Bob Zannelli responded to my questions about nuclear power plants. Thanks
Bob. Could you please respond to a few more questions.
My reactor experience comes from research reactors. I believe the largest
of these that I worked at was the 40 MW high flux beam reactor at Brookhaven
National Lab, which has now been shut down. Obviously, dealing with 40 MW
of power is a lot different from dealing with 3000 MW. Also, the research
reactors are not putting power onto the the grid, and therefore don't need
to worry about producing a lot of power for a load that could suddenly
disappear. Therefore I realize it’s a whole different ball game when
electric power reactors are compared to research reactors.
Here are some questions for which the answers would help me better
understand the electric power reactors.
[1] If a reactor is generating 3000 MW, of which 1000 MW is appearing on the
grid, how many turbines are there? Is it a single 1000 MW turbine, or four
250 MW units, or ten 100 MW units, or what?
[2] If it is not the case, why couldn't it be the case that there is a 10 MW
or 50 MW or 100 MW turbine (whatever would be the appropriate size) that
could keep running in order to power the nuclear controls and cooling system
in the event of grid loss?
[3] Depending on where the reactor was in its fuel cycle, if a SCRAM
occurred, wouldn't there be enough residual heat to run a 100 MW or smaller
turbine/generator? If this is true, why aren't power plants designed this
way?
[4] If a 3000 MW reactor system has the capability of getting rid of the
2000 MW of power that is not going to the grid, then it certainly has the
ability to get rid of 1500 MW of thermal energy (without using the grid) if
the reactor were shut down to 50% (or less) with only sufficient turbine
load to generate local power. So why shut it down all the way? Couldn't it
provide local power at reduced fission rate, and thereby be in sort of a
standby mode to come back online more quickly as the grid begins to recover?
[5] If most large nuclear power plants have only one very large
turbine/generator, I can see there would be little opportunity to operate at
reduced capacity. I can also see that it would be difficult to synchronize
this one large turbine/generator onto a grid that was not nearly fully
functional. So if there is only one turbine... why? Couldn't you more
easily synchronize and load four (or ten) turbines one at a time?
[6] There already has to be the capability of getting rid of the full
thermal output of the reactor in the event that the (all) turbine(s) are
shut down, so why would the reactor have to SCRAM if there were a
satisfactory way to maintain the cooling system even if there were no grid
power?
[7] Bob may answer some of these questions by repeating his comment that
current nuclear power reactors were designed as base load suppliers of
electric power. Okay, but is that the only way they could be designed?
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270 (office)
edmiston@bluffton.edu
-----Original Message-----
From: Spinozalens@aol.com
Sent: Saturday, September 10, 2011 6:40 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: 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.
))))))))))))))))))))))))))))))))))))))))))))
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.
)))))))))))))))))))))))))))))))))))
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