Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 30, 1990
Information Notice No. 83-44, SUPPLEMENT 1: POTENTIAL DAMAGE TO
REDUNDANT SAFETY EQUIPMENT
AS A RESULT OF BACKFLOW
THROUGH THE EQUIPMENT AND
FLOOR DRAIN SYSTEM
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This notice supplements information contained in NRC Information Notice
83-44 concerning potential damage to redundant safety equipment as a result
of backflow through the equipment and floor drain system and addresses the
effect of internal flooding of nuclear power plants on safety-related
equipment. It is expected that recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information
notice do not constitute regulatory requirements; therefore, no specific
action or written response is required.
Background:
The NRC issued Information Notice No. 83-44 to amplify the information
provided in IE Circular 78-06, "Potential Common Mode Flooding of ECCS
Equipment Rooms of BWR Facilities," in response to an event at a PWR
facility. IE Circular 78-06 applied specifically to Emergency Core Cooling
System (ECCS) equipment rooms and flooding of areas at lower elevations.
Information Notice No. 83-44 expanded this concern to other safety-related
compartments and to damage as a result of backflow through equipment and
floor drainage systems. Since the issuance of Information Notice No. 83-44,
the NRC has received an unexpectedly high number of reports from the nuclear
industry pertaining to actual and possible occurrences of internal flooding.
Description of Circumstances:
On July 7, 1990, the Quad Cities licensee, Commonwealth Edison, was
investigating a backed-up sump pump in the basement of the Unit 2 reactor
building. While attempting to clear the lines, the licensee discovered that
the check valves were inoperable in the floor drain lines from the ECCS
equipment rooms. Each drain line has both a flap-type and a ball-type check
valve in series between the ECCS equipment room and its respective sump.
Upon inspection, the licensee found that the flap-type check valves were
stuck open as a result
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IN 83-44, Supplement 1
August 30, 1990
Page 2 of 3
of debris and corrosion buildup and that three of the four balls in the
ball-type check valves had corroded and eroded away completely. In
addition, these eight check valves had been omitted from the licensee's
maintenance programs.
These inoperable check valves could have enabled flood water from the
following sources to render ECCS equipment inoperable: service water piping,
ECCS suction piping, the reactor building drain tank, and possibly the fuel
pool and suppression pool. As a corrective action, the licensee has
replaced and tested all the check valves in the ECCS room floor drains and
has included these valves in its maintenance programs. It should be noted
that this situation is not adequately covered in the Quad Cities Final
Safety Analysis Report (FSAR) or properly addressed in available
probabilistic risk assessment (PRA) analyses for similar plants.
Recently, the Office for Analysis and Evaluation of Operational Data (AEOD)
completed an Engineering Evaluation Report, AEOD/E90-07, "Effect of Internal
Flooding of Nuclear Power Plants on Safety-Related Equipment." This report
documents a review of operational events from 1980 to 1989 that resulted in
internal flooding in the secondary containment, auxiliary building, and
turbine building of nuclear power plants. The study also presents reported
conditions since 1984 that could have led to the flooding of safety-related
equipment compartments. Some of these events caused a loss of a significant
amount of safety-related equipment needed for decay heat removal, plant
shutdown, and plant cooldown, and have resulted in precursors that could
have resulted in severe core damage accidents. One plant probabilistic risk
assessment has identified internal flooding to be a significant contributor
to the risk of core damage.
Discussion:
Evaluation of the operational events identified errors by maintenance
personnel, procedural deficiencies, and inadequate design as the key
elements leading to internal flooding. Based upon the findings, the
following were identified as potentially generic inadequacies:
o Existing Flood Protection
The areas of particular concern include the adequacy of flood barriers
to prevent unisolable flooding and common-mode flooding, adequacy of
the floor drain system to prevent flooding of one area of the plant
from propagating to other areas, and adequacy of flood warning
instrumentation to provide the operators with an early signal of
flooding in the plant.
o Maintenance Procedures and Personnel Training
Some licensees have not always conducted adequate reviews of
maintenance procedures and personnel training to minimize the risk of
unanalyzed internal flooding events caused by single component failures
or personnel errors. In addition, the AEOD report indicated that
several significant flooding events occurred during cold shutdown or
refueling outages. This indicates weaknesses in maintenance planning
and administrative controls intended to minimize the risk of flooding
during such outages.
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IN 83-44, Supplement 1
August 30, 1990
Page 3 of 3
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: N. Su, AEOD
(301) 492-4434
J. Raleigh, NRR
(301) 492-1172
Attachment: List of Recently Issued NRC Information Notices
