Information Notice 2011-17, Calculation Methodologies for Operability Determinations of Gas Voids in Nuclear Power Plant Piping

Calculation Methodologies for Operability Determinations of Gas Voids in Nuclear Power Plant Piping

ML11161A111
Person / Time
Site:	Millstone, Point Beach
Issue date:	07/26/2011
From:	Laura Dudes, Mcginty T Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking
To:
Gall J
References
IN-11-017
Download: ML11161A111 (5)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001 July 26, 2011 NRC INFORMATION NOTICE 2011-17: CALCULATION METHODOLOGIES FOR

OPERABILITY DETERMINATIONS OF GAS

VOIDS IN NUCLEAR POWER PLANT PIPING

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 an early site permit, standard design certification, standard

design approval, manufacturing license, or combined license 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 recent instances of gas accumulation in safety-related systems in which the

resulting operability determination of the as-found condition relied on computer models that

were not demonstrated to be technically appropriate for the intended application.

Specifically, the computer models had not been sufficiently qualified by benchmarking against

test or plant data. The NRC expects that recipients will review the information for applicability to

their facilities and consider actions, as appropriate, to avoid similar problems. Suggestions

contained in this IN are not NRC requirements; therefore, no specific action or written response

is required.

BACKGROUND

Gas accumulation in systems that are designed to be full of water has been a longstanding

issue associated with commercial nuclear power plant operations. To address this problem, the

NRC issued Generic Letter (GL) 2008-01, Managing Gas Accumulation in Emergency Core

Cooling, Decay Heat Removal, and Containment Spray Systems, on January 11, 2008 (Agencywide Documents Access and Management System (ADAMS) Accession

No. ML072910759). GL 2008-01 asked addressees to submit information to demonstrate that

the subject systems were in compliance with the current licensing and design bases and

applicable regulatory requirements, and that suitable design, operational, and testing control

measures were in place for maintaining this compliance.

ML11161A111 NRC Inspection Manual Temporary Instruction (TI) 2515/177, Managing Gas Accumulation in

Emergency Core Cooling, Decay Heat Removal, and Containment Spray Systems (NRC

Generic Letter 2008-01), dated June 9, 2009 (ADAMS Accession No. ML082950666), provided

background information and guidance for NRC inspectors to verify that the onsite

documentation, system hardware, and licensee actions are consistent with the information

provided in the licensees response to GL 2008-01.

For parts of a nuclear plant system that are filled with water as part of their design basis, a gas

void is considered a degraded or nonconforming condition or both that could potentially render

the system inoperable. When a gas void is identified, the operability of the system in its

as-found condition can be evaluated using NRC Inspection Manual, Part 9900, Technical

Guidance, Operability Determinations & Functionality Assessments for Resolution of Degraded

or Nonconforming Conditions Adverse to Quality or Safety, dated April 16, 2008 (ADAMS

Accession No. ML073531346).

The NRC Office of Nuclear Reactor Regulation (NRR) prepared, Guidance to

NRC/NRR/DSS/SRXB Reviewers for Writing Temporary Instruction (TI) 2515/177 Suggestions

for the Region Inspections, Revision 11, dated May 23, 2011 (ADAMS Accession

No. ML111660749), to offer further technical guidance for use by NRC inspectors when

inspecting licensee operability determinations associated with gas voids. It covers topics such

as void transport behavior and pump response to voids. NRR revises this guidance as needed

to provide NRC inspectors with the most recent insights such as knowledge gained through

industry response to GL 2008-01 and plans to place any future revisions under ADAMS

Package Accession No. ML112070155.

DESCRIPTION OF CIRCUMSTANCES

Millstone Power Station Unit 3

On October 20, 2008, with Millstone Power Station Unit 3 at zero percent power, as part of

GL 2008-01 activities, the licensee discovered a 15-percent gas void in the accessible portion of

the 24-inch-diameter pipe connecting the refueling water storage tank (RWST) to the

emergency core cooling system (ECCS) pumps. Licensee engineering staff determined that

under some postulated loss-of-coolant accident scenarios, the gas void could have been

transported to specific ECCS pumps, rendering them inoperable. The licensee determined the

cause to be a latent design error, as the system design did not account for the as-built pipe

deviation from horizontal that trapped gas in the 24-inch-diameter section of pipe. The pipe

should have had either a greater slope towards the RWST or a vent valve installed in the

24-inch-diameter section. Licensee corrective actions included installing a vent valve on this

line to provide a venting location.

The licensees thermal-hydraulic and void size modeling to assess operability used the RELAP5 computer code (licensee test results and analysis are publicly available under ADAMS

Accession Nos. ML091170150, ML091170137, and ML091870829). The licensee qualified the

use of the RELAP5 computer model through benchmarking against test data obtained using

mockups of the specific Millstone Unit 3 ECCS configuration to demonstrate applicability of the

RELAP5 computer model to the Millstone Unit 3 ECCS piping application. Where necessary, the licensee performed additional calculations to supplement the RELAP5 calculations to provide results consistent with the test data. Additional information appears in Millstone Unit 3 Licensee Event Report 05000423/2008-004-00, dated December 19, 2008 (ADAMS Accession

No. ML090070031).

