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Top 4 reasons why nuclear fusion is an incredibly bad idea: 4. The power density of the Sun is surprisingly low, it's about 276 W/m3 [1]. If we would ever be a

Top 4 reasons why nuclear fusion is an incredibly bad idea: 4. The power density of the Sun is surprisingly low, it's about 276 W/m3 [1]. If we would ever be able to create fusion similar to the one that's happening in the Sun, we'd need a few million m3 to get to the power generated by a typical large power plant (1 GW). Imagine what type of containment vessel you need to enclose a few million m3 of hot (100 million degrees) stuff. For comparison, the largest oil tankers in the world can hold about 300,000 m3 of oil. 3. Even assuming we can build such a humongous fusion based power plant, what happens if, you know, something goes unplanned? Anti-nuclear people point to Chernobyl and Fukushima, what could happen with a fusion power plant? You would not have radiation, that's true, but you would have an explosion that would make the Tsar Bomba look like firecrackers. Why? All that hydrogen that is supposed to undergo fusion needs to first be heated to about 100 MM degrees. That's a lot of energy. How much? Here's very rough back of the envelope: density of Sun = 1 kg/m3; let's say we are looking at 10 MM m3 of hydrogen (we account for some power conversion inefficiency), that's 10 Megatons of hydrogen. At about 15 kJ/kg.K specific heat, it takes about 1.5 billion kj to heat one kg of hydrogen, or 1500 GJ. This is about 0.3 T TNT equivalent [2]. Overall, about 3 Gigatons TNT equivalent. That's 60 times larger than the largest nuke ever tried (the Tsar bomba mentioned before). Or 200,000 larger than the Hiroshima bomb. 2. The accident discussed above assumes nothing goes really bad. It's simply a rapid release of some thermal energy. But what if instead of this, we have an accelerated fusion reaction? Then we have at hand many orders of magnitude more fusion fuel than all the thermonuclear bombs ever build. I don't want to do any calculations, but there's a chance that could be the end of the humans, or maybe all multicellular life on earth. 1. All this hope for clean nuclear energy means a lot of people don't want to even consider practical solutions for the fission-based nuclear energy, such as waste reprocessing or next generation nuclear reactors (like Bill Gates's Terrapower). Why would they, if clean nuclear energy is just around the corner? That kills what is probably our most realistic tool to reduce carbon emissions. [1] https://physics.stackexchange.com/questions/370899/suns-power-density-compared-to-a-compost-heap https://physics.stackexchange.com/questions/370899/suns-powe... [2] https://en.wikipedia.org/wiki/TNT_equivalent https://en.wikipedia.org/wiki/TNT_equivalent

jcfrei 7y ago
Your numbers seem to be in contradiction to what ITER projects on its website (https://www.iter.org/FactsFigures https://www.iter.org/FactsFigures): 830 m3 plasma volume, expected power output ~500MW for 500-600 seconds.
pdonis 7y ago
> The power density of the Sun is surprisingly low That's because the Sun is very different from an Earthbound fusion reactor. It runs at a very different temperature and density, using very different reactions. The analogy you are trying to draw is invalid. > you would have an explosion that would make the Tsar Bomba look like firecrackers No, you wouldn't, because only a very small amount of fuel is inside the reactor at any given time. The inside of a fusion reactor is very different from a nuclear bomb. The analogy you are trying to draw is invalid. > what if instead of this, we have an accelerated fusion reaction? You can't. If anything goes wrong the injection of fuel into the reactor stops and the reactor shuts down.
- scottlocklin 7y ago
  > That's because the Sun is very different from an Earthbound fusion reactor. It runs at a very different temperature and density, using very different reactions. The analogy you are trying to draw is invalid. No, the analogy he draws is the correct one: the sun actually works. "Earthbound" controlled nuclear fusion doesn't in any meaningful way relating to energy production. There's no reason physics should cooperate with us and make it possible; "wouldn't it be nice" wishful thinking isn't good enough, and the results thus far are all complete and total crap. It isn't that we haven't spent enough: we simply don't know how to do this -there is no reason to believe it is possible. All the results from inertial confinement, tokomaks, fusors, whatever indicates we really don't know WTF we're doing. His other concerns are legit as well. If you can do controlled nuclear fusion, you have a really, really scary weapon.
  - pdonis 7y ago
    > the analogy he draws is the correct one: the sun actually works Under, as I said, very different conditions from those which Earthbound fusion reactors are attempting. The only meaningful similarity between the two is that both are fusion reactions. You certainly cannot use numbers for the Sun to make any valid estimates about Earthbound fusion reactors, as the poster I responded to was attempting to do. > Earthbound" controlled nuclear fusion doesn't in any meaningful way relating to energy production. It hasn't yet, but that doesn't mean it never will. If it does, it certainly will not be by reproducing conditions similar to those that produce fusion in the Sun. And if your criticisms of fusion power turn out to be valid and it never works, then everything the poster I responded to said is irrelevant anyway, since he was assuming fusion power would work. > If you can do controlled nuclear fusion, you have a really, really scary weapon. No, you don't, because, as I said, only a very small amount of fuel is in the reactor at any given time--enough for a few seconds' worth of energy production. That equates to the energy equivalent of a few tons of TNT, not a few megatons. And since the entire system is designed to remove that amount of energy at that rate, concerns about even that few tons of TNT energy equivalent "exploding" are invalid--that amount of fuel undergoing complete fusion and liberating all that energy is what the reactor is designed for, not an abnormal condition. And, as I said, all you need to do to stop energy production is stop injecting fuel, and the reactor shuts down. Again, a fusion reactor is very different from a fusion bomb.

