Information Notice 1990-10, Primary Water Stress Corrosion Cracking of Inconel 600

Primary Water Stress Corrosion Cracking of Inconel 600

ML053070392
Person / Time
Site:	Davis Besse
Issue date:	02/23/1990
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-90-010, NUDOCS 9002160093
Download: ML053070392 (4)

0 Q 0 0 0 9 0 3 h&@h. /-3!37 REG I V ED EXP-90i01536 UNITED STATES

MAR 0 5 1990 NUCLEAR REGULATORY COMMISSION

Dl-M OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D,C. 20555

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February 23, 1990

NRC INFORMATION NOTICE NO. 90-10: PRIMARY WATER STRESS CORROSION CRACKING

(PWSCC) OF INCONEL 600

Addressees

All holders o f operating licenses or construction permits for pressur zed water

reactors (PWRs) .

Purpose

This information notice is intended to alert addressees to potential problems

related to primary water stress corrosion cracking (PWSCC) of Inconel 600 that

has occurred in pressurizer heater thermal sleeves and instrument nozzles at

-several domestic and foreign PWR plants, It i s 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.

Description of Circumstances

During the 1989 refueling outage at Calvert Cliffs Unit 2 (CC-2), visual

examination detected leakage in 20 pressurizer heater penetrations and 1 upper-level/pressure tap instrument nozzle. Leakage was indicated by the

presence of boric acid crystals at the penetrations and on the nozzle. Non- destructive examinations (liquid penetrant and eddy current examinations)

were performed on 28 thermal sleeves and 3 instrument nozzles. Crack indi- cations were reported in 24 thermal sleeves, including the 20 originally

identified to be leaking as well as the leaking nozzle. No crack indications

were found in the two lower instrument nozzles. The examinations showed that

all cracks in the sleeves and the nozzle were axially oriented with a maximum

length not greater than 1.5 inches. The mode of failure for the thermal sleeves

was identified as PWSCC.

The heater sleeves and the instrumentation nozzles were made of Inconel 600

tubing and bar materials, respectively, supplied by INCO. All thermal sleeves

were made in a high strength heat (NX8878) with a reported yield strength of

63.5 ksi. No chemical contaminants were found on the sleeve fracture surfaces.

A review of the fabrication records showed that all 120 thermal sleeves in CC-2 were reamed 3.5 inches from the top before welding and all but two were also

IrJ90-10

9002160093 Y

IN 90-10

February 23, 1990

Page 2 of.4, , ,

reamed after welding to facilitate the insertion of the heater rods. All

cracks in the sleeves were reported to be located inside the reamed :r&g,ioflI,,

either above or below the J-groove weld.

All instrument nozzles were made from heat no, NX8297 and its yield strength -

was reported to be 36 ksi, The licensee indicated that four upper instrument

nozzles, including the leaking one, had been extensively reworked when the

faulty J-groove welds were repaired. Based on the results of the investiga- tions, the licensee, Baltimore Gas and Electric (BG&E), postulated that the

leaking in the pressurizer thermal sleeves and the instrumentation nozzle

was due to PWSCC, The licensee is in the process o f removing a metallurgical

samp?e from the leaking instrument nozzle for failure analysis to identify

the mode of failure.

On February 27, 1986, a small leak (about 0.15 gpm) was observed on a

3/4-inch-diameter upper pressurizer level instrument nozzle at San Onofre

Nuclear Generating Station (SONGS) Unit 3 while the plant was in hot standby.

An axial flaw about 5/8 inch in length was identified on the inside diameter

surface o f the nozzle. The flaw appeared to originate from the end of the

nozzle inside the pressurizer and extended beyond the attachment weld (1/2 inch

in depth) approximately 1/8 inch into the annulus area of the nozzle assembly.

'The flawed nozzle was cut out, including the entire attachment weld, The

results of the metallurgical examination performed on the flawed nozzle

assembly indicated that the cracking was PWSCC.

In spring 1989, leakage from instrument nozzles was observed in two foreign

PWRs (one from each 1300-MW plant) when the hydrostatic pressure testing o f

the primary system was performed durlng the first refueling outage. The

instrument nozzles were made of Inconel 600 material. The installation

of the nozzles included mechanically rolling a portion of the nozzle into

the pressurizer she1 1. Nondestructive examinations (NDEs) were performed

on the leaking nozzles and found the cracks to be principally axial in

orientation; however, some circumferential cracking was observed. Destructive

examlnation o f these two leaking nozzles to identify the root causes has not

been completed. Additional NDEs were performed on all the instrument nozzles

of five 1300-MW PWRs. Crack indications were found in 12 instrument nozzles.

