ML100640465

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR REACTOR REGULATION 

OFFICE OF NEW REACTORS 

WASHINGTON, DC 20555-0001 

June 16, 2010 

NRC INFORMATION NOTICE 2010-11: POTENTIAL FOR STEAM VOIDING CAUSING 

RESIDUAL HEAT REMOVAL SYSTEM 

INOPERABILITY 

ADDRESSEES

All holders of or applicants for an operating license or construction permit for a nuclear power 

reactor issued under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, “Domestic 

Licensing of Production and Utilization Facilities,” except those that have permanently ceased 

operations and have certified that fuel has been permanently removed from the reactor vessel. 

All holders of or applicants for a standard design certification, standard design approval, 

manufacturing license, or combined license issued under 10 CFR Part 52, “Licenses, 

Certifications, and Approvals for Nuclear Power Plants.” 

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform 

addressees of an issue at three pressurized-water reactor (PWR) plants where on multiple 

occasions, their residual heat removal (RHR) systems were inoperable because of the potential 

for steam voids at the RHR pump suction piping. Recipients should review the information for 

applicability to their facilities and consider actions to avoid similar occurrences. The 

suggestions contained in this IN are not NRC requirements, and no specific action or written 

response is required. 

DESCRIPTION OF CIRCUMSTANCES

In 2008 and 2009, the licensees at the Shearon Harris Nuclear Power Plant, Prairie Island 

Nuclear Generating Plants, and Wolf Creek Generating Station discovered that their RHR 

systems were potentially inoperable during shutdown periods because of elevated system 

temperatures at the RHR pump suctions. The elevated system temperatures resulted from the 

licensees’ lack of adequate procedures to ensure RHR system operability during all modes of 

operation. At each of these plants, the fluid in the piping between the reactor coolant system 

(RCS) hot leg to RHR system connection and the RHR minimum-flow line return connection 

remained stagnant and at elevated temperatures following forced cooling as a result of 

unrecognized system flow characteristics; namely, forced flow did not occur in that section of 

pipe. Consequently, each licensee concluded incorrectly that the RHR system was properly 

cooled, prior to shifting the RHR system to Emergency Core Cooling System (ECCS) injection 

mode, when the system temperature was actually such that the affected RHR systems could 

have incurred steam voiding if they had been used for emergency core cooling purposes. 
IN 2010-11 

Page 2 of 4 

Additional information is available on the Wolf Creek Generating Station in the NRC Focused 

Baseline Inspection Report 05000482/2009006, dated August 12, 2009; the Wolf Creek 

Licensee Event Report (LER) 50-482/2008-008-02, dated August 25, 2009; the Shearon Harris 

LER 50-400/2009-002, dated December 15, 2009; and the Prairie Island LER 50-282/2009-004, 

dated June 5, 2009. These documents can be found on the NRC’s public Web site under 

Agencywide Documents Access and Management System (ADAMS) Accession 

Nos. ML092240087, ML092450426, ML093580024, and ML091560611, respectively. 

BACKGROUND 

Gas accumulation in ECCS is an enduring issue associated with commercial nuclear power 

plant operations. To address this problem the NRC issued Generic Letter 2008-01, “Managing 

Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and Containment Spray 

Systems” (ADAMS Accession No. ML072910759). During system reviews in response to the 

generic letter, several PWR licensees discovered the potential for their RHR systems to become 

inoperable during certain shutdown cooling evolutions. 

When an RHR train is used for cooling the reactor coolant system, the temperature of the water 

in that RHR train can reach 350 degrees Fahrenheit (F) (Mode 4 upper temperature limit). With 

the temperature of the water in the RHR train as high as 350 degrees F, if its suction source is 

switched from the RCS hot leg to the refueling water storage tank (e.g., during ECCS operation 

in response to a loss-of-coolant accident) or to the containment sump (e.g., during extended 

response to a loss-of-coolant accident), conditions at the suction of the RHR pump would result 

in steam voiding since the temperature at the RHR pump suction would exceed the saturation 

temperature. Steam voiding can result in binding of an RHR pump and the refueling water 

storage tank discharge check valve, system flow interruptions, and water hammer; potentially 

inhibiting the capability of the RHR system to fulfill its ECCS function. Some PWR plants 

occasionally use multiple RHR trains to perform plant cooldowns. In such cases, multiple RHR 

trains can become simultaneously inoperable for emergency core cooling. 

The pressure and temperature at the suction of the RHR pump depends on the RHR system 

lineup and the as-built system configuration. For example, when the RHR system is aligned for 

shutdown cooling, the RHR pump suction pressure is the same as RCS pressure; during safety 

injection and containment sump recirculation operations, the pressure at the suction of an RHR 

pump is equal to the static head pressure created by the refueling water storage tank and the 

containment sump, respectively. In all system lineups, the as-built configuration also 

determines the head loss associated with different system configurations. The range of 

possible pump suction pressures makes the RHR system susceptible to steam voiding and 

water hammer during system lineup changes with suction temperatures above certain values. 

