3 ms·

Nuclear fusion reaction releases almost twice the energy put in

27 points by SkyMarshal 3y ago

simonblack 3y ago
Practical fusion reactors are about 20 years away - the same as they have been for the last 70 years.
- rowanG077 3y ago
  Well yeah. It's like saying the estimate of this issue is two weeks. It was two weeks a year ago as well. Because I just didn't start working on it in earnest yet.
credit_guy 3y ago
Here on HN people occasionally point out that the NIF's primary job is nuclear weapons maintenance. That's not actually the case. Or it is, because it receives money from the Congress for this reason, but it is, let's say, "complicated". The NIF was actually created for the purpose of achieving controlled nuclear fusion for civilian production of energy, unrelated to any weapons. They ran into various technical problems. Several times they projected that if they can build an ensemble of lasers to deliver xxx kJ or MJ, then they could achieve fusion. Each time they fell well short of their projections. Each time they asked for more funding for a bigger machine because "this time it's going to be different". After the end of the Cold War, the money was scarce, so they pivoted to "weapons maintenance". What weapons maintenance, you ask? Glad you asked, lots of people asked, but nobody would answer because it's classified. You can find on wikipedia [1] quotes like > Rick Spielman publicly stated that NIF had "virtually no internal peer review on the technical issues" and that "Livermore essentially picked the panel to review themselves". A retired Sandia manager, Bob Puerifoy, was even more blunt than Spielman: "NIF is worthless ... it can't be used to maintain the stockpile, period". Ray Kidder, one of the original developers of the ICF concept at LLNL, was also highly critical. He stated in 1997 that its primary purpose was to "recruit and maintain a staff of theorists and experimentalists" and that while some of the experimental data would prove useful for weapons design, differences in the experimental setup limit their relevance. One could say this is good news: a bit of white lies to get money from the military and spend on a worthy science project that could one day save the world. Less money for killing, more money for peace, or some similarly uplifting message. I would argue that it would be better to be honest with the people. Playing this type of games erodes people's trust in science. Conversely, the scientists that indulge in these games will end up thinking "people are too stupid to understand this stuff, we can't trust them to make the right decision. Only we are smart enough to make that decision". [1] https://en.wikipedia.org/wiki/National_Ignition_Facility#NIF,_1994 https://en.wikipedia.org/wiki/National_Ignition_Facility#NIF...
- p1mrx 3y ago
  > a worthy science project that could one day save the world What is the path from NIF to a practical power plant? Are they providing data that will help ITER, CFS, Helion and others progress toward that goal? None of them seem to be pursuing the 'lasers lasers n lasers' approach.
  - credit_guy 3y ago
    I don't think they have any concrete plans yet. But don't worry, ITER, CFS and Helion will never achieve economical fusion reactors either. The reason is quite simple: in order to heat a certain volume with matter to a certain temperature, you need to heat both the matter (vibration energy) and the vacuum (radiation energy). The energy to heat matter is proportional to the temperature (it's actually the definition of temperature). The energy to heat the vacuum is proportional to the fourth power of the temperature. If you draw a graph of T and T^4 you'll see that initially T^4 is negligible compared to T, and later T is negligible compared to T^4. If you want to heat 1m3 of vacuum to the temperature needed for sustained fusion, it takes more than 10 kT of TNT equivalent. In an actual nuclear bomb, this is available from the primary fission bomb; plus the volume of the part undergoing fusion is squeezed tremendously, so it's nothing close to 1 m3. ITER plans to have a volume of plasma of 840 m3. At the temperatures they talk about, the energy to heat 840 m3 exceeds the energy released by the Tsar bomba. Bottom line: the only possible way to get controlled fusion is the NIF way. But even that one is very, very far in the future.
    - p1mrx 3y ago
      Why does heating the vacuum require any energy at all? If the plasma is radiating at T^4, doesn't that mean they need something to reflect the radiation so it doesn't escape?
      - credit_guy 3y ago
        Radiation is electromagnetic waves. These waves, or photons if you wish, exist in vacuum, and have various frequencies from very low to very high. The energy of a photon or wave depends on the frequency, and is given by the Panck's formula; we don't care about this particular form. A volume of vacuum that is in equilibrium with its surroundings has a certain temperature. If you take the sum of the energies of all the photons inside that volume, you get a number that is proportional to T^4. This is called Stefan's law. If you take a volume of plasma with a certain average kinetic energy per particle (i.e. a certain temperature), then that average kinetic energy per particle may be higher or lower than the average energy of the photons in the same volume. If it's higher, then the plasma will radiate, and the average energy of the photons will go up, if it's lower, the photons will give heat to the plasma, and the plasma will get hotter. When they were working on making the fusion bomb, they initially did not think they can create an equilibrium between the plasma and the vacuum, in other words the vacuum was colder and the plasma will leak energy to the vacuum continuously. The hope was that enough fusion will happen so that the produced energy will make up for this heat loss and the reaction will continue for a while. That didn't happen. Fusion bombs that don't reach an equilibrium between the plasma and the vacuum were made and detonated, but their bang was underwhelming. Ulam and Teller came up with the breakthrough that resulted in an equilibrium, and a sustained fusion. The bomb they made (well, not the two of them, the other scientists and engineers) yielded 10 megatons of TNT, which was many times what one can obtain with either regular fission bombs, or with non-equilibrium fusion bombs. If their discovery was a peaceful one, they would have gotten a Nobel for sure.

