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The test that led to the Chernobyl accident was to see if they could keep cooling water circulating long enough in the event of a power cut for the generators to kick in.

So, why didn't they just power their own circulation system? Why be dependent on the grid, and not just use some of the power they were outputting to run their own systems? Or at least have it as an insta-backup?

Even more directly, why not just link the turbine and the pump together mechanically with some gearing to govern the speed-ratios?

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    $\begingroup$ As you said, it was a test. How do you test if your backup system is working if you never let your backup system work? Your question is also strangely worded. It seems to imply you think it was powered off the grid during normal operation. I seriously doubt that. If anything, it would power its own cooling system during normal operation, use the grid as backup, and use the generators as a backup for the backup. $\endgroup$
    – DKNguyen
    Commented Jul 13, 2021 at 14:49
  • $\begingroup$ Well, the nature of the test implies that it did take power from the grid, rather than just using its own output. The test was, "in the event of a power cut, can we keep the cooling going long enough before the generators to kick in?" So, it's not the necessity of testing I'm questioning - it's the necessity for external power during normal operation. I admit it seems odd, and I may have misunderstood, but that's why I'm asking. $\endgroup$
    – Tom Melly
    Commented Jul 13, 2021 at 15:45
  • $\begingroup$ "link the turbine and the pump together mechanically with some gearing" - fantastic idea... what is the speed of the turbine and what is the speed of the pump? $\endgroup$
    – Solar Mike
    Commented Jul 13, 2021 at 16:22
  • $\begingroup$ I assume, if the station is doing its job, "very fast" and "not very fast (in comparison)" - hence the need for gearing. $\endgroup$
    – Tom Melly
    Commented Jul 13, 2021 at 16:28
  • $\begingroup$ So what ratio would you need? $\endgroup$
    – Solar Mike
    Commented Jul 13, 2021 at 16:31

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Chernobyl does power its own cooling system during normal operation. It would be very strange if it did not. It uses the grid as backup and then uses diesel generators as a backup for the backup.

This report here supports that: https://www.rri.kyoto-u.ac.jp/NSRG/reports/kr79/kr79pdf/Malko1.pdf

On page 8, it states that before the incident, the four main pump were connected to the turbogenerator.

Wikipedia also has a reference (6) that states that the objective of the test was to see that if external backup power (i.e. grid power was lost) if the spinning down of the turbogenerators could provide enough power to hold the coolant system over until the diesel generators started up again. https://en.wikipedia.org/wiki/Chernobyl_Nuclear_Power_Plant#cite_note-techappr-6

So obviously if you are trying to test your diesel generators, you are going to have to disable your grid power. Remember: they weren't testing the backup. They were testing the backup to the backup.

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  • $\begingroup$ And even if they weren't , once the control rods did that 'bad thing', I rather doubt the peak coolant rate would have been sufficient to quench the incredibly massive energy output as the core "went critical" $\endgroup$ Commented Jul 14, 2021 at 14:46
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Your question stems partly from a common misunderstanding. The test was not meant to be conducted with the reactor running. In reality, the procedure called for the reactor to be SCRAMmed simultaneously with the turbine. So there was supposed to be no power available.

What happened was that there was a miscommunication, and the shift manager waited for confirmation from the turbine department or the deputy chief engineer. Then, (I'm speculating here) the reactor operator apparently noticed that the automatic rods were descending and brought this to his attention, and he finally gave the command to shut down the reactor.

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