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NRC Inspection Manual

IIPB

PART 9900: TECHNICAL GUIDANCE

SGTUBELKG.TG

STEAM GENERATOR TUBE PRIMARY-TO-SECONDARY LEAKAGE

A. PURPOSE

The purpose of this document is to provide guidance to inspectors on overseeing PWRs

with known steam generator tube primary-to-secondary leakage.

B. BACKGROUND

Steam generator (SG) tubes often leak before they fail or burst. This is not always the

case, and the possibility exists for burst with little or no observed leakage. For the many

cases where primary-to-secondary leakage can be detected, the licensees have an

opportunity to prevent tube burst by early detection of primary-to-secondary leakage and

to take corrective action. Routine leakage monitoring with adequate shutdown limits can

afford early detection and response to increasing leakage and, thereby, serve as an

effective means for minimizing the incidence of steam generator tube rupture. This can

be achieved by having near real-time leakage information available to control room

operators. Use of such monitoring capability, along with appropriate alarm set points and

corresponding action levels, can help operators respond appropriately to a developing

situation in a timely manner.

C. DISCUSSION

1. Sources of Primary-to-Secondary Leakage

Primary-to-secondary leakage is ordinarily caused by degraded tubes, leaking

plugs, leaking sleeves, or leak-limiting sleeves. To determine possible sources of

leakage, it is important to review what is known about the component materials

and condition of the steam generator. Reviewing the licenseeís report from their

last steam generator tube examination should provide details regarding the

condition of the steam generator and active degradation processes. Although

operating experience may provide insights as to possible sources of degradation,

sources of leakage cannot be reliably identified while the reactor is in operation.

Therefore, leakage should be treated in accordance with available guidance.

In older plants, itís more likely that leakage is due to some environmentally

assisted corrosion process (e.g.,outer diameter stress corrosion cracking (ODSCC)
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and primary water stress corrosion cracking (PWSCC)) or from a repair process

that exhibits some leaks (e.g., leak-limiting sleeves, leaking plugs). Mechanical

degradation due to wear, fatigue cracks from vibration, and damage from loose

parts are the most likely cause of leakage in newer plants and can also contribute

to leakage in the older SGs.

Components fabricated from mill-annealed Alloy 600 have been found to be highly

susceptible to environmentally-assisted degradation processes, such as ODSCC

and PWSCC. The in-service experience with thermally treated (TT) Alloy 600 and

Alloy 690 components has been uniformly good, especially with regard to

environmentally assisted degradation. 

Cracking has been reported for some Westinghouse plugs manufactured out of

Alloy 600. Industry experience with flawed plugs is discussed in NRC Information

Notice 94-87 and NRC Bulletin 89-01. Most licensees have replaced the Alloy 600

plugs with Alloy 690 plugs. It is also possible to have flaws in the welds that are

used to install the tube sleeves, and some sleeve designs are leak-limiting rather

than leak-tight.

2. Leakage Detection Methods

Most plants have radiation monitoring systems that monitor condenser off-gas,

steam generator blowdown, and the main steam lines. The condenser off gas is

monitored to identify the presence of radioactive gases removed from steam

condensate. The steam generator blowdown is monitored to identify non-volatile

radioactive species in the steam generator bulk water (excluding OTSGs). The

main steam is monitored to detect volatile gases, and in some cases N-16, carried

from the steam generator via the main steam lines. 

Grab samples are also commonly used, such as reactor coolant samples to

quantify the source term, steam generator blowdown to detect non-volatile

radioactive species in liquid and condenser off gas to detect noble gas and other

volatile species removed from steam condensate. In addition, other common grab

samples include condensed main steam to detect noble gas and other volatile

species carried over with main steam and condensate to detect soluble species

such as tritium and iodine. Also, blowdown filters and ion exchanger columns are

used to detect particulates and ionic species from liquid streams. 

Although no single monitor should be expected to fulfill all monitoring roles, some

monitoring methods have demonstrated particular value in certain situations.

Continuous control room display of key radiation monitor trends (e.g., blowdown,

condenser exhaust, Nitrogen-16 monitor of leakage rates and change in leak rate

over time) gives operators real-time information that can be used to safely respond

to the full range of primary-to-secondary leakage.

