Information Notice 2007-37, Buildup of Deposits in Steam Generators

Buildup of Deposits in Steam Generators

ML072910750
Person / Time
Issue date:	11/23/2007
From:	Michael Case NRC/NRR/ADRO/DPR
To:
References
IN-07-037
Download: ML072910750 (4)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001 November 23, 2007 NRC INFORMATION NOTICE 2007-37: BUILDUP OF DEPOSITS IN STEAM

GENERATORS

ADDRESSEES

All holders of operating licenses or construction permits for pressurized water reactors except

those that have permanently ceased operations and who 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 (IN) to alert

addressees to the potential for deposits to accumulate in their steam generators and potentially

affect steam generator performance and tube integrity. The NRC expects that recipients of this

IN will review the information for applicability to their facilities and consider taking actions, as

appropriate, to avoid similar problems. However, suggestions contained in this IN are not NRC

requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

Operating Experience at a Foreign Reactor

Between 2004 and 2006, three primary-to-secondary leaks occurred at the Cruas Nuclear

Plant, a multi-unit site in France. The last primary-to-secondary leak occurred at Cruas Unit 4 in February 2006 (Autorité de Sûreté Nucléaire 2006 Annual Report, http://annual-report.asn.fr/PDF/nuclear-power-plants-EDF.pdf), and it was detected through the

use of nitrogen-16 radiation monitors. The leak rate increased from very low levels to

approximately 3 gallons per minute [600 liters per hour] in 12 minutes.

All three leaks were a result of a circumferential crack in the tube at the location where the tube

passes through the uppermost tube support plate (TSP). All three tubes were near the center

of the tube bundle near an area where no tubes are installed. This tube-free area was intended

to facilitate secondary water/steam flow into the interior of the tube bundle. The U-bend region

of the affected tubes did not have any anti-vibration bar support since the radii of these

U-bends were small.

The cause of the cracks was determined to be high cycle fatigue as a result of flow-induced

vibration. The tubes became susceptible to vibration and fatigue as a result of the buildup of

deposits on the secondary side of the steam generator which changed the flow conditions in the

center of the tube bundle. The deposits had blocked water/steam flow through the

quatrefoil-shaped holes in the TSPs, forcing more water/steam into the center of the tube

bundle. The extent of the hole blockage was estimated based on visual examinations, analysis

of eddy current test results, and evaluation of the wide range feedwater level gauge readings.

High levels of deposits were determined to be present based on these inspections and

analyses.

Contributing factors to the buildup of the deposits were determined to be low secondary side pH

(less than 9.6), chemical intrusions (from main condenser cooling water leakage into the

secondary system), and the TSP hole design (quatrefoil-shaped holes).

To address this problem, several corrective actions came under consideration, including

evaluating the level of TSP hole blockage, removing the deposits through chemical cleaning, changing the chemistry conditions on the secondary-side of the plant to reduce the rate of

buildup of deposits, analyzing the consequences of the deposits on the flow through the steam

generator, and analyzing the stresses that could be imposed on the TSP during transients and

accidents as a result of the hole blockage.

Operating Experience at a U.S. Reactor

In the early 1990s, steam generator water level oscillations were observed at Surry Power Units

1 and 2, near Newport News, Virginia. Due to the severity of these water level oscillations, the

units operated at reduced power levels for varying periods of time. The cause of the steam

generator water level oscillations was severe deposit buildup in the TSP quatrefoil-shaped

holes. The licensee corrected the problem by performing chemical cleaning on all steam

generators at both units to reduce the extent of deposits.

BACKGROUND

Corrosion products can accumulate in the secondary side of the steam generator as a result of

the gradual erosion and corrosion of secondary side components in a pressurized water

reactor. This accumulation of corrosion products results in the buildup of deposits on the tubes, tubesheets, and other secondary side steam generator structures (including the holes through

which the tubes pass).

Harmful contaminants can concentrate in these deposits and result in corrosion of the steam

generator tubes. In addition, these deposits can affect the thermal performance of the steam

generator (i.e., the ability to transfer heat from the primary-to-secondary side of the steam

generator) and the thermal hydraulic characteristics of the steam generator (by changing the

flow patterns within the steam generator).

