Information Notice 2011-17, 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) | |
ML11161A111 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. 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..
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