Station blackout at Browns Ferry Unit One - Oak Ridge National ...

SUMMARY
This study describes the predicted response of Unit 1 at the Browns
Ferry Nuclear Plant to a hypothetical Station Blackout. This accident
would be initiated by a loss of offsite power concurrent with a failure of
all eight of the onsite diesel-generators to start and load; the only re
maining electrical power at this three-unit plant would be that derived
from the station batteries. It is assumed that the Station Blackout occurs
at a time when each of the Browns Ferry units is operating at 100% power
so that there is no opportunity for use of the batteries for units 2 or 3
in support of unit 1.
The design basis for the 250 volt DC battery system at Browns Ferry
provides that any two of the three unit batteries can supply the electri
cal power necessary for shutdown and cooldown of all three units for a
period of 30 minutes with a design basis accident at any one of the units.
It is further provided that the system voltage at the end of the 30 minute
period will not be less than 210 volts, and that all DC equipment supplied
by this system must be operable at potentials as low as 200 volts.
It is clear that the 250 volt system was not designed for the case of
a prolonged Station Blackout, and the period of time during which the DC
equipment powered by this system could remain operational under these con
ditions can only be estimated. It would certainly be significantly longer
than 30 minutes since all three batteries would be available, the equip
ment is certified to be operable at 200 volts, and there would be no de
sign basis accident. In response to AEC inquiry in 1971, during the pe
riod of plant construction, TVA estimated that the steam-driven High Pres
sure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC)
systems, which use DC power for turbine control and valve operation, could
remain operational for a period of four to six hours. A period of four
hours has been assumed for this study.*
Within 30 seconds following the inception of a Station Blackout, the
reactor would have scrammed and the reactor vessel would be isolated be
hind the closed main steam isolation valves (MSIV's). The initial phase
of the Station Blackout extends from the time of reactor vessel isolation
until the time at which the 250 volt DC system fails due to battery ex
haustion. During this period, the operator would maintain reactor vessel
water level in the normal operating range by intermittent operation of the
RCIC system, with the HPCI system available as a backup. Each of these
water-injection systems is normally aligned to pump water from the conden
sate storage tank into the reactor vessel via a feedwater line.
*It should be noted that the events subsequent to failure of the 250
volt DC system are relatively insensitive to the time at which this fail
ure occurs. This is because the only parameter affecting the subsequent
events which is dependent upon the time of failure is the slowly-varying
decay heat.
As part of a requested review of the results of this study, TVA per
formed a battery capacity calculation which shows that the unit batteries
can be expected to last as long as seven hours under blackout conditions.
In Appendix G, the effect of a battery lifetime of seven vice four hours
upon study results is shown to be limited to timing.