Why can't we use nuclear waste in nuclear reactors?

[mheslep]

The ocean is a giant heat sink.

The problem's not the ocean but preventing the radioisotope stack from melting in the same way that radioisotopes melted the Fukushima cores. It would require constant liquid cooling with a fairly large pumped flow, always running at sea and in port.

 

[mfb]

Well, a ship always has cooling water around while operational. 7.5 MW is ~20kg/s heated to boiling temperature or ~4kg/s boiled away.

 

[Evanish]

Direct heat is not an issue for global warming, as it does not change the equilibrium temperature (and the nuclear waste is there anyway).

Diesel has an energy density of ~50 MJ/kg, spread over one week this gives 80W/kg - a factor 800 above the value QuantumPion calculated for americium. Ships would not be happy if they had to carry 800 times more fuel, even if that fuel would last "forever".

Cargo ships burn between https://people.hofstra.edu/geotrans/eng/ch8en/conc8en/fuel_consumption_containerships.html of bunker fuel a day. Bunker fuel has an energy density of around http://www.people.hofstra.edu/geotrans/eng/ch8en/conc8en/energycontent.html. 50 ton* 907.185 kg/ton * 40 MJ/kg = 1,814,370 MJ a day

350 ton* 907.185 kg/ton * 40 MJ/kg = 12,700,590 MJ a day1,814,370 MJ * 0.277777777778 kwh/MJ = 503,992 kwh a day

12,700,590 MJ * 0.277777777778 kwh/MJ = 3,527,942 kwh a day503,992 kwh / 24h = 21,000 kw

3,527,942 kwh / 24h = 146,998 kwLets say the ships achieve a staggering 50% efficiency. 21,000 kw * .5 = 10,500 kw

146,998 kw * .5 = 73,499 kwSo cargo ships need between roughly 11 and 73 MW. From Wikipedia we get some power densities. Sr-90 = 0.46 kilowatts per kilogram

Pu-238 = 0.54 kilowatts per kilogram

Am-241 = ¼ Pu-238 = 0.135 kilowatts per kilogramSo if you got 100% efficiency you would need…10,500 kw / .135 kw/kg = 77,778 kg or 86 short tons

73,499 kw / .135 kw/kg = 544,437 kg or 600 short tonsOf course you won't get 100% efficiency. If you had a 10% efficiency you would need ten times that or 860 to 6,000, but it’s hard for me to believe more than 10% isn’t possible. With insulation you could increase the temperature difference and with it the efficiency. If you could get up to twenty percent then you would only need 5 times or between 430 to 3,000 tons. If you could manage 30 percent then you would only need between 287 and 2,000 tons. Now let's consider bunker fuel masses.A cargo ship going from Europe to the Us takes http://www.quora.com/Approximately-how-long-does-it-take-for-a-cargo-ship-to-go-from-Europe-to-the-USA depending on speed. Note that fuel use also varies with speed. So we get a range of...50 tons/day * 9 days = 450 tons

350 tons/day * 7 days = 2,450 tonsSo the range for bunker fuel is 450 to 2,450 tons.Compared to Am-241…

10% between 860 to 6,000 tons

20% between 430 to 3,000 tons

30% between 287 and 2,000 tonsThese masses seem to be in the same ballpark to me. If I made some mistakes please point them out.

 

[mfb]

Hmm, I relied on the values @QuantumPion used, apparently they are a factor 1000 too low. Okay, total mass would be acceptable. You still have the problem that you cannot have a single hot place where you burn fuel, because your whole fuel produces the heat throughout its volume. The large scale of a ship would make more than 10% efficiency plausible, but probably not the 50% the burning of fossile fuels gives.

And where do we get it from?
"A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes" according to Wikipedia - let's assume we just get the isotope we want and not the longer-living one - so we have to multiply those ~3000 tons by 10000 to get the initial fuel we have to burn to power a single ship from americium in its nuclear waste: ~30 megatons.

At ~40GWd/t that gives 1,200,000 TWd or 200 times the energy consumption of the world 2010 that has to be processed in nuclear power plants. If we satisfy the whole energy demand of the world (not just electricity, also transportation and heating) with nuclear power, and do so for 200 years, then we have enough americium to power a single ship. Well, not exactly, because 200 years is not short compared to the lifetime.

 

[Evanish]

Hmm, I relied on the values @QuantumPion used, apparently they are a factor 1000 too low. Okay, total mass would be acceptable. You still have the problem that you cannot have a single hot place where you burn fuel, because your whole fuel produces the heat throughout its volume. The large scale of a ship would make more than 10% efficiency plausible, but probably not the 50% the burning of fossile fuels gives.

And where do we get it from?
"A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes" according to Wikipedia - let's assume we just get the isotope we want and not the longer-living one - so we have to multiply those ~3000 tons by 10000 to get the initial fuel we have to burn to power a single ship from americium in its nuclear waste: ~30 megatons.

At ~40GWd/t that gives 1,200,000 TWd or 200 times the energy consumption of the world 2010 that has to be processed in nuclear power plants. If we satisfy the whole energy demand of the world (not just electricity, also transportation and heating) with nuclear power, and do so for 200 years, then we have enough americium to power a single ship. Well, not exactly, because 200 years is not short compared to the lifetime.

The whole fuel produces heat through its volume which is why you need some kind of coolant circulating through the fuel helping to keep the heat averaged out over the whole volume. Then you only remove heat from one non insulated area, but that heat is quickly replaced by heat from the whole volume by the circulating coolant. I don't imagine you would get 50%. Maybe 30%. It seems Am-241 wouldn't work, but I'm guessing you could do it with Sr-90. It has higher power density and a lot more of it is produced by nuclear reactors (the fission product yield from U-235 is 5.7%, from U-233 6.6%, but from Pu-239 only 2.0%).

 

[mheslep]

Well, a ship always has cooling water around while operational. 7.5 MW is ~20kg/s heated to boiling temperature or ~4kg/s boiled away.

Not enormous cooling volumes, granted. If existing LWR nuclear plant pumps are any guide, scaling down to 7.5 MW heat generation requires only a ~30HP pump, though it (or its backup) needs to be running continuously for the life of the vessel.

 

[mheslep]

I'm guessing you could do it with Sr-90. It has higher power density and a lot more of it is produced by nuclear reactors (the fission product yield from U-235 is 5.7%, from U-233 6.6%, but from Pu-239 only 2.0%).

Finding sufficient Sr-90 is also going to be impractical. If it were coming out of reactors in sufficient volume, the spent fuel casks would molten slag heaps and not merely warm to the touch cylinders.

 

[nikkkom]

Thanks for the link. It was very interesting. I used the numbers from the wiki article, and It seems like it might technically feasible to power cargo ships with Strontium 90. I think it would take around 50 to 350 tons

Sr-90 has a specific activity of 5.21 TBq/g. 100 tons of Sr-90 is about 500 exabequerels of activity.

It's unsafe to have so much activity on a vessel which can sink, be hijacked or sabotaged.

 

[Evanish]

Ok, I surrender already. I still think ships need to be nuclear powered. I hope fission reactors replace bunker fuel in the near future.