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

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The curves of Fig. 3.3 show that the maximum drywell average temper
ature can be kept below 148.9°C (300°F) provided that the operator takes
action to depressurize within one hour following the loss of drywell cool
ers concomitant with a Station Blackout.* A depressurization at the
Technical Specifications limit of 55.6°C/h (100°F/h) will produce a grad
ual reduction of the drywell ambient temperature; a rapid depressuriza
tion will produce a faster drywell temperature reduction, but is probably
not necessary. Certainly, if the operator perceives developing difficul
ties with the in-drywell equipment such as the relief valve solenoids, he
should convert an ongoing 55.6°C/h (100°F/h) depressurization into a more
rapid one.
Summary. Assuming no independent secondary failures of equipment,
reactor vessel level control and pressure control can be maintained during
a Station Blackout for as long as the DC power supplied by the unit bat
tery lasts. This is expected to be a period of from four to six hours,
and this portion of the Station Blackout severe accident sequence is
graphically displayed with commentary in Section 7. If AC power is re
gained at any time during this period, a completely normal recovery would
follow.
If AC power is not regained, and DC power is lost, remote-manual op
eration of the primary relief valves would no longer be possible and the
reactor vessel pressure would increase to 7.722 MPa (1105 psig), the low
est setpoint for automatic actuation of the relief valves. Thereafter,
the reactor vessel pressure would cycle between 7.722 and 7.377 MPa (1105
and 1055 psig) while the vessel level steadily decreased because injection
capability would be lost as well. The sequence of events following the
loss of injection capability is described in Sections 9 through 11.
*If the backup HPCI system were used for injection, the average re
actor vessel pressure after depressurization would have to be maintained
at about 1.14 MPa (150 psig), which corresponds to an average saturated
liquid temperature of 185°C (365°F). This is only 15°C (25°F) higher than
the average temperature associated with RCIC operation and would not
significantly increase the drywell temperatures shown in Fig. 3.3.