Use of N-16 monitors installed on or near steam lines has become increasingly

common in the industry as a supplemental means of monitoring leakage. These

monitors exhibit short time response to changes in leak rate and are very useful

to operators, provided their limitations are understood. However, the short half-life
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for N-16 presents some problems in the ability of the detector to measure leak

rate. Changes in power level and characteristics of the leak itself (location and

type of leak) will affect the N-16 concentration reaching the detector. Once the

reactor trips, N-16 quickly decays away and no longer provides a measure to

measure tube leakage. Also, due to the high energy of the gamma ray emitted by

N-16, detectors may be affected by nearby steam lines in addition to the one they

are mounted to. This can make it difficult to estimate total leakage or apportion

leakage among the generators on the basis of N-16 alone.

It is prudent for the monitoring program to also include provisions for detection of

primary-to-secondary leakage during low power or plant shutdown conditions. This

program should ensure that means are available to detect tube leakage whenever

primary pressure is greater than secondary system pressure. This includes hot

shutdown conditions and plant startup situations, when normal means of detecting

leakage might be limited or unavailable. For instance, the radionuclide mix is

altered following a period of plant shutdown so that condenser off-gas monitor

indications may be questionable during startup since they are calibrated for a

specific radionuclide mix based on power operation. Also, N-16 monitoring is not

considered reliable at low power since lower levels of N-16 are available to trigger

detector response during a tube leak.

Plants spend a relatively small fraction of time in low power or hot shutdown.

However, it is prudent to have techniques and procedures available to detect a

rapidly developing leak under those circumstances. In the event a tube failure

develops, operators should have reasonable time to respond to the situation

before the plant reaches full power operation, when the consequences of a tube

failure would be magnified.

The technical specifications (TS) include a limiting condition for operation (LCO)

limit with respect to the allowable primary-to-secondary leakage rate, beyond which

prompt and controlled shutdown must be initiated. As discussed in the next

section, the acceptable LCO limit will become 150 gallons per day as the licensees

change their TS to be consistent with a generic TS change package associated

with an industry SG initiative. 

As discussed in the next section, guidance to the industry is provided by EPRI

Report TR-104788, "PWR Primary-to-Secondary Leak Guidelines" (Ref. 11).

Detection capability and measurement uncertainties are discussed in the guidance,

as well as the characteristics of certain monitoring methods. This is useful to

licensees in determining the adequacy of specific parts of their monitoring system

and the effectiveness of the combination of methods used.

3. Guidance from Industry SG Initiative

The industry currently relies on industry-developed guidelines to evaluate the

significance of primary-to-secondary steam generator tube leakage. In the fall of

1997 the Nuclear Strategic Issues Advisory Committee, a committee consisting of

the chief nuclear officers from the nuclear utilities, voted to adopt the NEI 97-06,
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ìSteam Generator Program Guidelines,î industry initiative. This commitment is in

the form of an industry initiative and is an internal commitment between NEI

members to take the agreed upon position. The industry informed the NRC by

letter dated December 16, 1997, from the Nuclear Energy Institute, of their

commitment to implement the industry steam generator initiative described in NEI

97-06. Each licensee committed to evaluate its existing SG program and where

necessary, revise and strengthen program attributes to meet NEI 97-06 by no later

than the first refueling outage starting after January 1, 1999. 

In accordance with the industry commitment to NEI 97-06, the utility SG

management programs should explicitly provide direction on monitoring primary-tosecondary leakage in SG tubes. NEI 97-06 references a guidance document

prepared by the Electric Power Research Institute (EPRI), ìPWR Primary-ToSecondary Leak Guidelines,î TR-104788-R2, to assist the industry in developing

plant-specific procedures to manage small amounts of leakage within the context

of their SG management program. The guidelines address management

considerations, monitoring methods and equipment, leak rate calculations,

operational response and data evaluation. The guidelines were developed

consistent with industryís observed field experience with leakage, and where

possible, are intended to ensure the propagation of flaws to tube rupture is

minimized under normal and faulted conditions.

