Information Notice 2014-07, Degradation of Leak-Chase Channel Systems for Floor Welds of Metal Containment Shell and Concrete Containment Metallic Liner
| ML14070A114 | |
| Person / Time | |
|---|---|
| Issue date: | 05/05/2014 |
| From: | Michael Cheok, Kokajko L Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking |
| To: | |
| M Banic, 301-415-2771 | |
| References | |
| TAC MF3399 IN-14-007 | |
| Download: ML14070A114 (8) | |
ML14070A114 UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001
May 5, 2014
NRC INFORMATION NOTICE 2014-07:
DEGRADATION OF LEAK-CHASE CHANNEL
SYSTEMS FOR FLOOR WELDS OF METAL
CONTAINMENT SHELL AND CONCRETE
CONTAINMENT METALLIC LINER
ADDRESSEES
All holders of an operating license or construction permit for a nuclear power reactor under
Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of
Production and Utilization Facilities, except for those who have permanently ceased operations
and have certified that fuel has been permanently removed from the reactor vessel.
All holders of and applicants for a power reactor early site permit, combined license, standard
design approval, or manufacturing license under 10 CFR Part 52, Licenses, Certifications, and
Approvals for Nuclear Power Reactors. All applicants for a standard design certification, including such applicants after initial issuance of a design certification rule.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of issues identified by the NRC staff concerning degradation of floor weld
leak-chase channel systems of steel containment shell and concrete containment metallic liner
that could affect leak-tightness and aging management of containment structures. 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.
DESCRIPTION OF CIRCUMSTANCES
Containment Basemat Shell and Liner Leak-Chase Channel System
The containment basemat metallic shell and liner plate seam welds of pressurized water
reactors are embedded in 3- to 4-feet (0.9- to 1.2-meter) thick concrete floor during construction
and are typically covered by a leak-chase channel system that incorporates pressurizing test
connections. This system allows for pressure testing of the seam welds for leak-tightness
during construction and also in service, as required. A typical basemat shell or liner weld
leak-chase channel system is conceptually illustrated in elevation view in Figure 1. It consists of
steel channel sections that are fillet welded continuously over the entire bottom shell or liner
seam welds and subdivided into zones, each zone with a test connection. Each test connection
consists of a small carbon or stainless steel tube (less than 1-inch (2.5 centimeters) diameter)
that penetrates through the back of the channel and is seal-welded to the channel steel. The tube extends up through the concrete floor slab to a small steel access (junction) box
embedded in the floor slab. The steel tube, which may be encased in a pipe, projects up
through the bottom of the access box with a threaded coupling connection welded to the top of
the tube, allowing for pressurization of the leak-chase channel. After the initial tests, steel
threaded plugs or caps are installed in the test tap to seal the leak-chase volume. Gasketed
cover plates or countersunk plugs are attached to the top of the access box flush with the
containment floor. In some cases, the leak-chase channels with plugged test connections may
extend vertically along the circumference of the cylindrical containment shell or liner to a certain
height above the floor. The NRC staff notes that there are variations in the design and layout of
the leak-chase channel systems used in different containments.
Figure 1: Conceptual illustration of typical containment basemat shell/liner leak-chase system
Leak-Chase Channel Degradation Issues Identified
NRC inspectors recently identified degradation issues related to the containment floor steel shell
and liner plate leak-chase channel systems at several pressurized-water reactor (PWR) plants, examples of which are summarized below.
Virgil C. Summer Nuclear Station
During the RF-19 refueling outage (April 15-May 29, 2011), while conducting a routine
containment walkdown, the NRC inspectors noticed degradation to several metal cover plates
that were mounted flush with the concrete floor and had access boxes underneath. The access
boxes house the test connections of the containment liner weld leak-chase channel system.
pipe
tube
cap/plug
leak-chase
channel
embedded
liner/shell
seam weld
access box
cover plate
basemat
foundation
concrete
top of concrete floor
slab (floor level) The NRC inspectors determined that the licensee had no plan in place to perform visual
examinations of accessible parts of the containment liner plate leak-chase systems in
accordance with American Society of Mechanical Engineers Boiler and Pressure Vessel Code
(ASME Code)Section XI, Subsection IWE requirements. After the NRC inspectors raised the
issue, the licensee performed visual examinations of the 51 test connection plugs; four were
found with missing tube plugs, debris in the tubing, and water inside the channels. Based on
the limited visual examination possible using a borescope at one location, minor indications of
corrosion were evident. The licensee conducted pressure drop tests of each of these four
leak-chase channel zones. The leakage measured was minor, indicating confidence in the
overall leak-tightness of the liner weld areas. The licensee reassessed the conditions of these
four zones during the RF-20 outage as part of a formal ASME Code Section XI, Subsection IWE
augmented examination program, and took actions in its corrective action program to address
the issues associated with the problem. Additional information appears in Virgil C. Summer
Nuclear StationNRC Integrated Inspection Report 05000395/2011003, dated August 4, 2011, on the NRCs public Web site in the Agencywide Documents Access and Management System
(ADAMS) under Accession No. ML112160535.
