AN OVERWIEW OF STEAM GENERATOR AGING MANAGEMENT · PDF fileAN OVERWIEW OF STEAM GENERATOR...

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AN OVERWIEW OF STEAM GENERATOR AGING

MANAGEMENT

Fco. Javier Campaña Martín

Madrid, September 2014

2

Objectives

Introduction

Materials and

fabrication

Degradation

Mechanisms

Steam Generator

Program

Assessment of Degradation Mechanisms of Primary Components in Water Cooled Nuclear Reactors: Current Issues and Future Challenges

3

Objectives

KNOWING STEAM GENERATOR Description of different types of Steam Generator design

Introduction

Materials and

fabrication

Degradation

Mechanisms

Steam Generator

Program

4

Objectives

DESCRIBING MATERIALS AND FABRICATION Discussion of some of the more important steam generator

fabrication practices and materials of construction

Introduction

Materials and

fabrication

Degradation

Mechanisms

Steam Generator

Program


5

Objectives

MAIN DEGRADATION MECHANISMS IN PWR STEAM GENERATOR TUBES

Knowing the world experience

Introduction

Materials and

fabrication

Degradation

Mechanisms

Steam Generator

Program


6

Objectives

HIGHLIGHTS OF A STEAM GENERATOR PROGRAM Degradation Assessment and Integrity Evaluation

Introduction

Steam Generator

Program

Materials and

fabrication

Degradation

Mechanisms


7

Introduction

Introduction

Materials and

fabrication

Degradation

Mechanisms

Steam Generator

Program

KNOWING STEAM GENERATOR Description of different types of Steam Generator design


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Introduction

Steam Generator (SG) definition:

Large heat exchanger (shell-and-tube heat exchanger each with several thousands of tubes) that uses the heat from the primary reactor coolant to make steam in the secondary side to drive turbine generators. A typical plant has two to six SGs per reactor, although some units

have up to twelve SGs depending on the design.

SG functions:

Cold source of the primary system.

Confining radioactivity from neutron activation or fission products to the primary coolant during normal operation.

Producing suitable characteristic steam for the turbine.


PWR RECIRCULATING SG (RSG) (1/2):

Westinghouse (USA), Combustion Engineering (CE) (USA),

AREVA, former Framatome (France),

AREVA, former Siemens-Kraftwerke Union (Germany), and

Mitsubishi Heavy Industries (Japan).

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Introduction


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Introduction

Primary system coolant flows through U-tubes with a tubesheet at the bottom of the SG and U-bends at the top of the tube bundle.

Primary coolant enters the SG at 315-330°C on the hot-leg side and leaves at about 288°C on the cold-leg side.

Some design includes pre-heater, which are separate sections in the SG near the cold leg outlet.

The secondary system water (feedwater) is fed through a feedwater nozzle, to a feedring, into the downcomer, where it mixes with recirculating water draining from the moisture separators.

This downcomer water flows to the bottom of the SG, across the top of the tubesheet, and then up through the tube bundle where steam is generated.

PWR RECIRCULATING SG (RSG) (2/2):


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Introduction

CANDU RECIRCULATING SG:

Built by Babcock&Wilcox (B&W) Canada Ltd. or Foster Wheeler (only one SG).

Very similar to the PWR RSG with some differences in size, materials, operating temperatures and tube support structure.

Primary coolant in a CANDU reactor is heavy water (D2O).

Small tube size.

290°C to 310°C primary inlet temperature.

The smaller size of the primary (lower) head and tubes increases the difficulty in performing maintenance activities such as tube inspection, plugging, removal, etc.


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Introduction

PWR ONCE-THROUGH SG:

US design of SG.

They use straight heat exchanger tubes with tubesheets at both the top and bottom of the SG.

Primary coolant is pumped through the tubes from top to bottom while the secondary coolant moves around the outside of the tubes from bottom to top in a counter-flow direction.

