Information Notice 2002-11, Recent Experience with Degradation of Reactor Pressure Vessel Head
| ML020700556 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 03/12/2002 |
| From: | Beckner W NRC/NRR/DRIP/RORP |
| To: | |
| Dozier J, NRR/RLSB 415-1014 | |
| References | |
| IEB-01-001 IN-02-011 | |
| Download: ML020700556 (11) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 March 12, 2002 NRC INFORMATION NOTICE 2002-11: RECENT EXPERIENCE WITH DEGRADATION
OF REACTOR PRESSURE VESSEL HEAD
Addressees
All holders of operating licenses for pressurized-water reactors (PWRs), except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform
addressees about findings from recent inspections and examinations of the reactor pressure
vessel (RPV) head at Davis-Besse Nuclear Power Station. It is expected that recipients will
review the information for applicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is required.
Description of Circumstances
On February 16, 2002, the Davis-Besse facility began a refueling outage that included
inspection of the vessel head penetration (VHP) nozzles, which focused on the inspection of
control rod drive mechanism (CRDM) nozzles, in accordance with the licensees commitments
to NRC Bulletin 2001-01, ?Circumferential Cracking of Reactor Pressure Vessel Head
Penetration Nozzles, which was issued on August 3, 2001. These inspections identified axial
indications in three CRDM nozzles, which had resulted in pressure boundary leakage.
Specifically, these indications were identified in CRDM nozzles 1, 2, and 3, which are located
near the center of the RPV head. These findings were reported to the NRC on February 27,
2002, and supplemented on March 5 and March 9, 2002. The licensee decided to repair these
three nozzles, as well as two other nozzles that had indications but had not resulted in pressure
boundary leakage.
The repair process for these nozzles included roll expanding the CRDM nozzle material into the
surrounding RPV head material, followed by machining along the axis of the CRDM nozzle to
an elevation above the indications in the nozzle material. On March 6, 2002, the machining
process on CRDM nozzle 3 was prematurely terminated and the machining apparatus was
removed from the nozzle. During the removal process, nozzle 3 was mechanically agitated and
subsequently displaced in the downhill direction (i.e., tipped away from the top of the RPV
head) until its flange contacted the flange of the adjacent CRDM nozzle.
To identify the cause of the CRDM nozzle displacement, the licensee began an investigation
into the condition of the RPV head surrounding CRDM nozzle 3. This investigation included
removing the CRDM nozzle from the RPV head, removing boric acid deposits from the top of
the RPV head, and performing ultrasonic thickness measurements of the RPV head in the
vicinity of CRDM nozzles 1, 2, and 3. Upon completing the boric acid removal on March 7,
2002, the licensee conducted a visual examination of the area, which identified a large cavity in
the RPV head on the downhill side of CRDM nozzle 3. Followup characterization by ultrasonic
testing indicated wastage of the low alloy steel RPV head material adjacent to the nozzle.
The wastage area was found to extend approximately 5 inches downhill on the RPV head from
the penetration for CRDM nozzle 3, with a width of approximately 4 to 5 inches at its widest
part. The minimum remaining thickness of the RPV head in the wastage area was found to be
approximately d inch. This thickness was attributed to the thickness of the stainless steel
cladding on the inside surface of the RPV head, which is nominally d inch thick.
Background
The Davis-Besse Nuclear Power Station has an RPV head that is constructed from low alloy
steel, fabricated in accordance with the American Society of Mechanical Engineers (ASME)
specification SA-533, Grade B, Class1, and clad on the inside surface with stainless steel. Of
those 69 VHP nozzles, 61 are used for CRDMs, 7 are spare (empty) nozzles, and 1 is used for
the RPV head vent piping. Each of the 69 nozzles is approximately 4 inches in outside
diameter, with a wall-thickness of approximately e inch. Each is constructed of Alloy 600 and
is attached to the RPV head by a partial-penetration, J-groove weld using Alloy 82 and 182.
The distance from the center of one nozzle to the center of the next is approximately 12 inches.
The vessel head is insulated with metal reflective insulation, which is located on a horizontal
plane slightly above the RPV head (i.e., it is not in direct contact with the head). The minimum
distance between the RPV head and the insulation is approximately 2 inches at the center (top)
of the head. The CRDM nozzles pass from the RPV head through the insulation and terminate
at flanges to which the CRDM housings are attached.
