5 ms·
ARC will be about the size of JET, and JET was built in four years, with most of that being just for the buildings. The test reactor, SPARC, will be about half
by DennisP 5y ago
ARC will be about the size of JET, and JET was built in four years, with most of that being just for the buildings. The test reactor, SPARC, will be about half that size.
Tokamak scaling is very well established. The output scales with the square of plasma volume, and the fourth power of magnetic field strength. Stronger magnetic fields also make the plasma more stable. JET already demonstrated a five-second plasma, which they only had to shut down because they have copper coils that would melt if operated longer than that.
Because of all this, many independent fusion researchers think SPARC will succeed in getting 10X gain in 2025. After that, the larger ARC should easily reach commercial levels.
Of all the commercial fusion companies, CFS is the most conservative one. Tokamak Energy is a close second, with a very similar approach. The other fusion startups are attempting approaches that have more physics risk, though many of them would have fewer engineering and economic difficulties if the physics does work out.
- Retric 5y agoSimply scaling SPAR or ITER doesn’t result in commercial operation. First you need fuel, global Tritium supplies are tiny and DD fusion is much harder. Next stability is an open question, no Tokamak has ever operated near maximum capacity for even 1 hour. Add to that serious material science questions, etc etc and even Q>100 alone just doesn’t actually mean much. Now let’s just assume all of that is solved, you still need to actually ensure your design is economically viable. Simply producing energy from fusion alone isn’t enough which means you need to cheaply solve not only all the above but do so cheaply.
- akomtu 5y agoDoesn't Moon have immense supplies of tritium? I bet Musk, with his landing rockets, has bigger plans than just launching a bunch of sats.
- DennisP 5y agoTritium has only a 12-year half-life, and decays to He3. The Moon has large supplies of He3, which could be fuel for more advanced fusion reactors. He3 barely exists on Earth. However, pure deuterium fusion is easier than He3 fusion, and its waste is half He3, half tritium.
- pfdietz 5y ago3He on the moon occurs in regolith at a concentration of maybe 10 ppb. At that concentration, simply heating the regolith to drive out the 3He uses more energy than the 3He would produce in fusion, unless extreme heat recycling can be achieved (and good luck transferring heat into/out of powders in vacuum.) Even if it could be recovered, all the 3He on the entire moon might power the world for maybe 100 years.
- DennisP 5y agoThanks, didn't realize the energy balance was so bad. That strengthens the point I was trying to get at, which was that there's no reason to go to the moon for He3 when, if we can get net power from He3, we can just make it in a D-D reactor and gain energy in the process.
- pfdietz 5y agoMy favorite 3He source would be "Planet X", if it exists and is an Earth (or perhaps even Mars) size planet out far enough that the temperature at the exobase is low enough to retain 3He in its atmosphere. Sure, getting there would be difficult, but this scenario assumes we have D3He fusion, which we can handwave would get the miners out there in a reasonable amount of time.
- DennisP 5y agoThese are valid points. I was just responding to your statement that their time frame is "achievable if they don’t actually try and hit net energy gain." They are trying, and are likely to accomplish net gain about as fast as they say they will. They actually have a fairly practical-looking design though. ARC will use FLiBe for tritium breeding and coolant. That will surround an inner reactor wall that's 3D-printed and replaced annually. MIT already demonstrated joints in the superconducting tape, allowing the reactor to be opened up to replace the core. Economic viability is an open question for all tokamaks. Some of the other designs would have a better chance of achieving low costs if things work out, but there's a much bigger question mark on the plasma physics.
- TaylorAlexander 5y agoIt sounds like you might know the answer to this: I vaguely recall hearing them say the tritium is created in the reactor from duterium? Is that a thing?
- DennisP 5y agoIt is, deuterium fusion's final product is half tritium (and half He3). But D-T reactors would mainly breed tritium from lithium. The CFS reactor, for example, will immerse the reactor in a pool of FLiBe salt, which also functions as coolant. Each beryllium atom hit with a neutron emits two neutrons, and when neutrons hit lithium atoms they breed tritium. General Fusion does something similar with lead in place of beryllium.
- TaylorAlexander 5y agoThank you. So do they need to source tritium externally at all? I’ve seen people claim CFS will have to source tritium, making it expensive to operate. Can they breed all they need?
- DennisP 5y agoThey'll need tritium for startup. After that they shouldn't need more if they can get the breeding ratio high enough, but that might not be easy.
- TaylorAlexander 5y agoInteresting thank you!
- deleted 5y ago[deleted]
- ncmncm 5y agoIn practice it means the mix of Li-6 and Li-7 in your coolant pipes has to capture more than half of the neutrons emitted, and not too many of the slow neutrons released in intermediate reactions get stolen.
- pfdietz 5y agoThe other problem is that the thermal power density of ARC is about 1/40th that of a PWR reactor vessel. It's a much larger, much more massive/complex/expensive way to make heat.
- Retric 5y agoRemoving the need for large numbers of armed guards, giant concrete containment vessels, many many safety systems etc, should save some money. It’s really not clear which would end up cheaper in the long run.
- ncmncm 5y agoSpilling a thousand tons of melted lithium (and beryllium?) would be disruptive enough that containment, safety systems, and armed guards would still be needed. Also, not spending to protect your $100B facility from harm might make your insurance company nervous. (You did find an insurer, didn't you?)
- Retric 5y agoIf it actually cost 100B to built then your never going to break even. As to containment, the lack of high pressure steam makes much thinner walls completely viable. You still need shielding around the vessel, but not a completely redundant system capable of containing highly energetic steam explosions. In terms of risks, sabotage at a nuclear reactor can be vastly more expensive than just the cost of equipment. Modern reactors are reasonably safe, but not if people where actually trying to break them. Especially when you consider what someone could do with access to the fuel. It’s not weapons grade, but dirty bombs are horrific.
- pfdietz 5y agoYou do still need a very strong structure to resist outward pressure. It's isn't steam pressure, it's magnetic pressure, from JxB forces in the magnets. For ARC, the stainless steel supports for the magnets weigh 5300 tonnes, and comprise 3/4 of the mass of the entire reactor. The energy stored in that magnetic field is (I think) larger than would be stored in the steam of a PWR of the same thermal power output. The building housing an ARC will be very large. The reactor itself is 20 meters tall, and the entire top half of it has to be lifted off and moved aside when changing out the vacuum vessel. All that lifting and moving will have to be done remotely because of radiation from the activated vacuum vessel, which then will need yet another shielded area where it can be broken down for disposal and the debris from that cleaned up or at least contained.