Information Notice 2004-08, Reactor Coolant Pressure Boundary Leakage Attributable to Propagation of Cracking in Reactor Vessel Nozzle Welds
| ML041130396 | |
| Person / Time | |
|---|---|
| Issue date: | 04/22/2004 |
| From: | Beckner W NRC/NRR/DIPM/IROB |
| To: | |
| Dozier J, NRR/IROB 415-1014 | |
| References | |
| IN-04-008 | |
| Download: ML041130396 (7) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 April 22, 2004 NRC INFORMATION NOTICE 2004-08: REACTOR COOLANT PRESSURE BOUNDARY
LEAKAGE ATTRIBUTABLE TO PROPAGATION
OF CRACKING IN REACTOR VESSEL NOZZLE
WELDS
Addressees
All holders of operating licensees for nuclear power boiling-water reactors (BWRs), except
those who have permanently ceased operations and have certified that fuel has been
permanently removed from the reactor vessel.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert
addressees to cracking identified in the nozzle-to-cap weld of control rod drive (CRD) return line
penetration N10 at Pilgrim Nuclear Power Station (Pilgrim Station). The NRC expects
recipients to review the information in this notice for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems. However, suggestions contained in this
information notice do not constitute NRC requirements and, therefore, do not require any
specific action or written response.
Description of Circumstances
During a planned outage on September 30, 2003, the licensee for Pilgrim Station began
performing drywell inspections to identify and make necessary repairs to reduce drywell
leakage. On October 1, 2003, the licensees drywell inspections revealed leakage from the
nozzle-to-cap weld area of penetration N10. The licensee concluded that the leakage was a
contributor to the unidentified drywell leakage.
The licensee used a Performance Demonstration Initiative (PDI) qualified manual ultrasonic
testing (UT) technique to determine that the N10 nozzle-to-cap weld contained an unacceptable
flaw that was approximately 4.45cm (1.75 inches) long in the circumferential direction.
Observations by the nondestructive examination (NDE) inspector suggested that the flaw
initiated at the inner diameter (ID) of the weld, in the area of previous weld repairs. The
through-wall location appeared to be close to the centerline of the weld.
Root Cause
The reactor pressure vessel (RPV) nozzle is made of SA-508, Class 2 low-alloy steel, while the
CRD return line cap is made of Alloy 600. The subject weld is fabricated with Alloy 82/182 material, and the nozzle side of the weld is buttered with Alloy 182 material.
Section 2.2.1.2 of the BWR Vessel and Internals Project report BWRVIP-49, Instrument
Penetration Inspection and Flaw Evaluation Guidelines, states that there has been extensive
laboratory and field experience with stress corrosion cracking (SCC) of nickel based alloy, including wrought Alloy 600, Alloy 82 and Alloy 182 weld metal. Both Alloy 600 and Alloy 182 are potentially susceptible to SCC under normal water chemistry conditions in the BWR
environment. Alloy 600 is more resistant than Alloy 182 to crack initiation regardless of prior
fabrication history or metallurgical condition, particularly in the uncreviced condition. Consistent
with its higher chromium and lower carbon content, Alloy 82 weld metal is more resistant to
SCC than Alloy 182. Stress corrosion cracking in the base material is referred to as
intergranular SCC (IGSCC), while SCC in the weld material is referred to as interdendritic SCC
(IDSCC) because of the nature of the elongated grains (or dendrites) in the weld. Both
degradation mechanisms refer to essentially the same phenomenon in the base metal and weld
metal.
The licensee concluded that the root cause of the cracking in the nozzle-to-cap weld of CRD
return line penetration N10 was IDSCC, given that the flaw was completely contained within the
weld. The licensee asserted that the IDSCC was induced by a combination of a crevice
condition and weld repair stresses resulting from previous local weld repairs.
The licensee reviewed industry experience as part of its root cause evaluation. General Electric
(GE) and utility personnel who comprised the root cause team for a 1997 event at Hope Creek
concluded that the through-wall leak in the core spray nozzle to safe-end weld was attributable
to IDSCC in the Alloy 182 material. The root cause team also concluded that the crack growth
rate was influenced by the presence of fabrication defects and weld repair stresses (i.e. the
leak was in the area of a previous local repair using Alloy 182).
