Information Notice No. 96-55: Inadequate Net Positive Suction Head of Emergency Core Cooling and Containment Heat Removal Pumps Under Design Basis Accident Conditions

                                     UNITED STATES
                             NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555-0001

                                   October 22, 1996

                                    EMERGENCY CORE COOLING AND CONTAINMENT
                                    HEAT REMOVAL PUMPS UNDER DESIGN BASIS
                                    ACCIDENT CONDITIONS


All holders of operating licenses or construction permits for nuclear power reactors.


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert
addressees to recent discoveries by licensees that the available net positive suction head
(NPSH) requirements for emergency core cooling system (ECCS) and containment heat
removal pumps may not be adequate under all postulated design basis scenarios.  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

Haddam Neck

Insufficient NPSH for Residual Heat Removal Pumps (ECCS Recirculation Mode) 

In November 1986, the Haddam Neck licensee determined that the existing NPSH analysis
for the residual heat removal (RHR) pumps was in error.  This analysis indicated that
containment pressure in excess of the saturation pressure corresponding to the temperature
of the sump fluid was not needed to satisfy NPSH requirements for the RHR pumps in the
recirculation mode of ECCS operation.  The revised analysis conducted to correct the error
indicated, however, that credit for containment pressure above pre-event condition was
necessary to satisfy RHR pump NPSH requirements for recirculation operation.

A re-analysis conducted by the licensee in 1995 to reflect changing plant conditions indicated
that a required containment overpressure that was a significant fraction of peak calculated
containment design pressure was necessary to meet NPSH requirements.  Key assumptions
of the analysis were minimum design basis heat removal conditions, including minimum
service water flow, maximum service water temperature, and maximum fouling of the 

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containment air coolers.  A primary concern of the staff was the fact that the containment
overpressure relied upon was significantly greater than any previously approved calculation. 

On August 30, 1996, in Licensee Event Report 50-213/96-016, Accession Number
9609090320, the licensee stated that calculations performed in August 1996, to determine
the available NPSH to the RHR pumps operating in recirculation mode may not be adequate
under all postulated design basis scenarios.  The licensee indicated that the assumption of
sufficient containment overpressure to meet NPSH requirements used in previous analyses
could not be supported since recent sump temperature analyses cannot assure that the
necessary containment overpressure would be available.  In particular, for the preferred
recirculation flow path, the necessary overpressure would be approximately 136kPa [5 psig]
and would exist for the duration of the transient.  However, an alternate recirculation flow
path exists which is more restrictive, thus the necessary overpressure is greater and would
be unlikely to exist for the duration of short-term (single path) recirculation. The alternate path
exists to mitigate a potential failure of the preferred path.
The licensee attributed the apparent cause of the inadequate NPSH available to the failure to
fully analyze containment pressure and sump temperature response to support the NPSH
calculation.  The licensee intends to replace the piping between the containment sump and
the RHR pump suction with larger diameter piping to reduce the frictional losses so that
containment overpressure will not be relied on to satisfy NPSH requirements for the pumps.
Insufficient NPSH for Charging Pumps (ECCS Recirculation Mode) due to Inadequate

Another issue at Haddam Neck was reported on April 12, 1996, in Licensee Event Report 
50-213/96-06, Accession Number 9604190045, which involves inadequate NPSH for a single
centrifugal charging pump when the pump suction is aligned to the discharge of the RHR
pumps.  The postulated scenario would occur for a design basis loss of coolant accident
(LOCA) during the switchover to ECCS sump recirculation from the refueling water storage
tank (RWST) for the purpose of long-term recirculation cooling, with offsite power and only
one of the two centrifugal charging pumps available.  With one of the charging pumps
unavailable, the available pump would generate all of the flow, thereby requiring a greater
NPSH.  The licensee determined that under these conditions, the currently allowable
minimum RWST volume specified in the emergency response procedures would be
insufficient to provide the required NPSH as RWST level decreases during the switchover.

The licensee attributed the cause of the potential inadequate NPSH available to an error in
the analysis supporting the applicable emergency response procedures.  The minimum
allowable RWST volume was based on providing sufficient NPSH and protecting against
vortex air ingestion for the high pressure injection pumps.  The licensee incorrectly 
assumed that these requirements were more limiting than any associated with the charging
pumps.  Corrective actions included revising the emergency response procedures to .                                                                   IN 96-55
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caution the plant operators of the potential for charging pump cavitation and to advise the
operators to reduce charging pump flow.

Maine Yankee

Insufficient NPSH for Containment Spray Pumps (Sump Recirculation Mode)

Calculations performed in 1995 by the licensee for Maine Yankee indicate a worst case
condition where the available NPSH for the containment spray (CS) pumps would be
approximately 0.21m [0.7 ft] below the required NPSH specified by the manufacturer (4.66m
[15.3 ft] at 0.25m3/s [3900 gpm]) for the first five minutes following the switchover of pump
suction from the RWST to the recirculation sump after a design basis LOCA.  

In light of these recent calculations, the licensee discussed the results of the 1995 analysis
with the pump manufacturer to assess the impact of the results on long- and short-term pump
reliability.  The manufacturer agreed with the licensee's engineers that the pumps would not
be damaged during the five minute transient where minimum NPSH conditions exist and
would operate reliably following the transient.  In support of this assessment, the licensee
cited various tests conducted by the manufacturer which show:  (1) that similar pumps are
routinely operated at up to 50-percent degraded NPSH conditions for 1-3 minutes without
sustaining damage; (2) the installed CS pumps at Maine Yankee could operate indefinitely
with an available NPSH of 4.45m [14.6 ft] at 0.25m3/s [3900 gpm] without an adverse impact
on mechanical integrity; and (3) the installed pumps could operate for up to 15 minutes with
an available NPSH of 3.47m [11.4 ft] at 0.25m3/s [3900 gpm] with no impact on mechanical
integrity or long-term hydraulic performance.  

