This topic replicated from "News".
jsbiff wrote:Hello all.
In light of the recent events in Japan, I wanted to double check my understanding of fusion power safety characteristics.
This has been my understanding of fusion vs. fission, thus far:
In a fission reactor, you typically have a large fuel supply deposited into the reactor, once every 1-5 years (except for naval reactors which use higher-enriched uranium, and can go something like 10 years on a fuel load). This amounts to tons of material - most of which is not radioactive, but about 1-3 percent at any given time will be radioactive, and very highly radioactive immediately after reactor shutdown. Decay heat continues to generate a small fraction of the power of the reactor during regular operation, but since the reactor generates 2-5 Gigawatts of thermal energy during normal operation, that 'small fraction' is still over a Megawatt for a few weeks or month.
Additionally, the nature of the fuel is that it has a natural tendency towards fissioning on it's own - it doesn't need any active, outside forces, it simply needs enough fuel, in the right physical configuration, with moderator (like water) between it, then it will just spontaneously start fissioning (I believe to "start" a reactor they just withdraw the control rods, and the process will self-start?). Finally, the fuel has radioactive fission products which will continue to be significantly radioactive for several years, followed by weakly radioactive for centuries (a so called "long tail", e.g. asymptotically approaching zero).
Now, a fusion reactor, on the other, will generally be 'fed' fuel on a just-in-time basis, much like an internal combustion engine, which 'sips' gas or diesel as it operates. So, the amount of fuel in the reactor at any time is tiny. Fusion fuel doesn't even want to fusion - to achieve the fusion, you have to actively apply external forces to the nuclei to bring them together so violently that they have a good chance of fusioning.
The general approach to do this is by super heating the fuel into a plasma, either using a magnetic field (e.g. tokamak), or by using magnetic fields to trap a lot of electrons into a virtual (cathode? or is it Anode? Never can keep them straight), that is, a small, highly negatively charged electric field in space, then inject the fuel nucleii into the reactor (polywell), such that the strong negative field powefully attracts the nucleii towards the center of the node, super-heating the nucleii into a hot plasma, where the fusion can, hopefully, happen.
Questions: Are any of the fusion products of the proposed fuel mixes, radioactive. If they are radioactive, are they strong emitters, or weak emitters (e.g. Tritium is a weak emitter). What kind of half-lives do they have? I believe that, essentially, even if a fusion reactor exploded, there'd be very close to zero radiation released into the environment, in every possible case, because the amount of radioactive materials would be very small?
I believe I read somewhere that neutrons from the fusion reaction could be absorbed by some other materials, and make those other materials radioactive, but I think such 'activated' materials would be a fairly weak source of radioactivity?
If a disaster hits a polywell reactor, as an example, what would likely be the consequences? I believe that because fusion requires an active, external force to be applied, you can instantly (e.g. within microseconds or milliseconds) shut off the magnetic fields and the plasma will immediately dissipate? My understanding is, the plasma, during operation, is millions of degrees - does that pose any risk? My understanding is that there is such a tiny amount of plasma in the reactor at a time, that as it begins to interact with nearby matter (any air which might be in the reactor, reactor walls, etc), it will extremely quickly (within seconds) cool down to 'ambient' temperature, perhaps leaving the reactor at a temperature of a couple hundred degrees or less (because the mass of the reactor is hundreds of thousands or millions of times greater than the mass of the plasma)?
I believe once the fusion shuts down, there's no significant amount of decay heat, so that the reactor will cool off within like an hour or two, to 'room temperature'?
In a reactor which uses hydrogen fuel, there might be a small risk of hydrogen explosions if fuel lines/tanks got damages, and hydrogen started leaking, but it would be plain, old, 'boring' combustion which would only be a threat to the buildings and people on the plant premises?
pfrit wrote:First, this should be in general, not news.
Second, Neutron activation is no joke. It can make some really nasty stuff. Granted that this happens in a fission reactor as well, though my understanding is that it not as severe in fission as the neutron flux is not as intense. Also since the neutron flux would be higher, the need to build everything out of neutron safe materials would greater.
Assuming we are talking about a system that uses SC magnets, when one of these thing pop, they will pop awfully hard. Probably without breaching containment, but it would explosively destroy itself. I would expect no damage outside the reator building, but it would be truely trashed and this would be expected to be much more likely than a fission incident. However since the flux would be higher, the operators would be very well protected from any accident as the main neutron sheilding method (distance) would protect them from any explosion.
So, to sum up, much more likely to fail and expensive failure modes, but very low chance of harming anyone and move this to general.
shipjack wrote:I think it depends on the type of fusion reactor.
There will be much fewer fast neutrons in a PB11 burning reactor, so also less heavily irradiated reactor parts.
A reactor like the Dens Plasma Focus (if it works) would also not have any magnets that can explode.
I am wondering how much better or worse a Fusion- Fission- Hybrid like the guys at Helion are proposing, would have worked.
This thing is actually burning nuclear waste...
Oh and pfrit, compared to all the political crap that we have had in "news", I think that this is tollerable.
and in reply
Please continue from here.
jsbiff wrote:I don't think I can move the post. I thought about putting this in general, but since it's prompted by the news from Fukushima, I then thought it might be better in here. Basically, I wasn't really sure where to put this, and news seemed like as good a place as any.
Back to the question at hand, I just had a thought - with 'normal' electromagnets, you shut off power to them and their magnetic fields decay extremely quickly (to human perception, instantly).
Super Conductors, I believe, get a loop going, and because there is no resistance, can basically act a lot like 'permanent' magnets. I guess the 'shutoff mechanism' in that case is just to stop injecting fuel into the reactor, and it will 'burn itself out' very quickly? How quickly would it shutdown once you stop adding fuel - I guess a second or two?
Properly cite the original writer in any quotes.