Approach to Long Life
Operation and High Capacity
Factor of Nuclear PowerPlants
JINICHI MIYAGUCHI*1

KAZUYA SEKI*1

The problem of the global environmental load in recent years has increased the advantages of nuclear power plants, and theimportance of safe and effective operation of existing nuclear power plants over a long time period. Of the 23 PWR plants currently
in operation in Japan, seven have been operated for more than 30 years. Accordingly, it has become extremely important to takemeasures against aged structures and components in order to ensure the reliability of these plants. Further, in order to improve theoperation availability of existing plants, various measures have so far been taken such as reinforcement of preventive maintenance,and shortening the outage duration. For further effective use of the plants, however, the improvement of fuel combustion efﬁciency bydeveloping high burnup fuel and the improvement of capacity through extended cycle operations have become important.

with an operation period exceeding 30 years, in which
a comprehensive and cyclopaedical evaluation of theintegrity of the plant based on up -to-date knowledge
is carried out and a future 10 -year maintenance plan isdesigned based on the results of these evaluations.
The basic concepts in designing the maintenance plan
are given in Fig. 1. First, an evaluation of the life timeof the components currently in use is made on the basisof their current condition and the assumed degradation

1. For higher reliability of nuclear power plants(1) Evaluation of integrity of aged plants
In order to ensure the sa fet y and reliabilit y of aplant, it is important to make an adequate diagnosisof the degradation of the structures and components
caused by aging and to carry out appropriate preventive
ma intena nce. From this point of v iew, a technicalevaluation of aging is conducted every 10 years for plants

Plant life assumed to be 60 years
Confirmation of integrity throughinspection and monitoring

Based on the condition anddegradation mechanism ofeach component machine

Component Life >60 years

Confirmation of component integrity(evaluation of remaining lifetime)

Component life up to 60 years

Rationalization according to theimportance of the machine
Securing component life throughpreventive maintenance

Detailed inspection based on
inspection results

Component life <60 years

Securing reliability by easing
degradation mitigation and/or replacement
Component renewal

Consideration paid also to upgrading
Techniques
supporting
maintenanceactivities

Integrity evaluation
technology

Inspection andmonitoring

Evaluation of cost performance
Replacement

Degradation mitigation and repairingFig. 1 Concept of countermeasures against aging

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)*1 Kobe Shipyard & Machinery Works

49mechanism. According to the results of evaluation, thestructures and components are broadly divided into thefollowing three categories.
(a) Components found to have a sufficiently long lifeof over 6 0 years have their reliabilit y conf irmed
through inspection or monitoring their condition while
continuing a basic and normal maintenance program.
(b) Components found to have a life time up to 60 years
are suitable for the dedicated preventive maintenanceprog ra m such as deg radat ion mit igat ion a nd/orrepair in addition to inspection and monitoring theircondition.
(c) Components found to have a life time of less than 60years are candidates for replacement in addition to thedegradation mitigation and/or repairs.
In taking measures against aging structures andcomponents, it is impor t a nt to choose appropr iatemeasures from the maintenance menu based on thecomponent condition, and for this it becomes necessary
to provide the basic maintenance techniques in a timely
manner such as evaluation and analysis, inspection
and monitoring, degradation mitigation and repair, andreplacement.
(2) Maintenance techniques contributing to the safety andreliability of the plant
These individual maintenance techniques are actuallyused in maintenance activities in nuclear power plants,
for example, high-speed and high-accuracy inspection
equipment for non-destructive testing of components
and pipes, and repair equipment that removes detected
defects and carries out welding using reliable material byautomatic remote-control, and are effectively utilized inpreventive maintenance to prevent accidents.
In order to ensure the high-level safety and reliability

Maintenance engineering• Understanding
the degradation phenomenon• Evaluation of life time
• Development ofa maintenance plan

Development andmanufacturing• Inspection and repairs• Components
for replacement

On-site field work
• Inspections• Degradation mitigation• Repairing• Replacement oflarge components

Fig. 2 Three techniques supporting high-grade maintenanceof nuclear power plants, it is necessary to achieve highquality preventive maintenance, which is not only foraging issues. To this end the high-level integration of
the three technical fields of maintenance engineeringdevelopment and manufacturing and on-site ﬁeld work is
extremely important, and Mitsubishi Heavy Industries,
Ltd. (MHI) has the advantage of comprehensive technicalknowledge.

2. Improvement of capacity through optimized
maintenance program
The nuclear power plants in Japan are currently shut
down every 13 months to undergo periodical inspections.

