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Fukushima nuclear plant failure

some technical explanations on what's going on

  1. arclight
    Nuclear fuel in a BWR is composed of three parts: uranium oxide pellets (fuel), zirconium alloy tubes (cladding) & structural bits
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  2. arclight
    The fuel is sealed in the 12-15' long tubes, the tubes are arranged in a square grid & held together by a light frame & put in a shroud
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  3. arclight
    The shroud is basically a square sheet metal tube (also made of zirconium alloy) which keeps water flowing up along the fuel.
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  4. arclight
    The fuel is arranged in a roughly circular grid to form a big cylinder inside the reactor vessel.
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  5. arclight
    Cold water enters the side of the vessel, goes down to the bottom & up past the fuel where it's heated to boiling temperatures
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  7. arclight
    The steam water mix goes through a twisty path called a steam dryer to keep water droplets from being carried into the steam lines.
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  8. arclight
    The (dry) steam goes into the high pressure turbine, expands & spins the turbine, then goes into the low pressure side into the condenser.
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  9. arclight
    The condenser is a huge heat exchanger right under the turbine that carries waste heat out of the plant.
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  10. arclight
    Condensed steam is pumped back to the reactor and the whole cycle begins anew. This will thrill the Buddhists in the audience :)
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  11. arclight
    So in a quick shutdown, the control rods enter from the bottom of the core and latch in place so they can't fall out.
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  12. arclight
    Also, if the control rod drive (CRD) pumps fail, you can vent water which sucks the rods in. Very elegant & counterintuitive design.
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  13. arclight
    Why the bottom? (DUDE! GRAVITY!?) The steam dryer is in the way. 2000psi water pushes them up & in.
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  14. arclight
    So shutting down the reactor is pretty easy & foolproof; it has to be. The next thing that happens is shutting the valves to the turbine.
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  15. arclight
    Once you stop the reactor, you stop making steam and you'll start sucking water into the turbine which is Very Very Bad.
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  16. arclight
    How bad? 42" turbine blades spinning at 1800 RPM breaking apart and flying around the turbine building bad. Bye bye turbine.
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  17. arclight
    So when the rods go in, the main steam isolation valves (MSIVs) shut. But now there's no place for the water being pumped in to go.
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  18. arclight
    So at that point you switch to RHR (residual heat removal) which is a safety-related heat exchanger system to remove the decay heat
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  19. arclight
    Recapping from yesterday: Decay heat starts ~10% and drops over the next few days. It's this decay heat that damaged the fuel at Unit 1
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  20. arclight
    This long-winded exposition is getting to that. I've avoided using the term cladding because it's jargon for fuel tube.
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  21. arclight
    What has happened at Fukushima is that after an hour all AC power was lost at the plant and systems like RHR, pumps & valves had no power.
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  23. arclight
    So with no electricity & no cooling the water in the reactor began to boil away and pressurized the reactor vessel.
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  24. arclight
    The steam exits through pipes underwater in the big donut-shaped part of the containment. It's released underwater so the steam condenses
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