Fusion power and water depletion.

Assuming we ever get a workable fusion power system up and running how rapidly will we deplete the world’s water supply?

Three working assumtpions:

1)We can convert 10% of the energy released in the reaction into usable power.
2)All of the world’s current fossil fuel energy is replaced using fusion power.
3)World energy use will remain at current levels forever.

So how long until we see the ocean levels dropping? I’m assuming it will be millions of years, but I have absolutely no idea how much energy hydrogen fusion releases.

Well it’s not so easy. By splitting water into H and O and fusing the H we get a chain of radioactive particals, which are gases (well most of them), which also makes them very difficult to store safely and you don’t want to just emit a bunch of radioactive gasses into the atmosphere.

The only fusion reaction that produces energy AND does not create a radioactive byproduct is He3, the nearest place we can get this stuff is the moon. Is it worth the effort to mine the moon for it, well I have heard a estimate that if we send back a container the size of the space shuttle’s external fuel tank full of He3, it could supply current PEAK power demand for the USA for a time of 1 year.

Huh? When I was 8 years old splitting water with my toy electric train train transformer, I was making radioactive gasses?

Oh wait, you’re referring to the fusion of hydrogen. How can other heavier gasses besides helium be formed?

Maybe these other unwanted reactions occur in the sun, but not necessarily in (any potential) Earth bound reactions?

I don’t know if there are any radioactive byproducts of pure hydrogen fusion, but it’s far easier to fuse deuterium and tritium, which results in He4 plus a whole mess of neutrons which can induce radioactivity in the reactor vessel itself.

Helium-4 nuclei are alpha particles, and are therefore radiation. They are not, however, radioactive (that is to say, they don’t emit any further radiation). It’s very easy to stop an alpha particle, and once it’s stopped, it’s just harmless helium.

The reactor itself might be rendered radioactive, but that’s all solids, and in a fairly confined area. So that’d be no worse than the situation with fission reactors. Considerably better, in fact, since you only have the secondary radioactivity to worry about, not spent fuel.

And even if we only used deuterium (the easiest to use), there’s enough of that for (IIRC) hundreds of millions of years. Deuterium accounts for a little more than one hundredth of a percent of hydrogen, so when we’ve used it all up, the sea levels will be down by that amount. Frankly, I’m not too worried.

A back of the envelope calculation (if i did it right) suggests it would take 18 million years before the oceans dropped by one millimeter. My assumptions are 10 million electron volts of energy released per helium atom created, world energy consumption of 500 quadrillion BTU/year and ocean area of 300 million square kilometers.

True as far as it goes, BUT: the D-He3 reaction doesn’t produce neutrons or radioactive products, but any plasma hot enough to fuse deuterium and helium-3 will also produce tritium by D-D reactions.

P - B11 fusion also creates no radioactive byproducts. It’s orders of magnitude harder to accomplish than He3 fusion, but has the advantage of using fuel that we already have in effectively unlimited quantity.