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Plant Life Management and Maintenance Technologies for Nuclear Power Plants 84
Plant Life Management and Maintenance Technologiesfor Nuclear Power Plants
OVERVIEW: In Japan, nuclear power provides one-third of the electricpower used by the country and is consequently an important source of energy.To achieve efficient business operations in the electric power industry, thecost efficiency of nuclear power must therefore be improved. In addition, toensure a steady supply of energy at a time when plans for constructingnuclear power plants are being postponed, plants that are currently operatingmust be able to continue stable operations over the long term. For thesereasons, the Ministry of Economy, Trade and Industry, utility companies,scientists and engineers from academia and industry, and manufacturersare combining forces to develop technologies to deal with age-relateddegradation in nuclear power plants. Hitachi, Ltd. has been constructingnuclear power plants and maintaining operating plants for many years,and is applying the know-how accumulated through these experiences tothe development of new technologies to make currently operating plantscapable of operating over the long term in a stable and highly reliablemanner. In this report, we survey inspection technologies, preventivemaintenance, and repair/replacement technologies developed by Hitachi.
Tsukasa Ikegami
Takao Shimura
Masahiro Koike
INTRODUCTIONIN Japan, nuclear power has come to occupy animportant position in the supply of electric power—in fiscal year 1999, 35% of the electric power generatedin Japan came from nuclear power plants.
In addition to providing a stable supply of electricpower, nuclear power plants are called upon to reducethe cost of generating that power. In this regard,important issues in a maintenance plan for an operatingplant are (1) stable and continuous operation, (2) costreductions in operation and maintenance (O&M), and(3) improved facility reliability. In Japan, nuclearpower plants that reach 30 years of operation mustundergo a technical evaluation of age-relateddegradation (fitness and earthquake-resistanceevaluation of all plant facilities), and based on theresults obtained, establish a maintenance plan for theyears ahead. Such a maintenance plan must considercost efficiency and reliability as well as the particularcircumstances of the plant in question. After 30 yearsof operation, plant inspections are to be carried out asneeded according to this maintenance plan based onthe technical evaluation of age-related degradation.
A maintenance plan established after 30 years ofplant operation is then revised every 10 years. Needed
repairs and preventive maintenance are carried outsystematically according to this maintenance plan.
The following describes inspection technologies forage-related degradation and technical developmentsin repair and replacement work at Hitachi.
AGED PLANT ISSUES AND MANAGEMENTAmong the boiling water reactors (BWR) now
operating in Japan, 11 have been operating for 20 yearsor more as of November 2000. This number will almostdouble to 19 by 2010 (Fig. 1). To achieve a stablesupply of power and improve the cost efficiency ofgenerating electricity in the years to come,technologies that can make these aged plants operatefor an extended period of time are indispensable.
To improve the reliability and facility utilizationratio of nuclear power plants, Hitachi is engaged inthe development of new technologies relating to theprevention of equipment deterioration and themaintenance of equipment in general, and in thedigitization of inspection data through the introductionof information technology (IT). Hitachi is alsodeveloping base technologies required for plantmaintenance related, for example, to material integrity,water chemistry control, and occupational radiation
Hitachi Review Vol. 50 (2001), No. 3 85
exposure reduction, and technologies to deal withaging in relation to preventive maintenance andinspection, repairing and replacing, etc. (Fig. 2).
DEVELOPMENT OF INSPECTIONTECHNOLOGIES
Achieving 60-year operation in a nuclear powerplant requires that equipment used in such long-termoperation be evaluated and checked for fitness and
Fig. 3— Compact and Submersible ROV.Development of this compact and submersible ROV enablesvisual inspections to be performed in complicated/narrowstructures difficult for past inspections. The ROV mounts areattachable UT scanner.
Fig. 1— Number of Operating Yearsfor BWR Power Plants in Japan.Among BWR power plants now
operating in Japan, the number ofplants that have been operating for
20 years or more was 11 as ofNovember 2000. This number will
almost double to 19 by 2010.