The licensee concluded that the CS pumps remain capable of performing under postulated
LOCA conditions and that their NPSH calculations accurately reflect sump temperature at the
time CS pump suction is switched from the RWST to the recirculation sump.  The staff has
not yet completed its evaluation of the licensee's analysis.

Crystal River Unit 3

Insufficient NPSH for Low Pressure Injection Pumps (ECCS Recirculation Mode) due to
Inadequate Procedures  

On March 22, 1995, the licensee for Crystal River, Unit 3, indicated that for a given ECCS
configuration, it is procedurally possible to have inadequate NPSH for a low pressure 
injection (LPI) pump during design basis LOCAs, potentially resulting in LPI pump cavitation. 
The configuration consists of one LPI pump suction aligned to the reactor building sump with
its discharge directed to the reactor vessel, while the same pump simultaneously provides
flow to both high pressure injection pumps delivering their maximum flowrates.  The
configuration would occur as a result of the Emergency Operating Procedures (EOPs)
directing plant operators to cross-connect the high pressure injection piping when only one of
the two LPI pumps is available.  With just one LPI pump supplying both high pressure  .                                                                   IN 96-55
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injection pumps, the flow through the LPI pump would increase, resulting in a required NPSH
greater than that available from the sump.  The problem would not exist if the single LPI
pump were supplying both high pressure injection pumps from the borated water storage

The licensee indicated that the cause of the event was a procedural discrepancy resulting
from insufficient review during the EOP change process.  The change to allow one LPI pump
to be aligned to both charging pumps was not reviewed in terms of NPSH since it was not
thought that the flow demand of the available LPI pump would significantly increase.  Prior to
the change, the EOPs directed that two LPI pumps be aligned to the high pressure injection
pumps.  The EOPs were revised to address the concern.      

It is important that the emergency core cooling and containment spray system pumps have
adequate NPSH available for all design basis accident conditions such that the systems can
reliably perform their intended functions under these conditions.  Inadequate NPSH could
cause voiding in the pumped fluid, resulting in pump cavitation, vapor binding, and potential
common mode failure of the pumps.  Such failure would result in the inability of the ECCS
system to provide adequate long-term core cooling and/or the inability of the containment
sprays to maintain the containment pressure and temperature to within design limits.  Loss of
the containment spray pumps would also reduce the ability to scrub fission products from
containment atmosphere following a LOCA, and damage to ECCS or CS pump seals from
elevated fluid temperatures and cavitation induced vibration could result in increased leakage
of coolant outside containment.

For the analyses used to determine the available NPSH, NRC Regulatory Guide 1.1, "Net
Positive Suction Head for Emergency Core Cooling and Containment Heat Removal System
Pumps," issued November 2, 1970, establishes the regulatory position that ECCS and
containment heat removal system pumps should be designed so that adequate NPSH is
available assuming maximum expected temperatures of pumped fluids and no increase in
containment pressure from that present prior to postulated LOCAs.  Because containment
pressure can vary considerably depending on the accident scenario, the staff concluded in
the Regulatory Guide that sufficient NPSH should be available for all postulated coolant
accidents without crediting containment overpressure.

However, in the past, the staff has selectively allowed limited credit for a containment
pressure slightly above the vapor pressure of the sump fluid (i.e., an overpressure) on a
case-by-case basis for satisfying NPSH requirements.  In these cases, licensees have
typically been requested to calculate the peak containment pressure resulting from the most
limiting design basis LOCA using the models described in Branch Technical Position CSB
6-1.  The models in CSB 6-1 includes such provisions as maximizing heat transfer
coefficients to containment heat sinks, maximizing the containment free volume, and mixing
of subcooled ECCS water with steam in the containment, all of which effectively maximize
heat transfer from the containment atmosphere, thereby minimizing the calculated.                                                                   IN 96-55
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containment pressure and resulting in a conservative overpressure.  Generally speaking, this
minimum overpressure is substantially greater than the needed overpressure for assuring
adequate NPSH.

With regard to those cases where plant procedures would have directed system
configurations resulting in inadequate NPSH, the staff stresses the importance of ensuring
that the actions and the results of actions directed by the procedures do not result in
situations where safety-related equipment would be incapable of performing its intended
function, or of performing in a non-degraded manner.   

The events described herein highlight the importance of ensuring sufficient available NPSH
for ECCS and containment heat removal system pumps for the applicable spectrum of
postulated LOCAs or secondary/main steam line breaks, such that the ability for long-term
core cooling and containment heat removal are not compromised.  It is important that
licensees know the NPSH requirements of the pumps and the bases on which the NPSH
available is considered adequate under a spectrum of primary and secondary break sizes
and locations.  It is also important that licensees know the containment heat removal
conditions assumed in these analyses.  If credit has been taken for a containment over-
pressure above the vapor pressure of the sump fluid, it is important for licensees to know the
basis for the amount of overpressure credited, including the modeling assumptions of the
analysis used to determine it.  Finally, system configurations that result from following plant
procedures should not result in situations where the NPSH available would be inadequate
under design basis accident conditions.      

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 of the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.   

                                 Thomas T. Martin, Director
                                 Division of Reactor Program Management
                                 Office of Nuclear Reactor Regulation

Technical contacts: Howard Dawson, NRR
                    (301) 415-3138

                    William Long, NRR
                    (301) 415-3026

Page Last Reviewed/Updated Wednesday, March 24, 2021