Use Sun to dispose of radioactive waste?

[GarageTinker]

Hello All,
Just from the stand-point of the Sun itself and it's environment, is it possible or not to utilize the Sun to safely dispose of spent radioactive waste fuel from reactors and what would be the possible ramafications (positive or negative) for the Earth and it's environment as a result, if any? Basically, why can't something like this be done to get rid of all the crap instead of storing it forever here on Earth?

This is a question I've wondered about off and on for the past several years. Givens: Completely safe and successful loading of waste onto a payload carrying vehicle; perfect and uneventful launch of said vehicle into a trajectory (sp?) that would take it to the Sun.

 

[Janus]

You could never assure a perfect and uneventful launch every time; A failure will eventually occur.

Not only that, it takes a lot of delta v to "drop" something into the Sun, as you have to shed almost all of the Earth's orbital velocity(It would actually take less delta v to launch it out of the Solar system entirely). If you miss, you end up putting the waste into an orbit that crosses the Earth's path with a chance of returning to the Earth.

 

[D H]

If you want to get rid of it in space, it is much, much cheaper to send it on an escape trajectory than it is to send it into the Sun. You would have to nearly cancel all of the Earth's orbital velocity of the waste to make something crash into the Sun, but you would only have to increase the speed by 41% to put it on an escape trajectory. In other words, the delta-V cost to make the waste crash into the sun is 2.4 times that needed to place send it out of the solar system. The energy cost differential is even greater since increasing energy/delta-V highly non-linear.

So, can we send the junk out of the solar system, or even cheaper, make it crash into another planet? NASA's deep space vehicles sometimes include a small (on the order of a kilogram or less) of radioactive material for power generation. Every launch of such a vehicle is met with environmental protests. The environmental community would not stand for launch immense amounts of nuclear waste into space.

 

[Chronos]

Launching radioactive waste into space by rocket is far too risky, as Janus noted. A space elevator would perhaps lower the risk to an acceptable level.

 

[GarageTinker]

Thank you for your responses, but I think I need to clarify a little bit. This being a hypothetical question out of curiosity, so costs, protests, the eventuality of bad launches and such aside, what would the effect/s on the Sun be; and by extension what would be the effect/s on Earth; if any, of dumping such waste into it? Would it disrupt or change the Sun in any way? I realize how huge the Sun is; something on the order of 100,000 Earth diameters if I'm not mistaken, which I may well be; so that the physical amount of waste in comparison would be miniscule, but in some cases changes can be brought about with the introduction of the slightest amount of a material.

 

[ray b]

what all is in ''spent radioactive waste fuel ''
Plutonium can be used as fuel or other things
and in the near future other so called waste radioactive's may be valuable too

I think secure short tern storage is a better answer
and let progress find the answers to how to deal with the ''spent radioactive waste fuel ''
some years down the road

but throwing it away is a bad idea

 

[B. Elliott]

I believe it was Astronuc who brought up the point that it would never truly reach the sun. It would most likely just reach the corona, only to be melted down and dispersed (boiled?) and ride the solar wind back out again... possibly reaching Earth.

 

[CaptainQuasar]

I had what I thought was a nifty idea a little while ago: I wonder if at some point during the next several decades it will be possible to construct nanotechnological devices, perhaps no more complicated than, say, a cotton gin, that could mechanically purify uranium or plutonium out of pulverized ore by physically sorting the atoms?

I should think the same sort of technology, if practical, might be usable to clean / reduce the volume of, or possibly recycle, waste.

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[LURCH]

You could drop all the radioactive material on Earth into the Sun, and it would have no noticable effect at all. In fact, you could drop the entire Earth and everything on it into the Sun, and the Sun wouldn't even belch.

 

[gmax137]

Can you tell how politicized an issue/question is by counting the number of replies you get, before your original question is answered? This one took eight answers (well, seven if you don't count the OP's need to remind the group what his question was).

And I don't feel too bad about making this reply, since I think LURCH has already provided the correct response to the question.

 

[mgb_phys]

I had what I thought was a nifty idea a little while ago: I wonder if at some point during the next several decades it will be possible to construct nanotechnological devices, perhaps no more complicated than, say, a cotton gin, that could mechanically purify uranium or plutonium out of pulverized ore by physically sorting the atoms?

I should think the same sort of technology, if practical, might be usable to clean / reduce the volume of, or possibly recycle, waste.

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You can already do it - it's called a centrifuge.That's how you get the enriched uranium in the first place.

The problem isn't removing U and Pu from reactor waste - nobody really wants to do that, there is only one real market for Pu and we already have enough of that. Highly active reactor waste is actually a very good place to safely store Pu. You can get rid of it by burning it as Mox fuel.
A bigger problem is the large volume of intermediate waste you generate by dismantling old reactors / facilities - this is still fairly unpleasant plus it's big, heavy and long lived.

 

[montoyas7940]

there is only one real market for Pu and we already have enough of that.

Could Pu be used as fuel for a reactor?

 

[CaptainQuasar]

You can already do it - it's called a centrifuge.That's how you get the enriched uranium in the first place.

Yeah, I know that's how uranium is enriched now. But it seems like a pretty sizeable and heavy-duty industrial process that requires effort on a national scale. I'm imagining that at some point in the future it might be scaled down to an effort equivalent to, say, churning butter, and trying to imagine how that might be done.