Fig. 2— Aging Management Technologies.To improve reliability and cost efficiency of nuclear powerplants, Hitachi is developing base technologies related tomaterial integrity, water chemistry control, and occupationalradiation exposure reduction, and aging-managementtechnologies related to preventive maintenance, inspection,repairing and replacing, etc.
Operating years 2000/Nov. 2010/Jan.Over 21 years 11 1910-20 years 10 90-10 years 7 —
Tsuruga (#1)Fukushima Daiichi (#1)
Shimane (#2)Fukushima Daiichi (#2)
Hamaoka (#1)Fukushima Daiichi (#3)Fukushima Daiichi (#5)Fukushima Daiichi (#4)
Toukai (#2)Hamaoka (#2)
Fukushima Daiichi (#6)Fukushima Daini (#1)Fukushima Daini (#2)
Onagawa (#1)Fukushima Daini (#3)
Kashiwazaki (#1)Fukushima Daini (#4)
Hamaoka (#3)Shimane (#2)
Kashiwazaki (#5)Kashiwazaki (#2)
Shika (#1)Kashiwazaki (#3)
Hamaoka (#4)Kashiwazaki (#4)
Onagawa (#2)Kashiwazaki (#6)Kashiwazaki (#7)
Operating years
0 5 10 15 20 25 30 35
Plant information management
Material integrity
Water chemistry control
Occupational radiation exposure reduction
Preventive maintenance• Improved reliability
• Improved cost efficiency• Improved facility
utilization ratio
Inspection and diagnosis
Repair and replace
Aging management technologies
ROBOT
Scanner arm
Reattachable UT scanner UT scanner
controller
PC for data acquisition
ROBOT controller
Joy stick controller
UT: ultrasonic test
reliable operation. In this regard, an inspection anddiagnosis evaluation of an aged plant needstechnologies for (1) evaluating deterioration inmaterials, (2) improving the reliability of inspectionsand diagnoses, and (3) developing remote-controlequipment. In particular, inspections targeting theinternals of a nuclear reactor require tele-manipulationtechnology for accessing complicated/narrowstructures and technology for evaluating equipmentfitness.
Compact and Submersible ROVThere are complicated/narrow structures within a
nuclear reactor where visual inspections using a videocamera or other devices are difficult to perform. Hitachihas developed a compact and submersible remotelyoperated vehicle (ROV) (width: 200 mm; height: 200mm; length: 230 mm) that can perform visual
Plant Life Management and Maintenance Technologies for Nuclear Power Plants 86
inspections even in such hard-to-get-to places. ThisROV enables inspections to be performed incomplicated/narrow structures that are considereddifficult to access in conventional inspection work.
The configuration of this compact and submersibleROV and the layout of a test inspection of the lowersection of a reactor core using the onboard camera areshown in Fig. 3.
Flaw-Evaluation Technology Using UltrasonicHolography
To evaluate the fitness of reactor internals, the sizeof flaws must be measured with good accuracy. Hitachihas developed technology for displaying the size ofsuch flaws in three dimensions using ultrasonicholography1). This technology enables flaws to bedisplayed in a solid-like manner and for inspectionresults to be accurately evaluated.
The configuration of this ultrasonic holographysystem is shown in Fig. 4.
PREVENTIVE MAINTENANCE AND REPAIR/REPLACE TECHNOLOGIES
To maintain the reliability and cost efficiency ofan operating plant, appropriate repair and replacementwork must be carried out according to inspectionresults. We here present some examples of technicaldevelopments for repair and replacement of reactorinternals.
WJP TechnologyIn Japan, a technology has been developed called
water-jet peening (WJP) in which high-pressure wateris shot at a structure to ease residual stress on thesurface of a metallic material like stainless steel. Thistechnology has been applied to reactor internals ofBWR power plants.
In more detail, cavitation air bubbles are generated
when shooting high-pressure water in water, and theseair bubbles strike the surface of the material inquestion. Then, when the air bubbles collapse, animpact force of several thousand MPa is generatedgiving compressed residual stress to the metallicsurface and preventing stress corrosion cracking fromoccurring (Fig. 5).
