Fusion vs Fission - Feasibility and cost

[Lord Zentei]

The core of a fission plant is a lot more massive than the fusion variant, though most of the mass is going to be shielding if you want to use it in terrestrial applications. This would be less of an issue for interplanetary spacecraft, whose shielding only has to seperate the crew compartment from the reactor on one side.

For data: you might want to google "star power" -- an experimental reactor is in the works. As to how long it will take before it becomes competitive... 50 years is the usual formula.

As for specific impulse, the Daedalus project provided a concept design that allowed for a SI of roughly one million seconds. Thrust would be very low for any fusion rocket, however.

[WereGron]

2) Would fusion power offer advantages over fission in applications such as naval and space ships? If so, would it be better than fission in every way, or would there still be situations where good old uranium would still be a superior choice? How long do you think it would be until fusion is able to compete here?

Main fusion advantage: avaibility of fuel.

On Earth and otherwise, Fission fuel is scarce. In fact , energy stored in U-235 is hardly bigger than that in coal, oil and gas combined. The only alernative are brooding reactors gainings plutonium from U-238, but they are very risky as they can turn into an outright atom bombs. Which brings us to second issue - safety. Because fusion requires very hard pressure and so, any disruption would most likely result in stalling the process, not creatinmg a huge BOOM, an energy worth of a few seconds would be released at most. Thus, fusion is safer.
Also if a fusion reactor DOES blow, most outfall will be harmless - alpha and beta particles, but no constant source of them. Now the most important issue precluding use of nuclear power in vehicles is safety. Fusion promices to bypass this.

[Patrick Degan]

On Earth and otherwise, Fission fuel is scarce.

No, it isn't. There is enough uranium that can be extracted from seawater to last millions of years.

There is also the possibility of waste reprocessing techniques which would extend available uranium supplies on the order of several hundred millenia as well. Fission is a potentially viable power source for any forseeable future.

[Fingolfin_Noldor]

Fast Breeder reactors are designed such that they can produce lots of fissile material that can be later used for other reactors. Japan and Russia are studying the concept quite closely with Russia quite ahead of the pack I believe.

[Winston Blake]

Because fusion requires very hard pressure and so, any disruption would most likely result in stalling the process, not creatinmg a huge BOOM, an energy worth of a few seconds would be released at most.

Fission reactors don't blow up on failure either. There are already pretty safe designs out there, but even if it does fail, the core melts down, which isn't pretty, but is certainly isn't a bomb, and the damage from it can be contained quite well in modern reactors, and with future development, it can probably get even better.

Further, as far as I know, modern pebble-bed reactors cannot have a meltdown. On TV I recently saw a demonstration of a modern reactor being deliberately put under catastrophic failure conditions that would caused a meltdown in a pressurised water reactor, and it just reached a stable condition and nothing happened.

[QueerIngo]

1) Does nuclear fusion look feasible for potentially replace fission based power plants on the ground in the future?

With modern-day technology and resources, there's really no reason I can see to support fusion over fission.

2) Would fusion power offer advantages over fission in applications such as naval and space ships?

For space ships, it would allow a greater (factor of 2 or so) specific impulse, if we can actually shrink the damn things enough to get them on a spaceship. For naval reactors I can't see any point to it, although we should replace the current ones with ones that don't use such highly enriched uranium.

3) Are fusion based space rockets feasible? What kinds of specific impulse and thrust would be realistic for these?

For a pure-fusion system, you could get exhaust velocities around .1c (corresponding to a ~3,000,000 second specific impulse). Thrust really depends on what kind of system you use.

The core of a fission plant is a lot more massive than the fusion variant,

We don't even have a working fusion power plant yet, so I don't know how you're able to make size comparisons for nonexistent reactors. I'd be interested if you had anything on the physical size of ITER, the closest thing we currently have.

Main fusion advantage: avaibility of fuel.

Deuterium, one fusion fuel, is in seawater at a concentration of ~20 ppm by mass. Lithium, the other fuel, is found in the crust at around ~100 ppm by mass. Uranium, the current fusion fuel, is found at ~40 ppm by mass, and thorium (which is also fissile) is found at around ~120 ppm. So, I really don't see what the major advantage is.

