Safety Injection during Main Steam Line Break

[FermiAged]

For a Large Break LOCA the RCS would depressurize to the point where Accumulators discharge (about 600 psia) and ultimately where Low Pressure Safety Injection can take place (about 300 psia). Both of these systems would replenish inventory and provide boron.

For a Small Break LOCA, HPSI and charging are more important as the RCS pressure will level off above Accumulators and LPSI. This is Functional Recovery territory. I would first try heat removal with the SGs while sending someone to vent/start the HPSI pumps. If that is insufficient, the RCS would have to be depressurized using pressurizer relief valves. The idea would be to get the RCS pressure low enough for Accumulators/LPSI.

You are talking about losing 2 or 3 HPSI pumps and 2 or 3 Charging Pumps. Its hard to believe that the pump that had the most recent surveillance would be gas bound.

Can I ask what your background is? You sound like an engineering student.

 

[mudweez0009]

For a Large Break LOCA the RCS would depressurize to the point where Accumulators discharge (about 600 psia) and ultimately where Low Pressure Safety Injection can take place (about 300 psia). Both of these systems would replenish inventory and provide boron.

For a Small Break LOCA, HPSI and charging are more important as the RCS pressure will level off above Accumulators and LPSI. This is Functional Recovery territory. I would first try heat removal with the SGs while sending someone to vent/start the HPSI pumps. If that is insufficient, the RCS would have to be depressurized using pressurizer relief valves. The idea would be to get the RCS pressure low enough for Accumulators/LPSI.

You are talking about losing 2 or 3 HPSI pumps and 2 or 3 Charging Pumps. Its hard to believe that the pump that had the most recent surveillance would be gas bound.

Can I ask what your background is? You sound like an engineering student.

So basically, for small break LOCA, loss of HPSI/charging is more severe because the RCS pressure doesn't drop enough to allow other remedial operations to initiate?

What about my guesses of losing boron injection and RCP seal injection? It seems from your description that you only lose boron injection on a small-break? Are either of these major concerns that could result in further damage, or are they minor compared to pressure concerns?

And yes, I am an engineering student. I am currently halfway through grad school, on my way to a masters degree in mechanical engineering. Just took a nuclear engineering course out of curiosity and now realized that one course can't even cover the BIG picture items that I would like to know. I really am trying to learn the components of the plant and how they interact. (If situation A happens, what happens to B, C, and D) type of stuff..

 

[mudweez0009]

For a Large Break LOCA the RCS would depressurize to the point where Accumulators discharge (about 600 psia) and ultimately where Low Pressure Safety Injection can take place (about 300 psia). Both of these systems would replenish inventory and provide boron.

For a Small Break LOCA, HPSI and charging are more important as the RCS pressure will level off above Accumulators and LPSI. This is Functional Recovery territory. I would first try heat removal with the SGs while sending someone to vent/start the HPSI pumps. If that is insufficient, the RCS would have to be depressurized using pressurizer relief valves. The idea would be to get the RCS pressure low enough for Accumulators/LPSI.

Also, do accumulators deliver boron when there is a loss of all power?

 

[FermiAged]

Yes. They are essentially tanks of borated water driven by a charge of nitrogen (at a typical pressure of about 650 psia). They discharge to the RCS through check valves (like a door that only swings one way) that are closed or opened by the differential pressure across the valve. Under normal operating conditions the RCS pressure of 2250 psia keeps them closed. When RCS pressure falls below 650 psia, the valves open and the tanks discharge based upon the RCS pressure and level in the tank (which determines the elevation head and N2 pressure).

The accumulators (a Westinghouse term - CE designed plants call them Safety Injection Tanks - I don't know what B&W calls them) require no electrical power to function. Because there is no pump that requires time for power source alignment, start signal and start up, the accumulators can be the first source of safety injection in some accidents.

 

[mudweez0009]

That is what I figured. Thanks for confirming!

 

[jim hardy]

RCP seal cooling used to be PWR's Achilles heel.
Westinghouse came up with an improved one that's passive, but it was after I'd retired so I never saw one.. That would seem to be be a comfort.

http://www.westinghousenuclear.com/Products_&_Services/docs/flysheets/NS-FS-0106.pdf

We added some fittings (I think back in the 90's) whereby one could connect a portable diesel driven pump to cool the seals should the unthinkable happen.