Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 87-49
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 9, 1987
Information Notice No. 87-49: DEFICIENCIES IN OUTSIDE CONTAINMENT
FLOODING PROTECTION
Addressees:
All nuclear power reactor facilities holding an operating license or a con-
struction permit.
Purpose:
This notice is provided to alert recipients to a potentially significant
problem pertaining to the flooding of safety-related equipment as a result of
the inadequate design, installation, and maintenance of features intended to
protect against flooding. It is expected that recipients will review the
information for applicability to their facilities and consider action, if
appropriate, to preclude a similar problem. However, suggestions contained in
this notice do not constitute NRC requirements; therefore, no specific action
or written response is required.
Description of Circumstances:
Beaver Valley Power Station, Unit 2
In September 1986, during an Engineering Assurance Program, an incorrect, non-
conservative assumption in the outside containment flooding analysis was dis-
covered. After this assumption was corrected, a reanalysis showed that
without operator action, (1) higher flooding levels would occur in the
auxiliary and service buildings than were originally expected and (2)
essential Class 1E floor-mounted switchgear in the service building could be
affected, jeopardizing safe shutdown capability.
The analysis in question addresses a double-ended rupture of a main feedwater
line in the service building in which it was erroneously assumed that
100 percent of the process fluid above 212øF would flash to steam as it exited
the rupture. Reanalysis showed that, in fact, only about 25 percent of the
escaping feedwater would flash, leaving the remaining 75 percent in a liquid
form that would contribute to flooding.
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The Class 1E equipment threatened in this scenario is not located in the im-
mediate vicinity of the postulated feedwater line break, but is several levels
lower in the service building and would be flooded by water flowing through
door openings.
To limit the consequences of this postulated flooding event to an acceptable
level, the licensee changed plant operating procedures to require manual trip-
ping of the main feedwater pumps if a reactor trip occurs on steam flow/feed
flow mismatch. Since a reactor trip on this signal could indicate a feedwater
line break, securing the main feed pumps would minimize the amount of water
pumped out of the break. Also, the licensee installed gaskets on doors to
prevent the flood water from reaching Class 1E equipment. In addition, a
service building wall blowout panel, previously determined as necessary to
mitigate the temperature and pressure effects of a main steam line break, has
been installed at floor level to also provide for flooding protection.
Trojan Nuclear Plant
In March 1987, during a review of the turbine building flooding design basis
analysis it was found that the flood relief louvers in the turbine building
wall would not pass sufficient flow to prevent flooding of safety-related
equipment in the event of a break of the main circulating water system.
The major assumptions used in the main circulating water system break analysis
were that (1) full-diameter breaks would occur simultaneously in both circu-
lating water pipes at the condenser inlet and (2) the motor-operated valves at
the suction and discharge of both main circulating water pumps would fail to
close. It was originally believed that the flooding expected under these
conditions would be shunted to the exterior of the turbine building with no
detrimental effect. Further review has shown, however, that the flooding
relief capacity called for in the original plant design would be too small to
prevent overflow of flood barriers in the turbine building. Overflow of these
barriers would flood the auxiliary feedwater pumps and emergency diesel
generators.
In addition to the turbine building flooding concern described above, omission
or degradation of originally installed flood protection design features were
identified in several other areas: (1) drains in the auxiliary feedwater pump
room and service water strainer pit did not have required check valves
installed; (2) although the emergency diesel generator room drain line check
valve was properly installed, it was blocked open by a 1-foot long wooden
2 X 4; and (3) an auxiliary building pipeway flood barrier intended to protect
a centrifugal charging pump had not been installed. In each case, the
potential existed for equipment important to the plant's safe shutdown
capability to become disabled as a consequence of a flooding event.
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To correct these problems, the licensee installed the missing check valves and
flood barrier, and removed debris from drain lines as necessary. A preventive
maintenance program is being developed to ensure that flood protection
features are inspected periodically. The flood relief louver problem was
addressed by building up the turbine building flood barriers an additional
12 inches. Pending development of a modified design, the portion of the
turbine building wall that formerly housed the louvers will remain open.
Since the licensee's reassessment led to the conclusion that the assumption of
simultaneous rupture of both circulating water lines was overly conservative,
the turbine building flooding analysis in the Final Safety Analysis Report
will be revised to postulate the rupture of a single circulating water line.
This change, in conjunction with the programmatic and design changes described
above, will ensure that engineered safeguards features equipment will be
protected from a circulating water line failure.
Nine Mile Point Nuclear Station, Unit 2
In December 1986, it was discovered that a main circulating water piping
failure could disable Class 1E electrical equipment and both trains of the
service water system.
An electrical system manhole cover located in the decontamination area of the
turbine building was not watertight. Since electrical duct banks and conduit
running from the manhole also were found not to be fully sealed, flooding
paths existed that could make the safety-related service water pumps and Class
1E electrical equipment in the control building inoperable.
This flood protection deficiency was corrected by the installation of a
redesigned, watertight manhole cover, which would prevent entry of flood water
into the affected manhole.
Discussion:
The events described above illustrate the potential for the loss of safe
shutdown capability as a consequence of potential flooding of safety-related
equipment outside containment. A break in the main circulating water system
or main feedwater system has the potential to release an extremely large
volume of water in a very short period of time. Serious consequences may
result if the design features of the plant are not adequate to direct the
resulting flood water safely away from important equipment. Such design
inadequacies may result from (1) the inadvertent use of nonconservative
assumptions in the flooding design analysis, (2) the failure to recognize all
possible flooding flow paths, (3) the failure to install flood protection
features that have been determined to be necessary, or (4) the failure to
properly maintain installed flood protection features.
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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Kevin P. Wolley, AEOD
(301) 492-8373
Attachment: List of Recently Issued NRC Information Notices