As a result of the negative effects of these deposits, plant operators in the U.S. frequently

remove the deposits from the top of the tubesheet by a process referred to as sludge lancing.

Operators also occasionally remove these deposits from other areas in the steam generator

through chemical cleaning processes.

High cycle fatigue cracking caused by flow-induced vibration have been observed in U.S. steam

generators. Fatigue cracking in steam generators with U-shaped tubes was the subject of NRC

Bulletin 88-02, Rapidly Propagating Fatigue Cracks in Steam Generator Tubes. This cracking occurred, in part, because of two factors: (1) There was denting of the tubes at the tube-to-TSP

intersection and (2) non-uniform anti-vibration bar penetration into the tube bundle caused

locally high flow velocities. This cracking occurred in tubes with small U-bend radii. Fatigue

cracking in large radius U-bends was discussed in NUREG-1604, Circumferential Cracking of

Steam Generator Tubes. Fatigue cracking in steam generators with straight tubes has also

been observed in the U.S. This cracking is associated with localized high velocities near a

tube-free lane region in the steam generator.

DISCUSSION

Extensive deposit buildup in the TSP holes can increase the vibration of the tubes and

potentially lead to fatigue cracking of the tube. Deposits can also change the loading (stresses)

on the TSPs during transients and design basis accidents (e.g., a steam line break). Extensive

deposit buildup in the tube support holes can change the secondary water recirculation rate and

may result in water level oscillations within the steam generator.

The extent to which deposits can buildup in the steam generator (including the tube support

holes) will vary from unit to unit. It is a function not only of the tube support configuration (e.g.,

lattice grid, round hole, quatrefoil-shaped holes), but also of the design and operation of the

secondary side of the plant. Low secondary side pH levels may increase the corrosion rate on

the secondary side of the plant and lead to more impurity transport to the steam generators.

Chemical intrusions (e.g., from main condenser leakage) may also increase the transport of

impurities into the steam generator. The more impurities that enter the steam generator, the

more likely that deposits will buildup on the secondary side of the steam generator.

The potential for fatigue cracking of the steam generator tubes also varies from unit to unit. It

depends on how the tubes are supported and the secondary side flow conditions (which can be

affected by the buildup of deposits in the steam generator).

Operating experience demonstrates that once initiated, high cycle fatigue cracks can rapidly

propagate over periods ranging from hours to days to complete severance of the tube. As

discussed in NRC Bulletin 88-02, enhanced primary-to-secondary leakage monitoring can be an

effective tool at reducing the potential for a tube rupture.

Although there is no specific requirement for licensees to monitor, assess, or remove deposit

buildup, there are requirements for licensees to maintain steam generator tube integrity. These

integrity requirements are contained, in part, in plant technical specifications. Therefore, it is

important to assess the deposit buildup in a steam generator and the effects that deposits will

have on steam generator performance and tube integrity during normal operation and design

basis accidents. Promptly identifying and removing significant deposit buildup may prevent a

loss of tube integrity and may improve the thermal performance of the steam generator.

CONTACT

This information notice does not require any specific action or written response. Please direct

any questions about this matter to the technical contact listed below.

/RA by TQuay for/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Kenneth J. Karwoski, NRR

301-415-2752 E-mail: kjk1@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

CONTACT

This information notice does not require any specific action or written response. Please direct

any questions about this matter to the technical contact listed below.

/RA by TQuay for/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Kenneth J. Karwoski, NRR

301-415-2752 E-mail: kjk1@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

Distribution:

IN Reading File

ADAMS Accession Number: ML072910750

OFFICE DCI BC:CSGB:DCI TECH EDITOR D:DCI IP

NAME KKarwoski AHiser CHsu MEvans JJacobson

DATE 10/19/2007 10/25/2007 10/24/2007 11/8/2007 11/6/2007 OFFICE LA:PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DPR

NAME CHawes DBeaulieu MMurphy MCase

DATE 11/15/2007 11/15/2007 11/23/07 11/23/07 OFFICIAL RECORD COPY