Information Notice 1996-55, Inadequate Net Positive Suction Head of Emergency Core Cooling and Containment Heat Removal Pumps Under Design Basis Accident Conditions
UNITED STATES NUCLEAR REGULATORY
COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 October 22, 1996 NRC INFORMATION
NOTICE 96-55: INADEQUATE
NET POSITIVE SUCTION HEAD OF EMERGENCY
CORE COOLING AND CONTAINMENT
HEAT REMOVAL PUMPS UNDER DESIGN BASIS ACCIDENT CONDITIONS
Addressees
All holders of operating
licenses or construction
permits for nuclear power reactors.
Purpose
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 r 96101350005 pt E CooTlcaS 9a 20 2 IN 96-55 October 22, 1996 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 altemate 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
Procedures
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
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
v1-'mat.. .IN 96-55 October 22, 1996 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 0.25m 3/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 0.25m 3/s [3900 gpmj 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 0.25m 3/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 October 22, 1996 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 tank.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 concem.Discussion
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 October 22, 1996 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
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 J Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical
contacts:
Howard Dawson, NRR (301) 415-3138 Email: hfd@nrc.gov
William Long, NRR (301) 415-3026 Email: wol@nrc.gov
Attachment:
List of Recently Issued NRC Information
Notices
Attachment
IN 96-55 October 22, 1996 LIST OF RECENTLY ISSUED NRC INFORMATION
NOTICES Information
Date of Notice No. Subject Issuance Issued to 96-54 96-53 95-04, Supp. 1 96-40, Supp. 1 96-52 92-68, Supp. 1 Vulnerability
of Stainless Steel to Corrosion
When Sensitized
Retrofit to Amersham 660 Posilock Radiography
Camera to Correct Incon-sistency in 10 CFR Part 34 Compatibility
Excessive
Cooldown and Depressurization
of the Reactor Coolant System Following
Loss of Offsite Power Deficiencies
in Material Dedication
and Procurement
Practices
and in Audits of Vendors Cracked Insertion Rods on Troxler Model 3400 Series Portable Moisture Density Gauges Potentially
Sub-standard Slip-On, Welding Neck, and Blind Flanges 10/17/96 10/15/96 10/11/96 10/07/96 09/26/96 09/16/96 All materials
licensees All industrial
radio-graphy licensees All holders of OLs or CPs and vendors for nuclear power reactors All holders of OLs or CPs for nuclear power reactors All U.S. Nuclear Regulatory
Commission
portable gauge licensees
and vendors All holders of OLs or CPs for nuclear power reactors OL = Operating
License CP = Construction
Permit
IN 96-55 October22, 1996 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
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 William Long, NRR (301) 415-3138 (301) 415-3026 Email: hfd@nrc.gov
Email: wol@nrc.gov
Attachment:
List of Recently Issued NRC Information
Notices*SEE PREVIOUS CONCURRENCES
Tech Editor reviewed and concurred
on DOCUMENT NAME: 96-55.IN To receive a copy of this document, bidicate I the box: 'C' -Copy wlo ettachmenVenclosure
'E' -Copy w/attachment/enclosure
'N' -No copy OFFICE
CONTACT
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IN 96-55 October 21, 1996 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
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 William Long, NRR (301) 415-3138 (301) 415-3026 Email: hfdenrc.gov
Email: wol@nrc.gov
Attachment:
List of Recently Issued NRC Information
Notices*SEE PREVIOUS CONCURRENCES
Tech Editor reviewed and concurred
on DOCUMENT NAME: 96-55.IN To receive a copy of this document dIclete I the box: 'C' -Copy w/o attachment/enclosure
'E' -Copy w/attachmentlenclosure
'N' -No copy OFFICE
CONTACT
S C/PECB:DRPM
l /DRPM NAME HDawson* TAChaffee
% TMartin WLong* l DATE 09/23/96 10/iS796 10/ /96 OFFICIAL RECORD COPY
IN 96-55 October 18, 1996 containment
pressure and resulting
in a conservative
overpressure.
