"Radioactive waste will stay dangerous for hundreds of thousands of years"

clancy688

There's one statement which's always used by people opposed to nuclear power - that the waste will stay dangerous for thousands, ten thousands and even hundred thousands of years. But I'm wondering - is this accurate?

After following the Fukushima accident for over one and a half years, I came to the conclusion that stuff like Cesium, Iodine, Strontium, etc. is the most dangerous, but all those isotopes have half times of "not more" than several decades. Meaning that they'll be mostly gone after several hundred years.
Of course there's other stuff like Plutonium which's halftime is close to 25000 years, but it's a heavy metal and while it's also very poisonous, it won't be nearly as volatile as Cesium.

So... what hazard would a spent fuel cask pose after 1000, 10000 and 100000 years?

daveb

Depends upon whether it's damaged or not. Inatct, not much at all (see this.) Leaking, it would depend upon what was leaking and at what rate.

gmax137

Longer half life = less active

QuantumPion

The fission products decay to a non-hazardous level after around 200 years. The most prominent fission products are Cs-137 (half life = 30 years) and Sr-90 (half life 28 years). Most of the remaining radioactivity is due to long half life actinides which are not as dangerous as they are low activity alpha emitters. The exception is Tc-99 which is a beta emitter with a half life of 200,000 years. Tc-99 could be removed by reprocessing, or transmutation.

nikkkom

After 1000 years activity of Cs-137 and Sr-90 will fall by factor of about a billion. (It falls by a factor of 1000 every 300 years).

Thus, the activity will be dominated by various actinides and Tc-99.

Activity of actinides will continue to fall off, but Tc-99 has half-life of 200k years and stays essentially the same on the time scales of thousands of years.

30 MWd/kg burnup fuel will have ~500*10^6 Bq/kg activity when 100k years old. almost all of it from Tc-99.

nikkkom

IIUC current large-scale reprocessing in France and Britain only separates U and Pu, no further separation of actinides and fission products is done.

I can hardly blame them, though - the dissolved fuel is extremely radioactive, making all chemical operations more complex due to radiolysis - in addition to the expected PITA of having to do everything remotely. So they do the necessary minimum (recover most of fissionable and fissile materials) and vitrify the rest.

clancy688

Okay, thank you very much. How dangerous is Tc-99, compared to Cs-137 or Sr-90?

QuantumPion

Tc-99 is a beta emitter so its radiation is absorbed by the fuel cladding. Cs and Sr are gamma emitters, which can only be attenuated by shielding, not entirely blocked. Also the activity of Tc-99 is much lower due to its long half life. I don't know about the environmental or biological properties of Tc if it were released.

nikkkom

Not entirely true: Sr-90 emits 0.5 MeV beta, not gamma. But its daughter Y-90 also decays (64 hours, to stable Zr-90) and emits much stronger 2.2 MeV beta. Betas generate secondary gammas when they propagate through matter, more energetic betas give more gammas.

Thus, Sr-90 emits gammas as secondaries, while Cs-137 is a bona fide gamma emitter.

Tc-99 emits 0.3 MeV beta and decays to a stable daughter, making its secondary gamma emission rather weak (they are in fact X-rays, they aren't in gamma part of the spectrum).

And of course, Tc-99 has around ~6500 times lower specific activity (Bq/kg) than Sr-90, owing to its much longer half-life.

Still, standing next to a one ton metallic cube made of Tc-99 is not advisable :)

Angry Citizen

I would be more afraid of the chemical contamination than the radiological contamination. Uranium 238 is essentially harmless from a radiological standpoint, but if you ingest it, you and your children and your children's children are in for a raw deal.

nikkkom

Really? Radioactivity of spent fuel indeed comes mostly not from U, but it (radioactivity) is there. You think half a billion decays per second in each kilogram is not much?

nikkkom

My answer is: it would be enough of a hazard even after 100k years. Mostly because its contents isn't chemically immobilized.

Reprocessing *should be done*, anything else is much more irresponsible for the future generations.

With reprocessing:
(1) we recover fissionable and fissile material (spent fuel usually has ~95% of it compared to fresh fuel - IOW: most of it is still not burned)
(2) we reduce the volume of the waste
(3) we can vitrify the waste, making it insoluble

Then drop waste containers into boreholes reaching some subduction zone and voila.