Joseph M. Farley Nuclear Plant, Unit 1
While conducting an independent walkdown of the containment during the April 1-20, 2012, refueling outage, the NRC inspectors noticed degradation and misalignment of one of 49 floor
metal cover plates above access boxes that housed the test connections for the floor weld
leak-chase channel system. The inspectors determined that the licensee had no program in
place to inspect any portion of the leak-chase test connections for evidence of moisture
intrusion that could reach the containment liner, in accordance with ASME Code Section XI,
Subsection IWE requirements. The licensee subsequently conducted visual examinations of all
49 test connections and found two covers degraded and misaligned, and two covers missing
with blockages in the test connection piping and significant corrosion of the access box metal
plates. One test connection with blockage showed evidence of boric acid in the pipe and water
in the leak-chase channel, which was subsequently removed. The licensee further evaluated
these four connections to verify that containment integrity had been maintained. The licensee
planned to reassess the condition of these four zones during the next refueling outage as part of
a formal ASME Code Section XI, Subsection IWE augmented examination and initiated action in
its corrective action program to address the issues associated with this problem. Additional
information appears in Joseph M. Farley Nuclear PlantNRC Integrated Inspection Report 05000348/2012003; and 05000364/2012003, dated July 20, 2012, on the NRCs public Web
site under ADAMS Accession No. ML12202B078.
Sequoyah Nuclear Plant, Unit 2
During an independent walkdown of containment in the fall 2012 outage, NRC inspectors
observed several metal floor-covers underneath which were junction boxes that housed the test
connections for the liner weld leak-chase channel system. The NRC inspectors determined that
the licensee had no requirements in place, in its Subsection IWE containment inservice
inspection (ISI) program, to inspect any portion of these test connections for evidence of
moisture intrusion that could reach the containment liner. The licensee subsequently conducted visual examinations on all eight of the leak-chase test connection junction boxes that revealed
significant corrosion of the junction boxes, including one through-wall hole in the tubing leading
down to the leak-chase channels. Upon inspection of the channels using a borescope, the
licensee noted water in the channels and limited corresponding corrosion. The licensee added
the junction boxes to the containment ISI plan for future inspections. The licensee entered the
issue into their corrective action program to address the issues associated with this problem.
Additional information appears in Sequoyah Nuclear Plant - NRC Integrated Inspection Report 05000327/2012005, 05000328/2012005, dated February 13, 2013, on the NRCs public Web
site under ADAMS Accession No. ML13050A394.
BACKGROUND
Regulations in 10 CFR 50.55a, Codes and Standards, paragraph (g), Inservice Inspection
Requirements, requires that licensees implement the inservice inspection program for pressure
retaining components and their integral attachments of metal containments and metallic liners of
concrete containments in accordance with Subsection IWE of Section XI of the applicable
edition and addenda of the ASME Code, incorporated by reference in paragraph (b) of the
section and subject to the applicable conditions in paragraph (b)(2)(ix). The regulatory condition
in 10 CFR 50.55a(b)(2)(ix)(A) or equivalent provision in Subsection IWE of the ASME Code
(2006 and later editions and addenda only) requires that licensees shall evaluate the
acceptability of inaccessible areas when conditions exist in accessible areas that could indicate
the presence of or result in degradation to such inaccessible areas.
Paragraph (a)(1) of 10 CFR 54.29, Standards for Issuance of a Renewed License, requires
that applicants for a renewed license will manage the effects of aging, during the period of
extended operation, on the functionality of structures and components that require review under
10 CFR 54.21(a)(1). NUREG-1801, Generic Aging Lessons Learned (GALL) Report, recommends that the ASME Code,Section XI, Subsection IWE program, be used to manage
aging of metal containments and metallic liners of concrete containments for license renewal.
