8 ms·

I never quite understood the math behind power densities in a fusion reactor. In the sun, isn't energy production occurring at something like 100-1000 W/m3? So

I never quite understood the math behind power densities in a fusion reactor. In the sun, isn't energy production occurring at something like 100-1000 W/m3? So, if you want to build a multiple MW fusion plant, shouldn't these plants be ridiculously huge compared to, say, a wind turbine rated at a couple of MW? Is the density of the plasma so much higher in a fusion reactor? Also, something else I never grokked, how do you get the power out? The plasma heats up, but how do you turn that into useful electrical energy? Nevertheless of course I hope it does work as advertised... someday. Edit: thanks everyone for the thoughtful, insightful replies!

arthurcolle 6y ago
https://en.wikipedia.org/wiki/Thermoelectric_generator#:~:text=A%20thermoelectric%20generator%20(TEG)%2C,a%20form%20of%20thermoelectric%20effect https://en.wikipedia.org/wiki/Thermoelectric_generator#:~:te...).
08-15 6y ago
The sun "burns" ordinary hydrogen. The first step in this proton-proton-chain is the reaction of two protons to a deuteron, which requires conversion of one of them into a neutron. This is an interaction of the weak nuclear force, which has very small range, so it is slow. Reactions considered for terrestrial fusion don't require the weak force, and consequently won't run on pure hydrogen.
willis936 6y ago
The density is actually lower than the sun in magnetic confinement fusion (MCF) devices because we can’t squeeze plasma together as hard as the sun’s mass can. Inertial confinement fusion (ICF) can squeeze harder than MCF devices, but has serious unaddressed engineering issues. The trick is in higher temperature plasma. The sun fuses protium (lone protons). We don’t have the confinement necessary on Earth to do this, so we fuse deuterium (1p+1n) and tritium (1p+2n). This reaction is more energetically favorable and is achievable on Earth. Coupled with giant microwave ovens and clever geometry and electromagnetic tricks, we can make plasmas much hotter (faster moving particles) than the sun can. Once a plasma is fusing, it emits a lot of heat (alpha heating and fast neutrons). A plasma that requires no external heating (no microwave ovens) is said to be “ignited”. We don’t necessarily need or want ignition to have a successful reactor, but it’s a cool thought. The major trouble with fusion reactors is keeping particles in the bottle long enough to fuse. Since they’re leaving anyway they have to go somewhere. You can tune vessel geometry and magnetic fields to have designated strike points where most of the plasma will exit confinement. These are called divertors. Run some coolant through your divertors and you have a heat source that can boil water and spin a turbine. Here my knowledge gets shaky because I know that the fastest particles coming out of a D+T reaction are neutrons (they weigh much less than an alpha particle). Since neutrons are electrically neutral I think they are much less likely to become thermalized (they are not likely to bump into another particle on their way out). I’m not sure how neutron thermalization happens in reactor simulations, but I’m under the impression that it does.
- pfdietz 6y ago
  The trick is that deuterium (and tritium) are vastly more reactive than ordinary protons. The fusion of the latter is extremely slow, because it involves the weak nuclear interaction to convert a proton to a neutron.
- Zardoz84 6y ago
  lithium blankets traps neutrons. Sadly, the reactor chamber becomes radioactive (but a short period radioactivity that only requires to store the contaminated chamber for decades)
- deleted 6y ago
  [deleted]
- tuatoru 6y ago
  Project Rho has lay-person accessible detail on type of fusion reactions.[1] Well worth a read. The takeaway is, if we can't do D-T fusion, then don't even think about any of the other kinds. They're orders of magnitude harder. 1. http://projectrho.com/public_html/rocket/fusionfuel.php http://projectrho.com/public_html/rocket/fusionfuel.php
NikolaeVarius 6y ago
You boil water from the neutrons/heat it creates.
- coldtea 6y ago
  So in the end it's a glorified boiler!
  - adamesque 6y ago
    Same as a regular fission plant if I'm not mistaken!
  - willis936 6y ago
    A glorified boiler with dense and abundant fuel! We would have never entered the industrial era without boilers.
  - aaronblohowiak 6y ago
    as far as i am aware, solar power is the only prevalent power source that does not derive from spinning a dynamo. Besides wind (and perhaps there, too) almost all of the force used to spin dynamos includes some kind of boiling (eg: dams happen to use the force of the water after the "boil"/evaporation, nuclear, coal, natural gas, etc are all steam-based thermo-mechanical power plants.)
    - laurencerowe 6y ago
      As well as steam engines there are also small scale generators using internal combustion engines and gas turbines which spin the dynamo directly.
    - ben_w 6y ago
      Certainly the only major electrical generation method, but MHD and thermocouples can be the appropriate solution sometimes, and if you count storage systems as well as primary production, there are also fuel cells & batteries.
    - 6y ago
opwieurposiu 6y ago
The temperature at the center of the sun is about 15million kelvin. The plasma temperature of a fusion reactor is around ten times higher. Also, a rector uses deuterium–tritium (D–T) fusion, and the sun uses hydrogen fusion. D-T requires less confinement and is more energetically favorable. If DT is like setting off a fire cracker then hydrogen fusion is like igniting a damp log.
elil17 6y ago
You're correct, that's a good ballpark for the average heat generation rate in the sun. However, 99% of heat generation in the sun occurs in the core (approximately the center 25% by radius) and this area is much more energy dense. Further, fusion reactors could achieve an even higher volumetric heat generation rate then the sun's core does. To collect energy, heat would be transferred to a working fluid (e.g., molten salt) by exposing that fluid to the hot plasma. Then the working fluid would be used to boil water and spin a turbine.
- beervirus 6y ago
  There's something that feels so archaic about using a nuclear reactor to... boil water and spin a turbine. It's disappointing that we haven't figured out a better way to convert to electrical energy than what we were using in the 1800s.
  - ElFitz 6y ago