A bigger problem is the large volume of intermediate waste you generate by dismantling old reactors / facilities - this is still fairly unpleasant plus it's big, heavy and long lived.

What makes that kind of waste radioactive? I really don't know; my impression was that it's due to contamination by small amounts of U or Pu - is that correct?

My understanding is that some of the newer technologies, like pebble-bed reactors, have the problem that they produce more waste although they're safer and more manageable.

Also, when you say we have enough Pu, isn't that only due to limited use of nuclear power? If, say, China and the United States were both utilizing nuclear for power generation at the same rate as France does, for example, might we not end up in a situation where recycling was more important? (I don't know the answer to that one either, I'm really asking.)

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[mgb_phys]

Most industrial metalurgy extraction processes need large kit - a centrifuge isn't much worse than a copper mine, it certainly produces less waste.

Reactor materials etc aren't usually contaminated by Plutonium unless you had an accident, and Pu isn't that dangerous compared to some elements. The main problem is that all the decomissioning activities produce lots more waste, since anything used to decomission reactors also ends up becoming contaminated and so classified as waste.

The main use for Plutonium was weapons which is why a lot of waste is currently stored.
Either because there is valuable Pu in it and so if you ever needed to suddenly accelearate weapons production it's a good source, or because as it contains Pu it is politically difficult to move / process.
Sellafield (in the UK) was built as world leader for reprocessing reactor fuel from other countries (eg. Japan) that didn't have the technology to do it themselves but fear of a terrorist getting hold of the Pu has mostly stopped the trade.

If you had a very large increase in reactor use (and especially if you didn't have a local source of Uranium) you might look at breeder reactors. These burn used MOX fuel containing plutonium as waste from other reactors and at the same time create new U235 fuel from unwanted U238. in theory they are the best of all worlds but they are tricky to build and run, the source fuel isn't pleasant to handle and the current supply of natural uranium isn't a problem.

Pebble bed reactors have the advantage that they can't have a runaway reaction, and need much less complex support systems. They produce more waste while running but less at the end of life. The waste is less dangerous than from a fuel rod reactor and can't easily be reused in weapons (which is an advantage/disadvantage depending on who you are) but in the US at least it would still be classified as high level waste and so would ahve to be treated in the same way.

 

[gmax137]

Decommissioning reactors is not an easy task but it has been done numerous times. It takes a couple of years if you're in a hurry. Contamination doesn't cause the major disposal issue - the most radioactive parts (other than the spent fuel) are the various core intenals which have become activated by the neutron bombardment during reactor operation. These parts go to burial sites such as Barnwell in SC. Contamination is cleaned up by wiping everything with paper towels and then burying them. Liquids can be used and then evaporated, concentrating the contaminants into smaller volumes.

The notion that spent fuel from power reactors makes good weapons material is simply not true. The high burnup achieved for economic production of electricity leads to a preponderance of the "wrong kind" of plutonium. Just as there are several isotopes of uranium, there are isotopes of Pu, and those in the spent fuel produce a lot of neutrons spontaneously, which makes it very difficult to design & build a weapon device with this material. The AEC "production" reactors that created the plutonium for the bombs were operated for short cycles (weeks or a few months, I think), before the fuel was removed and the plutonium extracted.

The plutonium produced in a power reactor does make good reactor fuel though. In fact, before a reactor is shut down for refueling, a significant fraction of the power is produced by fission of the plutonium in the fuel. This Pu can be extracted and re-used, along with the U235 that remains in the fuel. The reason that refueling is necessary isn't so much because the uranium is gone, it's because the fission fragments have large neutron cross-sections, so they are absorbing more & more of the neutrons as the reactor operates and they build up. This eventually stops the chain reaction.

 

[mgb_phys]

Decommissioning reactors is not an easy task but it has been done numerous times. It takes a couple of years if you're in a hurry.

It's more difficult for some of the earlier sites that weren't really designed for decommisioning or have had accidents/leaks. It's also a little harder in the US where there is no intermediate waste category so everything that's not just gloves/paper towels is treated as fule assemblies.

The notion that spent fuel from power reactors makes good weapons material is simply not true

As is the idea that terrorists would want to make a complex Pu device instead of a simple U235 gun (or indeed that terrorists would bother when airliner+skyscraper is so effective). But Pu+terrorists+media is enough to stop shipments of recycled fuel to Japan.

 

[CaptainQuasar]

Contamination doesn't cause the major disposal issue - the most radioactive parts (other than the spent fuel) are the various core intenals which have become activated by the neutron bombardment during reactor operation.

Okay, this is the detail I think I don't understand properly - can non-radioactive material become radioactive, then, from neutron bombardment?

A friend of mine who was a reactor operator in the U.S. Navy said that this doesn't happen with the water used in a reactor, and that it actually stays safe to drink. But maybe it only happens to heavier elements than H and O?

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[mgb_phys]

Okay, this is the detail I think I don't understand properly - can non-radioactive material become radioactive, then, from neutron bombardment?

Yes if the element has a radioactive form with extra neutrons.
Light elements tend not absorb neutrons but merely scatter them that's why you don't turn the hydrogen in reactor cooling water into dueterium. You can create small amounts of tritium with a high enough neutron flux so it's probably not a good idea to drink lots of it.

 

[CaptainQuasar]

Thanks! I have wondered about that for such a long time.

(And, it probably explains why my friend is a bit odd. )

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