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MIT Designs Less Expensive Fusion Reactor That Boosts Power Tenfold
jan_jes writes: Advances in magnet technology have enabled researchers at MIT to propose a new design for a practical compact tokamak (donut-shaped) fusion reactor. The stronger magnetic field makes it possible to produce the required magnetic confinement of the superhot plasma — that is, the working material of a fusion reaction — but in a much smaller device than those previously envisioned (abstract). The reduction in size, in turn, makes the whole system less expensive and faster to build, and also allows for some ingenious new features in the power plant design.
From T(first)FA: the major radius is 3.3 m and the minor radius is 1.1 m.
If it weren't for deadlines, nothing would be late.
No, don't "see fukushima".
With fission, the challenge is stopping the reaction from running away. With fusion, the challenge is keeping it going. If you suddenly lose containment, what happens is that the hot plasma burns into the walls of the reactor, damaging them. Annnd.... that's it. There's a small amount of tritium there, but it's not a great amount, and tritium isn't that hazardous of a material compared to most radioactive elements. There's some induced radioactivity in the reactor, but it's quite limited because you can choose what to make the reactor out of (and iron's not all that bad for induced radioactivity anyway, it's generally the heavy stuff that's problematic). The lithium blanket is harmless (except for, again, breeding tritium - which is constantly removed). There's beryllium in there, but it's not dangerous when not in gas or dust form. Some work had looked into using lead as a neutron multiplier, which could have indirect breed polonium or other problematic compounds, but beryllium works a lot better than lead.
I'll never forget the last thing grandma said to me before she died: "What are you doing in here with that knife?!?"
TFA makes no mention of what happens if you stop supplying the energy required to confine the plasma.
Getting the right conditions for more-out-than-in fusion is REALLY HARD. So far it's pretty much only been done momentarily - using atomic fission bombs as working parts to apply enough heat and pressure.
So when there is ANY problem in the confinement, the fusion stops.
You're left with the energy in your plasma - several camera photoflashes' worth - and your superconducting magnet - which probably is unharmed and still running.
If the magnet is not properly quenched, at most it's got the energy of a large electrical fire or small bomb - on the rough order of a few hand grenades or laptop battery fires. This might be enough to throw around the small amount of low-level-radioactive material created by months or years of neutron bombardment of the reaction chamber walls and the like.
This is not in the same ballpark - by many orders of magnitude - as the few tons of molten, activated, coreium you'd get from an old-tech fission plant meltdown (all set to become an UNcontrolled, UNcooled, operating reactor if it manages to be puddled into a compact volume), or the fuel assemblies full of recent fission products still putting out, for months, heat enough to melt, ignite, or partially vaporize themselves if the coolant level drops enough to uncover them.
It's the difference between Fukushima or Chernobyl and, at most, a transformer fire in a warehouse with a substantial number of ionization smoke detectors installed.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
DAFUQ, solar is starting to get below COAL in cost and it's rapidly getting to Natural Gas territory. The cost is dropping faster than anyone expected now India and China are building plants as fast as possible. Oh and storage? It's being implimented, RIGHT NOW at costs comparable to coal. Why do you think thermal coal prices have dropped through the floor and existing coal mines are beginning to shut down?
Your opinion on renewables is from about 5 years ago.
You do understand that Chernobyl used a flammable material for the neutron moderator and poring water onto the plant, where necessary, caused a significant amount of radiation to become airborne, even after the steam explosion blew it apart. What eventually brought the situation under control was the partial burying of the core in lead and sand to reduce the radiation so a makeshift containment building could be hastily assembled over the blown apart reactor.
Also, the problem with Chernobyl was more about the lack of safety engineered into the system, than a fault of Nuclear power persay. In Soviet Russia times the imperative was to generate power cheaply, and NOW. They literally built a house of cards, with inadequate safety, cut corners on all kinds of safety systems, and had complex interactions between seemingly unrelated systems. Then they skimped on operator training and safety standards. It's no wonder that this reactor design didn't blow up more often. It truly was an accident waiting to happen.