27 points by SkyMarshal 3y ago

simonblack 3y ago

Practical fusion reactors are about 20 years away - the same as they have been for the last 70 years.

rowanG077 3y ago
Well yeah. It's like saying the estimate of this issue is two weeks. It was two weeks a year ago as well. Because I just didn't start working on it in earnest yet.

credit_guy 3y ago

Here on HN people occasionally point out that the NIF's primary job is nuclear weapons maintenance. That's not actually the case. Or it is, because it receives money from the Congress for this reason, but it is, let's say, "complicated". The NIF was actually created for the purpose of achieving controlled nuclear fusion for civilian production of energy, unrelated to any weapons. They ran into various technical problems. Several times they projected that if they can build an ensemble of lasers to deliver xxx kJ or MJ, then they could achieve fusion. Each time they fell well short of their projections. Each time they asked for more funding for a bigger machine because "this time it's going to be different". After the end of the Cold War, the money was scarce, so they pivoted to "weapons maintenance". What weapons maintenance, you ask? Glad you asked, lots of people asked, but nobody would answer because it's classified. You can find on wikipedia [1] quotes like > Rick Spielman publicly stated that NIF had "virtually no internal peer review on the technical issues" and that "Livermore essentially picked the panel to review themselves". A retired Sandia manager, Bob Puerifoy, was even more blunt than Spielman: "NIF is worthless ... it can't be used to maintain the stockpile, period". Ray Kidder, one of the original developers of the ICF concept at LLNL, was also highly critical. He stated in 1997 that its primary purpose was to "recruit and maintain a staff of theorists and experimentalists" and that while some of the experimental data would prove useful for weapons design, differences in the experimental setup limit their relevance. One could say this is good news: a bit of white lies to get money from the military and spend on a worthy science project that could one day save the world. Less money for killing, more money for peace, or some similarly uplifting message. I would argue that it would be better to be honest with the people. Playing this type of games erodes people's trust in science. Conversely, the scientists that indulge in these games will end up thinking "people are too stupid to understand this stuff, we can't trust them to make the right decision. Only we are smart enough to make that decision". [1] https://en.wikipedia.org/wiki/National_Ignition_Facility#NIF,_1994 https://en.wikipedia.org/wiki/National_Ignition_Facility#NIF...

p1mrx 3y ago
> a worthy science project that could one day save the world What is the path from NIF to a practical power plant? Are they providing data that will help ITER, CFS, Helion and others progress toward that goal? None of them seem to be pursuing the 'lasers lasers n lasers' approach.
- credit_guy 3y ago
  I don't think they have any concrete plans yet. But don't worry, ITER, CFS and Helion will never achieve economical fusion reactors either. The reason is quite simple: in order to heat a certain volume with matter to a certain temperature, you need to heat both the matter (vibration energy) and the vacuum (radiation energy). The energy to heat matter is proportional to the temperature (it's actually the definition of temperature). The energy to heat the vacuum is proportional to the fourth power of the temperature. If you draw a graph of T and T^4 you'll see that initially T^4 is negligible compared to T, and later T is negligible compared to T^4. If you want to heat 1m3 of vacuum to the temperature needed for sustained fusion, it takes more than 10 kT of TNT equivalent. In an actual nuclear bomb, this is available from the primary fission bomb; plus the volume of the part undergoing fusion is squeezed tremendously, so it's nothing close to 1 m3. ITER plans to have a volume of plasma of 840 m3. At the temperatures they talk about, the energy to heat 840 m3 exceeds the energy released by the Tsar bomba. Bottom line: the only possible way to get controlled fusion is the NIF way. But even that one is very, very far in the future.
  - p1mrx 3y ago
    Why does heating the vacuum require any energy at all? If the plasma is radiating at T^4, doesn't that mean they need something to reflect the radiation so it doesn't escape?
    - credit_guy 3y ago
      Radiation is electromagnetic waves. These waves, or photons if you wish, exist in vacuum, and have various frequencies from very low to very high. The energy of a photon or wave depends on the frequency, and is given by the Panck's formula; we don't care about this particular form. A volume of vacuum that is in equilibrium with its surroundings has a certain temperature. If you take the sum of the energies of all the photons inside that volume, you get a number that is proportional to T^4. This is called Stefan's law. If you take a volume of plasma with a certain average kinetic energy per particle (i.e. a certain temperature), then that average kinetic energy per particle may be higher or lower than the average energy of the photons in the same volume. If it's higher, then the plasma will radiate, and the average energy of the photons will go up, if it's lower, the photons will give heat to the plasma, and the plasma will get hotter. When they were working on making the fusion bomb, they initially did not think they can create an equilibrium between the plasma and the vacuum, in other words the vacuum was colder and the plasma will leak energy to the vacuum continuously. The hope was that enough fusion will happen so that the produced energy will make up for this heat loss and the reaction will continue for a while. That didn't happen. Fusion bombs that don't reach an equilibrium between the plasma and the vacuum were made and detonated, but their bang was underwhelming. Ulam and Teller came up with the breakthrough that resulted in an equilibrium, and a sustained fusion. The bomb they made (well, not the two of them, the other scientists and engineers) yielded 10 megatons of TNT, which was many times what one can obtain with either regular fission bombs, or with non-equilibrium fusion bombs. If their discovery was a peaceful one, they would have gotten a Nobel for sure.