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Re: [Phys-l] Nuclear Power and the Grid



Looking at the big picture, and trying to guess what motivates
the questions, I think the answer to the underlying question
is that the plants aren't particularly well designed. They
just didn't pay much attention to the blackout scenario. I
am not an expert, but I reckon that for very little cost they
could have added a 1 MW turbine (and a 1 MW dummy load) so
that as long as there was steam in the system they could
operate in safety *and comfort* while disconnected from the
grid.

I'm not saying this would be completely trivial. I can
think of several devilish details that would need to be
addressed, but that's what design engineers do for a living.

In show business, and in politics, there is a proverb that
says you should never believe your own press clippings. I
reckon the same rule should be applied to this industry. It
appears they believed their own hype and assumed blackouts
would be so rare as to be not worth worrying about. By way
of contrast, the airline industry assumes that crashes will
be very rare ... but they plan for them anyway. Not only
did Sully Sullenberger know how to ditch a plane in the
Hudson, the cabin crew knew exactly how to get everybody
off the plane afterwards in less than 90 seconds.

Turning now to the question that was actually asked:
On 09/10/2011 01:20 PM, Michael Edmiston wrote:

If this is not what happens now, then what does happen now? If the
generator load disappears suddenly, and the reactor can't cool down
suddenly, and there aren't valves to divert the steam around the turbine,
then why doesn't the turbine speed up to a much higher rotational speed
that would seem likely to cause mechanical damage? Surely there is not a
mechanical brake that can dissipate 1000 MW. So what is the current
practice for shutdown when a sudden disconnect from the grid is required?

I emphasize that I'm not an expert in this area, but it is
easy to find documents like this:
http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf

It says in part:

When the reactor is shutdown, the core will still continue to
generate decay heat. The heat is removed by bypassing the turbine
and dumping the steam directly to the condenser. The shutdown
cooling mode of the residual heat removal (RHR) system is used to
complete the cooldown process when pressure decreases to
approximately 50 psig.

Recall that I never said there wasn't a bypass valve; I
just said there was probably more to it than that ... which
there is.

As I understand it, there exist
*) the main steam system
*) the residual heat removal system
*) the emergency core cooling system, which consists of
-- the high pressure coolant injection
-- the automatic depressurization system
-- the low pressure coolant injection
-- the core spray system.
*) the standby liquid control system
*) the reactor core isolation cooling system
*) various fire suppression systems
*) who-knows-what else

It is alas quite easy to find conflicting information, so beware.
You could google for more information and decide for yourself:
http://www.google.com/search?q="residual+heat+removal+system"+"mark+i"+ge