Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
June 26, 1997
NRC INFORMATION NOTICE 97-40: POTENTIAL NITROGEN ACCUMULATION
RESULTING FROM BACKLEAKAGE FROM SAFETY
INJECTION TANKS
Addressees
All holders of operating licenses or construction permits for pressurized-water reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert
addressees to potential nitrogen accumulation in interfacing systems resulting from
backleakage from safety injection tanks (SITs). 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 are not
NRC requirements; therefore, no specific action or written response is required.
Description of Circumstances
Waterford
On November 19 and 21, 1996, Waterford Generating Station, Unit 3, experienced
waterhammer events on the low-pressure safety injection (LPSI) B train. On November 19,
1996, the control room pressure gauge was observed to spike to approximately 3.55 MPa
[500 psig] (normal operating pressure is approximately 1.2 MPa [160 psig]). On
November 21, 1996, the train B piping was vented, and a strip chart recorder was
installed to monitor pressure. A pressure of 4.65 MPa [660 psig] was observed following
the start of the pump. Shortly thereafter, the LPSI train B flow control valve was found
partially open because of a valve mispositioning error by the operations crew. The valve
was closed, and the pump started without a pressure transient. Apparently no structural
damage resulted from the waterhammer events.
On December 13, 1996, LPSI train A experienced a waterhammer event. During a routine
surveillance run of LPSI pump A, the pressure spiked to approximately 2.3 MPa [317 psig].
The licensee's investigation after this event utilized ultrasonic testing (UT) to inspect high
points in the LPSI piping. The licensee identified gas voiding in the horizontal run of the
two A train injection lines of 30.5 cm [12 inches] and 35.5 cm [14 inches] of arc. The
injection lines are 20 cm [8 inches] in diameter. The gas was sampled and found to be
approximately 97 percent nitrogen. The licensee concluded that nitrogen-saturated water
from the SIT (the
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SIT has a 4.6-MPa [650-psig] nitrogen blanket) was leaking back to the LPSI system. At
reduced pressures such as those found in the LPSI system piping, the nitrogen comes out
of solution and forms the gas voids.
The gas was vented from the A train of LPSI, and LPSI was declared operable on
December 14, 1996. On December 18, 1996, voiding was found in LPSI train B piping,
with arcs of 28 cm [11 inches] and 25.4 cm [10 inches]. Because of the location of these
gas pockets, the licensee was unable to vent these lines. The licensee performed an
operability evaluation and concluded that the B train remained operable but instituted a UT
surveillance program to inspect the voids every 3 days and identified acceptance criteria
for an acceptable arc length. Furthermore, the licensee was pursuing a design
modification to install high-point vents.
In February 1997, the licensee noted that one of its SITs was experiencing a lowering in
level. Upon investigation, the licensee found that a LPSI system containment isolation
valve was off its shut seat. The licensee surmised that water was leaking from the SIT
past check valves and the partially open isolation valve in the LPSI to the refueling water
storage pool (RWSP). On February 20, 1997, the licensee conducted testing on the
isolation valve to investigate why the valve was not shut. The licensee cycled the valve
several times during the course of this testing. At the completion of the testing, the
licensee performed UT of the A LPSI injection line to check for voiding.
The licensee found that the high horizontal run of piping was dry and that a void extended
203 cm [80 inches] down the vertical run of piping. In response to the large void, the
licensee used a vacuum pump to draw water into the line until the voiding was reduced to
a 12.7-cm [5-inch] arc. The reduced arc was within the licensee's acceptance criteria for
this line, and LPSI was declared operable.
The licensee also postulated that the shutdown cooling heat exchanger isolation valves
may be susceptible to waterhammer-induced pressure locking. The isolation valves had
been previously evaluated for a trapped bonnet pressure of 2.2 MPa [300 psig] and found
to be operable. However, there is no assurance that bonnet pressure would not exceed
2.2 MPa [300 psig] when subjected to a waterhammer pressure surge. There is no
evidence that this pressure locking has occurred in the past, and the licensee believes that
this scenario is unlikely. However, the postulated phenomenon could result in common-
mode failure of the heat exchangers, and the licensee has installed bonnet pressure relief
devices.