ActionImprovement

Plan

Development ofa maintenance plan

Check

Evaluation of maintenanceeffectivenessDo

Execution of inspection
and repair

Fig. 3 Maintenance PDCA

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)50

However, the capability of extended cycle operation, which
improves capacity by extending the shutdown intervals, hasbeen discussed in consideration of overseas experiences.
Of course, safe and reliable operation is fundamental
to extending the operation cycle, so that is necessary toevaluate the reliability of the structures and components
based on maintenance experience and the latest knowledge
and, if needed, to promote optimization by reviewing thespecifications and interval of individual activities. In thiscase, it is necessary from the safety point of view to carry out
maintenance based on the importance of the structures andcomponents. Further, in order to increase the effectivenessof maintenance activities, it is important to continuously
review them according to their results (performance), i.e.
to make effective use of the PDCA (plan, do, check, action)
cycle in everyday maintenance activities.
From these points of view, utilities are currently promoting
integrating their maintenance activities cyclopaedically andsystematically. And as a plant supplier MHI is providing
full support by employing its comprehensive technicalknowledge.

burnup level of more than 60 GWd/t.
One of the problems involving high burnup fuel is theincreased corrosion of the outside of the fuel cladding tube
caused by prolonged use of fuel in the core. The PWR fuelcladding tubes are conventionally made of Zircalloy-4. Since
the enhanced corrosion resistance of the cladding tube isindispensable for high burnup, new cladding with changed
alloying elements has been developed. MHI have developed
MDA (Mitsubishi developed alloy) cladding tubes for highburnup fuel. Step 2 (55 GWd/t) fuel adopts MDA cladding
as well as ZIRLO TM cladding developed by Westinghouse
Electric Corporation.
In order to comply with the higher burnup fuels, MHI has
developed M-MDATM (modiﬁed MDA) alloy by adjusting thealloying elements. M-MDATM exceeds MDA in corrosion
resist ance, while its other mechanical characteristics
remain equivalent to those of conventional alloys such as
MDA and Zircalloy-4. In order to conﬁrm the performanceof M-MDATM , an irradiation test was conducted in a PWR
plant in Spain, with the fuel rod burnup reaching a level of
70 GWd/t or more. As is clear from the maximum cladding
oxide thickness in Fig. 4, the thickness of corrosion of theM-MDA T M -SR cladding was found to be much less thanthat of MDA 1 (SR: stress-relief annealed, R X: recrystalizedannealed). Figure 5 shows fuel rod growth by irradiation,indicating higher dimensional stability than the conventional
material.1 MHI plans to continue development to comply
with higher burnup levels by confirming its performancethrough post irradiation examinations.
MHI is also engaged in a joint development with theJapanese PWR industry on J-AlloyTM, a binary alloy of Zr-Nbfor cladding tubes, which has excellent corrosion resistance,with irradiation tests being conducted in the PWR plant inSpain.
These claddings are planned to be adopted appropriately
according to the operating conditions such as the start ofoperations, the burnup level, and the linear power density.

3. Improvement of fuel efﬁciency

120
100
80
60

1.4

: MDA
: MHMDA-SR: MHMDA-RX: Zry-4 (*1)
: MDA (*1)
: ZIRLO (*1)(*1) The other program data

1.2
Fuel rod growth (%)

Maximum oxide thickness (µm)

Since the number of reloaded fuel assemblies is increased
when extended cycle operations are carried out under thesame burnup limit, i.e. under the same uranium enrichmentlevel, the fuel combustion efficiency deteriorates. Hence,efforts are being made to achieve higher discharge burnup
by increasing the uranium enrichment in order to improve
the fuel combustion efﬁciency. Higher burnup fuel generates
more energy and requires less replacement fuel assemblies
and therefore brings enhanced fuel combustion efﬁciency.
T he ma x imum bur nup of a f uel assembly for P W R
fuel has been improved from 39 GWd/t to 48 GWd/t, andcurrently fuel assemblies of 55 GWd/t are gradually being
introduced in Japanese PWR plants. To further improve fuelefficiency, MHI is striving to develop advanced fuel with a40200102030405060

1.00.80.60.40.20.0070: MHMDA-SR: MHMDA-RX: MDA: Zry-4 (*1)
: MDA (*1)
: ZIRLO (*1)(*1) The other program data

2468Average rod burnup (MWd/kgU)

Fast fluence (10 n/m ,E>1MeV)

Fig. 4 Cladding corrosion

Fig. 5 Fuel rod growth2510122

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)51

Reference

4. Conclusion

1 . S . Wa t a n ab e , et a l . , P ro c . 2 0 0 7 I nt e r n a t ion a l LW R F uel
Performance Meeting, San Francisco, October (2007)

Amidst mounting concerns over the global environmentalload, the safe and effective operation of existing nuclear
power plants has become an important challenge from theviewpoint of arresting global warming.
MHI is determined to maintain efforts to promote thereliabilit y of individual structures and components byusing state-of-the-art technology, and to ensure the safety
and reliability of existing nuclear power plants, and also toenhance their effectiveness, by utilizing comprehensive
maintenance technology.

Jinichi Miyaguchi

Kazuya Seki

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)52