Modern reactors can be designed to be fail safe. One design I saw claimed that you could literally walk away from it running at full power and it was both thermally and physically safe. It would insert the control rods if it got too hot and there was nothing that could stop it. At that point, even a total loss of coolant pumps would not result in a melt down as a number of plugs would melt, flooding the area around the containment vessel and allow the conduction/convection cooling of the core. Even then, if the core continued to heat, it would release the fuel assemblies which would fall into deep pools of reserved cooling water and end up far apart in the bottom of the containment building. All this didn't require ANY operator input, or power to accomplish, it was totally mechanical and automatic and only required the reactor containment system to remain in tact and right side up.
There are a number of very safe and practical designs for nuclear power today, it's just impossible to get a permit to actually build one because the environmentalists won't let that happen..
"File to fit, pound to insert, paint to match" - Aircraft Maintenance 101
Tokamaks work. Their flaw is that they are energy-negative.
Contrats, of all of the many thousands of radioactive isotopes created by man or nature, you picked the one with the 32nd longest known half life. Try compared to nuclides in general.
There's a balance in terms of half life. The shorter the half life, the more intense the radiation - but the shorter you have to deal with the problem. The longer the half life, the less intense the radiation, but the longer you have to deal with the problem. The only way around this is a product that has a very low energy in its radioactive decay. And indeed, that's just what tritium is .
Tritium's decay energy is only 18.591 keV, which is tiny by the standards of radioactive decay - by comparison, U235's decay energy is 4678 keV - 251 times more intense. Furthermore, alpha radiation, while harmless outside the body (like tritium's ultra-weak beta), is (unlike beta) terrible inside it - its biological effectiveness is 20x that of beta. Hence a decay from a atom of U235 inside of you is 5032 times more damaging than a 18.591keV electron (beta). On top of this, you have biological half lives. Uranium's is only slightly longer than tritium's, 15 days instead of 12. But, again, U235 is not normally a problematic radioisotope. 239Pu, 90Sr, 226Ra, 45Ca, etc have biological half lives so long that they're effectively with you until they decay or you die. Oh, and on top of all of this? All of the energy of beta decay doesn't go into the electron; a higher percentage goes into the muon antineutrino, which escapes harmlessly off into space. The average energy of the beta particle from tritium decay is only 5.694 keV. Net result? Before controlling for the difference in half life, U235 is 20540 times worse for the body than tritium.
Now, of course, due to 235U's incredibly long half life, its radioactivity rarely a problem - which is why fresh fuel rods are not considered very dangerous, but spent ones are. People's concerns in nuclear accidents center around the fission products: strontium, iodine, plutonium, etc - things with shorter (but still problematically long) half lives and strong biological effectiveness. Versus them, the ridiculously low energy tritium is almost irrelevant in terms of biological effect, even if present in similar quantities. Combined with the very small amount of tritium that's in the torus at any point in time, it's just simply not even remotely comparable.
Did I even bother to mention that gaseous tritium tends to rapidly escape wherever it is and ascend up and out of the atmosphere? Tritium in the form of heavy water can be problematic in higher quantities, but of course, there's no "higher quantities" of any form of tritium in the torus.
I'll never forget the last thing grandma said to me before she died: "What are you doing in here with that knife?!?"
Injection is relatively easy; one uses pellet injectors. They basically bore tiny pellets of a mixture of deuterium and tritium ice and shoot them into the middle of the core with a tiny gas gun.
Removing the helium "ash" is harder, and requires something called a divertor. The plasma naturally pushes the helium toward the outside, as it's heavier. The divertor basically juts out into the outer edge of the plasma stream and skims off the plasma, acting as sort of an exhaust system. But it's an incredibly hostile environment, and not just because the temperature (it has to operate continuously at thousands of degrees, and that's after water cooling!) - it's being pelted by high energy alphas all the time! Regardless, it provides not just a way to get rid of helium but takes up many megawatts of heat that are used for power generation.
I'll never forget the last thing grandma said to me before she died: "What are you doing in here with that knife?!?"
According to TFA it should be energy-positive, producing at least 3x the energy it consumes with room to expand that to around 5x.
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It depends. Building a wood-fired steam engine is pretty easy: the amount of heat is fairly small and a slow trickle of water can take it away (turned into steam, driven past turbines or used to power pistons), keeping the boiler at an equilibrium temperature. Move up to a denser fuel and the engineering becomes harder - you need higher water pressure and to get the steam out faster. Move up to fission and the coolant cycle can get quite large - remember, if you're moving the water past the nuclear reaction then stray neutrons are going to turn it into heavy water and you're not going to want to just dump it (though you might want to extract the tritium for other uses), so you need a closed cycle where you can cool the water down enough that you can feed it back over the reactor in a loop, taking the energy out in the turbines somewhere. You often do this with a couple of loops of coolant, where the coolant that's run over the reactor heats something else which then drives the turbines, so your turbines are not having to pass irradiated coolant.