Secondary-system water enters a feed annulus above the ninth tube support plate level where it mixes with steam aspirated from the tube bundle area and is preheated to saturation. The saturated water flows down the annulus, across the lower tubesheet, and up into the tube bundle where it becomes steam.


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Introduction

WWER SG:

Russian design.

They consist of a pressure vessel, a horizontal

heat exchange tube bundle, two vertical primary collectors, a feedwater piping system, moisture separators and steam collector.

Primary coolant enters the SG through a vertical collector, travels through the horizontal U-shaped submerged tubing, and exits through a second vertical collector.

The tube ends penetrate the collector wall (which performs the same function as the tubesheet in a PWR RSG) and are expanded using either a hydraulic or explosive expansion process and then welded at the collector inside wall surface.

The vertical hot and cold primary coolant collectors penetrate the vessel near its mid-point.

Position of feedwater depends on the model (WWER-440 or WWER-1000).

The tube bundle is completely submerged in both designs.


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Materials and fabrication

Introduction

Materials and

fabrication

DESCRIBING MATERIALS AND FABRICATION Discussion of some of the more important steam generator

fabrication practices and materials of construction

Steam Generator

Program

Degradation

Mechanisms


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Materials and methods used to fabricate SG components significantly affect their susceptibility to corrosion, especially to stress corrosion cracking (SCC).

Degradation of the SG tubing is also influenced by other aspects of design and construction, such as the tube support design and the method of tube installation.



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Heat Exchager Tubes (1/5):

Tube fabrication generally starts with extrusion of a shell from an ingot and then several cold reduction steps (by drawing or pilgering).

Each reduction step is followed by mill-annealing. Important parameters: temperature and initial carbon content.

Object: dissolve all the carbides and obtain a relatively large grain size and then cover the grain boundaries with carbides upon slow cooling in air.

Subsequent to the final mill-anneal, the tubing is passed through roll straighteners to produce a straight product. The straightening process plastically deforms the tubing, imparting

some residual stresses.

After straightening, the tubing may be abrasively polished (removing surface imperfections).


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Final steps for straight tubes involve visual, ultrasonic, and eddy-current inspections as well as various cleaning operations.

For RSGs, the straight tubes are bent to the desired U-tube configuration. For tight radius bends, internal

mandrels are often used to minimize ovality of the bent portion of the tube.


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SG Design Material Practice

RSG

Westinghouse Alloy600

Alloy600TT Alloy690TT

- Mill-annealed (low Temp.) - Thermal Treatment: 15 hours/705ºC followed by stress relief of tight radius U-bends (2 hours/700ºC)

CE Alloy600

- Annealed at 980-1065ºC (high Temp.)

Siemens/KWU Alloy800M - Thermal Treatment

Framatome Alloy600

Alloy690TT - Similar to Westinghouse design

AREVA Alloy600

Alloy690TT - Similar to Westinghouse design with polished


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SG Design Material Practice

RSG Mitsubishi-

Heavy Industries

Alloy600 Alloy690TT

- Similar to Westinghouse design

CANDU Alloy600-

Monel 400 (high Ni/Cu alloy) Alloy 800M

- Similar to Westinghouse design but using high temperatura mill-annealing (1065-1095ºC) and heat treatments of all the SG (15 hours/595ºC)

WWER Type 08X18N10T - Use stainless steel


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Units with RSG in Operation: 1968-2012



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Tubes Installation in the Tubesheet (1/2):

Different techniques for each design.

RSG and CANDU SG tubes have been installed in a thick tubesheet and WWER SG tubes have been installed in somewhat thinner walled collectors by mechanical rolling, hydraulic expansion, or explosive expansion.

For early PWR plants, tubing was connected to the tubesheet by: Hard rolling the tube into the bottom of the tubesheet.

Explosive expansion process.

Full-depth tube expansion accomplished in the shop using hydraulic methods.

Siemens/KWU were fabricated with either a three or two step mechanical hard roll.



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Tubes Installation in the Tubesheet (2/2):

Recent procedure, used by most of the RSG and CANDU: Performing a hydraulic expansion over nearly the entire tubesheet

thickness followed by a one (near the top) or two step mechanical hard roll near the top and near the bottom.