In fact , energy stored in U-235 is hardly bigger than that in coal, oil and gas combined.

And how do we know this? We made it up!

Quick reference check:

3.9 × 10^22 J energy in world's estimated total fossil fuel reserves (2003)

3.0 × 10^31 J energy in world's estimated recoverable U-238 reserves (2003)

Hmmm, that's a difference of nine full orders of magnitude. Even if you only consider the U235, it's still seven orders of magnitude.

but they are very risky as they can turn into an outright atom bombs.

We made this up too. It is physically impossible for any kind of nuclear power plant to become a nuclear explosive. The closest we ever got was Chernobyl, which was NOT a nuclear explosion- it was a steam explosion, with release of radioactive gases.

any disruption would most likely result in stalling the process, not creatinmg a huge BOOM,

Except that the supercooled magnetic coils used to store the fusion contain lots of circulating current, and that if coolant is lost, these coils will get very hot very quickly, creating a very nice "BOOM".

Also if a fusion reactor DOES blow, most outfall will be harmless - alpha and beta particles,

You forgot the radioactive tritium gas, which is used as fuel and is easily taken up into the body. Oops.

I'm not sure fusion in cars would work.

We know that fission in cars would work, at least mechanically- behold, the Ford Nucleon![/quote]

[Teleros]

1. Commercial fusion power is, and always has been, 40 years away . On a more serious note, well lets see what various upcoming projects on fusion yield.

2. With fusion reactors, only the reactor casing itself (if that makes any sense ) is left radioactive afterwards, unlike with fission, so a fusion reactor is easier to dispose of and I imagine maintain. Plus the various points mentioned above. Otherwise I don't see much point in replacing the navy's nuclear subs just because you've got fusion plants.

3. How feasible they are depends on size, weight etc - not going to try and predict the future (usually wrong ). It would also be a lot safer than fission-powered ones (less radioactive debris if all goes wrong at launch etc), so that would probably make it more attractive to people.

[kinnison]

Actually, fusion power does not necessarily produce neutrons and therefore radioactive waste, or need radioactive fuel. For example, the boron-11/proton reaction produces 3 alpha particles and a gamma ray - nothing else. Lithium-6/deuterium produces two, most of the time.

As for the size of the reactor, there seems to be some hope of a rather small one, using electrostatic confinement. This is quite easy to Google - Professor Bussard seems only to need a relatively small amount of money. This method, with a small amount of tweaking, also gives you a fusion rocket

[Fingolfin_Noldor]

Actually, fusion power does not necessarily produce neutrons and therefore radioactive waste, or need radioactive fuel. For example, the boron-11/proton reaction produces 3 alpha particles and a gamma ray - nothing else. Lithium-6/deuterium produces two, most of the time.

As for the size of the reactor, there seems to be some hope of a rather small one, using electrostatic confinement. This is quite easy to Google - Professor Bussard seems only to need a relatively small amount of money. This method, with a small amount of tweaking, also gives you a fusion rocket

However, fusion does produce enough neutrons that cause the surface of the tokomak reactor to be radioactive and that is one of the problems that has yet to be dealt with.

[rhoenix]

However, fusion does produce enough neutrons that cause the surface of the tokomak reactor to be radioactive and that is one of the problems that has yet to be dealt with.

True - and it was thinking along these lines that I created my new ITER thread in SLAM. According to their site, there is radioactive material created, but they stay radioactive for a much shorter time than with fission - less than 100 years in most cases. The rest would need to be disposed of as with fission materials, unfortunately.

( direct link, and the FAQ )

[Velthuijsen]

1) Does nuclear fusion look feasible for potentially replace fission based power plants on the ground in the future? How long in the future would you estimate until fusion technology is competitive with fission reactors, also assuming fission technology continues to mature?

Not yet. ITER is planned to do test runs that will generate 100 MW for 73 MW put into the reactor. This is to be increased to 500 MW (not sure of the amount to be put into the reactor at that level) output.
This is a test fusion reactor while we have fully functional fission reactors with the latest generations having very little excess reactivity.

2) Would fusion power offer advantages over fission in applications such as naval and space ships? If so, would it be better than fission in every way, or would there still be situations where good old uranium would still be a superior choice? How long do you think it would be until fusion is able to compete here?