Revision 2 to the guidelines was implemented October 14, 2000. The guidelines

direct the licensee to implement a monitoring program that accounts for plant

design, steam generator tube degradation, and previous leakage experience. In

addition, the EPRI guidelines recommend action levels defined by limits on the

leak rate and the rate of change of the leak rate. The action levels provide a

framework that licensees can use to formulate preplanned operator actions based

on specified leakage indications. The objective for the normal operating leak rate

limit or rate of change limit is to establish a reasonable likelihood that the plant is

shut down before the tube could rupture under either normal or faulted conditions.

The operating leakage experience together with the analytically based burst

pressure versus normal operating leak rate trends provide the bases for a

recommended leakage limit.

There is a generic TS change package under review by the NRC staff that is part

of the NEI 97-06 industry initiative. In the current TSs, the LCO limits can vary

from plant to plant with respect to the allowable primary-to-secondary leakage rate

through any one SG, beyond which prompt and controlled shutdown must be

initiated. The TS LCO limits in operating plants vary from 150 to 500 gallons per

day (gpd). The NEI 97-06 generic license change package will standardize that

value to 150 gpd for all PWRs.

4. Assessing the Significance of the Leakage

The EPRI guidelines developed under NEI 97-06 provide recommendations for

plant actions based on observed primary-to-secondary leakage rates. The

guidelines recommend increased monitoring at 5 gpd. Below 5 gpd during normal

operation, no specific actions are recommended. The guidelines suggest that the
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licenseeís grab sample program be able to quantify leakage at 5 gpd. At lower

leakage levels, the analysis is limited by a lack of analytical certainty at low

radiochemical concentrations. 

Above 5 gpd, the EPRI Primary- to-Secondary Leak Guidelines (Rev. 2)

recommend three action levels:

a. Action Level 1. This action level requires increased attention and monitoring.

This action level is entered when:

! leakage greater than or equal to 30 gpd, and

! less than 75 gpd in any SG. 

b. Action Level 2. This action level requires that the plant be in Mode 3 within

24 hours. This action level is entered when:

! leakage greater than or equal to 75 gpd in any SG, and

! sustained for greater than or equal to 1 hour, and

! the rate of increase in leakage is <30 gpd/hr in any SG. 

c. Action Level 3. This action level requires a prompt and controlled plant

shutdown to < 50% power within one hour and in Mode 3 within the next 2

hours. This action level is entered when:

! leakage is increasing by greater than 30 gpd/hr, and 

! is greater than or equal to 75 gpd in any steam generator. 

In addition, the guidelines recommend an action level at leakage of 30 gpd that

directs the licensee to increase their use of grab samples when no on-line

quantitative monitors are operable.

It is suggested that the NRC resident inspectors and regional staff use an informal

screening criteria of 3 gpd or greater for increased involvement by NRC

headquarters staff when steam generator primary to secondary leakage is

identified. This is not meant to be an absolute threshold, or requirement, because

there may be certain instances where there is something unusual about the

circumstances of the leakage, or other reason that the region would want

involvement by the headquarters staff before leakage reaches 3 gpd. If a licensee

reports levels of primary-to-secondary leakage exceeding 3 gpd to the resident

inspector or regional staff, Office of Nuclear Reactor Regulation (NRR) should be

informed through the morning phone calls. The Materials and Chemical

Engineering Branch (EMCB) staff is interested in being kept abreast of this

information by NRR, Division of Licensing Project Management. EMCB staff would

be concerned about the rate of change of the leakage, to assess how quickly the

situation is changing. The following section discusses some of the typical
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questions that inspectors can pursue with the licensee when leakage is reported.

EMCB staff is available if further direction is needed.

When leakage exceeds 3 gpd, parameters that can be considered are the

effectiveness of licensee procedures, equipment, and practices for monitoring and

responding to primary-to-secondary leakage. For example, the adequacy of

procedures and equipment to provide real-time information on leak rate and its rate

of change could be assessed. The appropriate setting of alarm setpoints on those

radiation monitors that are used for detecting primary-to-secondary leakage (e.g.,

condenser air ejector, N-16) to alert operators to any increasing leak rate could be

assessed. In addition, the adequacy of emergency operating procedures,

availability of systems and components, and operator training for response to

steam generator tube ruptures could also be assessed. For additional guidance,

refer to the Inservice Inspection Activities Procedure, IP 71111.08, from the NRC

Inspection Manual. 