The transition region is formed by the hydraulic expansion.

The WWER SG use two vertical cylindrical collectors. The tubes are embedded against the collector wall by explosion or

hydraulic expansion and welded at the collector inside surface using argon-arc welding.



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Tube Supports (1/3):


Type Schematic Materials SG model

Support plate

Ferritic or austenitic steel

RSG: Westinghouse,

CE, B&W

Egg-crate tube

support

Austenitic steel RSG: CE, AREVA,

Siemens/KWU

Staggered scallop bar

Austenitic steel CANDU


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Type Schematic Materials SG model

Antivibration bars (AVB)

Alloy600 (and are chrome

plated)

RSG: Westinghouse, CE (inlcudes

vertical, horizontal and

bat wing strips), B&W,

AREVA, Siemens/KWU

and Framatome


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Degradation Mechanims

Introduction

Degradation

Mechanisms

MAIN DEGRADATION MECHANISMS IN PWR STEAM GENERATOR TUBES

Knowing the world experience

Materials and

fabrication

Steam Generator

Program


27

SUMMARY OF DEGRADATION MECHANISMS FREQUENTLY IN RSG AND CANDU SG TUBES:

Degradation Mechanisms

DEGRADATION MECHANISM

POTENTIAL FAILURE MODE SG MODEL

Outside Diameter Stress Corrosion

Cracking (ODSCC)

Axial or circumferential craks at some locations (tube-to-tubesheet crevices, sludge pile, free span)

Loss of material

RSG, CANDU

Primary Water Stress Corrosion Cracking

(PWSCC)

Axial or circumferential craks at some locations (Inside sufrace U-bend, roll transition, dented tube

regions)

RSG

Fretting, wear (loose parts)

Local wear Depending on loose part geometry

RSG, CANDU

Denting Flow blockage in tube, may lead to cracks, decreases fatigue resistence

RSG


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PWR RSG and CANDU SG (1/7).


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PWSCC: Type of damage: cracks.

Conditions: susceptible tubing microstructure (alloy content or few intergranular carbides), high applied or residual tensile stress (near the yield strength), and a corrosive environment (high temperature water).

PWSCC is a stress-dependent process such that the damage rate increases as the stress to the fourth power. It is also a thermally activated process (Arrhenius).

A small decrease in SG operating temperature will significantly slow the initiation and growth.

Locations: inside surfaces SG tubing with high residual stresses (U-bend) and any dent locations at the tube support plate, tubesheet, or sludge pile elevations.


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PWSCC: This degradation has caused problems in plants of Belgium, France,

Japan, the Republic of Korea, Spain, Sweden, Switzerland, USA.

There are no cases with Alloy 800M tubing.


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ODSCC: Type of damage: cracks or volumetric degradation.

Includes two types: intergranular stress corrosion cracking (IGSCC, that requires the same conditions than PWSCC) and intergranular attack (IGA).

IGSCC: occur along the grain boundaries (crack).

IGA: local, corrosive loss of material on the grain boundaries (volumetric).

ODSCC strongly depends on the concentration of corrosive impurities at dryout regions in the SG.

Locations: most of this degradation takes place in the tube to tubesheet and tube to tube support plate crevices.

Other areas may be affected such as sludge pile and free-span.

This degradation is very extended and has caused problems around the world and with lots of desings and material: Alloy600TT, Alloy800M,…


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Fretting and wear: Type of damage: volumetric degradation.

These degradation are broadly characterized as mechanically-induced or –aided degradation mechanisms.

Fretting: small amplitude, oscillatory motion, between continuously rubbing surfaces.

Wear: tube vibration of relatively large amplitude, resulting in intermittent sliding contact between tube and support.

The major stressor in fretting and wear is flow induced vibration.

A wide range of factors have to be taken into account in initiation and growth of this degradation: support locations, stiffness and design, secondary flow velocities and directions, gap size tube-support…

These degradations has been noted to some degree in all major PWR SG designs.

