Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Jodie Rio on 29/01/2012 11:35:56
-
Jodie Rio asked the Naked Scientists:
Hi NakedScientist guys and gals,
I've been listening to your show (http://www.thenakedscientists.com/HTML/podcasts/) for about 6 months now and I love how you keep us all informed on current events in the scientific world, so thanks and keep up the good work!
I was wondering if you could answer a question that has been plaguing me for a while about nuclear waste. I understand that enriched Uranium/plutonium/thorium is used to generate radiation and heat, producing steam which drives turbines. After the fuel rods have been used they are then depleted but still radioactive and producing heat so they are put in cooling pools, or buried underground. What I don't understand is if these fuel rods are still generating heat why are we trying to find ways to dispose of them instead of continuing to use them?
I hope you can help as I feel I must be misunderstanding something fundamental about the process, if something that seems so obvious to me isn't an option in reality.
Yours Befuddedly,Â
Jodie Rio of Plymouth
What do you think?
-
Excellent question.
You can think of nuclear power as having two components.
Normal radioactive decay, or the loss of photons, electrons, positrons, and helium atoms. This generates a lot of low level heat, and is in fact what we use to fuel some of the deep space probes such as Voyager. However, it doesn't produce the very high temperatures that our nuclear power plants run with.
Fission. The atoms in the radioactive materials actually split more or less in half, or split into big chunks. This generates a lot of heat. It can be a self sustaining reaction requiring the release of neutrons. This is our main power source for nuclear power.
Unfortunately, some of the products of nuclear fission reactions are what are called nuclear poisons, or elements that absorb neutrons, without producing fission. And, thus counteracting the sustaining of a fission reaction. As the nuclear poisons build up, the fission reaction stops, even though the material remains very radioactive.
In the USA, the spent fuel rods are then considered waste.
Soon after removing the rods from a nuclear reactor, they are considered "too hot to handle". However, they stabilize after a year or so. Nuclear power plants are based on high energy levels, but one might at least expect that the cooling ponds could be designed to power their own pumps, but apparently this isn't the case.
After the initial "cooling", the rods still have about 95% fissionable material. There are methods to separate the fissionable materials from the nuclear poisons and recycle the rods. Even the poisons may be able to be used as part of the shielding in the reactors, and can be re-burnt. However, the USA has chosen not to utilize this technology. Japan, and other countries do recycle their nuclear waste.
One of the issues is that one may be able to enrich bomb-making materials from the nuclear waste. Yet, that would all depend on processing.
To many people, it just seems silly not to recycle the fissionable material, both to reduce the waste streams, as well as using less of our natural resources.
-
It of course a question of economics a power plant to use the relatively small amount of heat given of by radioactive decay would would be hopelessly un economical.
Fuel reprocessing has had a mixed success with a very expensive plant in the UK ending up mothballed it is again a matter of economics depending on the relative cost of Uranium compared with the reprocessing cost.
As pointed out by the previous correspondent reprocessing leads to the separating out of highly dangerous isotopes that are difficult to store and secure.
-
The highly dangerous fissionable products are sent back into the reactors, although they could be used for building bombs.
The highly dangerous non-fissionable products, are likely no more dangerous than the spent fuel as a whole.
As far as producing new fuel, it is cheaper, and safer to extract the fuel from uranium ore.
However, does it change the economics if one considers the long-term waste storage cost, on the order of 10,000 years!!!
The other thing to keep in mind is that uranium is a non-renewable resource.
I'm seeing an annual global consumption (http://en.wikipedia.org/wiki/Uranium_depletion#Uranium_production) of about 41,000 tons of Uranium.
Total estimated uranium deposits (http://en.wikipedia.org/wiki/List_of_countries_by_uranium_reserves) are estimated around 6,000,000 tons (greater if sea water is included).
So, somewhere around 100 to 150 years, the uranium deposits could be permanently depleted on Earth (although extended if it can be extracted from sea water).
If recycling could allow the fuels to be re-used... and reduce our consumption by a factor of about 1/20th, then it would extend the project lifespan of Uranium ore to over 2000 to 3000 years!
-
It's because a lack of imagination. My job had to do with nuclear power, so I have some experience about it.
The containers of high level waste glasses resulting from reprocessing (e.g. in Sellafield, U.K.) have a surface temperature so high that it has been necessary to get a special permission from the safety authority because the law doesn't allow that any equipment in a place where men can enter for work must not be higher of something like 60°C. This heat is lost for hundred of years, while with a suitable system it could be used for heating and air conditioning.
-
It is because of all the legal and logistical problems assosiated with handling radioactive materiel.
If I wanted to use them to heat the building I own can you imagine how many objections would be lodged, it is much cheaper and simpler to use gas or oil.