The NRR staff often receives notification of extremely low levels of leakage (<1

gpd), but these levels of leakage donít typically need to result in increased

interaction with the licensee. This is because many plants have experienced this

level of leakage during a full cycle, and itís difficult to definitively determine the

source of the leakage at that level. Often, small levels of leakage will persist for

the rest of the operating cycle for some plants. 

5. Questions to Gain Additional Information About the Leakage

Questions should focus on how the licensee is monitoring the leakage, evaluating

the potential sources of leakage, and what the past inspection results and in-situ

testing information tell them about the condition of their SGs. 

It is useful for the inspector to understand how the licensee detected the leakage,

and what the leakage history for this unit (and the specific steam generator) was

for previous outages. There are various advantages and disadvantages of various

monitoring techniques, which can affect the quantity of leakage reported. 

After shutdown, the licensee may observe leakage from post-shutdown visual

inspections of the tubesheet face. Additional information may be available from

secondary side tube leak tests performed early during outages to identify leaking

tubes. To conduct these tests, nitrogen pressure is applied to the water inventory

in the secondary side of the steam generators and maintained for an extended

period (often for days). If the visual inspections reveal any observed dampness

or drops of water from the tubesheet face, tubes in that area need to be evaluated

carefully with appropriate inspection methods. 

Sometimes plants experience very low levels of leakage in current and previous

cycles, with no clear cause identified. Small changes in low levels of leakage can

be due to changes in monitoring equipment, either putting new equipment in

service or recent calibrations of the existing equipment. The staff was recently
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informed of small changes of observed leakage that directly correlated to putting

new detection equipment in service. This led to a step increase in the very small

amount of leakage observed. This could also be observed after calibrating

equipment, or any other major change that would reset the baseline readings. 

The inspector should recognize that although it is not possible to reliably identify

the source of leakage while the plant is operating, insights can be obtained by

discussing with the licensee the SG tube examination findings from the eddy

current testing during the last outage, in situ pressure test results, and the

licenseeís knowledge of loose parts in the SGs.

The inspector can ascertain information on the degradation modes being

experienced by the SGs. For example, AVB wear can have a significant throughwall extent, even for replaced SGs that have not been in service many years.

Some of the plants have qualified sizing techniques for AVB wear, so indications

of wear are sometimes left in service for the next operating cycle. 

For any reported active degradation modes, the inspector can ask about their insitu pressure test results from previous outages. If the licensee has experienced

difficulty satisfying the in-situ pressure test performance criteria, it may indicate

that the flaws were deeper than their SG eddy current tests were capable of

detecting. 

Some plants also have known loose parts in the affected SG that they have not

been able to retrieve, that they had identified through techniques such as FOSAR

(foreign object search and retrieve). In some cases, the licensees will plug tubes

around a loose part that they are unable to retrieve, to reduce the chance of tube

rupture from the loose part during the next cycle. 

It should also be noted it just is not practical for licensees to shut down plants at

low levels of leakage. In fact, sustained leakage below 10 gpd in some older

plants is not unusual. As noted above, when plants shut down leakage tests are

used to identify leaking tubes. The experience of plants that have shut down with

low levels leakage has been that it is very difficult to determine the source of the

leakage.

In summary, obtaining background information about operating and inspection

experience may provide useful insights regarding the significance of ongoing

primary-to-secondary leakage. Accordingly, the staffís ability to influence the

actions of licensees with known primary-to-secondary leakage is limited. Because

of the inability to reliably identify the source of leakage while the plant is operating,

the NRC staffís primary role should be to ensure that the licensee is responding

to leakage in a conservative manner by monitoring the leakage and being prepared

to implement plant shutdown consistent with EPRI guidelines.

6. NRC Generic Communications and Regulatory Guidance

a. USNRC Information Notice No. 91-43: ìRecent Incidents Involving Rapid

Increases in Primary-to-Secondary Leak Rate,î (July 1991)
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b. USNRC Information Notice No. 94-43: ìDetermination of Primary-toSecondary Steam Generator Leak Rate,î (June 1994)

c. USNRC Regulatory Guide 1.45, ìReactor Coolant Pressure Boundary

Leakage Detection Systems,î (May 1973)

d. USNRC Regulatory Guide 1.97, ìInstrumentation for Light-Water-Cooled

Nuclear Power Plants to Assess Plant and Environs Conditions During and

Following an Accident,î (December 1980).

