4 ms·

I note that submarine reactors are typically on the order of 50Mw, and surface ships (CVNs -- aircraft carriers -- typically; also some Russian Arctic ice-break

by cstross 14y ago

I note that submarine reactors are typically on the order of 50Mw, and surface ships (CVNs -- aircraft carriers -- typically; also some Russian Arctic ice-breakers) are up to 100Mw per reactor. Anyone smell a [Naval] market?

potatolicious 14y ago
I doubt it, but my understanding of the topic is limited. AFAIK nuclear reactors are popular on naval warships because they dramatically reduce (and in some cases eliminate) the need to refuel. It's one huge factor crossed off of your logistics problems - modern CVNs can go literally decades without refueling, with the only resupply being jet fuel, munitions, and life support. Doesn't fusion demand constant refueling (like most power plants)?
- dalke 14y ago
  Yes, it needs constant refueling, but the rate is something like a milligram per second. A year's supply of fuel would weigh a few kilograms.
- jessriedel 14y ago
  Nope. > A 1 GW fusion plant will need about 100 Kg of deuterium and 3 tons of natural lithium to operate for a whole year, generating about 7 billion kWh. http://fusionforenergy.europa.eu/faq/ http://fusionforenergy.europa.eu/faq/ If it's possible to proportionally scale down that plant for the sub, then it would produce 50 MW for two decades on the same fuel. A nuclear sub weighs in the neighborhood of 10,000 tons, so 4 tons for fuel seems pretty easy to manage. Obviously, all these numbers are very rough, but it's probably within an order of magnitude.
  - mrweasel 14y ago
    I'm a bit surprised about the Lithium use, that seems like a lot. Assuming that we spend the entire early production of Lithium we would only be able to feed 11.000 1GW powerplants.
    - InclinedPlane 14y ago
      The Lithium is used to line shells near the inner walls of the reactor, in order to breed Tritium. Most of the Lithium does not become Tritium though.
      - jessriedel 14y ago
        Does that make it reusable, either in the reactor or for other uses?
        InclinedPlane 14y ago
        Potentially. In practice it'd just sit in the reactor for most of the time, it's basically an up-front construction cost, not an ongoing consumable. Out of several tons of natural Lithium you're going to breed some amount of Tritium, but the reactor will only use a few tens of kg of Tritium a year. So in terms of consumables you are only using up a few kg of Lithium per year, not tonnes. Depending on how effective Tritium breeding is it's possible that not all reactors would need to employ the technique. In any event converting a kilogram of Lithium which costs about $100 into fusion fuel which has an energy equivalent to over 5,000 tonnes of coal (worth about half a million dollars) which seems like a pretty good use of materials. Even the cost of tonnes of Lithium would be paid off in the first year of operation several times over.
        jessriedel 14y ago
        Awesome, thanks.
- critium 14y ago
  I believe that ocean water has plenty of deuterium.
- mrweasel 14y ago
  I think that, just like with fission, the point is that you get a huge amount of energi from very little fuel. So you take on a few kilos/liters of fuel and spend very, very little at a time. The difference between fission and fusion is that with fission you just load all of it into the reactor at once. With fusion you'll need a "fuel-tank" and feed the reactor continuously, but it's just a few atoms each time. This is how I understood the concept, it may very well be wrong.