DISCUSSION
The containment floor weld leak-chase channel system forms a metal-to-metal interface with the
containment shell or liner, the test connection end of which is at the containment floor level.
Therefore, the leak-chase system provides a pathway for potential intrusion of moisture that
could cause corrosion degradation of inaccessible embedded areas of the pressure-retaining
boundary of the basemat containment shell or liner within it. In addition to protecting the test
connection, the cover plates and plugs and accessible components of the leak-chase system
within the access box are also intended to prevent intrusion of moisture into the access box and
into the inaccessible areas of the shell/liner within the leak-chase channels, thereby protecting
the shell and liner from potential corrosion degradation that could affect leak-tightness.
The containment ISI program required by 10 CFR 50.55a to be implemented in accordance with
Subsection IWE, of the ASME Code,Section XI, subject to regulatory conditions, requires
special consideration of areas susceptible to accelerated corrosion degradation and aging, and
barriers intended to prevent intrusion of moisture and water accumulation against inaccessible areas of the containment pressure-retaining metallic shell or liner. The containment floor weld
leak-chase channel system is one such area subject to accelerated degradation and aging if
moisture intrusion and water accumulation is allowed on the embedded shell and liner within it.
Therefore, the leak-chase channel system is subject to the inservice inspection requirements of
10 CFR 50.55a(g)(4) and aging management requirements of 10 CFR 54.29(a)(1).
This IN provides examples of operating experience at some plants of water accumulation and
corrosion degradation in the leak-chase channel system that has the potential to affect the
leak-tight integrity of the containment shell or liner plate. In each of the examples, the licensee
had no provisions in its ISI plan to inspect any portion of the leak-chase channel system for
evidence of moisture intrusion and degradation of the containment metallic shell or liner within it.
Therefore, these cases involved the licensees failure to perform required visual examinations of
the containment shell or liner plate leak-chase systems in accordance with the ASME Code
Section XI, Subsection IWE, as required by 10 CFR 50.55a(g)(4). The moisture intrusion and
associated degradation found within leak-chase channels, if left uncorrected, could have
resulted in more significant corrosion degradation of the containment shell or liner and
associated seam welds. These examples and other similar previous industry operating
experiences highlight the importance of licensees recognizing the existence of leak-chase
channel systems in their containment floor. These experiences also highlight the importance of
understanding the system configuration and how the leak-chase system components interact
with the containment pressure-retaining metallic shell or liner plate within it to ensure that these
systems are appropriately included for required examinations in the containment ISI program
and the Subsection IWE aging management program.
For containments in which basemat shell/liner leak-chase channel systems exist with accessible
interface at the containment floor level, licensees are required to comply with the containment
ISI requirements of 10 CFR 50.55a(g)(4), in any one of the following ways:
1. Perform general visual examination of 100 percent of accessible components of the leak-chase
channel system during each inspection period in accordance with the ASME Code,Section XI,
Subsection IWE, Table IWE-2500-1, Examination Category E-A, Item E1.30 Moisture Barriers.
Note 3 for Item E1.30 under the Parts Examined column states: Examination shall include
moisture barrier materials intended to prevent intrusion of moisture against inaccessible areas
of the pressure retaining metal containment shell or liner at concrete-to-metal interfaces and at
metal-to-metal interfaces which are not seal-welded. Containment moisture barrier materials
include caulking, flashing, and other sealants used for this application. Note 3 provides the
definition of moisture barriers that are required to be examined in terms of intended function.
The accessible leak-chase channel system components, which include the cover plates and
countersunk plugs, and the access box and test connection components in it, prevent intrusion
of moisture into the containment shell and liner plate and welds within it; and therefore, also
function as moisture barriers under the Note 3 definition. Further, Table IWE-2500-1, second
column Examination Requirements/Fig. No. for Item E1.30, includes reference to Figure
IWE-2500-1 Examination Areas for Moisture Barriers. However, Figure IWE-2500-1 only
provides an illustration of typical moisture barrier examination areas for concrete-to-metal
interface moisture barriers. Figure IWE-2500-1 does not include illustration of metal-to-metal interface moisture barriers, which are included as parts to be examined in the Note 3 definition
of moisture barriers. Therefore, the NRC staff has determined that to meet the requirements of
10 CFR 50.55a(g)(4), the figure is not all-inclusive and does not provide a complete illustration
of the parts required to be examined under the scope of Item E1.30 in accordance with Note 3.
