Remote visual inspection - an eye for steam generator maintenance

In response to steam generator wrapper support structure failures in foreign plants and degradation of tube support plates at US facilities, the US Nuclear Regulatory Commission (USNRC) issued Information Notice 96-09 in February 1996 followed by a supplement in July 1996. In December 1997, Generic Letter 97-06 was issued concerning the same issue. These documents emphasise the importance of developing a maintenance plan, part of which includes thorough visual inspections of steam generator secondary side internals to evaluate structural integrity.

In addition, the Nuclear Energy Institute (NEI) issued an industry consensus document (NEW-9706) addressing steam generator tube degradation. This document, focusing on steam generator tubing degradation and the severity of a primary to secondary side breach, encourages utilities to develop a plan to proactively monitor and maintain its steam generators.

Recent advances in steam generator secondary side visual inspection and cleaning techniques, provide the means to view and clean essentially all areas within a steam generator – from the top of tubesheet to the moisture separators, from the tubelane to the outer periphery annulus region.

The figure, “Steam Generator Areas of Inspection,” details the areas accessible with current technology, as well as depicts typical inspection results.

Visual inspections are essential in evaluating steam generator conditions and making informed decisions on the appropriate maintenance action.

History

Westinghouse Electric Company, a leader in outage services, developed the Steam Generator Secondary Side Maintenance Guidelines in response to degradation concerns, as discussed above, and including:

• Structural integrity issues (outlined in NRC letters) – wrapper support structure degradation.

• Tube degradation (described in an NEI document).

• Loose parts.

• Corrosive deposits.

• Sludge accumulation.

• Fouling.

• Wear.

These conditions can result in a reduction in main steam pressure, hydrodynamic instabilities, stress corrosion cracking, lower steam pressure, tube rupture, replacement of steam generators prior to expected life, plant shutdown etc.

The maintenance guidelines emphasise the benefit of visual inspection as a way to support and confirm the results of other inspection techniques as well as the effectiveness of cleaning processes that may be used. With the use of this document, utilities have the tool to formulate strategies to proactively manage the maintenance and monitoring of steam generator internals. The resulting strategies can also serve to prevent future damage and the associated large repair costs.

Visual inspection – key to a proactive maintenance plan

Experiences in the visual inspection of steam generator internals have provided the foundation for the steam generator maintenance guide, its recommendations, and the necessity of a maintenance programme. Remote visual inspection has proven to be essential for a successful maintenance programme as well as the identifier of degradation issues.

The Electric Power Research Institute (EPRI) stated early in 1992 that “the most important thing is to monitor the performance of steam generators to identify degradation processes that can harm them. Delaying a fix can result in greater maintenance cost and power output reduction. The opportunity to view sludge accumulation shows the nature, degree and rate of build-up, providing the necessary information to intervene proactively, helping minimise unnecessary and costly repairs.” Enacting a periodic maintenance schedule incorporating visual inspection into the care of steam generator internals, will provide a more cost effective and safer way to monitor and service steam generators. EPRI went on to recommend that “utilities do secondary-side visual inspections regularly, in as many elevations of the tube bundle as possible, to determine tube fouling distribution, scale growth rates at various locations, and scale exfoliation tendencies.” Using the Westinghouse Steam Generator Secondary Side Maintenance Guide will assist in developing a maintenance programme that can lead to cost savings in repair and maintenance, valuable knowledge about steam generator conditions and maximise steam generator performance.

To address the industry’s concerns on structural integrity and component degradation, R Brooks Associates, Inc, which is involved in remote visual inspections and technology development, has developed several new technologies to access the areas of concern. Some of these are described in the panel, “Visual techniques and equipment”. In many cases, such visual inspection techniques have proven that the steam generator condition is significantly different than predicted by other non-visual techniques, providing data for deciding which maintenance action, if any, is needed.

Visual Inspection as a Diagnostic Tool

Many US utilities have reported decreases in output steam pressure of their steam generators. The cause of this was confirmed through the use of visual inspection and model boiler testing. Samples of a U-bend tube were removed from an in-service steam generator. The sample, with tube deposits present, was placed in the model boiler and operated under normal conditions. Pressures, temperatures, and flows were recorded and then the tube sample was removed and cleaned. Once the sample was placed back in the model boiler, a 34% increase in heat transfer occurred. Visual inspection has confirmed similar deposits exist at many plants that report a steam pressure reduction.

One instance where visual inspection played a role in determining the cause of a problem was when a plant reported steam generator level oscillations requiring a 10% reduction in power. Deposits were blocking the flow holes increasing the pressure drop across the upper support plates causing the level oscillations.

Because of the severity and composition of the deposits, chemical cleaning – a restorative procedure – was performed. Visual inspection, as shown below, confirmed the cleaning process was successful.

With the inspection and cleaning technology available today, preventative rather than restorative measures recommended in the Westinghouse guidelines can be employed sooner, significantly reducing the total cost to the utility.

Developing A Maintenance Programme

Using this guide, utilities can develop a maintenance programme accounting for various steam generator designs, assessment methods and NRC regulations. The information included in the guide covers:

• Baseline Profiles.

• Assessments.

• Preventative and Restorative Options.

• Recommendations for Inspection and Cleaning.

The diagram summarises the process involved in developing a steam generator maintenance plan.