The limited gap between the insulation and the RPV head does not impede the performance
of a visual inspection of the CRDM nozzles, as described in Bulletin 2001-01. This is because
the top of the RPV head is surrounded by a service structure that has 18 openings (referred to
as weep holes) near the bottom of the structure, through which small cameras can be inserted
to facilitate visual inspections of the RPV head.
During refueling outages in 1998 and 2000, the licensee performed visual inspections of the
RPV head surface that was accessible through the service structure weep holes. The scope of
these visual inspections covered the bare metal of the RPV head to identify the presence of
boric acid deposits, which would be indicative of primary coolant leakage. These inspections
also included checking for leakage from any of the CRDM flanges, located above the insulation, in response to Generic Letter 88-05, Boric Acid Corrosion of Carbon Steel Reactor Pressure
Boundary Components, which the NRC issued on March 17, 1988.
The visual inspections in 1998 showed an uneven layer of boric acid deposits scattered over
the RPV head (including deposits near CRDM nozzle 3). The outside diameter of the CRDM
nozzles had white streaks, which indicated to the licensee that the boric acid evident on the
head flowed downward from leakage in the CRDM flanges. During the refueling outage in 2000, the licensee also performed visual inspections of the
CRDM flanges and nozzles. Above the RPV head insulation, those inspections revealed five
CRDM flanges with evidence of leakage, including one flange that was the principal leakage
point. Boric acid deposits on the vertical faces of three of these five flanges and the associated
nozzles confirmed leakage from the flanges. Similarly, one of the other two leaking CRDM
flanges had boric acid deposits between the flange and the insulation, which indicated leakage
from the flange. All of these leaking flanges were repaired by replacing their gaskets. The
faces of the flange that was the principal leakage point were also machined to ensure a better
seal.
Visual inspections performed below the RPV head insulation during the 2000 refueling outage
indicated some accumulation of boric acid deposits on the RPV head. These deposits were
located beneath the leaking flanges, with clear evidence of downward flow from the flange area.
No visible evidence of CRDM nozzle leakage (i.e., leakage from the gap between the nozzle
and the RPV head) was detected. The licensee described that the RPV head area was cleaned
with demineralized water to the greatest extent possible, while trying to maintain the dose
as low as reasonably achievable (ALARA). Subsequent video inspection of the partially
cleaned RPV head and nozzles was performed for future reference.
A subsequent review of the 1998 and 2000 inspection videotapes in 2001 confirmed that there
was no evidence of leakage from the RPV head nozzles, although many areas of the RPV head
were not accessible because of persistent boric acid deposits that the licensee did not clean
because of ALARA issues (including the region around nozzle 3).
The inspections in 2002 did not reveal any visual evidence of flange leakage from above the
RPV head. However, as discussed above, three CRDM nozzles had indications of cracking
(identified by ultrasonic testing of the nozzles), which could result in leakage from the RPV to
the top of the RPV head.
Discussion
The following documents describe reactor operating experience with boric acid corrosion
of ferritic steel reactor coolant pressure boundary components in PWR plants:
- Information Notice 86-108, Degradation of Reactor Coolant System Pressure Boundary
Resulting from Boric Acid Corrosion, issued December 29, 1986
- Information Notice 86-108, Supplement 1, Degradation of Reactor Coolant System
Pressure Boundary Resulting from Boric Acid Corrosion, issued April 20, 1987
- Information Notice 86-108, Supplement 2, Degradation of Reactor Coolant System
Pressure Boundary Resulting from Boric Acid Corrosion, issued November 19, 1987
- Information Notice 86-108, Supplement 3, Degradation of Reactor Coolant System
Pressure Boundary Resulting from Boric Acid Corrosion, issued January 5, 1995
- Generic Letter 88-05, Boric Acid Corrosion of Carbon Steel Reactor Pressure Boundary
Components in PWR Plants, issued March 17, 1988 Several instances of boric acid corrosion discussed in these generic communications are
associated with corrosion of the RPV head. NRC Information Notice 86-108, Supplement 1, for example, described an instance in which boric acid had severely corroded three of the RPV
flange bolts, the control rod drive shroud support, and an instrument tube seal clamp. Similarly, NRC Information Notice 86-108, Supplement 2, described an instance in which boric acid
resulted in nine pits in the surface of the RPV head, ranging in depth from 0.9 to 1 cm
[approximately 0.4 inch] and ranging in diameter from 2.5 to 7.5 cm [1 to 3 inches].
