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

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HPCI system is actuated only when level is more than 0.25 m (10 in.)
below the 13.4-m (528-in.) lower limit of control room indication
under Station Blackout conditions. Suction for both systems is from
the condensate storage tank (CST). The operators actuate HPCI and
RCIC 1 min. after the blackout.
3. Following 1 min. of automatic SRV actuation, reactor pressure is con
trolled by remote-manual actuation of the SRVs. Control range dur
ing the first hour is 7.52 MPa (1090 psia) to 6.31 MPa (915 psia).
After 1 hour the reactor is depressurized to about 0.79 MPa (115
psia) at a rate consistent with the 55.6°C/h (100°F/h) cooldown rate
limit. After depressurization, pressure is controlled between 0.62
MPa (90 psia) and 0.86 MPa (125 psia).
Manual sequential SRV action is used to spread SRV discharge around
the suppression pool and thereby avoid localized heating of the pool.
4. The assumed reactor system leakage into the drywell is 0.0089 m /s
(4 gpm).
7.3.2 Normal Recovery — Results
Results for normal recovery are shown on Figs. 7.1 through 7.9.
Each system variable is discussed below.
7.3.2.1 Reactor vessel pressure. Figure 7.1 shows reactor vessel
pressure. During the first minute, reactor pressure cycles between 7.72
MPa (1120 psia) and 7.38 MPa (1070 psia) on automatic SRV actuation.
After 1 min. the operator opens one SRV to lower pressure to 6.31 MPa (915
psia). Since the HPCI is running, pressure continues to decrease to below
6.21 MPa (900 psia) until the HPCI is shut down. During the first hour a
single SRV is cycled open and shut seven times to keep pressure in the de
sired range.
After one hour, the depressurization begins. Initially, intermittent
operation of one SRV is sufficient to meet the target depressurization
rate [i.e., 55.6°C (100°F)/h], As pressure decreases, the rate of de
crease begins to slow until finally an additional SRV is opened. For
choked flow of dry steam, the mass flow rate is linearly proportional to
reactor pressure . Therefore an extra valve is required at the lower pre
ssure. When depressurization is complete, the operator controls pressure
between 0.62 and 0.86 MPa (90 and 125 psia).
After depressurization, the steam flow through one SRV is nearly
equal to the core steaming rate, resulting in much slower pressure change.
Under these conditions, the most rapid pressure change is caused by the
RCIC system. When RCIC comes on the 600 gpm injection of cold water tends
to lower the core steaming rate, causing pressure to decrease. This ef
fect can be seen in Fig. 7.1. At 256 min., the RCIC comes on, reducing
pressure until the open SRV is shut. When the RCIC is shut off 23 min.
later, pressure increases rapidly until the SRV is reopened.
Steam flow from the reactor vessel is shown in Fig. 7.2. The large
spikes are due to SRV actuation. Also included in the total steam flow
are the smaller amounts of steam flowing to the RCIC and HPCI turbines
when they are running.