I have been firing boilers for over 25 years. I don't know for sure what happens during a scram shut down, although I have a very good idea. It would go like this.

The boiler or steam generator is running at 90% load or higher, and something goes wrong. Sensors call for a shut down of the steam system feeding the turbines. The boiler is hot, the walls are over a foot thick to contain the heat inside.

When the steam stop valve is closed on the top of the boiler, all the energy contained in the water has to go someplace. That makes the safety valves lift. Where does this steam go after escaping from the boiler.

My boilers will lift the safety valves for around 8 hours, off and on. And the boiler will continue to generate steam for over 24 hours.

Now during a scram, if I am not mistaken, the steam stop valve on the top of the boiler will close, to contain the boiler. With the steam and the hot water in it.

Now they cannot add cool water to it to cool it down, or it would jump off of the concrete pad, it was sitting on.

You cannot just cool something this hot, down to a cool level without damaging the structure that supports it. The metal would have to move, if you will, or break.

I have seen the floor in a fire box explode, when the rear door to the fire box was opened by a young engineer, to cool the boiler down real quick. It takes at least a week to cool a boiler down, before you can work inside, and then only for short periods of time. After several weeks, the temperature inside is more tollerable, and easy to work on.

Looks like to me the nukes and the people who live near them are in a bad way.

It just cannot be done.

-- (Boilerman7@powerhouse.com), July 15, 1999

Answers

Do you mean it's never happened before? And if it does, they won't know what to do???

-- treading litely (rs@marketwatch.com), July 15, 1999.

No, I think what he is getting at is that there may be some "young engineer" who will do the wrong thing and cause massive damage. You know, like what happened at TMI and Chernobyl.

-- Gordon (gpconnolly@aol.com), July 15, 1999.

The steam gets dumped - depending on the plant design - to any of several different paths: the design differs between boiling water reactors, and pressurized water reactors.

While scram shutdowns are not routine events - the utilitiy gets penalized (fined by the NRC) big-time money for any failure that causes a scram - they DO occur rotuinely when electrical transients and other line faults trip the generator.

Contact me off-line - I'll give you more info to address specific quesions, or continue below with a specifc question. To politely disagree with you - it can be done, I've done it. To room temperature from operating temperature - in 12 hours. My brother still does it - in slightly longer times. My dad used to do the cooling systems design - by planning for even shorter times. Yes - they can be safely shutdown and cooled down.

For one thing - the max temperature is less - below 500 degrees in most cases. The initial cooldown is by forced water cooling (from several different normal and baclup systems) not by radiant convection to the air. The shutdowns are timed specifically to minmize the thermal shock you mentioned to pressurized components and vessels, and the piping and steam systems are thermally and physically shock-mounted specifically to avoid the thermal motion and stresses involved in cooldown. (Normal wall thickness is considerably less - so cooldown stresses are less than what you experienced too.)

You're correct - the metal and its supports move. (I've measured it.) You're incorrect - it doesn't break.

-- Robert A Cook, PE (Kennesaw, GA) (cook.r@csaatl.com), July 15, 1999.

Unless a weld seam crytalizes my friend...Been there,seen that... old certified welder on Palo Verde.

And there are only two can'ts on construction...you can't do it; you can't stay. Incidently wth each heating up of welded seams,you fateague the metal. As an engineer you know that better than I do..

Folks...in thirty some odd years of working on power houses, you get to have experence in all phases of the electrical end of it (we use welders in our tray and pipe work to) so I put in a couple of years at that also (welding).

Shakey

-- Shakey (in_a_bunker@forty.feet), July 15, 1999.

Boilerman mentioned the fire box. What about coal fired boilers?

Speaking of coal fired, GN said a month or so ago that a big coal fired plant uses "train loads" of coal every day. Is this true? I know I'm getting a little OT, but I never did get an answer on this. If it's true, where are they going to "stockpile" all that coal? <:)=

-- Sysman (y2kboard@yahoo.com), July 15, 1999.

Robert A. Cook Professional Engineer

Yes, they can't yun at higher temperatures. One of the neat design features of Indian Point No.1 was the separation of boiling water from superheating steam, which was accomplished in an oil fired unit to up the thermal efficiency of the overall plant.

Out here, Palo Verde Nuclear Generating Station uses brute force low temperature H2O boiling. So much heat must be dissipated in the Cooling Towers that on a clear day, the power plant .

.

Makes its own weather!!!

-- K. Stevens (kstevens@It's ALL going away in January.com), July 16, 1999.