A feature of this technology is that there is no needto worry about foreign materials entering the reactorsince the active fluid in this case is water, which alsomeans that WJP can be applied to complicated/narrowstructures within the reactor.
Technology for Cutting Reactor InternalsA structure inside a nuclear reactor may have to be
replaced as a countermeasure to stress corrosioncracking of stainless steel. This requires technologythat can cut into an activated structure inside the reactorremotely and under water. For this purpose, Hitachihas developed electric-discharged-machining (EDM)
Fig. 4— Ultrasonic HolographySystem.3D flaw imaging by ultrasonicholography enables non-destructiveinspection of flaw size andrecognition of flaw shape.
Probe
Display
Water butt
FD drive
Automatic scanning system
Signal processor
Scanner control unit
FD drive
Imaging unit
Mini-computer
Fig. 5— Principle of WJP.• High-pressure water shot into the water generates cavitation.• Collapse of cavitation produces high pressure on materialsurface.• Compressive stress appears on the surface because plasticdeformation is elastically constrained.
Cavitation
WJP nozzle Plastic deformation
Jet pressure: 60–70 MPaJet velocity: about 240 m/s
• Easy implementation• Applicable to complicated/
narrow structures• No foreign material gets
into the reactor
Pressure by cavitation collapse: about 1,000 MPa
Characteristics
Hitachi Review Vol. 50 (2001), No. 3 87
technology using a long-life electrode and has appliedit to shroud replacement work in Japanese reactorcores.
This technology adopts in particular a long-lifesilver-tungsten electrode for its disk-type electrode(Fig. 6). This reduces the number of electrodereplacements and minimizes the generation ofsecondary products like gaseous radioactive materialand radioactive particle material. As a result, operators’radiation exposure can be reduced and the operatingenvironment improved.
Chemical Decontamination TechnologyOccupational radiation exposure reduction must be
given consideration when inspecting, maintaining,repairing, or replacing facilities having a high doserate such as the reactor’s primary cooling system andwater clean up system. Chemical decontamination isone effective means of reducing occupational radiationexposure. Hitachi has developed a chemical-decontamination method called HOP (referring tohydrazine, oxalic acid, and potassium permanganate)having both low-corrosion properties and highdecontamination performance. Fig. 7 shows anexample of a dose-reduction effect when applying theHOP chemical-decontamination method to the primarycooling system. Decontamination operations areperformed over several days during which anextremely small amount of secondary radioactivewaste under 1 m3 will be generated. The HOP methodenables a significant exposure-reduction effect to beachieved.
RVR TechnologyReactor pressure vessel replacement (RVR)
technology aims to replace the reactor pressure vessel(RPV) and reactor internals simultaneously as onebody. This decreases the potential of stress corrosioncracking and enables the replacement work to becompleted in a short period of time. The RVR approachis receiving attention as a rationalized constructionmethod especially appropriate for a plant life of about60 years.
Making use of experiences gained in pastconstruction of nuclear power plants, RVR technologywill employ a large-size crawler crane to replace theRPV and reactor internals. This is expected to makeconstruction work safer and to shorten the work periodand to achieve an economically superior constructionmethod (Fig. 8).
Research on evaluating the feasibility of RVRtechnology to nuclear power plants in Japan is nowunderway in cooperation with utility companies.
Validation of Maintenance TechnologiesThrough Full-Scale Mock-up Tests
It is important that full-scale mock-up tests beperformed as part of the development of various typesof inspection and repair equipment and of constructionmethods needed for maintenance. The reliability ofsuch equipment and methods must also be checkedand operator training performed through tests of thiskind.
To this end, the BWR Maintenance TechnologyCenter was set up at a Hitachi site in 1994. The Center
Fig. 6— Electric Discharge Machining (EDM).EDM enables the generation of gaseous radioactive materialand radioactive particle material to be greatly reducedcompared to plasma cutting technology.
Fig. 7— Results of Applying the HOP Chemical-Decontamination Method to a BWR Nuclear Reactor.The HOP method achieved significant reduction in occupationalradiation exposure.