    You will love these four strips: http://www.geeksworld.org/strip_981.html http://www.geeksworld.org/strip_981.html http://www.geeksworld.org/strip_982.html http://www.geeksworld.org/strip_982.html http://www.geeksworld.org/strip_983.html http://www.geeksworld.org/strip_983.html http://www.geeksworld.org/strip_984.html http://www.geeksworld.org/strip_984.html
  - hderms 6y ago
    I'm not well versed in physics, but when I think about the fact that electric motors haven't changed all that much it sort of makes sense. Inducing a current by moving something in a magnetic field, or vice versa, may seem like simple designs, but perhaps that's why we haven't replaced them yet. Rotational motion seems easy to service and efficient. Same with water being particularly easy to turn into a gas using heat, as well as being super plentiful on Earth. There are just a lot of things going for a steam turbine
  - acchow 6y ago
    Photovoltaics (solar panels) don't rotate turbines.
rsynnott 6y ago
The actual fusion in the sun happens in only a small part At the middle. The rest is just there to provide mass :) > Also, something else I never grokked, how do you get the power out? The plasma heats up, but how do you turn that into useful electrical energy? Steam turbines; same as fission or coal. Or gas turbines, if you want to get fancy.
- alexpotato 6y ago
  There is a science fiction story by, I believe, Isaac Asimov which has as it's main story point that: - fusion is developed - b/c of fusion, the cheapest "rocket fuel" is basically water heated into steam by the fusion reactor - people on Earth get upset about the "spacers" taking all of the "earthers" water - the spacers then have to go to other sources of water in the solar system (I think it was either rings of Saturn or the asteroid belt) The thing that struck me at the time was the "water as propellant" without the extra step of breaking H2O into hydrogen and oxygen.
  - outworlder 6y ago
    > without the extra step of breaking H2O into hydrogen and oxygen Heat it sufficiently and it will be broken down.
mtzaldo 6y ago
I recommend the book "5 Equations that changed the world", it gives you the idea how modern world works and how that is direct influence of math. For example how electricity is being generated using dynamos.
Florin_Andrei 6y ago
> In the sun, isn't energy production occurring at something like 100-1000 W/m3? It's just an average. Fusion occurs only in the core, which is pretty tiny, relatively speaking. > The plasma heats up, but how do you turn that into useful electrical energy? The same way you can turn any kind of heat into useful energy.
- skykooler 6y ago
  It's not just that the fusion only occurs in the core, but the fusion rate is also very low; it's only because the sun is so incredibly massive that this slow reaction rate can self-sustain. With a much smaller fusion reactor, the reaction rate needs to be much higher to keep itself going and to create enough power to run the electromagnets that keep it confined.
beefman 6y ago
Even at the center of the sun's core, the power density doesn't go above 300 W/m^3. At the core boundary it's < 10 W/m^3, and it's < 2 W/m^3 overall. Terrestrial reactors try for much higher reaction rates. But most designs still have power density issues that would make them uneconomical even if they could make net power. Net power is actually a very low bar, corresponding to an EROI of 1.
ncmncm 6y ago
Exactly: there is no prospect or intent to actually, ever make a Tokamak that produces useful power. Besides the power density problem and the really absolutely enormous expense and already-sunk cost, the necessary neutrons would destroy the most expensive part of the reactor in short order, and make the rest radioactive. The whole research program gets its funding as what amounts to a jobs program to keep high-neutron-flux physicists employed and available to draw upon for weapons work. That is one reason why any fusion process that does not emit neutrons is not given any of the research funds: weapons work doesn't need high-alpha-flux physicists. (Secondarily, they have papers that purport to show e.g. p-B fusion could never work.) If we ever do get practical fusion, it won't be in a Tokamak, it probably won't be on the Earth's surface, and it certainly won't help resolve global climate disruption. The money being spent on Tokamaks, on the other hand, absolutely could help a great deal with global climate disruption. But not while also maintaining the all-important high-neutron-flux population.
Baeocystin 6y ago
>The plasma heats up, but how do you turn that into useful electrical energy? I appreciate that many people are commenting 'you couple the plasma to a working fluid', but I think the original comment was more along the line of how you couple a confined plasma to a working fluid. By definition the plasma is in a hard vacuum, magnetically bottled. What, then, is the coupling method? Thermal photons escaping confinement? I genuinely have no idea myself, but would really like to know.
- pfdietz 6y ago
  80% of the energy from DT fusion comes out as neutrons, which enter a blanket and deposit energy there by collisions and nuclear interactions.
  - Baeocystin 6y ago
    Thank you! Do you know how far along are we in understanding how to build an effective blanket that can withstand the neutron flux while maintaining its physical integrity?
    - pfdietz 6y ago
      It's not a solved problem (since really testing a wall design requires a working high power source of fusion neutrons), and it's not clear it can be solved in a sufficiently practical way. Indeed, the ability of the first wall to withstand the energy flowing through it will likely cause DT fusion reactors to have low volumetric power density, with sad effects on their economics.
      - Baeocystin 6y ago
        Thanks again for the reply.
    - DennisP 6y ago
      SPARC's approach is for the blanket to be a molten salt, and also function as a coolant. That will surround an inner wall which gets replaced annually. They tested joints in the REBCO superconductors, so the reactor can be opened up on hinges.
wmwmwm 6y ago
I chuckled/was staggered when I heard that the power density in the Sun's core is approximately the same as a compost heap! (albeit a rather large one...) https://en.wikipedia.org/wiki/Solar_core#Energy_conversion https://en.wikipedia.org/wiki/Solar_core#Energy_conversion
jacquesm 6y ago
> In the sun, isn't energy production occurring at something like 100-1000 W/m3? ~250W/m3.

