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Instead the pump relied on the diesel generators, why didn't it just use the residual heat to generate electricity?

P.s. The question is very specific to the residual heat generated after inserting the control rods and the cooling pump which failed to cool down the system since it has no electricity when the diesel generators failed.

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According to the Wikipedia entry:

[The earthquake] produced maximum ground g-forces of 0.56, 0.52, 0.56 (5.50, 5.07, and 5.48 m/s2) at units 2, 3, and 5 respectively. This exceeded the earthquake tolerances of 0.45, 0.45, and 0.46 g (4.38, 4.41, and 4.52 m/s2). The shock values were within the design tolerances at units 1, 4, and 6.

When the earthquake struck, units 1, 2, and 3 were operating, but units 4, 5, and 6 had been shut down for a scheduled inspection. Reactors 1, 2, and 3 immediately shut down automatically; this meant the plant stopped generating electricity and could no longer use its own power. One of the two connections to off-site power for units 1–3 also failed, so 13 on-site emergency diesel generators began providing power.


:EDIT:

OP asked:

yes, but isn't "immediately shut down automatically" means inserting Control rods? There is still unstoppable nuclear fission which create the residual heat, I'm just wondering why they don't utilize the residual heat to cool itself

Well, the "residual heat" (decay heat) is an effect that generates heat, but (speaking from military reactor experience, not civilian) one of the goals after a scram or fast insertion is to recover the plant, and that means not letting the primary cool down too much. This is assisted by automatically closing the main steam valves on the steam generators, which isolates the secondary plant and thus also isolates the turbine generators.

In order to recover the plant, you need to establish why the shutdown occurred, assess any damage, and then start bringing the plant back online. The entire time this is happening though, the secondary plant piping is cooling down. You can get into a scenario where you may need to do an accelerated secondary warmup, which still takes time despite being "accelerated."

To address your question directly, I'll make a couple points that are pure speculation on my part.

  1. Biggest point in my mind is that there was an assumption that backup power would be available regardless of current catastrophic conditions. There's no need to cool the plant down too quickly if you have the luxury of time thanks to a bank of diesel generators.
  2. Next biggest point - In the heat of the moment, you need to respond to what is happening. You can't take time to consider the "what ifs" that could happen down the line; you need to address the current casualty. You can say, "The reactor just tripped offline, what if the generators go down?" but you could also say, "The reactor just tripped offline, what if the main steam valves get stuck shut?" or, "..., what if the cooling water pumps won't restart?" The list of hypothetical questions goes on. It was up to the engineers that designed the power plant to consider the likely scenarios, and they codify the actions you should take in a book of emergency procedures. If they were doing their jobs correctly, the people running the plant were performing the actions dictated in a checklist that was written before the plant was commissioned. If the checklist said not to use decay heat for backup power (or failed to instruct them to use decay heat for backup power) then that would be why they didn't.
  3. Available decay heat may have been insufficient or borderline. Bear in mind that when you make enough power to "just cool the primary," that implies that you also need to make enough power to run all of the auxiliary machinery for the secondary, too. Depending on the plant, you need to run multiple primary coolant pumps - these are NOT small pumps! Picture at the end with people for scale. But you need a means to return the condensed steam back to the steam generator, so you need condensate pumps running, and booster pumps running, and main feed pumps running, you need to have the lubricating oil system online, so you need to run oil pumps (again, not small for a large commercial turbine), and you need to have cooling water to cool the condenser, so you need to have those pumps online, also. I don't have a number on the percentage of full power that the auxiliary machinery consumes, but it is not insignificant. All this is to say that using decay heat to cool the primary plant down may result in a rapid cooldown that exceeds safety limits. The reactor vessel itself has stainless steel walls that are 8-10+ inches thick (20-25+ cm), and the temperature gradient across the thickness there and in the primary piping could result in stresses that cause permanently bending (plastic deformation). This is generally the concern for rapid cooldowns; heating up too fast could lead to brittle fracture, where part of the primary plant could suddenly and completely fail.
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  • $\begingroup$ yes, but isn't "immediately shut down automatically" means inserting Control rods? There is still unstoppable nuclear fission which create the residual heat, I'm just wondering why they don't utilize the residual heat to cool itself $\endgroup$ – vk.edward.li Jul 27 '17 at 12:39
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    $\begingroup$ @vk.edward.li - Was typing a response in the comments here but it was too big. See the edit above. Also, keep in mind that decay heat generation is a function of operating power history and shutdown duration - the more recent, and the higher power, and the longer the reactor was running, the more decay heat there is. Half of the reactors were already shut down, implying that they would have had a significantly lower rate of decay heat generation. $\endgroup$ – Chuck Jul 27 '17 at 13:41

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