Why are Reators Still Built Above Ground?

Discussion in 'General Science & Technology' started by Carcano, Dec 2, 2012.

  1. Carcano Valued Senior Member

    The most enduring image of the Fukushima disaster is of emergency crews desperately using firehoses to spray cooling water high over the walls of the containment shell...which had been blown to smithereens by an internal hydrogen explosion.

    This of course, was largely futile considering that MOST of the water sprayed in simply flowed out the broken walls and into the ocean.

    Strangely enough, it appears that NOTHING has been learned from this practical lesson as the new AP1000 reactors being built in America still use this above ground design...instead of securing the reactor core 100 ft undergound in a natural pit that can hold more than enough cooling water in the event of a disaster.

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    Last edited: Dec 2, 2012
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  3. ElectricFetus Sanity going, going, gone Valued Senior Member

    Why are reactors still built with water as the coolant? Why do they still us uranium instead of thorium? The answer is all the same: cut corners, be cheap now by forgoing research on more advance, safer, clear and cheaper reactor designs by simply upgrade a now 60 year old reactor design. As a result nuclear power never became cheap and safer because they have trap themselves in a technological dead end.
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  5. leopold Valued Senior Member

    what would the advantage of thorium be?
    according to the following wiki article it must be converted to uranium to be used.

    i believe there are some lessons learned from fukishima.
    relocating or hardening emergency power supplies for example.
    if that was done in the first place fukishima might not have happened.


    throughout this ordeal you must remember that fukishima was the result of 2 massive disasters back to back.
    as a matter of fact the delayed explosion of plant 4 could be seen as a triumph of reactor hardening.
    a reactor of lesser design might have folded during the tsunami.
    Last edited: Dec 2, 2012
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  7. ElectricFetus Sanity going, going, gone Valued Senior Member

    So? uranium needs to be enriched, a harder processes actually, since a reactor can both convert and produce energy in one package and not require hundreds of gas centrifuges! Thorium is hundreds of times more plentiful then U235, Even 3 times more plentiful then all uranium isotopes combined. It produces only far less transuranic radioactive waste that uranium or plutonium does and its resistant to being used for making nuclear weapons.

    The Lesson should be to have reactors with passive safety, at the very least the reactor is passively cooled when shut off! A molten lead or salt cooled reactor would not have these problems, would not need active cooling systems, would not be capable of steam explosions (Chernobyl) or devastating coolant leaks (3 mile island), but those solutions were never developed because they just figured they could keep upgrading the inherently flawed and unstable water cooled reactor design for cheaper.

    And a reactors of greater design might not have melted down and leaked any radiation at all! And people would be like "yeah nuclear power is not so bad" and relies huffing burnt coal dust is in fact worse!
  8. leopold Valued Senior Member

    maybe future reactors will have an internal sheathing of lead.
    when a certain temp is reached the lead melts and floods the reactor.
    the problem is getting a high enough melting point.

    i believe most of fukishima could have been prevented by an adequate emergency power supply.
    that's the real mystery.
    why were these generators located underground at a sea shore?
    what's up with that?
  9. billvon Valued Senior Member

    The containment building of a reactor can also hold more than enough cooling water for a short time. But then all that water starts boiling. And what do you do then? Vent the boiling radioactive water? Drain it somewhere to hold it safely? That's hard to do if it's in a pit and you have no power. The one benefit of the pit is that you can seal it off and just let the core melt, then hope you don't get hydrogen explosions or tunneling of the core material.

    Modern intrinsically safe reactors are built aboveground with sufficient surface area to cool for several weeks via convection alone, releasing the heat through the containment to the outside air. You can't do that underground (no convection.)
  10. billvon Valued Senior Member

    Cheap, good moderator AND good coolant, nontoxic, nonflammable.

    Same answer. Cheaper, safer, more proven. Thorium reactors cannot start up on their own without a plutonium or uranium seed, so you need those materials anyway.
  11. billvon Valued Senior Member

    Well, better protection against air attacks, storms, lightning, etc - everything except floods. It is ironic indeed that even a fairly small solar power system could have saved the day by providing intermittent power to run pumps and controls.
  12. leopold Valued Senior Member

    in my opinion the emergency generators should have been located at least 10 feet higher than the tsunami walls.

    another lesson learned:
    the reactors should have been shut down and flooded as soon as the tsunami walls where breached.
    the technicians wasted valuable time trying to ascertain the condition of the cores.
    the condition was frikken obvious, IT'S A MAJOR DISASTER ! !
  13. billvon Valued Senior Member

    The reactors were shut down as soon as the tsunami warning went out.
    The reactor is always flooded with water.

