Station blackout in nuclear power plants

I'm not familiar with that exact terminology, but I would imagine that the Blackout diesel gens are capable of 'black start', meaning they can start and supply the EPS bus with absolutely no other power available to them. I'm not really all that familiar with these, but normal generators would require fuel pumps, and cooling fans, and other electrical permissives to start up, whereas blackstart does it all from nothing.

 

Maybe when you say "station blackout diesel" you really mean "alternate AC source." The reason AACs are smaller than the emergency diesel gens is because they were added to the plants' designs after original construction, in response to the blackout rule (10CFR50.63 ?). That rule addresses the very unlikely situation in which both of the EDGs fail to start following a loss of offsite power. The rule requires each plant to show that they can cope with the blackout for a predetermined time (usually 4 or 8 hours). The rule also allows credit for an AAC, for many units it was easier to add this smaller diesel (or in some cases a gas turbine, or other designs) than to show coping for 8 hours without any AC. Since no concurrent accident is postulated the engineered safety features components are not auto started, hence the AAC capacity is lower than that of the EDGs.

 

Your question cant be answered without some more info about the design of the plant you are considering. I'm not sure what kind of plant design you are thinking about, that would have four emergency diesels. I'm familiar with the plants in the US, these all (?) have two "trains" of engineered safeguards and therefore two emergency power trains and two diesel generators. Your plant, with 3 or 4 diesels wouldn't make sense unless there are 3 or 4 trains of safeguards (safety injection pumps, contanment sprays, vital HVAC, etc...). And depending on how many trains of ESF you have, that's how many diesels you need.

To reiterate, my understanding of what you mean by "station blackout diesel" is another power source, installed for postulated failures of all of the emergency busses. Since AFAIK, no one postulates an Loss of Coolant accident coincident with these failures, the SBO diesel only needs to power the normal RCS makeup and some HVAC loads (with the emergency feedwater powered by steam driven turbine(s), independent of any AC power). So, the SBO diesel is much smaller than the EDGs.

If the plant you are considering is based on a different design, then you can likely be guided by two ideas: (1) the EDGs each power a single train of safety features, and the number of trains required depends on the single failure criterion plus consideration of allowed equipment outages, and (2) the SBO is a "beyond design basis" event that postulates multiple failures in the AC system but does not assume a coincident accident.

Does that help, or is it more confusing?

 

In very simple terms an emergency generator is capable of suppling the critical items of plant on site that are required to keep things safe; for example cooling pumps and basic station lighting.

To start any power station up from cold it requires power and quite a lot at that. Normally when a power station is started from cold they are able to obtain the start up power from the grid. The problem that station back up generators address is what happens when there are no power stations at all operating on the grid. It is this load that the black out generator is sized for. The load being different from one power staion to the next.

The chances of no power station operating on the grid is actully higher than it first seems. The example that comes to mind is if there was a major earthquake the power stations arcoss a whole region will shut down, this on its own is not a problem as power can be drawn from outside the region accross the HV grid. However if there was also a couple of HV lines comprioposed by the earthqauke that supplied the region then theer would be no power available to restart the stations until the comprosied part of the HV grid was repaired.

One thing in the power supply networks is that they love and addicted to reduncancy of systems and for good reason.