jcfrei 7y ago

Your numbers seem to be in contradiction to what ITER projects on its website (https://www.iter.org/FactsFigures https://www.iter.org/FactsFigures): 830 m3 plasma volume, expected power output ~500MW for 500-600 seconds.

pdonis 7y ago

> The power density of the Sun is surprisingly low That's because the Sun is very different from an Earthbound fusion reactor. It runs at a very different temperature and density, using very different reactions. The analogy you are trying to draw is invalid. > you would have an explosion that would make the Tsar Bomba look like firecrackers No, you wouldn't, because only a very small amount of fuel is inside the reactor at any given time. The inside of a fusion reactor is very different from a nuclear bomb. The analogy you are trying to draw is invalid. > what if instead of this, we have an accelerated fusion reaction? You can't. If anything goes wrong the injection of fuel into the reactor stops and the reactor shuts down.

scottlocklin 7y ago
> That's because the Sun is very different from an Earthbound fusion reactor. It runs at a very different temperature and density, using very different reactions. The analogy you are trying to draw is invalid. No, the analogy he draws is the correct one: the sun actually works. "Earthbound" controlled nuclear fusion doesn't in any meaningful way relating to energy production. There's no reason physics should cooperate with us and make it possible; "wouldn't it be nice" wishful thinking isn't good enough, and the results thus far are all complete and total crap. It isn't that we haven't spent enough: we simply don't know how to do this -there is no reason to believe it is possible. All the results from inertial confinement, tokomaks, fusors, whatever indicates we really don't know WTF we're doing. His other concerns are legit as well. If you can do controlled nuclear fusion, you have a really, really scary weapon.
- pdonis 7y ago
  > the analogy he draws is the correct one: the sun actually works Under, as I said, very different conditions from those which Earthbound fusion reactors are attempting. The only meaningful similarity between the two is that both are fusion reactions. You certainly cannot use numbers for the Sun to make any valid estimates about Earthbound fusion reactors, as the poster I responded to was attempting to do. > Earthbound" controlled nuclear fusion doesn't in any meaningful way relating to energy production. It hasn't yet, but that doesn't mean it never will. If it does, it certainly will not be by reproducing conditions similar to those that produce fusion in the Sun. And if your criticisms of fusion power turn out to be valid and it never works, then everything the poster I responded to said is irrelevant anyway, since he was assuming fusion power would work. > If you can do controlled nuclear fusion, you have a really, really scary weapon. No, you don't, because, as I said, only a very small amount of fuel is in the reactor at any given time--enough for a few seconds' worth of energy production. That equates to the energy equivalent of a few tons of TNT, not a few megatons. And since the entire system is designed to remove that amount of energy at that rate, concerns about even that few tons of TNT energy equivalent "exploding" are invalid--that amount of fuel undergoing complete fusion and liberating all that energy is what the reactor is designed for, not an abnormal condition. And, as I said, all you need to do to stop energy production is stop injecting fuel, and the reactor shuts down. Again, a fusion reactor is very different from a fusion bomb.