- AnthonyMouse 14y ago
  >Doesn't fusion demand constant refueling (like most power plants)? I don't think the half-life of deuterium/tritium is particularly long, so even if you could store it, it might not last decades before use. Though on the other hand, it's conceivable that you could design the reactor so that it produces deuterium/tritium on site from hydrogen cracked from seawater. But realistically the reason we want fusion over fission in the first place is the proliferation concern. All of the other arguments against fission are NIMBY nonsense that could be applied equally to a whole swathe of chemical processing facilities that nobody seems to care anything about because they're not OMG nukular. And there is hardly a major proliferation risk from a reactor surrounded by military personnel on a vessel designed to securely carry actual nuclear bombs, so it makes the specifically military application kind of questionable. We really need fusion for two reasons. 1) There is a better chance that we can actually build a large number of reactors without roving gangs of idiots destroying their cost effectiveness with gratuitous and intentional red tape, and 2) we can build them in countries we don't trust without making it easier for them to blow up the world, which will help to fight global warming vs. those countries continuing to burn oil and coal.
  - InclinedPlane 14y ago
    Deuterium doesn't decay, the half-life of tritium though is 12 years, which could be a problem. Though it's possible to breed Tritium from Lithium under neutron bombardment, which could potentially be a feature of a reactor design.
  - cstross 14y ago
    Deuterium is stable. Tritium has a half-life of 12.3 years. Like all proposed fusion reactors, you have to feed fuel in constantly rather than firing it up with a critical mass in situ, but you could store enough Tritium to run an aircraft carrier for a year in a smallish liquid gas cannister. (Smallish by aircraft carrier fuel tank standards.) ((With lead shielding. Lots of lead shielding. Tritium belts out about 9650 Curies of radiation per gram in the shape of beta radiation, which is enough to kill you really quick if you get it on or in you.))
- InclinedPlane 14y ago
  Fusion is unlike fission in that it will require constant resupply of fusion fuels into the reactor during operation, whereas in a fission reactor the fuel can just sit in the reactor for potentially a very long time. However, this is a distinction without a difference. Fusion fuel has an enormous energy density. For example, the fusion fuel capsule in a half megaton nuclear bomb is the size of a soda can, give or take. Deuterium-Tritium fuel has an energy density of around 3.4e14 Joules/kg or 94 million kilowatt-hours/kg. Even at a paltry 1% efficiency that means that operating a 50 megawatt output fusion reactor continuously for 10 years will only use about 50 kilograms of fuel, which is trivial to store.
DennisP 14y ago
Fusion reactors at that scale would revolutionize naval operations. Right now the carrier is nuclear-powered and can cruise at high speed for as long as it wants. But in practice it can't because the rest of the carrier group runs on oil, and can't keep up. It would change things a lot if the whole group could keep up with the carrier. This is probably why the Navy has been funding polywell research.

