Re: 80 minutes?
I wrote a reply to this, and I must have pressed Preview twice, rather than Preview-Submit, and it has vanished into the ether. Which is irritating.
The short answer is, I wondered that too, but unfortunately after the tour. To be fair, the guys giving the courtesy tour didn't say it was a single tank - they just said the storage was under the car park (which was large), so it could conceivably have been multiple tanks with some means of transferring diesel between them allowing you periodically to empty one and clean it before moving on to the next. There were multiple large generators in the machine hall with a schedule of being started and run for a period of time to make sure they worked when needed. It struck me as a rather professional operation, and I'm sure they had some way of keeping the fuel 'sweet'. Of course, I can't verify they had the reserves they claimed: I didn't have my audit hat on at the time, but they didn't have any particular reason to lie or exaggerate.
As well as the claimed fuel reserves, they had independent connections to more than one area of that country's equivalent of the National Grid.
Sorry to be so vague. I'd be interested in the mechanics/process too, but it's not something I could find out now - it was a while (rather too many years than I like to remember) ago. I expect nuclear power stations have similar issues. Fukushima had fossil-fuelled backup generators that were swamped by the tsunami, but I'd expect them to have had enough fuel to keep them going through shutdown and provide adequate cooling for a reasonable period afterwards. Since you don't tend to use it day to day, they must have some method of fossil-fuel management to prevent things like sludging. It's not my area of expertise, and it'd be great if someone commented here from actual knowledge. I know fuel tank biocides are available commercially (ask a boat owner) - looking at an MSDS sheet for one shows it contains 3,3'-methylenebis[5-methyloxazolidine] which German Wikipedia has details for (I used Mozilla Firefox's built in translation). I'm sure there are plenty of other effective chemicals that could be used.
NN