This may cause confusion to the user with regard to scope of moisture barriers to be examined
under Item E1.30. Licensees have the option of including applicable accessible leak-chase
channel system components as moisture barriers for general visual inspection subject to this
provision. If such general visual examination detects evidence of degradation or presence of
moisture or water in the accessible areas (e.g., coverplates, interior of the access box and the
test connection) of the leak-chase system, further examination and evaluation of the
containment shell and liner within it are required pursuant to Table IWE-2500-1, Examination
Category E-C, Item E4.11, and the regulatory condition for inaccessible areas in 10 CFR
50.55a(b)(2)(ix)(A) (or equivalent code provision). In developing the general visual inspection
plan, licensees are alerted to consider the fact that allowing moisture intrusion through the cover
plates into the access boxes could result in degradation of the test connection tube and the cap
or plug on it, even if the caps and plugs are seal welded, and allow moisture intrusion into the
embedded containment shell or liner and welds.
2. Perform augmented VT-1 visual examination of 100 percent of the accessible containment
shell/liner and welds within the leak-chase channel in accordance with the ASME Code, Section
XI, Subsection IWE, Subarticle IWE-1240 and Item E4.11 in Table IWE-2500-1, Examination
Category E-C.
Subsection IWE, Subarticle IWE-1240 Surface Areas Requiring Augmented Examination, paragraph IWE-1241 Examination Surface Areas states, in part, that:
Surface areas subject to accelerated degradation and aging require the augmented
examinations identified in Table IWE-2500-1, Examination Category E-C. Such areas include
the following:
(a) interior and exterior containment surface areas that are subject to accelerated corrosion
with no or minimal corrosion allowance or areas where absence or repeated loss of
protective coatings has resulted in substantial corrosion and pitting. Typical locations of
such areas are those exposed to standing water, repeated wetting and drying, persistent
leakage, and those with geometries that permit water accumulation, condensation, and
microbiological attack. Such areas may include penetration sleeves, stiffeners, surfaces
wetted during refueling, concrete-to-steel shell or liner interfaces, embedment zones,
leak-chase channels, drain areas, or sump liners. [emphasis added]
Paragraph IWE-1242 Identification of Examination Surface Areas states in part that: Surface
areas requiring augmented examination shall be determined in accordance with IWE-1241, and
shall be identified in the Owners Inspection Program.
Since leak-chase channels have a configuration that could permit moisture intrusion and water
accumulation into inaccessible containment shell and liner areas, and are explicitly mentioned in
IWE-1241 as an example of an area subject to accelerated degradation and aging, the Owner
has the responsibility to identify and consider including containment basemat leak-chase channel systems in their containment ISI program as areas requiring augmented examination
every inspection interval, in accordance with Table IWE-2500-1, Examination Category E-C,
Item E4.11 Visible Surfaces. This would involve VT-1 visual examination of the accessible
areas of the embedded containment shell/liner and welds within the leak-chase channels.
Remote visual examination techniques (e.g., borescope) allowed by IWA-2210 may have to be
used to accomplish this examination. If the VT-1 examination determines that degradation
and/or moisture are present, such conditions shall be corrected or evaluated in accordance with
IWE-3520. Further, if it is determined that conditions exist in accessible areas of the leak-chase
channel system that could indicate the presence of, or result in, degradation in inaccessible
areas, the regulatory condition in 10 CFR 50.55a(b)(2)(ix)(A) (or equivalent code provision)
requires that an engineering evaluation shall be performed to determine the acceptability of the
inaccessible area. Such an evaluation may include performing a leak test by pressurization of
the affected leak-chase zone to demonstrate leak-tight integrity of the affected area.
3. Perform inservice inspection of leak-chase channel system using licensee-proposed
alternatives to code requirements under 10 CFR 50.55a(a)(3).
The leak-chase channel system may be examined or inspected using licensee-proposed
alternatives to the ASME Code requirements in 1 and 2 above. The proposed alternatives must
be submitted to and authorized before implementation by the Director, Office of Nuclear Reactor
Regulation (NRR), NRC, under 10 CFR 50.55a(a)(3).
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below or the appropriate NRR project manager.
/RA by Sher Bahadur for/
/RA/
Lawrence E. Kokajko, Director
Michael C. Cheok, Director
Division of Policy and Rulemaking
Division of Construction Inspection
Office of Nuclear Reactor Regulation
and Operational Programs
Office of New Reactor
Technical Contact:
301-415-6181
George.Thomas2@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
ML14070A114 *concurred via email
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