Most important to developing a maintenance plan is gathering all information possible about the targeted steam generators. This is called a baseline profile. The baseline profile is an overall assessment of the current state of the steam generator. It should be established prior to the steam generator’s initial service data. However, if the profile is created later in the generator’s life, information in addition to what is listed for inclusion in the profile may be necessary. This additional information could be historical data such as any visual inspections, cleaning, maintenance performed, and any operational information and events that have occurred. Baseline profiles should be updated at every outage to maintain an accurate history of the steam generator and provide current information for trending analysis.

The information to be included in the baseline encompasses:

• Deposit inventory.

• Foreign object search and retrieval (FOSAR) data for both the annulus and in bundle regions.

• Steam generator construction and materials (any important or unique design and operating characteristics).

• Operating history.

• Structural integrity assessment.

A steam generator maintenance plan should not be developed without first creating this baseline profile. The profile will contain all pertinent information necessary in setting up the maintenance plan. Once the baseline profile has been completed, the owner can determine whether or not conditions call for preventative or restorative processes.

The type of cleaning process used depends on the amount of deposit accumulation, composition of build-up, complexity of the location and cost. As summarised in Table 3, sludge lancing, pressure pulse cleaning, UBHC, and reduced concentration chemical cleaning are preventative measures, while targeted lancing and pressure pulse cleaning after scale conditioning agent (SCA) treatment and chemical cleaning are considered restorative.

Various lessons have been learned through past utilisation of these methods. These lessons have helped improve the methods themselves so that they can be administered to provide the most effective results possible. One of these lessons includes performing a visual inspection prior to cleaning to target areas and once the cleaning process has been applied, to verify the process effectiveness.

Recommendations

Visual inspections are recommended as a regular part of a proactive steam generator maintenance programme. Criteria for conducting such inspections are included in the guidelines and they address the NRC requirements for management of steam generator integrity. This criteria also provides information necessary to determine the steps to be taken in the management of the maintenance programme. Specific recommendations are made for visual inspections that provide information for inclusion in the baseline profile, as well as for each outage.

Table 1 summarises the inspection recommendations detailed in the Westinghouse Steam Generator Secondary Side Maintenance Guidelines July 1998.

Visual Inspection Techniques

An initial assessment and routine visual inspection of the condition and cleanliness of the secondary side steam generator components is a vital part of a maintenance programme. Several inspection options are available to provide detailed visual data at specific locations within the steam generator.

Tubesheet Region

The top of the tubesheet, tubelane and annulus region can be a collection point for foreign material. If not removed, these foreign objects can cause tube wear and potentially a primary to secondary leak.

Approach: Several techniques are available to perform a tubesheet region inspection. Manually operated fibre optic probes offer a basic and limited inspection capability for looking at the annulus region and tubelane area. The use of this technique requires an operator at the hand-hole to manipulate and control the probe and is the least ALARA efficient process with marginal picture quality.

Remote visual inspection can be used to access essentially all parts of the tube bundle at the top of the tubesheet. The In-Bundle Inspection System (IBIS) utilises a small diameter camera head attached to an extendible inspection wand and provides very good picture quality. The wand operates from the tubelane and is deployed between selected columns of tubes to locate foreign objects and determine the location of sludge accumulations and scale collars.

Remote inspection of the annulus can be performed to locate and subsequently retrieve foreign objects. The equipment available to perform this service is the Shell/Wrapper Annulus Transport System (SWATS).

SWATS is a remotely operated system that delivers high resolution CCD cameras in positions to inspect the top of the tubesheet in the annulus region and the upper and lower wrapper support structures and provides superior picture quality.

Upper Bundle Region

Visual inspections are required in the upper bundle region to determine their general condition of the tubes and tube support plates. Plant operating data has confirmed approximately 80% of corrosion product transport deposits in the tube support plate/upper tube bundle region of the steam generator. These deposits can lead to the concentration of potentially aggressive chemical species or form a ledge type structure blocking flow holes which increases pressure drop.

Approach: Currently there are three techniques available to perform a remote visual inspection of the upper bundle region. A Support Plate Inspection Device (SID) is installed through the hand-hole and uses a vertical delivery system to access the tubelane region through the flow slot openings. Penetration into the tube bundle is limited to the first two or three rows on both the hot and cold leg sides of the steam generator. In-bundle access can be obtained using the Upper Bundle In Bundle (UBIB) inspection system. UBIB is mounted and deployed in a similar manner to SID. However, with UBIB the inspection is performed using a high resolution CCD camera mounted on a telescoping wand that traverses between the tube columns while maintaining a constant elevation above or below the tube support plate. Additionally, position feedback on row, column and elevation is obtained. The egg crate lattice visual inspection system (ELVIS) is a remotely operated inspection tool that inspects the periphery region at all elevations on the secondary side of combustion engineering (CE) steam generators.

Cleaning

Cleaning recommendations have also been provided based on field experiences. Consideration has been given to severity of deposits as well as conditions such as materials used in the design of the generator and the best method to most effectively clean, given these factors. Recommendations are given under each category of cleaning: Preventative and Restorative, as defined earlier.

Tables 2 and 3 summarise these recommendations.

CONCLUSION

The monitoring and maintenance of steam generator internals are important tasks in the operation and care of steam generators. Employing the Westinghouse Steam Generator Secondary Side Maintenance Guideline as a way to develop a maintenance plan provides invaluable information on how to minimise repair costs, increase and maintain efficiency, and minimise unplanned shutdowns.

The use of remote visual inspection is the key factor in achieving these objectives and in establishing an effective maintenance plan