Information Notice 1996-55, Inadequate Net Positive Suction Head of Emergency Core Cooling and Containment Heat Removal Pumps Under Design Basis Accident Conditions

Inadequate Net Positive Suction Head of Emergency Core Cooling and Containment Heat Removal Pumps Under Design Basis Accident Conditions
	ML031050598
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	10/22/1996
From:	Martin T; Office of Nuclear Reactor Regulation
To:	;
References
	IN-96-055, NUDOCS 9610150005
	Download: ML031050598 (11)
	v • d • e

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

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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 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

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.

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 0.25m3/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 0.25m 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 0.25m3 /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 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 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.

J Thomas T. Martin, Director

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 Vulnerability of Stainless 10/17/96 All materials licensees

Steel to Corrosion When

Sensitized

96-53 Retrofit to Amersham 660 10/15/96 All industrial radio- Posilock Radiography graphy licensees

Camera to Correct Incon- sistency in 10 CFR Part 34 Compatibility

95-04, Excessive Cooldown 10/11/96 All holders of OLs or CPs

Supp. 1 and Depressurization and vendors for nuclear

of the Reactor Coolant power reactors

System Following Loss

of Offsite Power

96-40, Deficiencies in 10/07/96 All holders of OLs or CPs

Supp. 1 Material Dedication for nuclear power reactors

and Procurement

Practices and in

Audits of Vendors

96-52 Cracked Insertion 09/26/96 All U.S. Nuclear Regulatory

Rods on Troxler Commission portable gauge

Model 3400 Series licensees and vendors

Portable Moisture

Density Gauges

92-68, Potentially Sub- 09/16/96 All holders of OLs or CPs

Supp. 1 standard Slip-On, for nuclear power reactors

Welding Neck, and

Blind Flanges

OL = Operating License

CP = Construction Permit

IN 96-55 October22, 1996 containment pressure and resulting in a conservative overpressure. Generally speaking, this