Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 12, 1991
Information Notice No. 91-19: STEAM GENERATOR FEEDWATER DISTRIBUTION
PIPING DAMAGE
Addressees:
All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).
Purpose:
This information notice is intended to alert addressees to potential
problems resulting from degradation of feedwater distribution piping in
steam generators due to thermal stress, cracking, erosion and corrosion.
Depending on the design of the steam generator feedwater system, these
problems may affect operation of the auxiliary feedwater system. 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 do not
constitute NRC requirements; therefore, no specific action or written
response is required.
Background:
The degradation noted below of the feedwater distribution system piping in
the steam generators at San Onofre Units 2 and 3 may be applicable to
Combustion Engineering steam generator designs predating the System 80
design and to similar designs in other steam generators at other nuclear
power plants. This matter is considered safety-significant because the
feedwater distribution system piping degradation may affect the delivery of
auxiliary feedwater flow in some of these steam generators and because of
the potential for consequential damage to the steam generator tubes from
resulting debris. The NRC has issued several generic communications dealing
with one or more aspects of such degradation (Attachment 3).
At San Onofre Units 2 and 3, both main feedwater and auxiliary feedwater
enter the steam generators through a feedwater nozzle. The feedwater enters
a distribution box and 12-inch diameter piping (feedring) that distributes
the flow through top-mounted discharge elbows (J-tubes) around the periphery
of the steam generator shell (Figure 1). The feedring is attached by two
U-bolts at each of four supports that are welded to the shell wall. A
3-inch elbow and tee vent assembly is attached to the upper portion of the
innermost (toward the interior of the steam generator) end of the
distribution box (Figure 2).
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IN 91-19
March 12, 1991
Page 2 of 3
Combustion Engineering originally designed the distribution box without the
vent assembly. However, in 1980 during preoperational testing at San Onofre
Unit 2, a test of the auxiliary feedwater system caused a partial vacuum
within both halves of the feedring in one of the steam generators, and the
feedring collapsed. The licensee, Southern California Edison Co.,
determined that the inadequate flow area of the discharge elbows and the
relatively thin-walled Schedule 40 piping constituting the feedring had
contributed to the feedring collapse. Corrective actions included replacing
most of the feedring with Schedule 120 piping (except for 9-inch segments on
each side of the distribution box), enlarging the diameter of the discharge
elbows from 1.5 inches to 3.5 inches, and installing the vent assembly on
the distribution box.
Description of Circumstances:
San Onofre Unit 3:
On May 10, 1990, the licensee found several pieces of carbon steel debris
during a routine inspection of the secondary side of the tubesheet of one
steam generator (LER 50-362/90-05-01). During further inspection of the
internal components of this and the other steam generator, the licensee
found material missing from the lower portion of the feedring at its
intersection with the distribution box, surface cracks in the heat-affected
zone at the toe of the weld at that intersection, erosion and corrosion
indications on the interior surfaces of the distribution boxes, erosion of
the vent assemblies, "T" section tops missing from the vent assemblies, and
deformation of several U-bolt supports.
San Onofre Unit 2:
On July 23, 1990, the licensee shut down Unit 2 to perform a similar inspec-
tion. The damage found was significantly less than on Unit 3. No material
was found missing from the distribution box-feedring junction. One U-bolt
was fractured.
Discussion:
The licensee determined the root cause contributing to the degradation of
the feedwater distribution system piping to be inadequate design of the
feedring and feedring supports. The design did not adequately consider the
thermal stresses resulting from normal operating conditions, in particular
the batch process of auxiliary feedwater addition during startup operations.
In addition, the design of the vent assembly had not properly considered the
potential for erosion and corrosion resulting from localized high velocity
flow. The corrective actions taken by the licensee included replacing the
remaining Schedule 40 piping material with Schedule 120 piping material,
replacing the distribution box-feedring weld configuration with weld-o-let
forgings, removing the distribution box vents from the design, repairing
local thinning of the distribution box by weld buildup and removal of local
interior surface discontinuities, modifying the feedring supports to provide
flexibility for thermal expansion, and using stronger U-bolts. The licensee
had previously modified the auxiliary feedwater system to provide continuous
feeding of the steam generator rather than the batch feeding that was used
during startup operations.
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IN 91-19
March 12, 1991
Page 3 of 3
On September 20, 1990, the steam generator vendor, Combustion Engineering,
issued an information bulletin (Combustion Engineering Infobulletin 90-04,
"Feedwater Distribution System Degradation"), recommending that its client
utilities perform a baseline inspection during their next refueling outage
to detect wall thinning in the feedwater distribution system.
This information notice requires no 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 NRR project
manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Vern Hodge, NRR
(301) 492-1861
Lawrence E. Kokajko, NRR
(301) 492-1380
Attachments:
1. Figure 1. Top View of the Feedwater Distribution Piping
2. Figure 2. Side View of the Feedwater Distribution Piping
3. List of References
4. List of Recently Issued NRC Information Notices
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Attachment 3
IN 91-19
March 12, 1991
Page 1 of 1
LIST OF REFERENCES
1. Bulletin No. 87-01: "Thinning of Pipe Walls in Nuclear Power Plants,"
July 9, 1987
2. Bulletin No. 79-13: "Cracking in Feedwater System Piping," June 25,
1979
3. Ibid., Revision No. 1: "Cracking in Feedwater System Piping,"
August 30, 1979
4. Ibid., Revision No. 2: "Cracking in Feedwater System Piping,"
October 16, 1979
5. Generic Letter No. 89-08: "Erosion/Corrosion-Induced Pipe Wall
Thinning," May 2, 1989
6. Generic Letter No. 79-20: Untitled, on Cracking In Feedwater Lines,
May 25, 1979
7. Information Notice No. 88-17: "Summary of Responses to NRC Bulletin
87-01, 'Thinning of Pipe Walls in Nuclear Power Plants,'" April 22,
1988
8. Information Notice No. 87-36: "Significant Unexpected Erosion of
Feedwater Lines," August 4, 1987
9. Information Notice No. 86-106: "Feedwater Line Break," December 16,
1986
10. Ibid., Supplement 1: "Feedwater Line Break," February 13, 1987
11. Ibid., Supplement 3: "Feedwater Line Break," November 10, 1988
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