    (I assume you don't mean 'the containment building should have been flooded with water.' It's not designed for that and could have caused the very thing you want to avoid.)

    ?? Not at first it wasn't. Indeed, while they had power for the control systems it was a non-event. The problem was that they lost control power and could not restore it in time. The valves/pumps went to their unpowered state, and that state did not allow enough water to keep the cores cool.
  14. leopold Valued Senior Member

  15. leopold Valued Senior Member

    i don't know if it would be a "lesson learned" but the uranium core design is a poor one for intuitive thermal control.
    there is also a lag time which might be too long for emergency situations.
    full insertion of the control rods would seem to be "complete shutdown" but raises its own thermal problems.
  16. billvon Valued Senior Member

    In what way?

    I think you are talking about decay heat here. When you insert the control rods on a modern reactor the reaction stops; the reactor is no longer critical, and neutron production goes down to almost nothing. However, when uranium fissions it produces new isotopes that then decay to other isotopes, then to others etc. Some of them have half-lives of minutes; these generate a lot of heat but go away quickly. Some of these have longer half-lives; these generate less heat but take a long time to decay.

    When you first shut down a reactor these decay products mean that you're still generating about 6% of your original energy - and 6% of 500 megawatts of heat is still a lot of heat. After a minute you are down to 4%, after an hour you're at 1.5% and at a day you are at .5%, or 2.5 megwatts. That's not hard to deal with, but you can't just shut off all cooling and have the core not have problems.
  17. X-Man2 We're under no illusions. Registered Senior Member

  18. ElectricFetus Sanity going, going, gone Valued Senior Member

    Why not just use lead as a the coolant? Lead is liquid between 328°C and 1749°C but Lead-Bismuth (44.5/55.5) is liquid between 123.5°C and 1670°C just 23.5 degree above the boiling point of water! That is a reasonable temperature in fact lead cooled reactors have been built before and are being pursued by both a Russian and a American firm for cheaper/safer nuclear power. Such reactors are for more prone to "freezing up" then melting down as Lead-Bismuth coolant won't boil away, spill out, and can withstand large temperature spikes, designing a reactor to be able to restart after freezing up is a more economical prospect in theory then building for a melt down! Safety systems include passive dropping control rods that drop if the reactor gets to hot by action of physics and boron balls glued to the bottom of the reactor that detach and float into the reactor core killing nuclear reaction if the reactor gets to hot. Also lead cooled reactors could use passive circulation, no need for active pumping to keep the reactor cool as it can convect heat with molten Lead-Bismuth all by its self! Hot coolant rises spills over the reactor core cools along the inner wall of the reactor and the heat exchangers and falls to the bottom where is sucked up into the reactor core, no pumps required. The only problem is the Lead cooled reactors are fast neutron reactors (Lead is opaque to slow neutrons) which need 80% U235 or plutonium and fuel and thus would run on a plutonium cycle, merely changing the schedule for fueling the reactors could breed prodigious amounts of weapons grade plutonium. On the other hand fast neutron reactors could breed their own fuel (fuel grade plutonium) allowing all uranium (the 99.3% that is U238, not just the 0.7% that is U235) to be utilized as fuel, and can "burn" 99% of the transuranics nuclear waste.

    It could have been prevent by a passively cooled reactor! They lacked enough battery power to keep the active cooling system running. The state-of-the-art Gen 3+ reactors (still using water cooling) implement passive cooling by building a titanic pressure dome around the reactor that then is cooled by convention with the outside air and 3 days supply of water that rains down over the dome from storage at the top. Lead cooled reactors on the other hand would not need a containment dome, they can self cool once they reach a nominal over heat temperature (900°C) and bleed heat by thermal radiation, or by thermal conduction by being buried in the ground and using passive convention to pump heat from the reactor into the surround medium.

