Information Notice No. 85-94: Potential for Loss of Minimum Flow Paths Leading to ECCS Pump Damage During a LOCA
SSINS No.: 6835
IN 85-94
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
December 13, 1985
Information Notice No. 85-94: POTENTIAL FOR LOSS OF MINIMUM FLOW PATHS
LEADING TO ECCS PUMP DAMAGE DURING A LOCA
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
Purpose:
This notice is provided to alert licensees of recent instances where it was
discovered that minimum flow requirements might not or could not be met for
some emergency core cooling system (ECCS) pumps under small-break loss-of
coolant-accident (SBLOCA) conditions. It is suggested that recipients review
this information for applicability to their facilities and consider actions,
if appropriate, to preclude similar problems at their facilities. However,
the suggestions contained in this notice do not constitute NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances:
In three recent instances, it was discovered by licensee personnel or by NRC
site resident inspectors that minimum flow paths for ECCS pumps were jeopar-
dized by design errors or personnel errors.
Brunswick Units 1 and 2: On or about May 23, 1984, while performing local
leak fate testing of the primary containment penetrations, operations
personnel at Brunswick Unit 2 observed that the minimum flow valve for the
2A core spray pump would not stay in the closed position following receipt
of a "close" signal from the remote manual operator in the control room.
(These valves do not receive a "close" signal upon actuation of the
containment isolation system.) Figure 1 shows the location of the minimum
flow valves. Engineering personnel determined that the control logic for the
minimum flow valves was such that the valves would reopen after closure
whenever a low flow condition was sensed in the core spray line, including
conditions where the core spray system (CSS) pumps were not running.
On June 1, an engineering review determined that the minimum flow valve
control logic did not appear to meet design criteria for containment isola-
tion. Based on this determination, the normally open minimum flow valves for
the CSS trains for both units were declared inoperable. Operations personnel
then complied with the plant technical specification action statement for
inoperable primary containment isolation valves (PCIVs) by closing and
deactivating the valves. At the time, Unit l was operating at power and Unit
2 was defueled.
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IN 85-94
December 13, 1985
Page 2 of 4
The licensee established procedures intended to ensure effective operator
action to minimize the potential for pump damage in the event of a CSS pump
start.
From subsequent discussions with the pump vendor, the plant operating staff
learned that damage to the CSS pumps could occur in as little as 1 minute of
operation at shutoff head without the minimum required flow. The plant staff
reevaluated the situation and concluded that the risk of pump damage with
the valves closed was unacceptable. After shutdown on June 12, the minimum
flow valves were reopened and actuator power was restored. During the sub-
sequent Unit 1 startup on June 13, administrative controls and special pro-
cedures were implemented to ensure closure of the valves when required for
containment isolation. The licensee's planned permanent corrective action is
to modify the logic to allow remote isolation capability for the valves when
their associated pumps are not running, so that minimum flow and containment
isolation functions can both be ensured.
Peach Bottom Unit 3: During an NRC inspection at Peach Bottom 3, from July
16 to August 31 1984, the site resident inspectors performed a review of the
plant procedures and practices for ensuring minimum flow protection for the
ECCS pumps. They determined that the minimum flow valves for separate trains
of the residual heat removal (RHR) system had been closed on two different
occasions while the reactor was at power. Figure 2 shows the locations of
the minimum flow valves. The first event occurred on April 27, 1982, when
the "D" RHR pump minimum flow valve was closed and deactivated for several
hours. The second event occurred between June 22 and 25, 1984, when the "A"
RHR pump minimum flow valve was closed and deactivated for approximately 3
days. In neither case could the inspectors determine why the minimum flow
valves had been closed and deactivated. However, the inspectors believe
t,hat the actions were taken to perform maintenance on the valves: or their
motor operators. The inspectors also found that, in both cases, the licensee
did not consider (or declare) the associated low pressure coolant injection
(LPCI) system train to be inoperable while the minimum flow valve was
closed.
These events were brought to the attention of the licensee by the resident
inspectors. To prevent recurrence, operations personnel were instructed not
to deactivate any of the ECCS minimum flow valves when system operability is
required. Additional corrective measures are still under review.