by cstross 14y ago

potatolicious 14y ago

I doubt it, but my understanding of the topic is limited. AFAIK nuclear reactors are popular on naval warships because they dramatically reduce (and in some cases eliminate) the need to refuel. It's one huge factor crossed off of your logistics problems - modern CVNs can go literally decades without refueling, with the only resupply being jet fuel, munitions, and life support. Doesn't fusion demand constant refueling (like most power plants)?

dalke 14y ago
Yes, it needs constant refueling, but the rate is something like a milligram per second. A year's supply of fuel would weigh a few kilograms.
jessriedel 14y ago
Nope. > A 1 GW fusion plant will need about 100 Kg of deuterium and 3 tons of natural lithium to operate for a whole year, generating about 7 billion kWh. http://fusionforenergy.europa.eu/faq/ http://fusionforenergy.europa.eu/faq/ If it's possible to proportionally scale down that plant for the sub, then it would produce 50 MW for two decades on the same fuel. A nuclear sub weighs in the neighborhood of 10,000 tons, so 4 tons for fuel seems pretty easy to manage. Obviously, all these numbers are very rough, but it's probably within an order of magnitude.
- mrweasel 14y ago
  I'm a bit surprised about the Lithium use, that seems like a lot. Assuming that we spend the entire early production of Lithium we would only be able to feed 11.000 1GW powerplants.
  - InclinedPlane 14y ago
    The Lithium is used to line shells near the inner walls of the reactor, in order to breed Tritium. Most of the Lithium does not become Tritium though.
    - jessriedel 14y ago
      Does that make it reusable, either in the reactor or for other uses?
      - InclinedPlane 14y ago
        Potentially. In practice it'd just sit in the reactor for most of the time, it's basically an up-front construction cost, not an ongoing consumable. Out of several tons of natural Lithium you're going to breed some amount of Tritium, but the reactor will only use a few tens of kg of Tritium a year. So in terms of consumables you are only using up a few kg of Lithium per year, not tonnes. Depending on how effective Tritium breeding is it's possible that not all reactors would need to employ the technique. In any event converting a kilogram of Lithium which costs about $100 into fusion fuel which has an energy equivalent to over 5,000 tonnes of coal (worth about half a million dollars) which seems like a pretty good use of materials. Even the cost of tonnes of Lithium would be paid off in the first year of operation several times over.
        jessriedel 14y ago
        Awesome, thanks.
critium 14y ago
I believe that ocean water has plenty of deuterium.
mrweasel 14y ago
I think that, just like with fission, the point is that you get a huge amount of energi from very little fuel. So you take on a few kilos/liters of fuel and spend very, very little at a time. The difference between fission and fusion is that with fission you just load all of it into the reactor at once. With fusion you'll need a "fuel-tank" and feed the reactor continuously, but it's just a few atoms each time. This is how I understood the concept, it may very well be wrong.
AnthonyMouse 14y ago
>Doesn't fusion demand constant refueling (like most power plants)? I don't think the half-life of deuterium/tritium is particularly long, so even if you could store it, it might not last decades before use. Though on the other hand, it's conceivable that you could design the reactor so that it produces deuterium/tritium on site from hydrogen cracked from seawater. But realistically the reason we want fusion over fission in the first place is the proliferation concern. All of the other arguments against fission are NIMBY nonsense that could be applied equally to a whole swathe of chemical processing facilities that nobody seems to care anything about because they're not OMG nukular. And there is hardly a major proliferation risk from a reactor surrounded by military personnel on a vessel designed to securely carry actual nuclear bombs, so it makes the specifically military application kind of questionable. We really need fusion for two reasons. 1) There is a better chance that we can actually build a large number of reactors without roving gangs of idiots destroying their cost effectiveness with gratuitous and intentional red tape, and 2) we can build them in countries we don't trust without making it easier for them to blow up the world, which will help to fight global warming vs. those countries continuing to burn oil and coal.
- InclinedPlane 14y ago
  Deuterium doesn't decay, the half-life of tritium though is 12 years, which could be a problem. Though it's possible to breed Tritium from Lithium under neutron bombardment, which could potentially be a feature of a reactor design.
- cstross 14y ago
  Deuterium is stable. Tritium has a half-life of 12.3 years. Like all proposed fusion reactors, you have to feed fuel in constantly rather than firing it up with a critical mass in situ, but you could store enough Tritium to run an aircraft carrier for a year in a smallish liquid gas cannister. (Smallish by aircraft carrier fuel tank standards.) ((With lead shielding. Lots of lead shielding. Tritium belts out about 9650 Curies of radiation per gram in the shape of beta radiation, which is enough to kill you really quick if you get it on or in you.))
InclinedPlane 14y ago
Fusion is unlike fission in that it will require constant resupply of fusion fuels into the reactor during operation, whereas in a fission reactor the fuel can just sit in the reactor for potentially a very long time. However, this is a distinction without a difference. Fusion fuel has an enormous energy density. For example, the fusion fuel capsule in a half megaton nuclear bomb is the size of a soda can, give or take. Deuterium-Tritium fuel has an energy density of around 3.4e14 Joules/kg or 94 million kilowatt-hours/kg. Even at a paltry 1% efficiency that means that operating a 50 megawatt output fusion reactor continuously for 10 years will only use about 50 kilograms of fuel, which is trivial to store.

DennisP 14y ago

Fusion reactors at that scale would revolutionize naval operations. Right now the carrier is nuclear-powered and can cruise at high speed for as long as it wants. But in practice it can't because the rest of the carrier group runs on oil, and can't keep up. It would change things a lot if the whole group could keep up with the carrier. This is probably why the Navy has been funding polywell research.