7. Operating Experience: Primary-to-Secondary SG Tube Leakage

A summary of information in Table 1 on past forced outages from 1990 to 1998

due to SG tube leaks. References to generic communications are given for many

of the events, if more information about the event is needed.

Table 1: Tube Leak Forced Outages at US PWRs

Plant Date Leak Rate

(gpd)

Cause Reference

St. Lucie 1 Jan. 1990 3 Foreign Object

TMI 1 Mar. 1990 1440 Fatigue

Millstone 2 May 1990 Cracked Plug

North Anna 2 Aug 1990 40 Cracked Plug

Oconee 2 Nov. 1990 130 Fatigue

Shearon Harris Nov. 1990 50 Loose Part

Maine Yankee Dec. 1990 1440 PWSCC

San Onofre 1 Apr. 1991 150 Sleeve Joint EN 20860

Millstone 2 Apr. 1991 70 U-bend SCC PN 1-91-030

Millstone 2 May 1991 70 Tube Sheet Circumferential Crack EN 21077

McGuire 1 Jan. 1992 250 Freespan Crack PN 2-91-002

ANO 2 Mar. 1992 360 Tube Sheet Circumferential Crack PN 4-92-018, -

081A, 

EN 22975

Prairie Island 1 Mar. 1992 144 Roll Transition Zone Axial Crack

McGuire 1 May 1992 5 MR 3-92-0255

PN 23400

Prairie Island 1 Sep. 1992 187 MR 3-92-0255

PN 3-92-048

McGuire 1 Nov. 1992 250

Trojan Nov. 1992 200 Sleeve Weld Circumferential

Crack

PN 5-92-035

EN 24569
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Palo Verde 2 Mar. 1993 240 Upper Bundle Freespan Inter

Granular Stress Corrosion

Cracking

PN 5-93-009, -

009A, -009B, -

009C, -009D

EN 25255

Kewaunee Jun. 1993 100 Leaking Plug MR 3-93-0167

McGuire 1 Aug. 1993 185 (?) Sleeve Failure PN 2-93-038

EN 25990

Palo Verde 3 Sept 1993 Freespan crack MR 5-93-0066

PN 5-93-017

McGuire 1 Oct 1993 185 Circ. crack in sleeved tube PN 2-93-053

Braidwood 1 Oct. 1993 300 Freespan Cracks PN 3-93-061

San Onofre 3 Nov. 1993 50 Loose parts degradation and

leaking welded plugs

MR 5-93-0081

PN 5-93-020

Farley 2 Nov. 1993 MR 2-93-0132

McGuire 1 Jan. 1994 100 Leaking Sleeve PN 2-94-003

EN 26665

Oconee 3 Mar. 1994 144 Fatigue PN 2-94-014

EN 26967

S. Texas Mar. 1994 160 Leaking Plug PN 4-94-005A

EN 26859

Zion 2 Mar. 1994 1440 Tubesheet Crevice Inter Granular

Attack OD

EN 26901

Oconee 2 Jul. 1994 144 Fatigue PN 2-94040

Maine Yankee Jul. 1994 50 Circumferential Crack PWSCC MR 1-94-0079

EN 27587

Zion 1 Feb. 1996 Foreign object PN 3-96-009

Byron 2 Aug. 1996 120 Loose Part PN 3-96-049

MR 3-96-0106

Vogtle 1 May 1996 Foreign object PN 2-96-041

EN 30555

ANO 2 Nov. 1996 65 Axial Crack PN 4-96-061

EN 31344

McGuire 2 June 1997 66 ODSCC at TSP PN 2-97-033

Oconee 1 Nov. 1997 400 2 Welded Plugs PN 2-97-065, -

065A

EN 33458

Farley 1 Dec. 1998 90 2 Freespan Cracks

Information from EPRI TR-106365-R14 and NRC documents

END