Sometimes problems have been solved by changing the supporting structures (for example, the AVB).


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Fretting due to loose parts/foreign objects: Type of damage: volumetric degradation.

These degradation depends on the type of loose part.

Loose parts and other debris have been found on the secondary side of the SGs at a large number of PWRs over the years.

These parts includes: tools, valve and pump parts, equipment used for previous inspections, broken SG material, debris left from previous modifications and repairs, etc.

Although most loose parts damage has occurred on the secondary side of the SG, there have also been cases of primary side damage, mainly to protruding tube ends and tube-to-tubesheet welds.

Prevention is essential to avoid the inclusion of loose parts within the SG, either primary or secondary.


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Denting: Type of damage: mechanical deformation or constriction of the tube

at a carbon steel tube support plate intersection caused by the buildup of deposits and the growth of a voluminous support-plate corrosion product in the annulus between the tube and support plate.

Denting does not involve cracks or volumetric degradation, but can lead to other degradation mechanisms such as PWSCC or IGSCC.

Denting is a concern because even small dents can induce tensile stresses above yield strength in the tube wall.

Although modifications and attention to secondary-side water chemistry have reduced denting to a lesser concern, denting is still considered a degradation concern, particularly if a plant:

has experienced one or more major secondary-side intrusions of contaminants, or

is constructed with low-temperature mill-annealed tubing and is, therefore, susceptible to PWSCC even at small-size dents.


Consequences of degradation:

Loss of efficiency: plugs.

Augmented scope or expansions of initial samples.

Potential loss of reactor coolant pressure boundary.

Potential accident of SG tube rupture.

SG replacement:

Usually due to excessive number of plugged tubes (or with sleeves).

Economic analysis vs. thermal efficiency analysis.

Incorporated design enhancements to address degradation.

World experience.

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Incidents:

Ginna (1982) (NRC GLS 1982-07, 08, 11): Steam side of SG filled with water. Water entered the steam lines.

Water was discharged trough steam relief valves into atmosphere.

Incidents with leakage from SCC: Arkansas Nuclear 2 (1996), McGuire Unit 2 (1997), Farley Unit 1

(1998) and Comanche Peak Unit 1 (2002).

Incidents with leakage with fatigue cracks: North Anna Unit 1 (1987), Oconne Unit 2 (1994) and Unit 3 (1994)-

San Onofre: Mitsubishi replacement RSGs

Unit 2 wear: loose parts, tube supports and retainer bar, tube-to-tube wear. This Unit operated full cycle and maintained tube integrity.

Unit 3 wear: tube supports and retainer bar, tube-to-tube wear. This Unit shut down half way through cycle due to primary-to-secondary leakage.

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Steam Generator Program

Introduction

HIGHLIGHTS OF A STEAM GENERATOR PROGRAM Degradation Assessment and Integrity Evaluation

Steam Generator

Program

Materials and

fabrication

Degradation

Mechanisms


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What is a SG Program? Objective: preventing SG tube rupture

accident or exceed the licensing basis of other limiting accident.

Evidence of commitment to the nuclear industry for the reliable and safety operation of SG.

Technical support for ensuring the integrity of the SGs, especially tubes against degradations. It includes:

Limits for operating chemical conditions.

Performance Criteria. Primary to Secondary leaks monitoring. Calculations for allowable degradations. Quantifying uncertainties in calculations. Reinspection frequency calculations. Maintenance Recommendations.

Establishes a set of preventive measures, inspection, evaluation, repair, and leakage monitoring.


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Origin: NRC concedes initiative to industry (NEI):

NEI 97-06: Framework that has been developed. Definition of Performance Criteria. Description of content and basic elements.

TSTF 449 Rev.4 ó TSTF 510 Rev.2 Provide information to include SG Program in Technical Specifications.

Other issues.

