar 23, 2011 5:28 pm

If a fusion reactor shuts down, the shutdown will be very quick as it takes a maintained vacuum and ion sources to sustain the fusion reaction. There will be very little decay heat. The big problem with D-T fusion is the neutron flux, which is much higher than fission per amount of energy produced. This, of course, is the reason for the focus on developed advanced fuel fusion such as pB11 or He3. The other technical issue with D-T fusion is the need to breed Tritium. It seems to me that if we are stuck with D-T fusion, it make sense to make it a part of a fusion/fission hybrid reactor.

It seems to me that if advance fuel fusion is impossible, that Gen IV fission concepts such as LFTR, IFR, and traveling wave reactors are the way to go. Traveling wave reactors, fueled by waste and depleted Uranium, will give us 500 to 1000 years time to develop fusion power.

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Postby Skipjack » Wed Mar 23, 2011 5:54 pm

Kurt, what do you think about fusion- fission hybrids such as the ones proposed by Helion? I would think that they could be quite save, but I am not so sure about the decay products with this type of reactor. Do you think that they could have caused simillar problems to Fukushima?
I think that since they are able to burn nuclear waste and burn nuclear material more completely (at least to my understanding), there would probably be a lot less radioactive material on site. Of course the question is how much that would be mitigated by the fact that there would be many parts of the reactor that would be quite irradiated by fast neutrons...
The concept by Helion is at least much less complex than a Tok, so AFAIK, there are less parts that need to be replaced (and that would be easier also).

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Postby Aero » Wed Mar 23, 2011 6:33 pm

Aero

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Postby happyjack27 » Wed Mar 23, 2011 6:45 pm


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Postby D Tibbets » Wed Mar 23, 2011 7:40 pm

To error is human... and I'm very human.

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Postby D Tibbets » Wed Mar 23, 2011 8:05 pm

To error is human... and I'm very human.

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Postby Aero » Wed Mar 23, 2011 10:31 pm

Aero

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Postby chrismb » Thu Mar 24, 2011 12:09 am

biff,

the answer to your question on the comparison of dangers from fission to fusion waste is actually a question of chemistry.

When U-235 fissions, it splits into binary pairs of isotopes. These isotopes range in atomic mass from around 70 to 160. This means that in a used, depleted reactor rod, you will have every chemical element in the middle of the periodic table. This is a toxic chemistry set of near-unlimited potential, particularly if you then add in some hydrogen and oxygen from a cooling system!!

The distribution of isotope masses from U-235 fission has a distinctive double hump, with isotopes of masses ~95 and ~135 being more prevalent that other atomic masses.

We find that amongst the jungle of atomic species around 135 mass that the ones that stick out - toxic chemistry wise - are isotopes of iodine, caesium. In the ~95 masses we find strontium and technetium.

Also, you will note Xenon and Krypton fit in here, and such isotopes [due to their lack of chemistry reactivity] will simply drift off to be breathed in somewhere.

There are now two scenarios of waste risk to consider - short term and long term. For long term risks, once this stuff is locked up in a sealed cask and buried in a thick granite seam 1 mile underground, the reality is that the only risks are to political reputations.

But there are serious risks from the short term waste of spent rods, as you are currently seeing in Japan: Iodine will readily vaporise once hot and drift away, and caesium oxidises very energetically, and may also further cobine with iodine to make CsI aerosol. These are particularly dangerous because they are a) the most common emissions from 'distressed' spend rods and b) are very active in the body, with Cs displacing any K and with I being soaked up readily by the thyroid gland.

These two isotopes are quite short lived, so you'd not find them in 1000 year old spend rods, but their short life also means they are more radioactive. Isotopes with short lives means they disintegrate more readily, so are far more radioactive, per mass, that the likes of U that takes billions of years.

I would suggest [I'm not a 'nuclear heath' person, so this is a bit of speculation/suggestion of my own] there is a simple way to look at this chemical+nuclear risk; if you can protect yourself against these two insidious and high radioactive isotopes, then you are already over the worst and the others in the menu of nasties are 'relatively benign' by comparison with how readily these are taken up by the body. But if you can't protect yourself from Cs and I, then the other nasties are the least of your problems.

Now we turn to fusion, which I will close by simply saying the rather obvious statement that no-one has yet build a working reactor so we can't yet know what types of materials will be essential for its operation. But what we can say is that we won't have every chemical element in the periodic table floating around inside the reactor, unlike a fission reactor, so in that sense any radioactive waste can be 'designed for' and will arise in a controlled manner by selection of reactor materials (which will be chosen by their low rate of absorption by humans, and their low activation cross-sections). So, no nasty Cs and I

Fusion is also very difficult to do and for thermonuclear plasma devices, like tokamak, the plasma could easily be [deliberately] disrupted, killing the reaction in microseconds. For other devices, many are pulsed machines anyway [Polywell has not been operated for any longer than a few milliseconds] so it would simply be a case of not powering it for its next pulse.

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Postby hanelyp » Thu Mar 24, 2011 1:22 am

For a fusion reactor we have only a few likely fuels to choose from, most of which produce lots of neutrons. But we can choose the inner walls of the reactor for low neutron activation. Boron, Carbon, and Silicon are all decent on that front.

pfrit
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Postby pfrit » Thu Mar 24, 2011 3:26 am

The big difference between the neutrons produced in a fission reactor and a fusion reactor is where they would be absorbed. In a fuel rod the neutrons are in a dense material of heavy atoms, so most of neutrons (ok, many) will be absorbed by the surrounding fuel. Thus the chain reaction. In a fusion reactor, the neutrons would be in a vacuum and fly out of the reactor without hinderance. Thus, a higher neutron flux. And aneutronic fusion is a misnomer. The side reactions will produce many neutrons. I know this has been heavily discussed. Now I don't think that this would be a problem except in how the SC magnets are effected. We do know how to build things in a neutron flux.
As far as the magnets, quenching is a major issue and they would be a major part of the expense in building a polywell. And they will fail. The flux demands that they will. Earthquakes and tsunamis will do them in as well. And when they quench, they will explode. Pehaps not doing much damage to anything besides the magnets themselves (I kinda doubt that), but the magnets will be destroyed. Expensively. The actual fusion waste products are meaningless in terms of danger, but the waste at decommisioning could be considerable.
All sums up to expensive, but not dangerous
What is the difference between ignorance and apathy? I don't know and I don't care.

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Postby chrismb » Thu Mar 24, 2011 7:19 am


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Postby D Tibbets » Thu Mar 24, 2011 4:23 pm

To error is human... and I'm very human.


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