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



In a message dated 9/9/2011 6:38:41 P.M. Eastern Daylight Time,
edmiston@bluffton.edu writes:

Under the "all-too-predictable blackout" thread, John Denker mentioned
that all power plants should have "black-start" capability. He also mentioned
that nuclear plants will generally shut down during a cascading blackout,
and then be the last to come back online.

I strongly agree that all plants should have black-start capability, and I
have been saying that for years.

I also know that nuclear plants tend to SCRAM when there is trouble on the
grid, and I wonder if someone can explain that to me. If grid power is
lost, and the reactor SCRAMs, and there is no black-start capability, the
reactor probably cannot be restarted for a pretty ling time. That means
reactor cooling and spent-fuel cooling has to be provided by diesel generators,
or something similar... and this might be required for a time longer than
there is sufficient fuel to run the backup power.
)))))))))))))))))))))))))))))))))))

BZ

Commercial Nuclear reactors are never started in a black out condition.
These plants are designed to feed power into a live grid. It's very difficult
if not impossible to maintain frequency and voltage stability with a large
plant feeding into a power grid with virtually no load.

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It's not obvious to me why the reactor has to SCRAM in the first place.
Although grid trouble might indicate some sort of nasty thing coming (like
an earthquake), why not wait to SCRAM until you know there is something
nasty coming. It's difficult to imagine that waiting for a few minutes after a
grid anomaly before initiating the shutdown would make much safety
difference.
))))))))))))))))))))))))))))))))))

BZ

A typical nuclear reactor produced over 3000 MW of thermal power, which
becomes about 1000 MW sent out on the grid. If the ability to send power to
the grid is lost, the reactor still producing 3000 MW of thermal power will
rapidly heat up and raise the water pressure ( in light water reactor
designs) to dangerous level absent a scram. You just can't swing power fast
enough to prevent this from happening. Nuclear power plants are used for
base load, that is they aren't designed to rapidly swing power levels,
basically the nature of nuclear power plants makes this impossible.

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Anyway, if a SCRAM eventually does occur, and if the nuclear plant has
black-start capability, once it is ascertained that nothing at the nuclear
plant is damaged, the reactor could be restarted, the generators could be
brought back up (because black-start is provided) and the nuclear plant can
provide its own energy for cooling itself and the spent fuel pools.

)))))))))))))))))

BZ

This is never done, see above.

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So what's the point of a reactor shutdown when the external grid power is
lost? Indeed, what in the world would happen in a system where *all* power
is provided by nuclear plants, and there is a grid problem? They all
shutdown, and you can never get any of them running again if external grid
power is a condition of restart. Sounds like you'd have the proverbial
"catch-22."

)))))))))))))))))

BZ

Grids are very big, they can tolerate a certain percentage of units
tripping off line and still maintain stable operation. But of course if something
goes wrong on the grid, it's quite possible you will get a cascade effect
with all the power units tripping off the line over a large area.
Engineers attempt to design power grids to minimize this possibility but in
engineering shit happens.

Bob Zannelli

Retired Nuclear Engineer

)))))))))))))))))))))))))




Michael D. Edmiston, PhD.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
Bluffton, OH 45817
Office 419-358-3270
Cell 419-230-9657


-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Friday, September 09, 2011 3:58 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] an all-too-predictable blackout

One more thing:

According to
http://www.sce.com/PowerandEnvironment/PowerGeneration/SanOnofreNuclearGener
atingStation/default.htm?goto=songs

The twin reactor units at ... San Onofre ... are Southern California's
largest and most reliable sources of electricity.

Most reliable? Really?

It is entirely predictable that during a cascading blackout, all nuclear
power plants in the area will shut down ... and they will be more-or-less
the last to come back up.

As of now, the San Onofre reactors are still shut down:
http://www.songscommunity.com/index.html?from=SONGS

I have long argued that every power plant should have black-start
capability.
The lack of such capability makes it harder to recover from a blackout.

In this regard, nuclear power plants are a big step in the wrong direction.
Not only do they require external grid power for startup, they require
nice stable external grid conditions while running. In a system containing
/only/ nuclear power plants, a blackout would result in deadlock; no
recovery would be possible.