arthurcolle 6y ago

https://en.wikipedia.org/wiki/Thermoelectric_generator#:~:text=A%20thermoelectric%20generator%20(TEG)%2C,a%20form%20of%20thermoelectric%20effect https://en.wikipedia.org/wiki/Thermoelectric_generator#:~:te...).

08-15 6y ago

The sun "burns" ordinary hydrogen. The first step in this proton-proton-chain is the reaction of two protons to a deuteron, which requires conversion of one of them into a neutron. This is an interaction of the weak nuclear force, which has very small range, so it is slow. Reactions considered for terrestrial fusion don't require the weak force, and consequently won't run on pure hydrogen.

willis936 6y ago

The density is actually lower than the sun in magnetic confinement fusion (MCF) devices because we can’t squeeze plasma together as hard as the sun’s mass can. Inertial confinement fusion (ICF) can squeeze harder than MCF devices, but has serious unaddressed engineering issues. The trick is in higher temperature plasma. The sun fuses protium (lone protons). We don’t have the confinement necessary on Earth to do this, so we fuse deuterium (1p+1n) and tritium (1p+2n). This reaction is more energetically favorable and is achievable on Earth. Coupled with giant microwave ovens and clever geometry and electromagnetic tricks, we can make plasmas much hotter (faster moving particles) than the sun can. Once a plasma is fusing, it emits a lot of heat (alpha heating and fast neutrons). A plasma that requires no external heating (no microwave ovens) is said to be “ignited”. We don’t necessarily need or want ignition to have a successful reactor, but it’s a cool thought. The major trouble with fusion reactors is keeping particles in the bottle long enough to fuse. Since they’re leaving anyway they have to go somewhere. You can tune vessel geometry and magnetic fields to have designated strike points where most of the plasma will exit confinement. These are called divertors. Run some coolant through your divertors and you have a heat source that can boil water and spin a turbine. Here my knowledge gets shaky because I know that the fastest particles coming out of a D+T reaction are neutrons (they weigh much less than an alpha particle). Since neutrons are electrically neutral I think they are much less likely to become thermalized (they are not likely to bump into another particle on their way out). I’m not sure how neutron thermalization happens in reactor simulations, but I’m under the impression that it does.