General peaking, this minimum overpressure
is substantially
greater than the needed overpress
e for assuring adequate NPSH.With regard to those cases where plant procedures
would have dircted system configurations
resulting
in inadequate
NPSH, the staff stresses t importance
of ensuring that the actions and the results of actions directed by the pro dures do not result in situations
where safety-related
equipment
would be incapab of performing
its intended function, or of performing
in a non-degraded
manner.The events described
herein highlight
the importance
f ensuring sufficient
available
heat removal system p ps for the applicable
spectrum of postulated
LOCAs or secondary/main
steam line reaks, such that the ability for long-term core cooling and containment
heat removal ar ot compromised.
It is important
that licensees
know the NPSH requirements
of t pumps and the bases on which the NPSH available
is considered
adequate under a nectrm of primary and secondary
break sizes and locations.
It is also important
that Ii nsees know the containment
heat removal conditions
assumed in these analyses.
f credit has been taken for a containment
over-pressure above the vapor pressure the sump fluid, it is important
for licensees
to know the basis for the amount of overpressu
credited, including
the modeling assumptions
of the analysis used to determine
it. Firily, system configurations
that result from following
plant procedures
should not result in ituations
where the NPSH available
would be inadequate
under design basis accident nditions.This information
notice re ires no specific action or written response.
If you have any questions
about the info ation in this notice, please contact one of the technical
contacts listed below of the ap priate Office of Nuclear Reactor Regulation (NRR) project manager.Thomas T. Martin, Director Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical
co acts: Howard Dawson, NRR William Long, NRR (301) 415-3138 (301) 415-3026 Email: hfdenrc.gov
Email: wolenrc.gov
Attachmnt:
List of Recently Issued NRC Information
Notices SEE PREVIOUS CONCURRENCES
Tech Editor r viewed and concurred
on DOCUMENT NA: 96-55.IN To receive a copy othis document.
Indicate In the box: 'C- -Copy w/o attachment/enclosure
'E' -Copy wlattachmentlenclosure
'N' -No copy;OFFICE
CONTACT
S C/PECB:DRPD/DRPM
NAME HDawson* AChaffe TMartin____WLong*
-_________DATE 09/23/96 10/1i/96 10/ /96 OFFICIAL RECORD COPY vc)
IN 96-September , 1996 This information
notice requires no specific action or written res se. If you have any questions
about the information
in this notice, please conta ne of the technical
contacts listed below of the appropriate
Office of Nuclear Reactor gulation (NRR) project manager.omas T. Martin, Director ivision of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical
contacts:
Howard Daw n, NRR (301) 415 138 Email: denrc.gov Wili Long, NRR (3 ) 415-3026 ail: wol@nrc.gov
Attachment:
List o ecently Issued NRC Information
Notices*SEE PR IOUS CONCURRENCES
DOCUMENT N E: G:MEJB1\NPSH.IN
To receive a opy of this document, Indicate In the box: "C" = Copy without attachment
closure "E" = Copy with attachment/enclosure "N" = No copy OFFICE ,ontactsi
BCSS:SA DIDSSAJ ICPECB:DRPM
IDIDRPmL NAMEA HDawson* CBerlinger*
GHolahan*
AChaffee TMartin WLong*DAT 09/23/96 09123/96 09/30/96 / /96 / /96 09/23/96 1 _ __/ OFFICIAL RECORD COPY
IN9 9 Se ember XX, 1996 ge 6 of 6 This information
notice requires no specific action or ritten response.
If you have any questions
about the information
in this otice. please contact one of the technical
contacts listed below of the propriate
Office of Nuclear Reactor Regulation (NRR) project manager.Th s T. Martin, Director Di ision of Reactor Program Management
fice of Nuclear Reactor Regulation
Technical
Contacts:
Howard Daws , NRR (301)415-
38 Internet fd@nrc.gov
Willi Long, NRR (301 415-3026 In rnet:wol@nrc.gov
DOCUME NAME: G:\EJB1\NPSH.IN
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