As discussed in Information Notice 86-108, Supplement 2, the primary effect of boric acid
leakage onto the ferritic steel RPV head is wastage or general dissolution of the material.
Pitting, stress corrosion cracking (SCC), intergranular attack, and other forms of corrosion are
not generally of concern in concentrated boric acid solutions at elevated temperatures such as
those that may occur on the surface of the RPV head. The rate of general corrosion (wastage)
of ferritic steel from boric acid varies and depends on several conditions, including whether the
boric acid is dry or in solution. If the boric acid is dry (i.e., boric acid crystals), the corrosion rate
is less severe; however, boric acid crystals are not completely benign to carbon steel. During
operation, the temperature of the RPV head is sufficiently high that any leaking primary coolant
would be expected to flash to steam, leaving behind dry boric acid crystals.
Given the wide range of conditions around reactor primary coolant leakage sites and the wide
variation in boric acid corrosion rates, the deleterious effects of boric acid on ferritic steel
components indicate the importance of minimizing boric acid leakage, detecting and correcting
leaks in a timely manner, and promptly cleaning any boric acid residue.
The investigation of the causative conditions surrounding the degradation of the RPV head at
Davis-Besse is continuing. Boric acid or other contaminants could be contributing factors.
As discussed above, factors contributing to the degradation might also include the environment
of the head during both operating and shutdown conditions (e.g., wet/dry), the duration for
which the RPV head is exposed to boric acid, and the source of the boric acid (e.g., leakage
from the CRDM nozzle or from sources above the RPV head such as CRDM flanges).
Related Generic Communications
Bulletin 2001-01, Circumferential Cracking of Reactor Pressure Vessel Head Penetration
Nozzles, August 3, 2001.
Bulletin 82-02, Degradation of Threaded Fasteners in the Reactor Coolant Pressure Boundary
of PWR Plants, June 2, 1982.
Generic Letter 88-05, Boric Acid Corrosion of Carbon Steel Reactor Pressure Boundary
Components in PWR Plants, March 17, 1988.
Generic Letter 97-01, Degradation of Control Rod Drive Mechanism Nozzles and Other Vessel
Closure Head Penetrations, April 1, 1997.
Information Notice 80-27, Degradation of Reactor Coolant Pump Studs, June 11, 1980.
Information Notice 82-06, Failure of Steam Generator Primary Side Manway Closure Studs, March 12, 1982. Information Notice 86-108, Degradation of Reactor Coolant System Pressure Boundary
Resulting from Boric Acid Corrosion, December 29, 1986.
Information Notice 86-108, Supplement 1, Degradation of Reactor Coolant System Pressure
Boundary Resulting from Boric Acid Corrosion, April 20, 1987.
Information Notice 86-108, Supplement 2, Degradation of Reactor Coolant System Pressure
Boundary Resulting from Boric Acid Corrosion, November 19, 1987.
Information Notice 86-108, Supplement 3, Degradation of Reactor Coolant System Pressure
Boundary Resulting from Boric Acid Corrosion, January 5, 1995.
Information Notice 90-10, Primary Water Stress Corrosion Cracking of INCONEL 600,
February 23, 1990.
Information Notice 94-63, Boric Acid Corrosion of Charging Pump Casing Caused by Cladding
Cracks, August 30, 1994.
Information Notice 96-11, Ingress of Demineralizer Resins Increases Potential for Stress
Corrosion Cracking of Control Rod Drive Mechanism Penetrations, February 14, 1996.
Information Notice 2001-05, Through-Wall Circumferential Cracking of Reactor Pressure
Vessel Head Control Rod Drive Mechanism Penetration Nozzles at Oconee Nuclear Station, Unit 3, April 30, 2001.