pfdietz 6y ago
The trick is that deuterium (and tritium) are vastly more reactive than ordinary protons. The fusion of the latter is extremely slow, because it involves the weak nuclear interaction to convert a proton to a neutron.
Zardoz84 6y ago
lithium blankets traps neutrons. Sadly, the reactor chamber becomes radioactive (but a short period radioactivity that only requires to store the contaminated chamber for decades)
deleted 6y ago
[deleted]
tuatoru 6y ago
Project Rho has lay-person accessible detail on type of fusion reactions.[1] Well worth a read. The takeaway is, if we can't do D-T fusion, then don't even think about any of the other kinds. They're orders of magnitude harder. 1. http://projectrho.com/public_html/rocket/fusionfuel.php http://projectrho.com/public_html/rocket/fusionfuel.php

NikolaeVarius 6y ago

You boil water from the neutrons/heat it creates.

coldtea 6y ago
So in the end it's a glorified boiler!
- adamesque 6y ago
  Same as a regular fission plant if I'm not mistaken!
- willis936 6y ago
  A glorified boiler with dense and abundant fuel! We would have never entered the industrial era without boilers.
- aaronblohowiak 6y ago
  as far as i am aware, solar power is the only prevalent power source that does not derive from spinning a dynamo. Besides wind (and perhaps there, too) almost all of the force used to spin dynamos includes some kind of boiling (eg: dams happen to use the force of the water after the "boil"/evaporation, nuclear, coal, natural gas, etc are all steam-based thermo-mechanical power plants.)
  - laurencerowe 6y ago
    As well as steam engines there are also small scale generators using internal combustion engines and gas turbines which spin the dynamo directly.
  - ben_w 6y ago
    Certainly the only major electrical generation method, but MHD and thermocouples can be the appropriate solution sometimes, and if you count storage systems as well as primary production, there are also fuel cells & batteries.
  - 6y ago

opwieurposiu 6y ago

The temperature at the center of the sun is about 15million kelvin. The plasma temperature of a fusion reactor is around ten times higher. Also, a rector uses deuterium–tritium (D–T) fusion, and the sun uses hydrogen fusion. D-T requires less confinement and is more energetically favorable. If DT is like setting off a fire cracker then hydrogen fusion is like igniting a damp log.

elil17 6y ago

You're correct, that's a good ballpark for the average heat generation rate in the sun. However, 99% of heat generation in the sun occurs in the core (approximately the center 25% by radius) and this area is much more energy dense. Further, fusion reactors could achieve an even higher volumetric heat generation rate then the sun's core does. To collect energy, heat would be transferred to a working fluid (e.g., molten salt) by exposing that fluid to the hot plasma. Then the working fluid would be used to boil water and spin a turbine.

beervirus 6y ago
There's something that feels so archaic about using a nuclear reactor to... boil water and spin a turbine. It's disappointing that we haven't figured out a better way to convert to electrical energy than what we were using in the 1800s.
- ElFitz 6y ago
  You will love these four strips: http://www.geeksworld.org/strip_981.html http://www.geeksworld.org/strip_981.html http://www.geeksworld.org/strip_982.html http://www.geeksworld.org/strip_982.html http://www.geeksworld.org/strip_983.html http://www.geeksworld.org/strip_983.html http://www.geeksworld.org/strip_984.html http://www.geeksworld.org/strip_984.html
- hderms 6y ago
  I'm not well versed in physics, but when I think about the fact that electric motors haven't changed all that much it sort of makes sense. Inducing a current by moving something in a magnetic field, or vice versa, may seem like simple designs, but perhaps that's why we haven't replaced them yet. Rotational motion seems easy to service and efficient. Same with water being particularly easy to turn into a gas using heat, as well as being super plentiful on Earth. There are just a lot of things going for a steam turbine
- acchow 6y ago
  Photovoltaics (solar panels) don't rotate turbines.

rsynnott 6y ago

The actual fusion in the sun happens in only a small part At the middle. The rest is just there to provide mass :) > Also, something else I never grokked, how do you get the power out? The plasma heats up, but how do you turn that into useful electrical energy? Steam turbines; same as fission or coal. Or gas turbines, if you want to get fancy.