Corrective Action
The Pilgrim Station licensee performed a weld overlay repair to stop the leakage. The
licensees repair technique is an alternative to the requirements in Section XI, IWA-4000, of the
Boiler and Pressure Vessel Code promulgated by the American Society of Mechanical
Engineers (ASME). The repair was based on the use of Code Case N-504-2, Alternative
Rules for Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping (with modifications), and
Code Case N-638, "Similar and Dissimilar Metal Welding Using Ambient Temperature Machine
GTAW Temper Bead Technique. (See ADAMS Accession No. ML032870328.)
Background:
The N10 nozzle is a 10-cm (4-inch) diameter RPV penetration that was previously used to
return CRD system flow to the reactor vessel. In 1977, the licensee modified the N10 nozzle to
prevent cracking attributable to the cyclic thermal stresses resulting from the return of cooler
water to the reactor vessel from the CRD system. That modification consisted of cutting and
isolating the existing CRD system return line to nozzle N10 and rerouting the CRD return line to
the CRD cooling water header. The modification also included removing the safe end and
thermal sleeve from nozzle N10 and installing an Alloy 600 cap. The final configuration of the nozzle was composed of an Alloy 82/182 nozzle-to-cap butt weld from the forged steel nozzle
to the Alloy 600 cap. Radiographic examination following the modification identified defects in
the weld, which the licensee subsequently repaired. The final testing of the modification was
performed in 1977 using NDE and hydrostatic testing.
The NRC subsequently issued Generic Letter (GL) 88-01, NRC Position on IGSCC in BWR
Austenitic Stainless Steel Piping, to address the subject of IGSCC cracking in BWR piping.
During that same time period, GE recommended that BWR owners inspect nozzle-to-safe-end
welds containing Alloy 182 or a combination of Alloy 182 and Alloy 82 and, wherever practical, these inspections should be performed using automated UT scanning. Past inspections of
dissimilar metal piping welds at Pilgrim Station were completed using the guidance in GL 88-01, which was superseded by guidance in BWRVIP-75, Technical Basis for Revisions to Generic
Letter 88-01 Inspection Schedules. (See ADAMS Accession Nos. ML003688842 and
ML021350645.) In accordance with BWRVIP-75, the N10 nozzle-to-cap weld was classified as
a Category D weld, meaning that it is made of susceptible materials that have not been treated
with an IGSCC remedy and in which cracks have not been reported. The N10 nozzle is located
2.1m (84 inches) above the top of the active fuel and is not protected by hydrogen water
chemistry (HWC). (The purpose of HWC is to protect components from SCC.) Category D
welds have a 6-year inspection frequency. Prior to the Fall 2003 inspection, the licensee
performed its last inspection of the N10 nozzle-to-cap weld during the Spring 1999 refueling
outage. As part of that inspection, Inservice Inspection/ Nondestructive Examination personnel
reviewed data sheets, but did not discover any recordable indications of SCC.
Other related generic communications involving weld inspections and degradation in BWR
systems include the following NRC information notices (INs):
IN 1990-30: Ultrasonic Inspection Techniques for Dissimilar Metal Welds
IN 1992-50: Cracking of Valves in the Condensate Return Lines of a BWR Emergency
Condenser System
IN 1998-44: Ten-year Inservice Inspection (ISI) Program Update For Licensees That Intend
to Implement Risk-Informed ISI of Piping
Discussion:
The licensees root cause for the cracking in nozzle N10 is consistent with the available
evidence and industry experience. The weld metal is susceptible to IDSCC, and there is
minimal protection (i.e., no HWC) from SCC mechanisms because of the location of the nozzle
cap and stagnant flow conditions.
In conducting the Spring 1999 inspection, the licensee used manual ultrasonic inspection
techniques with qualified inspectors. The 2003 examinations were performed to the updated
requirements of Appendix VIII to Section XI of the ASME Code and the PDI program.