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 one of the technical contacts
listed below or the appropriate project manager in the NRCs Office of Nuclear Reactor
Regulation (NRR).
/RA/
William D. Beckner, Program Director
Operating Reactor Improvements Program
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts: Allen Hiser, NRR Ken Karwoski, NRR
(301) 415-1034 (301) 415-2752 E-mail: alh1@nrc.gov E-mail: kjk1@nrc.gov
(301) 415-1014 E-mail: jxd@nrc.gov
Attachment: List of Recently Issued NRC Information Notices Information Notice 86-108, ?Degradation of Reactor Coolant System Pressure Boundary
Resulting from Boric Acid Corrosion, December 29, 1986.
Information Notice 86-108, Supplement 1, Degradation of Reactor Coolant System Pressure
Boundary Resulting from Boric Acid Corrosion, April 20, 1987.
Information Notice 86-108, Supplement 2, Degradation of Reactor Coolant System Pressure
Boundary Resulting from Boric Acid Corrosion, November 19, 1987.
Information Notice 86-108, Supplement 3, Degradation of Reactor Coolant System Pressure
Boundary Resulting from Boric Acid Corrosion, January 5, 1995.
Information Notice 90-10, ?Primary Water Stress Corrosion Cracking of INCONEL 600,
February 23, 1990.
Information Notice 94-63, ?Boric Acid Corrosion of Charging Pump Casing Caused by Cladding
Cracks, August 30, 1994.
Information Notice 96-11, ?Ingress of Demineralizer Resins Increases Potential for Stress
Corrosion Cracking of Control Rod Drive Mechanism Penetrations, February 14, 1996.
Information Notice 2001-05, Through-Wall Circumferential Cracking of Reactor Pressure
Vessel Head Control Rod Drive Mechanism Penetration Nozzles at Oconee Nuclear Station, Unit 3, April 30, 2001.
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 one of the technical contacts
listed below or the appropriate project manager in the NRCs Office of Nuclear Reactor
Regulation (NRR).
/RA/
William D. Beckner, Program Director
Operating Reactor Improvements Program
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts: Allen Hiser, NRR Ken Karwoski, NRR
(301) 415-1034 (301) 415-2752 E-mail: alh1@nrc.gov E-mail: kjk1@nrc.gov
(301) 415-1014 E-mail: jxd@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
DISTRIBUTION:
PUBLIC IN Reading File
ADAMS ACCESSION NO.: ML020700556 Template: NRR-052
- See previous concurrence
OFFICE RSE:RORP:DRIP RSE:EMCB:DE BC:EMCB:DE (A)SC:RORP:DRIP PD:RORP:DRIP
NAME IJDozier* KJKarwoski* WHBateman* TKoshy* WDBeckner
DATE 03/11/2002 03/11/2002 03/11/2002 03/11/2002 03/12/2002 OFFICIAL RECORD COPY
Attachment 1 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2002-10 Nonconservative Water Level 03/07/2002 All holders of operating licenses
Setpoints on Steam for nuclear power reactors, Generators except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.
2002-09 Potential for Top Nozzle 02/13/2002 All holders of operating licenses
Separation and Dropping of for nuclear power reactors, and
Certain Type of Westinghouse non-power reactors and holders
Fuel Assembly of licenses for permanently
shutdown facilities with fuel
onsite.
2002-08 Pump Shaft Damage Due to 01/30/2002 All holders of operating licenses
Excessive Hardness of Shaft for nuclear power reactors, Sleeve except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.
2002-07 Use of Sodium Hypochlorite for 01/28/2002 All holders of operating licenses
Cleaning Diesel Fuel Oil for nuclear power except those
Supply Tanks who have ceased operations and
have certified that fuel has been
permanently removed from the
reactor vessel.
2002-06 Design Vulnerability in BWR 01/18/2002 All holders of operating licenses
Reactor Vessel Level or construction permits for boiling
Instrumentation Backfill water reactors (BWRs).
Modification
2002-05 Foreign Material in Standby 01/17/2002 All holders of licenses for nuclear
Liquid Control Storage Tanks power reactors.
2002-04 Wire Degradation at Breaker 01/10/2002 All holders of operating licenses
Cubicle Door Hinges for nuclear power reactors.
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit