1

In-service inspection of reinforced concrete cooling towers – EDF’s

feedback

Alexis COURTOIS

EDF DTG, Lyon, France (alexis.courtois@edf.fr)

Yves GENEST

EDF DTG, Lyon, France (yves.genest@edf.fr)

Abstract : natural draft cooling towers provide the heat sink in 9 power stations of the EDF nuclear

fleet in France. An in-service inspection program is implemented for each tower, including visual

examination, topographic measurements and surface mapping of the shell. Monitoring outputs feed

data bases which are used to compare, rank and build a maintenance plan over its buildings fleet.

This paper focuses on monitoring techniques used by EDF and on the subsequent data analysis.

Some outcomes from recent development program are also presented.

Key-words : cooling tower, visual inspection, monitoring, ageing, maintenance.

1. Introduction

Cooling tower structural failures have recently become a focus area for the nuclear industry based

on events that have resulted in lost generation as well as high repair costs and personal injury (Boles

2011). Besides, cooling tower structures similar to those used in nuclear plants have collapsed at

fossil plants. Environmental concerns have also recently increased the need for guidance concerning

cooling tower inspection and maintenance.

Although no natural draft towers have collapsed at any nuclear power plants, such towers have

suffered collapse throughout the world. Notable collapses are as follows:

− In 1965, three out of a group of eight reinforced concrete natural-draft cooling towers were

blown down in strong winds at Ferrybridge Power Station in the United Kingdom.

− In 1973, a single 137 meters tall natural-draft cooling tower collapsed under moderate winds at

Adeer Nylon Works power plant just off the southwestern coast of Scotland.

− In 1979, one natural-draft tower at Bouchain in France collapsed under minimal winds. This

tower was known to have had serious dimensional errors from the beginning, and it is now

suspected that the collapse could have been caused by progressive deterioration.

− In 1984, one natural draft tower under construction at the Fiddlers Ferry Power Station collapsed

under high wind gusts, in the United Kingdom.

In France, EDF operates a fleet of 28 reinforced concrete towers have been built from 1977 to the

middle of the 1990’s on 9 of its nuclear power plants (NPP). These structures are not safety related,

but considering the investment and their role in the plant operation, EDF aims at anticipating the

effects of the ageing phenomena. Since the beginning of operation life, structural survey comprising

in-service inspection and monitoring have been implemented to feed analysis supporting

maintenance management processes.

The attempt of the paper is to propose an overview of EDF’s practices and feedback concerning in-

service inspection and monitoring of natural draft cooling towers shells. Surveillance techniques,

early diagnosis and on-going developments are addressed to yield to further needs as well for

2

monitoring systems as for data processing in order to improve structural assessment necessary for a

cost effective maintenance plan.

2. Design basis and ageing of reinforced concrete cooling towers

Contrary to popular belief, the shape of a hyperbolic natural draft cooling tower has nothing to do

with the airflow through it. The airflow through the tower is generated by the difference in density

of the warm, humid air inside the tower, versus the relatively cool, dense ambient air outside the

tower. Originally, natural draft cooling towers were cylindrical in shape. As the design of these

types of towers evolved, and the towers were made increasingly larger, the cylindrical shape was

changed to hyperbolic, which offers superior structural resistance to ambient wind loadings with an

optimized volume of concrete.

2.1. Design basis

EDF’s standards for reinforced concrete cooling towers require a minimum duration of 30 years for

operations (EDF 1991). The lessons learnt from collapses or heavily damaged towers in the years

1960-1980 are taken into account in these technical specifications.

Reinforced concrete cooling towers may be subjected to a variety of loading conditions. Most

commonly, these are dead load, wind load, earthquake load, temperature variations, construction

loads, and settlement.

The height of current cooling towers can reach 180 m with a diameter at the base of about 140 m.

The thickness at the throat level is about 20 cm, this is why it has often been claimed that this kind

of buildings is thinner than an egg shape. The tower is supported by a large number of columns

(diagonals, straight vertical or X-shape) linked together by annular footing which transmits loads to

the ground (Figure 1).

Figure 1. Natural draft cooling tower shell (Krätzig et al. 2000).

2.2. Ageing phenomena

It is well known that concrete structures are not built for ever. In normal exposure conditions, they

deteriorate over time. Cooling towers environment can speed up expected ageing phenomena or

entail serious damage that would not occur otherwise. Overviews of the main degradation

mechanisms of concrete cooling towers are periodically provided (Guimaraes 2010). The most

3

frequently addressed phenomena are thermal and moisture through-wall gradients, freeze-thaw, ice

formation, rebar corrosion (due to chloride ingress or carbonation) and alkali-silica reaction.

As a matter of fact, all of these degradation mechanisms are more or less linked with cracking and

rebar corrosion. Rebar corrosion has been identified by EDF’s researchers and engineers as the

main threat for cooling towers integrity. Decrease of the overall bearing capacity is not the unique

concern for the operator. Extensive cracking or spalling can also result in concrete blocks falling,

damaging surrounding buildings or equipment (for instance, distribution basins or canopy beams),

or being dangerous for people working nearby.

3. Overall maintenance strategy

As other nuclear power plant operators, EDF undertakes a preventive and long-term maintenance program for all of the civil engineering structures relevant for safety and for power generation.

3.1. Routine maintenance

The objective of the routine maintenance is to preclude any major expansive, disrupting and time consuming repairs. Standard guidelines and maintenance rules are periodically applied for all the cooling towers. Relying on collected data, a risk analysis is undertaken to determine if repairs are necessary to plan and which would be the most appropriate way to prevent any further degradation or to retrofit the structure.

3.2. Long-term operation

If the owner aims at extending the service life of the cooling towers, more extensive investigations and thorough analyses can be performed to determine the remaining operating life and the current safety margins. Nevertheless, there is currently no generic accepted method for cooling towers service life assessment. Then, opinion of in-house experts and external consultants can help managers and investors to determine whether a new tower or major reparation will be required in a period of 5 to 10 years.

3.3. Guidelines

EDF ageing management is mainly based on IAEA Saftey Guide NS-G-2.12 (IAEA 2009). This

standard is not specific to civil structures but it provides a general methodology to address ageing

issues. Monitoring is clearly identified in the overall organization for implementing efficient ageing

management.

According to the same principles, EPRI and EDF have issued an “end user’s flowchart” for life time

management (Le-Pape et al. 2009), to help NPP operators (considered as “end users”) in building an

appropriate strategy for the civil structure ageing assessment. Such an assessment requires to use a

cross-disciplinary analysis that takes advantage of knowledge of materials science, non destructive

testing, visual inspection and, where appropriate, computational mechanics (Figure 2).

4

Figure 2. Example of ageing assessment strategy (Le-Pape et al. 2009).

4. Surveillance system for cooling towers and diagnosis procedure

As a result of the cooling towers collapses that occurred within the years 1960 and 1980, and

although these structures are not safety related, EDF decided to inspect all the remaining shells and

to carry out measurements of the real shape of their outer surface by geodetic and photogrammetric

techniques. Subsequent inspection measurements were carried out in the aim of mapping all defects

and deformations and to record and follow up the adverse ageing effects.

EDF follows a detailed procedure for theses inspections, which consists in:

− measuring the differential settlement and potential tilting of the structure using traditional

topography equipment;

− performing detailed visual inspections of the concrete surface to map the shape and size of the

cracks;

− mapping the real shape of the shell by planimetry at different levels and by photogrammetric

measurements or by using a 3D laser scan survey.

Moreover, embedded sensors, such as vibrating wire strain gages or temperature probes, have been

set within some shells walls, providing additional information on the structural behavior.

4.1. Topographical survey

Vertical displacements of the bearing columns are measured by topographical leveling since the

beginning of the erection of the towers. A network of markers fixed on the circular ring footing

enabled a periodic monitoring with leveling instrument.

EDF decided to carry on with this survey also in operation, to check if observed settlements do not

exceed design hypothesis. Simple plotting over time is useful to detect any local or global

unexpected or abnormal trend in settlement. The frequency of these surveys is adapted to

foreseeable and observed movements, between 1 and 4 years. Uncertainty measurement is about 1

millimeter. Generally, observed settlements are almost stabilized within a few years of operation.

4.2. Visual inspections

A visual inspection is a throughout examination of the concrete to identify the various conditions and the possible outward signs of ageing that may be encountered during the service life of a building. It is quite simple to undertake but yet limited to the areas of the concrete structure that are visually accessible. Requirements for in-service inspection of nuclear civil structures entail visual inspection of visible concrete parts (Naus 2009).

5

To undertake reliable visual inspections for ageing management, a convenient way consists in

searching for categorized patterns, distresses and imperfections, with known features to record. For

example, IAEA and the ACI proposed some guidance to support such a survey (IAEA 2002). EDF

has issued its own specifications, with standardized checklists, outputs formats and reports. For

example, a crack is documented with its width, length, direction and pattern. Distresses and defects

linked with corrosion marks are carefully mapped. As long as a defect is recorded, it is stored in a

database with its characteristics and a minimum of one picture of it.

For simple building, a visual inspection can be undertaken with rather simple devices, but for

structure like cooling towers, surveyors need remote systems. The surface of the shell is scanned by

a long focal length telescope coupled to a video camera driven by an accurate geodesic system, to

acquire reliable data on defects positions and features. An image storage system is used to record

the results. These devices are able to estimate cracks length and opening with a 0.1 millimeter

resolution.

The frequency of visual inspection of cooling towers ranges between 3 and 6 years. For each

campaign, a final report provides an overview of the observed ageing patterns. For example,

number of cracks, corrosion marks or spalling are computed, statistical analysis are performed for

different areas of the shells and comparisons with previous inspections are made to assess ageing

kinetics.

4.3. Shell shape monitoring

If visual inspection and foundation displacement monitoring seem to be included in most of the

routine inspection guidelines, shell shape monitoring is required in some countries which have

experienced failures due to huge geometrical imperfections, as reported in Bamu & Zingoni (2005).

EDF’s practice comprises such measurement and analysis, some towers in France having been

demolished in the past for too excessive distortion, so that they posed a high risk of collapse (fossil

power stations of Pont-sur-Sambre and Ansereuilles).

Up to a recent time, photogrammetry was the main technique used to control the real shape of the

outer surface of the cooling towers and to record geometrical imperfections. Photogrammetry is a

practice of determining 3D coordinates of points on an object, from photographic images. These are

determined by measurements made in two or more photographic images taken from different

positions, by calibrated cameras. The geometric reconstruction is derived from a triangulation

processing of several common points identified on each image.

The photogrammetric survey performed for a draft cooling tower on EDF’s nuclear site involves a

network of sixteen camera stands surrounding the structure situated at a distance between 50 and

100 meters of the footing. Sophisticated algorithm and software enable the integration of other data,

such as theoretical geometry of the objects. The deviation between the theoretical and the measured

shell shapes can be analyzed and followed up over time.

Nowadays, photogrammetry has been replaced by 3D scanning survey. EDF has chosen laser

devices, which collect positioning data as point clouds, using reference markers but also natural

features on the structures. These techniques enable to acquire an almost unlimited number of points,

which entail a more accurate description of the cooling towers. Moreover, the acquisition and

processing are less time consuming than photogrammetric survey (see an example of processing

Figure 3).

6

Figure 3. Mapping of difference between theoretical and measured shape with a horizontal sectional drawing

The frequency of shell shape controls ranges between 4 and 6 years. Uncertainty measurement is

evaluated between 10 and 30 millimeters at the top of the structure, taking into account the

performance of the devices themselves but also the layout of the benchmarks and the distance from

the stations to the structure.

4.4. Annual synthesis for maintenance management

A synthesis report is issued every year for the entire cooling towers fleet. It gathers all the main

outputs of previous measurements and inspections which are presented in an individual sheet for

each tower. The document is concluded by a ranking of all the towers. This ranking is based on

experts’ judgments to determine if some towers need further investigations (for example, local non

destructive test for corrosion detection, core sampling for laboratory analysis) or if repairs are to be

planned within the next years.

5. Needs for new surveillance systems

The current surveillance system for EDF’s cooling towers shells is considered as reliable and

robust, but only for the purpose for which it has been designed. Nowadays, the extensive use of

finite element modeling to analysis the adverse ageing effects on the structural behavior entails new

needs in the monitoring field. For example, engineers would like to know how a tower shape is

modified by the effect of sunshine in a day or by a plant start in cold winter time. They search for

more information on the temperature and moisture gradients through the concrete wall of the shell

too. Moreover, monitoring can be used to optimize innovative repair processes (better knowledge

of the state of the surface, identification of the better time to apply a coating…).

5.1. Embedded sensors

Embedded temperature or strain sensors appear as a convenient way to monitor the structure for

such aims. One of the EDF’s fleet towers has been equipped by these kind of sensors embedded in

their concrete wall (temperature, humidity probes or vibrating wire strain gages). Theses sensors

can help to better understand concrete behavior during erection and in operating conditions. This

kind of devices is accurate and robust. In view of long term operation, EDF intend to require such

instrumentation for future towers, to ensure a better anticipation of possible delayed adverse ageing

phenomenon.

7

5.2. Monitoring to optimize repair conditions

Recently, EDF has tested a wireless system to monitor temperature air humidity at the top of a

tower (Figure 4). The sensors were installed at the four cardinal points and connected to a data

acquisition system. The system communicates through GSM transmission with a database, which

can be queried via the internet (Figure 5).

Figure 4. Example of wireless instrumentation

Figure 5. Example of temperatures graph

6. Conclusions

Reinforced concrete draft cooling towers are huge and slender structures, which are essential for power generation. A structural health monitoring is carried out by EDF, to routinely detect any abnormal trend in the overall behavior and to anticipate repairs with long-term operation in prospect. This approach involves dedicated systems but also processing techniques and expert’s judgment to rank the different towers of the fleet. However, further developments in monitoring are necessary to improve the management of maintenance for these structures.

In the field of instrumentation, new sensors could help to better understand the phenomena involve

in the aging. If engineers decide to handle with numerical modeling to reach this goal, it is obvious

that EDF’s surveillance system could not provide all of the expected measurements and that

8

additional specific survey could be undertaken to feed structural analysis. Due to the size of cooling

towers, wireless sensors networks would offer a significant gain for the surveillance system

implementation. Besides, fiber optics is an attractive tool to provide distributed strain and

temperature measurement over a cooling tower shell. Furthermore, for new built structures,

embedded sensors appear as suitable tools to better understand concrete behavior or detect

corrosion.

Concerning data post-processing, progress could be achieved by an extended use of symbolic data

analysis which allows summarizing huge and heterogeneous database information in manageable

datasets. This tool enables quick routine analysis to consolidate expert’s judgment and to determine

the priority for maintenance operations.

7. Acknowledgments

EDF would like to thank SITES S.A.S. for their long-term and valuable contribution in the area of

remote visual inspections.

8. References

Bamu P.C. & Zingoni A. 2005. Damage, deterioration and the long-term structural performance of

cooling-tower shells: A survey of developments over the past 50 years. Engineering Structures 27

(2005): 1794-1800.

Boles G. 2011, Nuclear Maintenance Applications Center: Guideline for Cooling Tower Inspection

and Maintenance. Technical Report 2011.1021060. Palo Alto : EPRI.

EDF 1991. Ouvrages en béton des réfrigerants atmosphériques humides à contre courants et tirage

naturel. Clauses Générales. Technical Specifications 56.C.005.00. Paris: EDF.

Guimaraes M. 2010. Program on Technology Innovation: Assessment of needs for concrete

research in the Energy Industry. Technical Report 2010.1022373. Palo Alto: EPRI

IAEA 2002. Guidebook on non-destructive testing of concrete structures, Training Courses n° 17,

Vienna: IAEA.

IAEA 2009. Ageing Management for Nuclear Power Plants. Safety Standards Series n° NS-G-2.12,

Vienna: IAEA.

Krätzig W.B., Petryna Y.S. & Stangenberg F. 2000. Measures of structural damage for global

failure analysis. International Journal of Solids and Structures 37: 7393-7407

Le-Pape Y., Wall J. & Salin J. 2011, A general stratgey to provide an effective owner-orientated

toolbox to support long term operation of civil infrastructure. EDF report H-T25-2009-00877-EN.

Moret-sur-Loing: EDF

Naus J.-L. 2009, Inspection of nuclear power plant structures – overview of methods and related

applications. Report ORNL/TM-2007/191. Oak Ridge, Tenesse, USA: Oak Ridge National

Laboratory.