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

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of heat transfer to the drywell at that instant from the hot [287.8°C
(550°F)] reactor vessel and associated piping is about 0.98 MW (3.35 x
10° Btu/h). Under this impetus, the drywell temperature rapidly in
creases. After a few minutes, the drywell temperature will have signifi
cantly increased so that the rate of heat transfer from the reactor vessel
is reduced while a substantial rate of heat transfer from the drywell at
mosphere to the relatively cool drywell liner has been established, as
discussed in Section 4.
The design temperature for the drywell structure and the equipment
located therein is 138.3°C (281°F). The response of this structure and of
certain safety components within the drywell to higher temperatures has
been previously investigated7 and the results are summmarized below:
It was determined that the drywell liner will not buckle under liner
temperatures as high as 171.1°C (340°F) nor would this temperature produce
higher than allowable stress intensity.
The drywell electrical penetrations were purchased with a specified
short term (15 min) temperature rating of 162.8°C (325°F) and a long term
rating of 138.3°C (281°F).
It was determined that the stresses in the drywell piping penetra
tions would not exceed the allowable stress intensities at a temperature
of 171.1°C (340°F).
A DC solenoid control valve used for the remote-manual operation of
the primary relief valves was tested at 148.9°C (300°F) for ten hours with
118 actuations during the test period. An AC-DC solenoid control valve
for the Main Steam Isolation valves was tested at 148.9°C (300°F) for 7
hours with 83 actuations each on AC, DC, and the AC-DC combination. The
maximum temperature achieved during each of these solenoid tests was
153.3°C (308°F).
The electrical cable which feeds the safety equipment inside the drywell
has a 600-V rating with cross-linked polyethelene insulation. It has
seven conductors of number 12 wire with no sheath. It is estimated that
the ten-hour temperature rating is in excess of 160.0°C (320°F).7
The results of these tests and investigations indicate that it can be
confidently predicted that the primary relief valves will remain operable
and the drywell penetrations will not fail if the drywell ambient tempera
ture is prevented from exceeding 148.9°C (300°F) during the normal recov
ery phase of a Station Blackout.
With the operator acting to maintain reactor vessel level by use of
the RCIC system and to control vessel pressure between 7.688 and 6.309 MPa
(1100 and 900 psig) by remote-manual relief valve actuation as previously
discussed, the drywell ambient temperature would reach 148.9°C (300°F) in
one hour (curve A of Fig. 3.3). Before this occurs, the operator should
act to reduce the reactor vessel pressure by blowdown to the pressure sup
pression pool. This will reduce the temperature of the saturated liquid
within the reactor vessel and thereby reduce the driving potential for
heat transfer into the drywell.
The vessel pressure reduction would be by remote-manual actuation of
the primary relief valves, and should proceed at the Browns Ferry Techni
cal Specifications limit of a rate equivalent to a 55.6°C/h (100°F/h) de
crease in saturated liquid temperature. The response of the drywell ambi
ent temperature to such a reactor vessel depressurization begun at one
hour after the inception of a Station Blackout is shown by curve B of Fig.
3.3.