4 ms·

I understand that this is the promise, but it all starts to sour once you look at the details. The most commonly proposed fusion reactors use D+T fusion, which

I understand that this is the promise, but it all starts to sour once you look at the details. The most commonly proposed fusion reactors use D+T fusion, which produces hugely energetic neutrons, the most dangerous kind of radiation that you can think of. You need hugely thick walls to contain these neutrons and be heated up by them, and some kind of system to extract the energy from the walls. And yes, the walls and the extraction system end up radioactive, and now you have this huge radioactive mass to take care of somehow, instead of tiny pellets of spent fuel in their cladding. You have other problems for other reactions. Again, the devil is in the details here, an individual fusion reaction might look good, but by the time you include everything else you need, not so much. Also, any system of safety and containment that will make this look good, will make fission look even better.

demallien 15y ago
yaakov, you are fighting strawmen. When we look at nuclear fission reactors, we note 4 major types of problem. 1) Meltdown. Chernobyl, Fukushima, TMI. This type of problem can't happen with fusion reactors, as the reaction stops as soon as reactant stops being fed in. This is not the case for fission, because of the need to attain near-critical mass before the chain reaction can occur. 2) Waste disposal. As fissile material is consumed by a fission reactor, the resulting waste products are still highly radioactive. This waste has to be stored safely (the famous cooling pools from Fukushima were the first step in this process), for very long periods of time, generally measured in decades and centuries. 3) Fuel availability - there are not huge quantities of fuel-quality fissile material available on Earth. This limits it's usefulness going into the future 4) Theft of fuel - although reactor fuel is not generally very useful for bombs, sometimes the waste is. Some reactors, eg breeder reactors were in fact designed to generate bomb material, more so than to generate electricity. Have a look at that list, and think about how it applies to nuclear fusion. Meltdown is impossible, waste disposal is far easier (it's just the reactor's shield that gets radiated, which is a much more contained problem than that of radioactive reaction by-products. The amount of fuel for a fusion reactor on this planet is just huge, and you can't make bombs out of the material coming in or out of a fusion reactor. These are the reasons that fusion reactors are highly interesting, not because of the possibility to generate more power.
- yaakov34 15y ago
  I am arguing with the sort of vision of fusion power that's presented in popular articles, like the NYT one that started this thread. "Clean power that powers the Sun" - yup, he said that. A senior scientist who knows the numbers. See below in the thread how relevant this is. "Relies only on seawater as fuel" - check. "Offers no chance of a catastrophic accident" - oh come on, even a coal powerplant offers that. Articles like this are a strawman, and Stewart C. Prager, director of the Princeton Plasma Physics Laboratory, knows better and should be ashamed of himself. To take up just the matter of catastrophic accidents: it is said correctly that a fission powerplant can never explode like a fission bomb - the atoms of fuel are in the wrong arrangement for that. But any continuous system for producing fusion power is theoretically capable of "running away" - it is a self-reinforcing process, after all - and producing an actual nuclear explosion. They will design it to make this spectacularly unlikely? OK, sure they will, but let's not talk about this being danger-free. Or the "fuel from seawater" thing. Seawater contains 3.3 ppb of uranium, and it would be commercially feasible to extract it if the prices were higher. It's actually easier to do this than to design fusion reactors. And you can use more advanced fission cycles, especially the thorium ones, to extend the energy you get out of fission fuels. EDIT: let me clarify the actual quote from Dr. Prager that I am complaining about: Harnessing nuclear fusion, the energy that powers the sun and the stars, has been a goal of physicists worldwide since the 1950s. It is essentially inexhaustible and it can be created using hydrogen isotopes — chemical cousins of hydrogen, like deuterium — that can readily be extracted from seawater. This is so incredibly misleading in many ways, without containing an actual false statement. This kind of manipulation of popular opinion makes me angry. Does anyone want to defend this, even as a popularization?
  - uvdiv 15y ago
    Or the "fuel from seawater" thing. Seawater contains 3.3 ppb of uranium, and it would be commercially feasible to extract it if the prices were higher. ... And you can use more advanced fission cycles, especially the thorium ones, to extend the energy you get out of fission fuels. Indeed. There's about a factor of 100,000x right there -- 500x for increased uranium supply (5e9 tons in oceans, vs. 1e7 tons in "conventional" <$130 kg/ton reserves (IAEA est.)), and 200x for increased fuel efficiency. But any continuous system for producing fusion power is theoretically capable of "running away" - it is a self-reinforcing process, after all - and producing an actual nuclear explosion. Don't think this is true. Fusion plasmas need active confinement -- a hotter plasma is no longer confined, and will no longer fuse. This is so incredibly misleading in many ways, without containing an actual false statement. For example, the limited resource is not deuterium but tritium (a misdirection). The plan to create this is by fissioning lithium, which is actually a rather scarce element (comparable with uranium, ironically). To get more interesting sustainability you need to switch to something like D+D fusion, which is much harder to achieve. http://en.wikipedia.org/wiki/Tritium#Lithium http://en.wikipedia.org/wiki/Tritium#Lithium

