DejaVU wrote:
so, Gunner and his survivalist buddies have a point after all!

but I still fail to understand why the nuclear plants shut themselves
down? anyone care to explain?

swarf, steam and wind

Here's the short simplistic story. I'm sure Gary C. or
Peter H. will elaborate now that I've replied (:

1. Main circuit breaker on plant output lines trip
because of overload.

2. Generator now has no load for turbine so turbine
overspeeds and trips.

3. Heat from reactor now has no place to go so reactor
trips. Heat also causes pressure relief valve to
open and blowdown part of cooling water into hotwell.

At this point, the emergency generators should come
on and the operators are faced with about 150 red lights
and they have to start sorting out what went wrong.
Their first job will be to stablize the reactor, getting
coolant temperatures, pressures and levels to normal
idling levels. Once that is done, they have to determine
whether they can reconnect the plant to the grid and
ramp up power.

To get some idea of the amount of energy involved
here, a good sized nuke can put out 1.2 gigawatts of
power. Now a modern 3-truck locomotive can put out
about 6 megawatts. So we have the energy equivilent
of 200 big locomotives running at full throttle.

There was much talk after the '75 blackout about modifing
the power plants to trip to house load, meaning that
the plant would continue to generate power after the
circuit breaker to the grid tripped. Looks like it
didn't work.