In a message dated 9/10/2011 4:09:19 P.M. Eastern Daylight Time,
RLAMONT@providence.edu writes:
This may be an incredibly simple minded question, but when the grid
collapses, why is it necessary to scram the reactor. Since the reactor generates
steam for a turbine, why can't it be vented out the cooling stack until the
grid has stabilized?
Bob at PC
))))))))))))))))))))
It's not so simple minded. In theory you can design a condenser steam dump
system to handle full load rejection. So why is this not normally done.
First you need a power source for the condenser circulating water pumps and
these are high power units.You would need a diesel to supply these pumps but
it would be next to impossible to transfer power for these pumps during a
loss of grid power. You don't want to dump over a 1000Mw of energy in a
condenser with no circ pumps, not even for few seconds. You never want to vent
steam to the environment if you can avoid it. In a BWR this is steam right
from the reactor, you don't want to send this to the environment. In
addition this is vital water inventory. what you lose has to be replenished
with pumps. ( in BWR steam can be dumped to a suppression pool in the
containment but this is primarily for pressure control of both reactor and
containment in the event of a loss of reactor load. ) For PWR you have steam
generators but this again is vital water inventory, you don't want to lose it.
These load rejection systems are expensive and they don't buy you much. All
Nuclear plants have some load rejection capability but almost never , that
I have seen , 100 % rejection capability. Even if you designed a plant
with 100 % rejection load capability you still need to scram it and get in a
safe shutdown state in the event of loss of off site power. In other words
you have to push the scram buttons anyway.