Ex-Nuke here. Not positive but I think the original intent was to always have a reserve that could be used in a tight. theoretically a day tank would have enough fuel for the emergency diesel to run 24 hours, but that was not the case with these, I think they'd run about 1/2 day fully loaded (more if less loaded). They were also fed from a tank farm that was 'uphill' from them which allowed for fuel xfer w/o pumps but also provided opportunity to flood the world w/fuel oil if things were left open. We monitored levels religiously. In addition to a 'day' tnk we had '7-day' tnks that fed into the 'days', more redundancy for the nukes.