Enhanced ultrasonic examinations using PDI-qualified inspectors have improved the capability
to detect flaws related to SCC mechanisms, including those that occur entirely within the weld
metal. With respect to future inspections of this weld, after the qualified ISI examination of the nozzle
N10 weld, which is scheduled for the 2009 outage, the weld will be examined in accordance
with the schedule for Category E welds in BWRVIP-75. BWRVIP-75 defines Category E welds
as those that have weld overlay repairs made with an IGSCC-resistant, nickel-based alloy (such
as Alloy 52) and have received one qualified ISI since the initial post-overlay examination. After
the initial examination, Category E welds with weld overlays are successively examined in
accordance with BWRVIP-75, and related NRC comments, in order to ensure that there is no
new cracking or crack growth. The Category E welds are then examined at a rate of 25 percent
of the population every 10 years for normal water chemistry.
The staff and the licensee discussed expanding the scope of the Fall 2003 inspection to include
all other Category D welds. The licensee used the following factors to consider this expanded
scope based on the attributes of the cracked N10 weld:
- weld at a reactor vessel nozzle
- Category D weld
- low HWC protection
- dissimilar metal weld (Alloy 82/182)
- significant weld repair during original installation
- ID grinding and/or radiographic defects
The other Category D welds were, for example, protected by HWC, had improved inspections in
the past (i.e., automated UT, rather than manual UT), had no weld repairs, and had no
radiographic defects. Therefore, the licensee did not expand the scope of the inspection.
The leakage from the penetration N10 nozzle-to-cap weld and other leak sources in the drywell
was less than the limit allowed by the plants technical specifications (TS) for unidentified
leakage and total leakage (combined unidentified and identified). The staff found that the
licensee had mitigating procedures, routine inspection activities, operable leakage detection
equipment and TS requirements designed to detect low levels of leakage from the reactor
coolant system (RCS) and minimize the potential that a flaw could remain undetected. On that
basis, the staff determined, qualitatively, that the N10 pressure boundary leakage was of very
low safety significance.
Generic Implications:
Based on the information currently available, such as other capped BWR CRD return lines and
prior industry experience with IDSCC, the degradation that occurred at Pilgrim Station may be
relevant to other BWR facilities. The licensee for Pilgrim Station used guidance from
BWRVIP-75 to determine the appropriate inspection method and frequency for this weld. This information notice does not require any specific action or written response. If you have
any questions about the information in this notice, please contact the technical contact identified
below or the appropriate project manager in the NRCs Office of Nuclear Reactor Regulation
(NRR).
/RA/
William D. Beckner, Chief
Reactors Operations Branch
Division of Inspection Program Management
Office of Nuclear Reactor Regulation
Technical Contact:
Andrea D. Lee, NRR Jerry Dozier, NRR
(301) 415-2735 (301) 415-1014 Email: adw1@nrc.gov Email: jxd@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
ML041130396 OFFICE OES:IROB:DIPM Tech Editor OCIO:IRSD:PSS EMCB:DE LPD4:DLPM
NAME IJDozier JWF* PKleene* PAGarrity* ADLee* ABWang*
DATE 04/20/2004 03/01/2004 03/01/2004 03/03/2004 04/07/2004 OFFICE BC:EMCB:DE OES:IROB:DIPM SC:OES:IROB:DIPM C:IROB:DIPM
NAME WHBateman* CDPetrone* CJackson WDBeckner
DATE 04/15/2004 04/15/2004 04/22/2004 04/22/2004 / /
Attachment LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2004-07 Plugging of Safety Injection 04/07/2004 All holders of operating licenses
Pump Lubrication Oil Coolers or construction permits for
with Lakeweed nuclear power reactors, except
those who have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel.
2004-06 Loss of Feedwater Isokinetic 03/26/2004 All holders of operating licensees
Sampling Probes at Dresden for nuclear power reactors except
Units 2 and 3 those who have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel.
2004-05 Spent Fuel Pool Leakage to 03/03/2004 All holders of operating licensees
Onsite Groundwater for nuclear power reactors (except
those who have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel) and for research
and test reactors, and all holders
of fuel storage licenses and
construction permits.
2004-04 Fuel Damage During Cleaning 02/24/2004 All holders of operating licenses
at a Foreign Pressurized Water for light-water reactors, except
Reactor those who have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor.
Note: NRC generic communications may be received in electronic format shortly after they are
issued by subscribing to the NRC listserver as follows:
To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following
command in the message portion:
subscribe gc-nrr firstname lastname
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit