ML20216F544
| ML20216F544 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 09/07/1999 |
| From: | Fredrickson P NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | Scalice J TENNESSEE VALLEY AUTHORITY |
| References | |
| 50-327-99-04, 50-328-99-04, NUDOCS 9909220086 | |
| Download: ML20216F544 (3) | |
See also: IR 05000327/1999004
Text
'
I
t
4
.
September 7, 1999
Tennessee Valley Authority
ATTN: Mr. J. A. Scalice
Chief Nuclear Officer and
Executive Vice President
]
6A Lookout Place
J
1101 Market Street
Chattanooga, TN 37402-2801
SUBJECT:
SEQUOYAH FLOODING EVENT RISK DETERMINATION (NRC INTEGRATED
INSPECTION REPORT NO. 50-327/99-04 AND 50-328/99-04)
,
Dear Mr. Scalice:
The purpose of this letter is to provide you with the results of our risk evaluation of a June 30,
1999, flooding event at your Sequoyah 1 & 2 reactor facilities. The evaluation was conducted
under the NRC's Significance Determination Process (SDP). This event is documented in
Inspection Report 50-327/99-04 and 50-328/99-04 and was transmitted to you in a letter dated
August 13,1999, in our letter, we informed you that one potentially safety significant issue had
been identified. Specifically, on June 30,1999, inadequate performance of the storm drain
system resulted in water from a heavy rainfall backing up and flooding the turbine building
railroad bay where the 6.9kv unit boards were located, creating the potential for a loss of offsite
power. At the completion of the inspection, the risk significance of this event had not been
determined.
'
The NRC has completed a preliminary risk determination of this event using the SDP. Our
evaluation has determined that performance for this event is in the White regulatory response
band, which indicates issues with some increased rlsk to safety, and which may require
additional NRC inspections. The assumptions used by the NRC in determining the significance
of this event are provided in the enclosure.
Although we believe we have sufficient information to make a final risk determination on this
issue, we are providing you an opportunity to provide us with the following information within 14
days of the date of this letter:
-
The results of your analysis of the risk significance of this issue, if different than the
NRC's significance level
-
A description of the assumptions you used and consider applicable to the analysis
in addition, if you choose to provide us with this additional information and believe it would be
,
beneficial to discuss your risk significance evaluation with us, you should contact me within 14
1
days of the date of this letter to schedule a meeting.
\\
9909220086 990907
ADOCK 05000327
,
O
$0l
j
.
2
in accordance with 10 CFR 2.790 of the NRC's " Rules of Practice," a copy of this letter and its
enclosure will be placed in the NRC Public Document Room.
!
Sincerely,
(Original signed by)
Paul E. Fredrickson, Chief
Reactor Projects Branch 6
Division of Reactor Projects
Docket Nos. 50-327, 50-328
Enclosure: NRC Assumptions for Sequoyah
Flooding Event
cc w/enci:
Senior Vice President
Nuclear Operations
Tennessee Valley Authority
Electronic Mail Distribution
Jack A. Bailey, Vice President
Engineering and Technical Services
Tennessee Valley Authority
>
Electronic Mail Distribution
Masoud Bajestani
Site Vice President
Sequoyah Nuclear Plant
Electronic Mail Distribution
General Counsel
Tennessee Valley Authority
Electronic Mail Distribution
N. C. Kazanas, General Manager
Nuclear Assurance
Tennessee Valley Authority
Electronic Mail Distribution
cc w/enci cont'd: See page 3
.
.