alexpotato 6y ago
There is a science fiction story by, I believe, Isaac Asimov which has as it's main story point that: - fusion is developed - b/c of fusion, the cheapest "rocket fuel" is basically water heated into steam by the fusion reactor - people on Earth get upset about the "spacers" taking all of the "earthers" water - the spacers then have to go to other sources of water in the solar system (I think it was either rings of Saturn or the asteroid belt) The thing that struck me at the time was the "water as propellant" without the extra step of breaking H2O into hydrogen and oxygen.
- outworlder 6y ago
  > without the extra step of breaking H2O into hydrogen and oxygen Heat it sufficiently and it will be broken down.

mtzaldo 6y ago

I recommend the book "5 Equations that changed the world", it gives you the idea how modern world works and how that is direct influence of math. For example how electricity is being generated using dynamos.

Florin_Andrei 6y ago

> In the sun, isn't energy production occurring at something like 100-1000 W/m3? It's just an average. Fusion occurs only in the core, which is pretty tiny, relatively speaking. > The plasma heats up, but how do you turn that into useful electrical energy? The same way you can turn any kind of heat into useful energy.

skykooler 6y ago
It's not just that the fusion only occurs in the core, but the fusion rate is also very low; it's only because the sun is so incredibly massive that this slow reaction rate can self-sustain. With a much smaller fusion reactor, the reaction rate needs to be much higher to keep itself going and to create enough power to run the electromagnets that keep it confined.

beefman 6y ago

Even at the center of the sun's core, the power density doesn't go above 300 W/m^3. At the core boundary it's < 10 W/m^3, and it's < 2 W/m^3 overall. Terrestrial reactors try for much higher reaction rates. But most designs still have power density issues that would make them uneconomical even if they could make net power. Net power is actually a very low bar, corresponding to an EROI of 1.

ncmncm 6y ago

Exactly: there is no prospect or intent to actually, ever make a Tokamak that produces useful power. Besides the power density problem and the really absolutely enormous expense and already-sunk cost, the necessary neutrons would destroy the most expensive part of the reactor in short order, and make the rest radioactive. The whole research program gets its funding as what amounts to a jobs program to keep high-neutron-flux physicists employed and available to draw upon for weapons work. That is one reason why any fusion process that does not emit neutrons is not given any of the research funds: weapons work doesn't need high-alpha-flux physicists. (Secondarily, they have papers that purport to show e.g. p-B fusion could never work.) If we ever do get practical fusion, it won't be in a Tokamak, it probably won't be on the Earth's surface, and it certainly won't help resolve global climate disruption. The money being spent on Tokamaks, on the other hand, absolutely could help a great deal with global climate disruption. But not while also maintaining the all-important high-neutron-flux population.

Baeocystin 6y ago

>The plasma heats up, but how do you turn that into useful electrical energy? I appreciate that many people are commenting 'you couple the plasma to a working fluid', but I think the original comment was more along the line of how you couple a confined plasma to a working fluid. By definition the plasma is in a hard vacuum, magnetically bottled. What, then, is the coupling method? Thermal photons escaping confinement? I genuinely have no idea myself, but would really like to know.

pfdietz 6y ago
80% of the energy from DT fusion comes out as neutrons, which enter a blanket and deposit energy there by collisions and nuclear interactions.
- Baeocystin 6y ago
  Thank you! Do you know how far along are we in understanding how to build an effective blanket that can withstand the neutron flux while maintaining its physical integrity?
  - pfdietz 6y ago
    It's not a solved problem (since really testing a wall design requires a working high power source of fusion neutrons), and it's not clear it can be solved in a sufficiently practical way. Indeed, the ability of the first wall to withstand the energy flowing through it will likely cause DT fusion reactors to have low volumetric power density, with sad effects on their economics.
    - Baeocystin 6y ago
      Thanks again for the reply.
  - DennisP 6y ago
    SPARC's approach is for the blanket to be a molten salt, and also function as a coolant. That will surround an inner wall which gets replaced annually. They tested joints in the REBCO superconductors, so the reactor can be opened up on hinges.

wmwmwm 6y ago

I chuckled/was staggered when I heard that the power density in the Sun's core is approximately the same as a compost heap! (albeit a rather large one...) https://en.wikipedia.org/wiki/Solar_core#Energy_conversion https://en.wikipedia.org/wiki/Solar_core#Energy_conversion

jacquesm 6y ago

> In the sun, isn't energy production occurring at something like 100-1000 W/m3? ~250W/m3.