Re: Nuclear Reactors & Blackout

["Robert I. Eachus" <rieachus@attbi.com>]

Robert C. Leif wrote:
> According to the US press, the reactors in New York State and other areas
> had to be shut down because there was a risk of an incident if the auxiliary
> power from the rest of the grid was lost. This approach to hazard analysis
> should be named Fail-For-Sure.

Yes and no.  The real problem is that the Northeast power grid is a 
collection of separately designed power plants, distribution lines, and 
substations.  It is an emergent property of this system that under high 
load conditions, it becomes an amplifier.  The next transient in the 
system, even if it comes from outside the grid, will get amplified to 
the danger point for connected power stations and even substations. 
These will then blow fuses to protect the equipment from meltdown. 
(Even though the generator casings will probably contain all that once 
rotating, now-molten copper, the generator will be so much scrap.)

For nuclear power plants, the threatened meltdown is of the generators, 
not of the steam supply system.  But once there is no external load, the 
reactor has to be shut down to reduce the amount of heat generated to 
something the cooling system can handle with no generator load.

Couldn't the breakers just interrupt the power for a few milliseconds? 
No, that won't work.  All the firecrackers going off create more 
transients to be amplified by the power grid, and everything disconnects 
from it.  (If you have ever heard one of these breakers blow, it doesn't 
sound like a firecracker.  More like a tank firing a supersonic main gun 
round.)  Eventually, after a few seconds, the reactor could reconnect, 
but by then there is no load connected to the grid anywhere.  The grid 
then has to be reconnected to an "island" with both one or more 
generating stations and a large load.  Then individual substations and 
generating stations can be reconnected in a co-ordinated fashion keeping 
the load balanced with the available power.  It is this balancing act 
that took most of the day that it took to restore power.

The only way to avoid this problem is to keep sufficient "reserve" 
capacity on-line to avoid the instability.  For decades this number has 
been known to be 15%.  But when the Federal government got into the 
power "deregulation" business, they decided that that guideline was too 
conservative.  Guess what, it isn't.  It may be that, with computers in 
charge, 12% is manageable.  In a few months we will probably know what 
the numbers were for New York State.  The problem of course is that a 
10% margin for the grid as a whole can result in some areas with 
negative reserve.  When such an area gets large enough--read New York 
City and suburbs, the local amplification effects can overwhelm the 
balancing effect of reserve capacity elsewhere.

The solution, of course, is to treat the grid as a whole as a system, 
and manage it to keep these areas of amplification from developing.  But 
try and explain to the environmental extremists that those old coal 
burning plants in NYC have to be kept on line in these conditions.  They 
don't actually need to be generating much, if any, power.  It is the 
reserve capacity in terms of generators idling on-line that is needed.

For example, this has never been a problem in Philadelphia, because of 
Conowingo Dam http://www.fieldtrip.com/md/0a457501.htm just a few miles 
down Route 1 from the city.  The dam is used more as a peak load 
facility than base load.  But the fact that it is so close to the city, 
and almost never run at full capacity, keeps the area relatively safe 
from the type of disruption that hit NYC.

I say relatively safe, because when NYC goes, it puts a lot of stress on 
all the surrounding power grids.  In 1965, one area in Northeast 
Philadelphia did lose power for about twenty minutes.  It was too far 
from the moderating influence of Conowingo which is south of the city. 
Incidently, part of the moderating influence of Conowingo is that it is 
ancient, and the generators and turbines are overbuilt by modern 
standards.  So there is all that rotating inertia on-line.

--
                                                 Robert I. Eachus

"As far as I'm concerned, war always means failure." -- Jacques Chirac, 
President of France
"As far as France is concerned, you're right." -- Rush Limbaugh