Point Beach Units 1 and 2: On July 24, 1985, the licensee notified the NRC,
pursuant to the provisions of 10 CFR 21, of a potential defect in the design
of the control circuit for the safety injection (SI) pump recirculation flow
path isolation valves. During a post-implementation review of the Emergency
Operating Procedures, the licensee discovered that the failure of the power
supply breaker in the remote control circuitry for either of the isolation
valves in the recirculation line constituted a single failure that could
cause the loss of both SI pumps upon subsequent actuation of the pumps with
reactor coolant system (RCS) pressure remaining above pump shutoff head for
a short period.
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IN 85-94
December 13, 1985
Page 3 of 4
The SI pumps have a common return pipe from their discharges to the
refueling water storage tank (RWST) to provide a test flow path and a
recirculation path for minimum flow when the RCS pressure exceeds pump
shutoff head (see figure 3). There are two (one for each train) isolation
valves in series on the return line. They are air-operated and fail closed
when their control circuits lose electrical power or upon loss of air
pressure, which is not supplied from a safety related system. The purpose of
these valves is to Isolate the RWST (outside containment) from the
containment sump during the recirculation phase of emergency core cooling
following a LOCA. The valve position indication and the valve closed
annunciation in the control room are powered from the same breaker as the
valve control circuit. Therefore, the single failure of the breaker
associated with either train would isolate the minimum flow path for both SI
pumps, defeat the control room annunciation of the valve closure, and cause
the loss of valve position indication. Because the valve position indication
is not on a front panel, this condition could remain unnoticed for as much
as a month (the surveillance interval). Subsequent start of the SI pumps
could then result in the failure of both pumps within a short period unless
RCS pressure dropped below the 1470 psi shutoff head.
The licensee's short-term corrective action is to use the manual handwheel
operators on the isolation valves to override the remote operators and
maintain the valves in the open position in the event of loss of electrical
power or air pressure. This' action defeats the interlock between the
isolation valves in the RWST return line and the motor-operated containment
sump isolation valves. The interlock is designed to close the recirculation
line isolation valves when either of the containment sump isolation valves
is not fully closed. To ensure that contaminated containment sump water will
not be inadvertently pumped into the RWST during the recirculation phase of
safety injection following a design-basis accident, the remote containment
sump isolation valve controller switches have been fitted wi$h plastic
covers and plaques explaining the requirement for manually closing the RWST
return valves before the containment sump isolation valves are opened. The
local operators have similar plaques and have been administratively "red
locked" with plastic tab devices. Long-term options for permanent corrective
actions are still under investigation.
Discussion:
These events all demonstrate failures of licensees to recognize in a timely
manner that the operability of various ECCS pumps was jeopardized by loss or
potential loss of recirculation flow paths. In the event of a LOCA which did
not depressurize the RCS below the shutoff head of these pumps within a very
short period, the pumps could become damaged and unavailable to perform
their safety functions.
The events at Brunswick and Peach Bottom illustrate that deficiencies exist
in training and procedures with respect to the relationship between the
availability of minimum flow paths and the operability of ECCS pumps.
The events at Brunswick and Point Beach illustrate the difficulties in
designing systems to meet multiple criteria. The minimum flow valve control
logic at the
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IN 85-94
December 13, 1985
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Brunswick plant was inadequate to meet its containment isolation function.
The RWST return line control circuitry at the Point Beach plant was
inadequate to ensure minimum flow through at least one SI pump in the event
of a single failure.
The importance of minimum flow recirculation systems to ECCS pump
operability may not yet be fully reflected in design and operation because
adequate attention has not been focused on the effects of SBLOCA sequences
until relatively recently (i.e., following the Three Mile Island accident).
It appears that the minimum flow system for the two SI trains at Point Beach
was not designed to sustain a single failure. In addition, the Point Beach
defect illustrates the problem with reliance upon administrative controls to
ensure minimum flow through the ECCS pumps following a failure of the
minimum flow path. Timely manual action in the control room can be
compromised by a variety of difficulties. In the case of the defect reported
by Point Beach, the same failure that could block the minimum flow path
would also compromise control room indications needed by the operators.
Other, more simple indication failures could also compromise the ability to
detect mispositioned valves or inadequate flow. For this reason,
administrative controls are poor substitutes for adequate design.
No specific action or written response is required by this notice. If you
have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contacts: S. M. Long, IE
(301) 492-7159
E. J. Leeds, AEOD
(301) 492-4445
Attachments:
1. Figure 1, BWR Low Pressure Core Spray
2. Figure 2, BWR Low Pressure
3. Figure 3, Diagram of Point Beach ECCS
4. List of Recently Issued IE information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021