Scale it up more and it becomes an even more difficult engineering challenge. For comparison, look at a 100W lightbulb and a 100W Pentium 4. Both need to dissipate 100W, but one is doing it over the surface of a large bulb, the other over about a square centimetre on the top of the package - the total heat is the same, but it's a lot harder to keep the P4 cool than it is to cool the lightbulb.
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Fusion's 17 MeV neutrons are nothing compared to spallation's neutrons, which can approach (or in some designs even exceed) a GeV. 17MeV neutrons are most eminantly stoppable. Yes, they have a longer penetration distance, and yes, there are some differences in behavior (they tend to cause (n,2n), (n,alpha), (n,d) etc reactions a lot more often while lower energy neutrons usually only do (n,gamma) transmutation), But these are not fundamental differences nor fundamental problems.
Fusion reactors do not use "layers of lead" as shielding. You have some misconceptions about how shielding works. Lead is an excellent shielding material for gamma and beta, but it's terrible for neutrons. It does not moderate them down at any relevant rate due to its high atomic mass, it has a low (n, gamma) gross section, and when it does undergo neutron capture it breeds bismuth - which is fine, except when bismuth undergoes (n, gamma) it breeds polonium, which is really, really nasty stuff. There's also a variety of other neutron reactions lead can undergo which lead to other radioactive products. You don't use lead for neutron shielding. Quite to the contrary, lead is used as a coolant in some types of nuclear reactors because of how little it interferes with neutrons.
Neutrons by contrast are generally best blocked by light elements. Hydrogen is the most effective moderator, although you want both to moderate down the neutron energy and have a high neutron cross section. And of course you don't use pure hydrogen because that's an explosion hazard. So if you want liquid shielding, something like borated water is your best bet. For solids, borated plastics are best.
However, the neutrons in a fusion reactor are not seen as an undesirable thing, but as a critical part of the process to keep it going. Because you need tritium to run it, and tritium doesn't grow on trees, you have to breed it. D-T gives one neutron and it takes one neutron to make one tritium, so if you didn't have any neutron multiplication, the *best* you could possibly do (with no losses and 100% capture) would be breakeven. The reality is that you have to do neutron multiplication to get enough to operate. So the reactors use a lithium-beryllium blanket, of a thickness to absorb the overwhelming majority of the neutrons. Outside of this there will always be stray neutrons that escape, you're not going to want to just stand next to the thing, but it's not going to be a Glowing Ball of Death.
Now, obviously, for structural materials, you're not going to be building it out of borated water, borated plastic, or lithium. Beryllium, mind you, is light and an excellent structural material, but it's super-expensive and difficult to work with, so it's only generally used structurally in key areas. Aluminum (better, lithium-aluminum) is great and undergoes almost no induced radioactivity, but its low melting point limits its use in high temperature applications. Graphite would be great, and is great in some cases - but it undergoes Wigner energy problems if not operated at high enough of a temperature. Composites, which aren't as Wigner energy sensitive, usually can't take the heat. So altogether, one generally deals with iron alloys (steels), with the alloying agents chosen based on what gives the desired properties while undergoing the least problematic transmutation reactions. With proper design, the level of transmutation can be kept pretty low.
Why would it be low? Well, the vast majority of iron is 56 iron. There are also a few percent of 54Fe, 57Fe, and a fraction of a percent of 58Fe. Let's trace the neutron capture paths here.
54Fe becomes 55Fe. This is radioactive, but the half life is only 2,7 years - hardly "forever". It decays to 55Mn, which is stable. If during the 2,7 years average it captures another neutron, it becomes the common 56Fe. If the 55Mn captures a neutron, it becomes 56Mn. 56Mn is radioactive but only has a halflife of 2,6 hours. It decays into 56Fe. So either way we get back to 56Fe with no long-lived product
I'll never forget the last thing grandma said to me before she died: "What are you doing in here with that knife?!?"