Information Notice 1991-19, Steam Generator Feedwater Distribution Piping Damage

Steam Generator Feedwater Distribution Piping Damage
	ML031190553
Person / Time
Issue date:	03/12/1991
From:	Rossi C; Office of Nuclear Reactor Regulation
To:	;
References
	IN-91-019, NUDOCS 9103060156
	Download: ML031190553 (8)
	v • d • e

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UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON,'D.C. 20555 March 12, 1991.

NRC 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 facil.ities and consider

actions,-as appropriate, to avoid'similar problems.

However, suggestions

contained in this information notice'do not constitutekRC 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 consequen- tial 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 feedwoter enter.

the steam generators through a feedwater nozzle. The feedwater enter'sa.

distribution box and 12-inch diameter pipi'ng (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 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 distri- bution 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, NT" 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 addi- tion, 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.

&

IN 91-19 March 12, 1991 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 distri- bution 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 addi- tion, the design of the vent assembly had not properly considered the potential

for erosion and corrosion resulting from localized high velocity flow.

The