The NRC inspected the licensees operability determination and identified no findings of

significance. Additional information appears in Millstone Unit 3 NRC Special Inspection Team

Report 05000423/2008010, dated March 23, 2009 (ADAMS Accession No. ML090820433).

Point Beach Nuclear Plant

On September 30, 2010, the NRC completed an inspection of Point Beach Units 1 and 2, which

included an inspection using TI 2515/177 as documented in Point Beach Units 1 and 2 NRC

Integrated Inspection Report 05000266/2010004; 05000301/2010004, dated November 9, 2010

(ADAMS Accession No. ML103130057). The NRC inspectors reviewed the licensees

procedures for conducting surveillances and determining void volumes to ensure that the void

criteria were satisfied and would be reasonably ensured to be satisfied until the next scheduled

void surveillance. The licensee established void volume acceptance criteria for piping system

high points to be used during field verifications. The void volumes were derived based on pipe

internal diameter and as-built slope, and internal height of the void. In addition, the licensee

relied on the use of the computer software GOTHIC to perform two-phase and two-component

analysis of gas movement to predict how a void volume in piping is translated into a transient

void fraction at the entrance of a pump following pump start. The licensee provided supporting

information that did not rely upon the GOTHIC computer model to demonstrate acceptability.

The NRC inspection report describes several issues related to whether the GOTHIC computer

model had been sufficiently qualified through benchmarking against test or plant data to

demonstrate the applicability of the computer model to the type of analysis being conducted, and the applicable terms, conditions and limitations for its use. While the licensee referenced

some testing to qualify the GOTHIC computer model for use in predicting quantitative void

transport behavior, the NRC inspection report provides examples of how the test configuration

and conditions differed from the actual plant configuration and conditions. The NRC inspection

report states that the inspectors discussed these observations with NRR and that it was

determined that these observations required further evaluation by NRR to better understand the

acceptability of the application of the test results. The licensee subsequently provided

supporting information that did not rely upon GOTHIC.

DISCUSSION

During the NRC staffs reviews of licensee responses to GL 2008-01 and subsequent NRC

inspections, the NRC reviewed instances of gas accumulation in safety-related systems in

which the resulting operability determination of the as-found condition relied on a computer

model that was not demonstrated to be technically appropriate for the intended application.

Specifically, the computer model had not been acceptably qualified by benchmarking against

test and plant data to demonstrate its applicability to the type of analysis being conducted, and

the applicable terms, conditions and limitations for its use. This, along with basing analyses on

inappropriate pump suction void criteria, could result in licensees establishing inappropriate or

unsupported values for gas void volumes that could impact system operability. The above example of Millstone Unit 3 illustrates a case where a licensee acceptably qualified

the use of a computer model (RELAP5) to assess past ECCS operability in response to a gas

void the licensee discovered. This benchmarking, and therefore the applicability, of the

RELAP5 computer model for predicting gas transport was limited to the Millstone Unit 3 piping

application. Conversely, the above example at Point Beach describes issues related to whether

the GOTHIC computer model had been acceptably qualified through benchmarking to

demonstrate applicability for the specific application. Qualification issues also exist for other

calculation methodologies such as other computer programs and manual calculation methods.

CONTACT

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

matter to the technical contacts listed below or to the appropriate project manager.

/RA/ /RA/

Timothy J. McGinty, Director Laura A. Dudes, Director

Division of Policy and Rulemaking Division of Construction Inspection and

Office of Nuclear Reactor Regulation Operational Programs

Office of New Reactors

Technical Contacts: Warren Lyon, NRR Jennifer Gall, NRR

301-415-2897 301-415-3253 E-mail: warren.lyon@nrc.gov E-mail: jennifer.gall@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library..

ML11161A111 TAC ME6570

OFFICE NRR/DSS/SRXB NRR/DSS/SRXB ADM/DAS/PB NRR/DSS/SRXB

NAME JGall WLyon KAzariah-Kribbs AUlses

DATE 7/18/11 7/18/11 7/12/11 e-mail 7/18/11 OFFICE NRR/DSS NRO/DSRA/SRSB NRR/DPR/PGCB NRR/DPR/PGCB

NAME SBahadur JDonoghue IFrankl for DBeaulieu CHawes CMH

DATE 7/19/11 7/13/11 7/25/11 7/26/11 OFFICE NRR/DPR/PGCB NRO/DCIP NRR/DPR

NAME SRosenberg LDudes TMcGinty

OFFICE 7/26/11 7/26/11 7/26/11