    And that just lead cooled reactors, I have not begun to talk about molten salt cooled/fuel reactors.
  19. Captain Kremmen All aboard, me Hearties! Valued Senior Member

    I like where they put the control room. Underneath the reactor vessel.
    That must concentrate the mind wonderfully in the event of an emergency.

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    Message from Chief of Control Room: "Estimated time to meltdown now five minutes and counting...........
    Permission is granted to start running round in circles."

    Possibly the extra cost of building could be recouped by cutting down the cost of disposal when the reactor's life is over.
    If the reactor can be built in an area which is geologically stable over millions of years,
    then disposing of the reactor when it has finished its life would be easy. Fill it with super tough concrete.
    Site possibly would be in ancient beds of granite. Could you cut a hole that big in granite?

    In the UK we would have to build them in Scotland.
    They have rocks 3 Billion years old up there.
    They'd bitch about it though.
    Last edited: Dec 4, 2012
  20. leopold Valued Senior Member

    why not a coolant of carbon?
    think outside the box here.
    maybe that's the problem.
    we are too busy trying to remove heat when in fact we need to figure out ways to stop producing it.
    carbon would be the perfect "coolant" if it can be used.
    not only would it remove the heat it would stop the reactor from producing it.

    lead has at least one major drawback.
    one it has solidified it could take months before the reactor could be brought back online, if ever.
    maybe nanotechnology can produce a liquid lead made of suspended particles.

    i doubt if we could even have a "passively cooled reactor".
    small ones maybe, but the ones as large as fukishima will probably need some pressure to overcome the pressure inside the core.
    Last edited: Dec 4, 2012
  21. ElectricFetus Sanity going, going, gone Valued Senior Member

    because its melting point is too high and it is combustible? Similar problem with why molten sodium reactors never took off, because it was a extremely dangerously stupid idea to use a coolant that burst into flames on contact with air, explodes in contact with water and become a powerful gamma emitter when exposed to neutron radiation, oh but its all worth it for that extra 5% of neutron transparency over lead-bismuth and 30°C lower melting point... NO IT WAS NOT, hence why only 2 of such reactors are still in operations on earth.

    When a reactor is shut off residual heat is produced mainly by radioactive waste in the core that is decaying, carbon not going to stop that decay, nothing will. Best just to design a reactor that can cool it self passively until the waste as decayed enough for the reactor to freeze-up

    yeah its call "freeze up" already covered that, far less serious problem then melt down. "nanotech" is not likely to survive intense radiation, fission fragments would tear it apart like cannon balls through a person! In fact radiation cuts nanoscopic holes and crack through metal as is.

    Actually the larger the reactor the more likely passive safety will work for it, small reactors can't convect naturally, there is simply not enough space for a temperature gradient. Lead as a matter of fact convects easier then water and allows for smaller reactors that operate on passive cooling.
  22. billvon Valued Senior Member

    In general the opposite is true. The volume of a reactor determines its power potential; the surface area determines its passive cooling potential. Volume goes up as the cube of size, surface area goes up as the square of size. Thus a reactor that is twice as tall has four times the surface area - but eight times the active material producing heat (scaling linearly in all dimensions.)

    However there are certainly economies of scale that can be used to make complex passive cooling systems less expensive per megawatt-hour on larger reactors.
  23. kwhilborn Banned Banned

    I would be happier to think that every nuclear power plant was required to recognize what happened in Japan, and was forced to replicate devices that would have prevented any radioactive leaks under similar scenarios even if not located on the ocean.

    I feel confident that our power plants are very safe and live within 200km of several nuclear facilities.

    I understand radiation levels are monitored in the surrounding air, if a unit was buried I would be skeptical about measures preventing land degradation. Especially over time. I would also want much more accurate and detailed mapping of underground resources should they exist. I doubt anyone would ever want to live at the site anyhow, but the possibility/results of continuous radioactive degradation would increase with the agglomeration of radioactive soil over time in a subterranean setting.

    Nobody would be satisfied knowing an old reactor was filled with cement, unless the reactor was already in a lower populated area, which is not frequent. I have seen truckloads of earth hauled from various sites because of possible chemical contamination, and a nuclear facility would need to be hauled away along with a meter of soil it stood upon.

    I think more safety protocols can never hurt though. Have water towers all over. Divert rivers. It doesn't matter as long as the public stays safe.

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