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Thread: Spent fuel rods

  1. #1 Spent fuel rods 
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    We've heard all about the water boiling away in spent fuel rod pools recently. From what I've read these rods generate heat for years after they're removed from the reactor and can happily boil 2000 tonnes of water away. Curiosity compels me to ask what happens to this heat in normal circumatances. It is captured and used in some profitable way (i.e. through heat exchangers), or just pushed out into the atmosphere? (Note I'm talking about the waste heat here, not radiation leaks or anything).
    Secondly, if the rods can produce useful heat, why are they termed 'spent'?


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  3. #2  
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    In normal circumstances cooling water is circulated through a heat exchanger to remove the heat from the fuel pool. The heat is just dissipated into the sea water or river water cooling system. It is called spent fuel because it doesn't have enough fissionable material left to make enough power in the reactor core for generating a useful amount of electricity.

    The spent fuel generates heat for years, but a little less each year as time goes by and the radioactive isotopes decay off.


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  4. #3  
    Forum Freshman whoisjohngalt's Avatar
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    For your second question, spent fuel is also the result of the way a core is mapped. During different life-cycles of the core, there is a different rod withdraw sequence to bring the core critical. This takes into account the fuel density in the core. Control rods are inserted and withdrawn in groups. Some groups are major groups and some are minor. In order to safely control a reactor, you don't necessarily want your major groups all the way withdrawn or all the way at the bottom. We also don't want fission/heat generated to be concentrated in one specific area of the core. We want it to be spread out.

    All of these concerns lead to a more average fuel consumption over the core and makes it impossible to completely exhaust the fuel. Because Uranium still exists after the core is 'exhausted', it will continue to generate heat for decades.

    There are facilities that reclaim spent fuel and reuse them, and in the future, I think it will play a bigger part in building new power plants.

    For your first question, my guess is the water that is being boiled off does not generate a high enough pressure to perform any useful work to be economically viable to attempt to use. Also, the lower demand items like, say, heating domestic water probably don't want to use that heat source because of the nuclear aspect of it and the proximity required.
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  5. #4  
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    Thanks for your helpful replies.

    Still seems such a waste, pumping all that energy (heat) into the sea, given that the rods produce it for years, but can see the difficulties in reclaiming it.

    I did hear of a power station (in Sweden I think) where waste heat was sent via very heavily insulated pipes to heat homes in nearby towns and villages but this could have been a non-nuclear facility - honestly can't remember. Don't even remember where I read/heard about it now.

    Cheers.
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  6. #5  
    Forum Isotope Bunbury's Avatar
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    The system you read about is called district heating, or combine ehat and power (CHP) and is widely used in Scandinavia and is in use in some large US cities which pipe low pressure steam from fossil plants around city buildings. The cooling water is too low in temperature to be useful but condensate from the turbines and LP steam from the HRSG can be used this way. Nukes don't use HRSGs but I would think there is still the possibuility of using turbine condensate.
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    As far as delivering water or coolant to the spent fuel containers has any one though of using essentially a water bomb?

    It seems radiation has gotten so bad they cant get close enough or not long enough to make a difference in adding water to the spent fuel rods containers.

    What if some one developed a laser guided munition where instead of a payload, it split apart and released Water or coolant directly into the targeted area.

    I don't know if any one has considered this option, but it seems the helicopter drops and fire truck runs aren't working very well.

    Not sure where to suggest an idea like this. Seems like a good place to start.
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    Forum Radioactive Isotope MagiMaster's Avatar
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    Can you post sources when you say stuff like "it seems the radiation has gotten so bad"? There's a lot of really bad reporting out there.

    Anyway, why would a missile work at all? Water's very heave compared to explosives and the like. Helicopters and planes can carry a lot of the stuff. A missile couldn't. If airdrops won't work, a missile certainly won't. Also, hitting a pile of radioactive material at high speed with anything, even water, is probably a bad idea.
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  9. #8  
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    The radiation levels are keeping people from getting a close look at the fuel pools. I think they have probably had success with the fire hoses. The temperatures measured from helicopters by infrared thermometers are 100C or less. They have put enough water in to fill the pool 3 times over, but it's a bit difficult to tell where the water is going, due to the mass of twisted metal from the explosions.

    I think probably they will be able to fill the fuel pools using pumps once they get power back. I don't know for sure, though. It might require somebody to get close enough to open some valves.
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