GL 2006-01: It shows the concern of NRC by SG tube integrity requirements contained in the existing Technical Specifications at the moment of issue, 2006. It requires plants to provide a description of their SG Program to ensure the integrity of the tubes in the interval between inspections, or adopt some alternative requirements in the Technical Specifications to ensure the integrity of the SG tubes.


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Application: Each part of the SG Program has an

associated EPRI Guidelines. These guidelines explain how each section has to develop the content of the program.

Highlights Guidelines: #1013706 Examination Guidelines. #1019038 Integrity Assessment Guidelines. #1025132 In-Situ Pressure Testing (ISPT) Guidelines.

#1016555 Secondary Water Chemistry Guidelines.

And other EPRI documents: Specific Management Flaw Handbook.

Great interplay between “engineering” and “field” work.


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Degradation Assessment (DA): DA report prior to each refueling outage:

Identifying existing and potential degradation mechanism and the type of defects that are generated: Outstanding Operational Experience (own/ outside).

Choosing suitable techniques for inspection and “sizing”.

Establishing the number of tubes (initial sample) which are to be inspected and its scope extensions.

Establishing structural limits to meet Performance Criteria with degradations and appropriate load conditions: Condition Monitoring (CM) limits and limits to contrast with Operational Assessment (OA).

Identify degradation mechanisms for the rest of the primary and secondary components:

Identify additional activities in the primary or secondary, supports, loose

parts, plugs, etc.



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Tube Inspection (usually eddy current test): It must be performed in accordance to the provisions of DA.

Sampling as supported by the DA.

Obtaining the information necessary to develop degradation, CM and OA.

Qualifying the inspection program by determining the accuracy and defining the elements for enhancing NDE system performance, including technique, analysis, field analysis feedback, human performance and process controls.


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Integrity Assessment: CM y OA. Condition Monitoring:

A backward-looking assessment which confirms that adequate SG tube integrity has been maintained during the previous inspection interval.

Screening: define which verification level should be used depending on the severity of the indications to assess Performance Criteria.

If CM analysis results are not acceptable, should be performed an In Situ Pressure Test (UNUSUAL):

- Too high uncertainty.

- Overcome direct criteria of indication amplitude (Volts).

- For detecting primary to secondary leakage.

- For not having qualified inspection technique for some of the degradation mechanisms found in the inspection.


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Integrity Assessment: CM y OA. Condition Monitoring:

IF CM IS NOT PASSED, THEN IT IS NECESSARY TO PREPARE SOME CORRECTIONS FOR THE DEGRADATION MECHANISMS AFFECTED IN THE SG PROGRAM.

Suitable technique?

Reinspection frecuency.

Ignorance of the real growth rate.

Corrections and reporting to Regulator.


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Integrity Assessment: CM y OA. Operational Assessment:

A forward-looking assessment which demonstrates that the tube integrity performance criteria will be met throughout the next inspection interval.

Applicable to 100% of tubes with indications.

Similar calculating process than those used to integrity limits, taking into account the growth.

Document the evaluation and calculations with all available information.

Establish the most degradated tube maintained on service (detected or not).

- Confirm preventive plugging limit for reinspection period desired.


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CONTINUOUS LEARNING PROCESS.

DA CM OA

Confirm limits for Performance Criteria

compliance

Confirms projetion was right for

inspection interval

Set projections

Set limits

Previous Future Refueling Outage



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Other contents: Tube plugging and repairs (DA):

The qualification of the plugging and repair techniques considers the specific SG conditions and mockup testing

Maintenance of secondary side integrity (DA): Identify secondary-side SG components that are susceptible to degradation are monitored if their failure could prevent the SG from fulfilling its intended safety-related function. Chemical cleaning and sludge lancing. Remote visual inspections of secondary site.

Plug and channel head inspections (DA).

Primary-to-secondary leak monitoring.

Primary and Secondary water chemistry.

Foreign material exclusion: Procedures to prevent the inclusion of loose parts in the primary or secondary when the SG is open.

Contractor oversight.

Reporting.

Self assessment.



THANKS FOR YOUR ATTENTION

Ref.Cod.: xxxxx 48

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