EDM cutting machine for shroud
EDM: electric discharge machining
Orbit rail
Shroud
EDM actuator
EDM cutting machine for shroud
Core plateCutting trolley
start
BWR PLR valve disk
BWR RPV bottom
BWR PLR pipe (1)
BWR PLR pipe (2)
7
6
5
4
3
2
1
0after
1st cycle
Dos
e ra
te (
mSv
/h)
after2nd cycle
after3rd cycle
aftercleaning
Plant Life Management and Maintenance Technologies for Nuclear Power Plants 88
has a test pit 11 m in diameter and 36 m in depth andcan simulate at full scale a reactor building from theoperation floor to the lower drywell floor (Fig. 9). Thecenter is already being used for validating technologiessuch as for replacing reactor internals in Japanesenuclear power plants, and is producing effective results.
CONCLUSIONSThis report has described technologies for
managing aging in nuclear power plants and hasdiscussed development trends and future directions.
Aging management for nuclear power plants hasjust begun. From here on, as plants continue to age,our goal is to achieve a level of plant maintenance thatexcels in cost efficiency while maintaining safety andreliability. To this end, Hitachi will work to developsystematic maintenance technologies and to enhancemaintenance services in close collaboration with utilitycompanies and other concerned organizations.
REFERENCES(1) F. Takahashi et al., “Sizing and Recognition of Cracks and
Porosities in Weld Metals Using Acoustical HolographicInspection,” 1st Int. Conf. on NDE in Relation to StructuralIntegrity for Nuclear and Pressurized Components, pp. 557-561 (Oct. 1998).
(2) T. Yoshimura et al., “Reduction of Residual Stress by Peeningwith Reflected Water Jet,” JSME Materials and MechanicsDivision Meeting, pp. 469-470 (Oct. 2000).
(3) H. Takashima et al., “Chemical Decontamination Experienceson Shika Unit-1,” 10th Int. Cong. of IRPA, P-6a-300 (May2000).
Fig. 8— Reactor Pressure Vessel Replacement Technology.This technology will use a large-size crawler crane having amaximum lifting load of 1,000-1,700 t to shorten thereplacement period.
Fig. 9— BWR Maintenance Technology Center.The center is provided with full-scale facilities of a reactorpressure vessel and in-core structures, enabling the developmentof preventive-maintenance technology and processes forinspection and modification of major BWR equipment.
The old RPV, which houses reactor internals, is carried out with a covering attached.
The new RPV housing reactor internals is carried in.
(1) Carrying the old RPV out (2) Carrying the new RPV in
Facility
Test pit
Test room
Lower pedestal
Water treatment
Auxiliaries
Main specifications
Diameter 11 m × Depth 36 m
W. 20 m × L. 35 m × H. 10 m
Diameter 6.4 m × Depth 10 m
Water conductivity 2–3 S/cm (reactor water)
Water inventory 710 t
Crane (2.8 t), working bridge, elevator
Total height46 m
ABOUT THE AUTHORS
Tsukasa IkegamiJoined Hitachi, Ltd. in 1974, and now works at thePreventive Maintenance Engineering Section ofNuclear Plant Service Engineering Department,Nuclear Systems Division, Power & IndustrialSystems. He is currently engaged in the preventivemaintenance engineering for nuclear power plants.Mr. Ikegami can be reached by e-mail [email protected].
Takao ShimuraJoined Hitachi, Ltd. in 1980, and now works at theReactor Internal Maintenance Engineering Section ofNuclear Plant Service Engineering Department,Nuclear Systems Division, Power & IndustrialSystems. He is currently engaged in the preventivemaintenance engineering for nuclear power plants.Mr. Shimura can be reached by e-mail [email protected].
Masahiro KoikeJoined Hitachi, Ltd. in 1982, and now works at theReactor Preventive Maintenance Group of Power &Industrial Systems R&D Laboratory, Power &Industrial Systems. He is currently engaged in theR&D of non-destructive inspections and diagnosistechnologies. Mr. Koike can be reached by e-mail [email protected].