,
'TVA
3
cc w/ enc!: Continued
Mark J. Gurzynski, Manager
Nuclear Licensing
Tennessee Valley Authority
'
Electronic Mail Distribution
Pedro Salas, Manager
Licensing and Industry Affairs
Sequoyah Nuclear Plant
Tennessee Valley Authority
i
Electronic Mail Distribution
D. L. Koehl, Plant Manager
Sequoyah Nuclear Plant
Tennessee Valley Authority
Electronic Mail Distribution
Debra Shults, Manager
i
Technical Services
Division of Radiological Health
Electronic Mail Distribution
l
County Executive
Hamilton County Courthouse
i
Chattanooga, TN 37402-2801
Distribution w/ encl:
j
R. W. Hernan, NRR
H. N. Berkow, NRR
PUBLIC
l
OFFICE
RII/pR_5 g ,RII g
51bMATURL
yf gg 0 g []
NAML
R54 [qnr0 N
Mbr gnoW
DATE
f)
q
W/
g /W9
W/
/vv
v/
/WW
9/
/v3
W/
/vy
w/
/uv
,
turYf
1 E5*
ND
YE5
NO
YE5
NO
YES
NO
YES
NO
I
YE5
NO
YE5
NO
OFFICIAL RECORD C PY
DOCUMENT NAME; 1:\\RPB6\\5Q\\ REPORTS \\5q9904-8-r2.wpd
Ws
<
NRC Assumptions for Sequoyah June 30,1999 Flooding Event
,
The NRC has determined through examination of Sequoyah's Individual Plant Examination for
External Events (IPEEE), submitted to the NRC in June of 1995, that the impact of rain induced
external flood on risk was screened out in the IPEEE as not an important contributor to plant
risk.
/
The NRC used the site's Individual Plant Examination (IPE) to characterize the impact of Loss of
Offsite Power (LOOP) events caused by the flooding of the 6.9 kV switchgear. Revision 1 of the
Sequoyah IPE lists LOOP as contributing 10.5% of the plant's internal risk frequency with a
contribution to risk of 4E-6 per year. The LOOP initiator frequency is 4.85E-2 per year. This
results in a risk to frequency ratio for LOOP of 8E-5. An additionalinitiating event frequency
contributor to LOOP of greater than 1.23E-2 per year would result in a contribution to risk that
would exceed the Green threshold of the NRC Significance Determination Process (SDP) of 1E-
6 per year. Thus a LOOP-inducing rainfall event would be a significant contributor to core
damage frequency (CDF) if it were predicted to occur at a frequency greater than once every 80
,
years.
Flooding of the 6.9kV panels, to sufficient depth, could result in a dual unit LOOP event. The
6.9kV panels have experienced rainfall-induced water intrusion to a depth of approximately 2
inches, twice in the last five years. These rainfall events did not result in a LOOP event but were
estimated to be at the 2-year and 5-year storm intensities. The factors that determine how high
the intruding water level must peak to cause a LOOP event and the relationship between peak
water level and rainfall intensity are numerous. Rainfall studies for the area around Sequoyah
indicate that rainfall intensity increases by a factor of two when comparing a 50-year to a 5-year
rainfall.
The capacity and condition of the storm and internal turbine building drains, the volume and rate
of run-off into yard drains in the turbine building railroad bay (TBRB) area, and the elapsed time
between onset of flooding and its isolation through closure of the roll-up door and/or placement
of other barriers have a significant impact on how high the resulting flood waters in the building
will peak. The impact of the June 1999 rainfall was limited by closure of the turbine building roll-
'
up door. This was evidenced by the peak water depth outside the door (approximately 18
inches) compared to the peak level inside the TBRB (approximately 4-6 inchesy. Timely
isolation of the TBRB would not be certain for future rainfalls because the flooding is not alarmed
and the required response time can be very short.
A LOOP event caused by flood-damaged electrical panels would result in a longer expected
recovery time than that assumed in the LOOP initiating event recovery curves, which are based
on grid centered losses, and would require adjustments to the analysis recovery assumptions.
Longer expected recovery times would lead to a higher CDF contribution and would have the
effect of increasing the calculated CDF.
Given the information and assumptions presented above, that two rainfalls of less than 5-year
storm intensity have resulted in partial flooding of the 6.9 kV panels in the last five years, and
given the likelihood of rainfalls of higher intensity during the plant's lifetime, the NRC has
determined that Sequoyah's performance for this event is in the White regulatory response band
of the NRC SDP.
Enc!osure
-
~