The benefit would be that it is easier to harvest fuel. But until we have working production type fusion reactors fission will be the winner.
That said fusion is better used as a propulsion source. That would simplify the design, especially if you can come up with a way to have the magnetic bottle (partially) outside the vehicle. Another problem you most likely won't have (or in a reduced form) is removal of waste products since they are used for propulsion.

You forgot the radioactive tritium gas, which is used as fuel and is easily taken up into the body. Oops.

Which a reactor like ITER will be using 54 grams of in a cycle, a cycle is 400 seconds, of which 1 gram is estimated to undergo fusion. And a worst case scenario of another 350 grams of tritium 'stored' as deposits, that is the thing blows just before a clean up of the deposits is mandated.

A bigger hazard would be the beryllium lining of the entire reaction chamber.

[metavac]

Questions:
1) Does nuclear fusion look feasible for potentially replace fission based power plants on the ground in the future? How long in the future would you estimate until fusion technology is competitive with fission reactors, also assuming fission technology continues to mature?

If by feasible you mean applicable in any predictable time frame, no. The research is primarily focused on discovering science that will properly bound the engineering of workable fusion powerplants. That said, there's no physical reason why we can't expect a useful fusion reactor sometime in the future. I suspect, though, even if fusion comes to dominate industrial energy it will be thoroughly complemented by fusion powerplants.

2) Would fusion power offer advantages over fission in applications such as naval and space ships?

That's a hard question to answer. Ideally, a perfectly burning fusion plant that's the same size and mass of its fission counterpart would produce more power per pound. A reaction can be chosen to be aneutronic. An ideal magnetic confinement plant pitted against a modern day fission reactor would further be able to magnetohydrodynamically capture thermal energy and convert it into electricity, a far more efficient method than heating a working fluid.

On the other hand, we don't know all the obstacles that may constrain what we can expect from a practical fusion plant. Until we do, it's impossible to say whether fusion can ever supplant naval nuclear power as we currently know it. Fusion research may even have implications that make fission powerplants more compact and burn more efficiently; the resulting cost reductions might even beat out a the best fusion alternative available. Gas core and nuclear salt water rockets might be pretty lightweight in comparison. Both give you pretty decent specific impulses and thrusts--enough for say a vibrant economy in Earth's orbits or in the inner solar system.

The point is that without an engineered idea of where fusion is headed, we can't say where and when fusion will supplant fission or any other power generation scheme. If history is any guide, it simply may end up complementing every other energy source mankind's discovered or innovated, from fire all the way through antimatter.

[Starglider]

With modern-day technology and resources, there's really no reason I can see to support fusion over fission.

One standard reason is 'fusion is less likely to be derailed by the enviro-cretins and NIMBYs'. Given that Greenpeace is protesting about ITER (I doubt most of them even comprehend anything beyond NU-KU-LAR = BAAAAHD!) I'm not sure this holds any more.

3) Are fusion based space rockets feasible? What kinds of specific impulse and thrust would be realistic for these?

For a pure-fusion system, you could get exhaust velocities around .1c (corresponding to a ~3,000,000 second specific impulse). Thrust really depends on what kind of system you use.

You can also use fusion to power an ion accelerator with near-c exhaust velocities at the cost of very low thrust and high engine mass. Fusion will do this with a fuel mass (and possibly lower engine mass) than fission.

Deuterium, one fusion fuel, is in seawater at a concentration of ~20 ppm by mass.

Refining deuterium in large quantities is rather easier and less environmentally disruptive than mining uranium. Does anyone know if it's practical to produce deuterium by exposing tanks of liquid hydrogen to a strong neutron flux? I would've thought that would work in principle but I don't know if it's practical - not that it's ever likely to be relevant, it would take thousands of years of consumption for it to be even vaguely necessary and at that point I'd be very surprised if we were limited to earth's resources.

Also if a fusion reactor DOES blow, most outfall will be harmless - alpha and beta particles,

You forgot the radioactive tritium gas, which is used as fuel and is easily taken up into the body. Oops.

Which will immediately escape into the upper atmosphere, harming no one.