Since the pressure and corresponding saturation temperature at the suction of an RHR pump 

depends on RHR system design and as-built configuration, it is important that each licensee 

ensure that its particular RHR operating procedures are tailored to their specific systems and 

include parameters validated as plant-specific to ensure RHR systems required by Technical 

Specifications remain operable. 
IN 2010-11 

Page 3 of 4 

DISCUSSION 

Industry operating experience and guidance has shown that effective methods of maintaining 

RHR system temperature within appropriate limits exist. For example, licensees can isolate the 

RHR system, or a single train of the RHR system, from the RCS at a low enough temperature 

so that the fluid at the RHR pump suction remains below the saturation temperature 

corresponding to the pressure at the suction of a running RHR pump. 

Another method of maintaining RHR system temperature is forced cooling through the 

minimum-flow line. The main function of the RHR minimum-flow line is to allow RHR pump 

operation during a safety injection signal when RCS pressure is still above the pump’s shutoff 

head. A typical minimum-flow line flow path takes suction from the discharge of the RHR pump, 

after flow has passed through the RHR heat exchanger, and returns flow to the suction of the 

RHR pump. Neglecting minor conduction heat transfer, the amount of RHR piping that can be 

cooled through the minimum-flow recirculation method is dependent on the location of the 

minimum-flow return connection. Specifically, the RHR pipe upstream of this connection would 

not be cooled due to there being little to no flow through this section of pipe. 

Some licensees incorrectly assumed that using the minimum-flow recirculation method of 

cooling their RHR system was sufficient when, in fact, the as-built configuration of the plant did 

not allow for complete RHR system cooldown using this method. For example, approximately 

140 feet of Wolf Creek’s RHR system piping is not subject to minimum-flow recirculation 

because it is located upstream of the minimum-flow return connection. The stagnant water 

inside this 140 foot section of RHR pipe can only cool through ambient losses; therefore, it 

remains at elevated temperatures for extended periods of time, possibly exceeding 24 hours. 

Nevertheless, station procedures allowed the RHR system to be realigned to the ECCS injection 

mode of operation under these conditions and resulted in both trains of the RHR system being 

inoperable during periods of operation in Modes 3 and 4. Sharon Harris and Prairie Island 

plants discovered a similar condition occurred in their plants with the length of affected pipe 

being the primary variable. 

Other licensees may be susceptible to a similar issue in that their plants may contain piping runs 

that are not able to be effectively cooled using the forced cooling method; yet their station 

procedures may allow them to shift the RHR system to ECCS injection mode during elevated 

system temperatures. 

Licensees should consider this operating experience to ensure that their RHR system 

procedures address their plant specific configurations and that they provide adequate methods 

for satisfying the plant’s RHR system operability requirements as stated in the plant’s Technical 

Specifications. 
IN 2010-11 

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CONTACT 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor 

Regulation (NRR) project manager. 

/RA/ /RA by MShuaibi for/ 

Timothy McGinty, Director Glenn Tracy, Director 

Division of Policy and Rulemaking Division of Construction Inspection and 

Office of Nuclear Reactor Regulation Operational Programs 

 Office of New Reactors 

Technical Contacts: David Garmon, NRR 

301-415-3512 

E-mail: david.garmon-candelaria@nrc.gov 

Warren Lyon, NRR 

301-415-2897 

E-mail: warren.lyon@nrc.gov 

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 
IN 2010-11 

Page 4 of 4 

CONTACT 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor 

Regulation (NRR) project manager. 

/RA/ /RA by MShuaibi for/ 

Timothy McGinty, Director Glenn Tracy, Director 

Division of Policy and Rulemaking Division of Construction Inspection and 

Office of Nuclear Reactor Regulation Operational Programs 

 Office of New Reactors 

Technical Contacts: David Garmon, NRR 

301-415-3512 

E-mail: David.Garmon-Candelaria@nrc.gov 

Warren Lyon, NRR 

301-415-2897 

 E-mail: Warren.Lyon@nrc.gov 

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 

ADAMS Accession Number: ML100640465 TAC ME3411 

OFFICE DIRS/IOEB Tech Editor BC/DIRS/IOEB DSS/SRXB BC/DSS/SRXB 

NAME DGarmon KAzariah-Kribbs JThorp WLyon AUlses 

DATE 5/13/10 04/05/10 e-mail 05/14/10 e-mail 05/13/10 5/14/10 e-mail 

OFFICE D/DSS BC/NRO/SRSB DPR/PGCB DPR/PGCB BC/DPR/PGCB 

NAME WRuland JDonoghue CHawes DBeaulieu SStuchell (Acting) 

DATE 05/26/10 05/18/10 e-mail 6/10/10 05/27/10 6/11/2010 

OFFICE D/NRO/DCIP D/DPR/PGCB 

NAME GTracy (MShuaibi 

for) 

TMcGinty 

OFFICE 6/15/2010 6/16/2010 

OFFICIAL RECORD COPY