demallien 15y ago

yaakov, you are fighting strawmen. When we look at nuclear fission reactors, we note 4 major types of problem. 1) Meltdown. Chernobyl, Fukushima, TMI. This type of problem can't happen with fusion reactors, as the reaction stops as soon as reactant stops being fed in. This is not the case for fission, because of the need to attain near-critical mass before the chain reaction can occur. 2) Waste disposal. As fissile material is consumed by a fission reactor, the resulting waste products are still highly radioactive. This waste has to be stored safely (the famous cooling pools from Fukushima were the first step in this process), for very long periods of time, generally measured in decades and centuries. 3) Fuel availability - there are not huge quantities of fuel-quality fissile material available on Earth. This limits it's usefulness going into the future 4) Theft of fuel - although reactor fuel is not generally very useful for bombs, sometimes the waste is. Some reactors, eg breeder reactors were in fact designed to generate bomb material, more so than to generate electricity. Have a look at that list, and think about how it applies to nuclear fusion. Meltdown is impossible, waste disposal is far easier (it's just the reactor's shield that gets radiated, which is a much more contained problem than that of radioactive reaction by-products. The amount of fuel for a fusion reactor on this planet is just huge, and you can't make bombs out of the material coming in or out of a fusion reactor. These are the reasons that fusion reactors are highly interesting, not because of the possibility to generate more power.

yaakov34 15y ago
I am arguing with the sort of vision of fusion power that's presented in popular articles, like the NYT one that started this thread. "Clean power that powers the Sun" - yup, he said that. A senior scientist who knows the numbers. See below in the thread how relevant this is. "Relies only on seawater as fuel" - check. "Offers no chance of a catastrophic accident" - oh come on, even a coal powerplant offers that. Articles like this are a strawman, and Stewart C. Prager, director of the Princeton Plasma Physics Laboratory, knows better and should be ashamed of himself. To take up just the matter of catastrophic accidents: it is said correctly that a fission powerplant can never explode like a fission bomb - the atoms of fuel are in the wrong arrangement for that. But any continuous system for producing fusion power is theoretically capable of "running away" - it is a self-reinforcing process, after all - and producing an actual nuclear explosion. They will design it to make this spectacularly unlikely? OK, sure they will, but let's not talk about this being danger-free. Or the "fuel from seawater" thing. Seawater contains 3.3 ppb of uranium, and it would be commercially feasible to extract it if the prices were higher. It's actually easier to do this than to design fusion reactors. And you can use more advanced fission cycles, especially the thorium ones, to extend the energy you get out of fission fuels. EDIT: let me clarify the actual quote from Dr. Prager that I am complaining about: Harnessing nuclear fusion, the energy that powers the sun and the stars, has been a goal of physicists worldwide since the 1950s. It is essentially inexhaustible and it can be created using hydrogen isotopes — chemical cousins of hydrogen, like deuterium — that can readily be extracted from seawater. This is so incredibly misleading in many ways, without containing an actual false statement. This kind of manipulation of popular opinion makes me angry. Does anyone want to defend this, even as a popularization?
- uvdiv 15y ago
  Or the "fuel from seawater" thing. Seawater contains 3.3 ppb of uranium, and it would be commercially feasible to extract it if the prices were higher. ... And you can use more advanced fission cycles, especially the thorium ones, to extend the energy you get out of fission fuels. Indeed. There's about a factor of 100,000x right there -- 500x for increased uranium supply (5e9 tons in oceans, vs. 1e7 tons in "conventional" <$130 kg/ton reserves (IAEA est.)), and 200x for increased fuel efficiency. But any continuous system for producing fusion power is theoretically capable of "running away" - it is a self-reinforcing process, after all - and producing an actual nuclear explosion. Don't think this is true. Fusion plasmas need active confinement -- a hotter plasma is no longer confined, and will no longer fuse. This is so incredibly misleading in many ways, without containing an actual false statement. For example, the limited resource is not deuterium but tritium (a misdirection). The plan to create this is by fissioning lithium, which is actually a rather scarce element (comparable with uranium, ironically). To get more interesting sustainability you need to switch to something like D+D fusion, which is much harder to achieve. http://en.wikipedia.org/wiki/Tritium#Lithium http://en.wikipedia.org/wiki/Tritium#Lithium