-
I don't think I'll volunteer to keep any "too hot to handle" spent fuel rods in my living room to heat the house [:0]
I wonder if one could build a second tier nuke plant that would take the spent rods, and put them into a low level reactor, perhaps producing a few hundred KW, or a MW or so.
It would be excellent for a small town. And, in principle a lot safer than the big nuke plants. Except for the NIMBY concept.
Issues, of course, would be the cost. Is it cost effective? Transportation of the spent fuel rods, especially those just out of the reactor (which would be the hottest in more ways than one) would still be a problem.
And, there is always issues of control of the nuclear materials.
Perhaps an alternative would be to build a low-energy reactor as part of the main reactor complex, where the spent fuel rods would be sent rather than just storing them in a cooling pond.
One issue, of course, is that if the lifespan of the "new" rods is only 6 months or so, but the "used" rods might be more or less good for a decade or more, then there would be a glut of the "used" rods. But, I suppose one would then just figure out the proportions of each type of reactor. Would dense packing in a reactor cause the production of new isotopes that would not be produced in a cooling pond?
With any luck, after a decade of "cooling" in a low level reactor, the rods would be much better feeder stock for a reprocessing plant.
-
I think if there was the slightest chance of it being economical it would have been done by now , I think the integrate of the cooling arrangments is much more important than trying to extract the last small amount of power (look what happened in Japan).
-
I think if there was the slightest chance of it being economical it would have been done by now
Well... In Fact...
http://en.wikipedia.org/wiki/Spent_fuel_pool#Other_possible_configurations
http://aiche.confex.com/aiche/2005/techprogram/P21198.HTM
China to utilise spent fuel for heating and desalination.
A Chinese group has agreed to build a 200 MWt nuclear reactor run on used fuel from nuclear power stations to provide district heating and desalination. The US$ 42 million project at Yingkou in Liaoning province will heat 5 million square metres of buildings over 4-6 months each year and initially desalinate 3000 tonnes (3 megalitres) of seawater per day in warmer months, rising to 80,000 tonnes (80 megalitres) later. The deep-pool reactor will operate at atmospheric pressure, which will reduce the engineering requirements for safety.
Comtex 25/6/02 (via Wilmington).
-
Ok, here is a chart of decay heat in KW per ton of Uranium.
For isolated rods in a cooling pond.
[ Invalid Attachment ]
One gets about 100KW per ton of Uranium for fresh "spent:" fuel. And, over the first year, it drops down to about 10KW per ton of Uranium. Over a decade, it is down to about 1KW per ton.
I'm not quite sure what the actual capacity is of the reactors. Perhaps 100Tons, with 1/3 of the fuel changed out each year.
A few KW is a drop in the bucket when considering a GW plant, and thus generally isn't considered as part of the energy production of the plant.
One of the questions that came up with Japan, now that you mention it, is why the spent fuel ponds could not produce enough energy to at least power their own pumps.
I would also ask whether the spent fuel can only be used for decay fuel, or if it could actually be maintained at a low level of fission, and perhaps provide long-term low-grade energy.
-
"I would also ask whether the spent fuel can only be used for decay fuel, or if it could actually be maintained at a low level of fission, and perhaps provide long-term low-grade energy."
Spent fuel is just that it has aquired so much fission inhibiting materiel that it is no longer usfull in a reactor of the the design where it was initialy working.
Perhaps it could be transfered to a reactor of different design where fission could still take place which seems to be what you are suggesting but I do not think that providing a second reactor where poisened fuel rods could be used to provide a small amount of additional power could ever be economical even if such a reactor was technicaly possible.
I note also the article concerning the generation of Hydrogen this was also the cause of an accident in a German wartime reactor that may well have seriously delayed the development of nuclear weapons there and of course added to the recent problems in Japan.
It probably as well to prevent the generation of Hydrogen as much as possible!.
I have just noticed your newsletter 2002 I wonder if it ever got built, China is a strange place they can do things there that are not possible in other parts of the world (such as moving hundreds of thousands of people to builds huge dams).
-
I have just noticed your newsletter 2002 I wonder if it ever got built, China is a strange place they can do things there that are not possible in other parts of the world
The link came from Wikipedia, above, but may be a dying link, so I copied the text. Perhaps I'll try to get more info on it.
Yes, the NIMBY syndrome.
People in China might not be informed of the heat source for their schools.
$42 Million seems to be quite a bit for a small heat plant. However, to put it into perspective, the University of Oregon is planning to spend on the order of $80 million for upgrades to their already existing natural gas steam plant. Of course, the Nuke plant would have fuel handling costs and perhaps higher labor costs. However, if it considered to be using a waste product, then it might be cheaper than a fossil fuel source.