Discussion:

Extensive laboratory testing has shown that intergranular stress corrosion

cracking (IGSCC) requires the presence of the following three key elements:

an aggressive environment, susceptible material, and 'sufficient tensile

stresses for crack initiation and propagation. PWSCC refers to IGSCC in

the primary water environment of PWRs. The laboratory demonstration of

PWSCC in Inconel 600 was first reported by Coriou almost 30 years ago.

The studies of PWSCC in Inconel 600 have been documented in numerous

published reports. However, the mechanism for PWSCC in Inconel 600

is still not well understood. In PWRs, PWSCC o f Inconel 600 was first

reported in steam generator tubing.

I M 90-10

February 23, 1990 !

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The cracking to date in the thermal sleeves and the instrument nozzles of the

domestic PWRs has been reported as being only axially oriented. The safety -

I

1 implication of an axial crack is not considered a significant threat to the

!

structural integrity of the components and most likely will result in a small

leak. However, limited circumferential cracking was reported in the instrument

<

nozzles of several foreign PWRs. The difference in the cracking morphology has

been attributed to the different type of mechanical working (rolling vs. reaming)

being performed on these nozzles and thermal sleeves. The appearance of the

crack in the rolled instrument nozzle is consistent with that observed in the

roll-expanded region of steam generator tubing, Circumferential cracking

poses a more serious safety concern because if it were to go undetected it

could lead to a structural failure of a component rather than to a limited

leak.

The reported cracking of the pressurizer thermal sleeves and the instrument

nozzle at Calvert Cliffs Unit 2 and the instrument nozzle at SONGS Unit 3 are the most current PWSCC events for Inconel 600 in domestic PWRs besides

the cracking problem associated with the steam generator tubin and plugs.

7

.The pressurizer thermal sleeves in Calvert Cliffs Unit 1 (CC-1 were also

made of the same heat of susceptible material, but the recent inspection

of the CC-1 pressurizer did not reveal any leaking or cracking of the thermal

sleeves. The licensee indicated that the major difference in the fabrication

of thermal sleeves between CC-1 and CC-2 is that the pre-installation reaming

operation was not performed on CC-1 sleeves. The Combustion Engineering Owners

Group (CEOG) performed an evaluation of pressurizer heater sleeve susceptibility

to PWSCC for plants designed by Combustion Engineering. The CEOG recomnended a

visual inspection program for the thermal sleeves. The inspection program for

the thermal sleeves varied, depending on the degree of susceptibility of the

sleeve materials. The sleeve susceptibility was rated by the elements described

z

above. The staff notes that at CC-2 the yield strength of the thermal sleeve

material is higher than that of the instrument nozzle material. However, PWSCC

occurred in both heats o f materials. This circumstance may indicate that the

yield strength of the material is not necessarily a reliable screening criterion

for PWSCC susceptibility. The CEOG is performing additional work to address.

PWSCC of Inconel 600. The CEOG programs include the following activities:

evaluations to gain better understanding of the cracking mechanism in

pressurizer thermal heater sleeves and instrument nozzles; an analytical

determination of a temperature profile for the heater sleeves; review of

the fabrication history of all Inconel 600 penetrations in the primary system

components; conduct of a test that is primarily a system leakage test on a

mock-up of the flawed components; and improvement of NDE methods for cracking

eva 1 ua t ion.

PWSCC of Inconel 600 is not a new phenomenon. However, very little special

attention has been given to the inspect.ion for PWSCC in Inconel 600 applica- tions other than that associated with the steam generator tubing. As a result

of the recently reported instances of PWSCC in the pressurizer heater thermal

sleeves and instrument nozzles in several domestic and foreign PWRs, it may be

prudent for licensees o f all PWRs to review their Inconel 600 applications in

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the primary coolent pressure boundary, and when necessary, to iniplement an

augmented inspection program.

I N 90-10

February 23, 1990

Page 4 o f 4 This information n o t i c e r e q u i r e s no s p e c i f i c a c t i o n o r w r i t t e n response. I f

you have any questions about the i n f o r m a t i o n i n t h i s n o t i c e , please contact one

o f t h e t e c h n i c a l contacts l i s t e d below o r the appropriate NRR p r o j e c t manager.

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W 5 - e '

h a r l e s E. Rossi. D r e c t o r

D i v i s i o n of Operational Events Assessment

O f f i c e of Nuclear Reactor Regulation

Technical Contacts: W i l l i a m H. KOO, NRR

(301) 492-0928 .

Robert A. Hermann, NRR

(301) 492-0911 A,,achment: L i s t o f Recently Issued NRC I n f i r m a t i o n Notices