Sequoyah
In January 1995, during a Sequoyah Unit 1 scheduled quarterly residual heat removal
(RHR) pump test, a loud metallic noise was heard and movement of the refueling water
storage tank (RWST) suction piping was observed. At the time when the noise was heard
and pipe movement observed, personnel were performing a walkdown of a previously
identified damaged RHR pipe hanger. The RHR system piping was subsequently inspected
for gas voids and approximately .23 cubic meters [8 cubic feet] of gas was identified. A
sample taken of the vented gas determined the gas to be approximately 98 percent
nitrogen. The license believed that the source of the gas was one or more of the
Emergency Core Cooling System (ECCS) Cold Leg Accumulators (CLA) and that there were
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paths that water could travel from the CLAs to the RHR injection piping where the nitrogen
gas came out of solution: through the Safety Injection System (SIS) test header and/or
through the secondary check valves on the RHR cold leg injection lines to the Reactor
Coolant System (RCS). The licensee performed an operability evaluation which determined
that the gas did not affect operability of the RHR system either in normal or accident
conditions.
Over the course of the next several months the licensee periodically vented the RHR
piping, monitored the rate of gas accumulation and calculated the gas volume in the RHR
piping, and tracked the level decrease of the CLAs. To date, the licensee has not been
successful in identifying and correcting leaking isolation valves in the SIS test header. The
Sequoyah RHR system design and layout may have contributed to the gas accumulation
due to the lack of high point vents in the RHR system and the difficult accessibility of
those vents which were available. In November 1996, the licensee installed a manual
vent in the Unit 1 A train RHR system in a 49 meter [160 foot] section of piping which
previously was not completely ventable.
In October 1996, an analysis of the Unit 1 RHR piping system was performed to
determine the imposed forces resulting from system operation with various volumes of gas
in the piping system. The analysis concluded that the hydraulic forces built up are higher
with increasing volumes of gas but level off when the gas volumes go beyond more than
.28 cubic meters [10 cubic feet]. The measured volumes of gas in the Unit 1 RHR system
have varied from approximately .23 cubic meters [8 cubic feet] to .4 cubic meters [14
cubic feet]. The licensee also calculated that for each gallon of water released from the
CLAs, approximately 2.1 liters [0.073 cubic feet] of gas is released.
Discussion
If SITs or CLAs lose water level, nitrogen gas accumulation could be occurring in
interfacing systems. The presence of large amounts of gas in discharge piping creates the
possibility of waterhammer with potentially significant consequences. Waterhammer of
sufficient magni- tude can cause common-mode failure of safety injection trains.
Structural failure of the discharge piping could create a containment bypass release path in
addition to preventing safety injection flow. Additionally, a waterhammer event could
potentially cause pressure locking in some valves. A plant's susceptibility to nitrogen
accumulation in the ECCS lines is dependent on a number of plant-specific factors,
including operating pressure of the SITs or CLAs, elevations and orientation of ECCS
injection lines, and the elevation of the RWSP or RWST. Licensees may wish to evaluate
the possibility and effect of gas accumulation on potentially susceptible systems.
Licensees may also wish to evaluate the design of these systems to ensure adequate
venting capability.
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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 Office of Nuclear Reactor Regulation (NRR) project
manager.
signed by S.H. Weiss for
Marylee M. Slosson, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Robert D. Starkey, RII Christopher Jackson, NRR
(423) 842-8001 301-415-2947
E-mail: [email protected] E-mail: [email protected]
Phillip Harrell, RIV John Tappert, NRR
(817) 860-8250 301-415-1167
E-mail: [email protected] E-mail: [email protected]
