E-Journal of Advanced Maintenance Vol.2 (2010) 110-119
Japan Society of Maintenology

A Study for the Practical Methods and Evaluations for the On-LineMaintenanceNaoki CHIGUSA and Takahiro OGAMI*Kansai Electric Power Co., Inc. 13-8, Goichi, Mihama-cho, Mitaka-gun, Fukui-ken, 919-1141, JapanABSTRACT
For planning and enforcing the On Line Maintenance (OLM) in the near future, we should take care of keeping
a reasonable balance between the degree of risk increase and the task volume equalization. In this paper, thecurrent status of studying for the OLM introducing on Japanese utilities is described. The planning approachof OLM is examined on the assumption that planned OLM can be carried out. The practical methods andevaluations for the OLM are described, and the trial evaluation for this method is also described.
KEYWORDS

ARTICLE INFORMATIONOn Line Maintenance, Planning Approach, Risk Minimization, Task Volume
Equalization

Article history:
Received 25 December 2009
Accepted 18 November 2010

1. Introduction
Fig. 1 compares the ratio of maintenance tasks between U.S. and Japan’s Nuclear Power Plants(NPPs). As shown in this figure, approximately 20% of maintenance tasks are conducted at power
while 80% are during the outage, indicating concentrated maintenance tasks during the outage.
The much higher ratio of maintenance tasks during the outage at Japan’s NPPs than that in theU.S. is partially attributable to the governing laws and regulations. However, the main contributors to
the high ratio of maintenance tasks during the outage at Japan’s NPPs are the shorter outage intervals
specified by the regulations and a large number of time-based maintenance tasks, mainly consisting of
the overhaul inspection, conducted during outage regardless of the time dependency or contingency ofdegradation modes.

【U.S.】【Japan】 （KEPCO）
Routine maintenance at power(Walkdowns, surveillance tests)

Routine maintenance at power(Walkdowns, surveillance tests)

Maintenanceduring outage

8%
12%

29%

27%

Maintenance atpower during AOT

Maintenance at power
during AOT(maintenance inadvance of outage, etc.)

Maintenance during outage
80%

44%

Fig. 1. Ratio of Maintenance Tasks (at Present)*

Corresponding author, E-mail: oogami.takahiro@c3.kepco.co.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line MaintenanceOn the other hand, at the U.S. plants, condition-based maintenance is predominant. In otherwords, careful maintenance conducted at Japan’s NPPs has resulted in a world low record ofunplanned reactor shutdowns (Fig. 2).
Under such circumstances, the new inspection system was made effect in January 2009 requiring
the licensees to make an effort to improve the nuclear safety by conducting the evaluation ofmaintenance effectiveness through the review of their maintenance system so that maintenance tasks
will be performed in an appropriate timing by an appropriate method and to implement optimized
maintenance in accordance with plant-specific characteristics.
Fig.3 shows the number of workers engaging in the maintenance work at Japanese representativeplant (4 units plant). As can be seen in this figure, there is an imbalance in the number of workers
between at power and during outage. To address this situation, it is planned to achieve better resource
load leveling through the year by transferring part of the outage work to the maintenance tasks atpower (i.e., on-line maintenance: OLM) and by enhancing the plant condition monitoring at power.
These measures are expected to contribute to the assurance of quality engineers for the maintenancework and to reduce workers’ exposures, eventually leading to the improved reliability of maintenance.
As mentioned in the above, the licensees are increasingly required to introduce more OLM as apart of their maintenance program. However, the internal rules for the review of Tech. Spec. currentlyprovides a limit on the implementation of OLM.
Presently, Reactor Safety Subcommittee Operation Management Working Group is discussing
the plant safety assuming that a single system is out of service to implement maintenance tasks atpower (OLM).
Unplanned automatic scrams per 7000 hours critical for major counties
2008

Data source: IAEA/PRIS

Japan
U.S.
FranceRussia
GermanySouth Korea
Finland
Spain
Sweden

UA7=

Number of unplanned automatic scrams while critical x 7000
Total number of hours criticalFig. 2. Ratio of Unplanned Automatic scrams

Fig. 3. Past Performance of Task Volume111

N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line MaintenanceThis study is to establish the planning approach for OLM and to evaluate the effectiveness of theOLM planning approach by performing simulations to validate the planning processes in view of thefull-scale introduction of OLM at Japan’s NPPs for the components subject to limiting conditions foroperation (LCO) as well as for other components.

2. Planning approach for establishing OLM program
The OLM planning approaches vary according to its objectives. For example, the U.S. plantsgenerally launch preparations for the OLM program approximately 12 weeks in advance, and thenplan, implement and evaluate the maintenance tasks by changing the system configuration every week.
This work management process is described in INPO AP-928 document. [1]As mentioned above, the way of implementing OLM at Japan’s NPPs is different from that in theU.S. partially because of the difference in the governing laws and regulations. Therefore, it is difficult
to directly apply the lessons learned from the U.S.-Japanese NPP benchmarking study to Japan’s NPP.
Considering the circumstances, we have come up with an OLM planning approach applicable toJapan’s plants while referring to the U.S. plant experience.
2.1 Outline of our OLM planning approach
We have studied the concrete procedure in implementing OLM at a nuclear power plant.
The procedure of OLM implementation consists of the processes, “initial setting prior to theplanning process”, “planning OLM” and “validation of OLM plan”, as shown in Fig. 4. The details ofeach process will be discussed later.
Initial setting (1)Select the components subject
to maintenance at power.

(2)

Determine the scope of isolation.
(3)
(13)

Review of initial
setting

Consider the working hours to conduct
maintenance at power for the scope of isolation.

(4)

Perform safety assessment forthe scope of isolation.

(5)

Planning

Plan the work schedule.

(6)
Amendment(12)

Plan the maintenance details.

(7)

Make preparations for maintenance at power.

Amendment of plans(8)
Occurrence of unexpected trouble
Implementation

Evaluation

Yes

(9)

No
Implement maintenance at power.

(10)

Verify the validity of maintenance at power.

“Amendment of maintenance plans” (11)Study the necessity of “review of initial setting”
is needed.
and “amendment of maintenance plans.”“Review of initial setting” is needed.

Fig. 4. Procedure of OLM implementation

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N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line Maintenance2.2. Initial setting prior to the planning process
The initial settings of fundamental data, which should be organized in preparation for planning
OLM, are defined as follows:
1) Selection of components to which OLM is applicableFirst of all, it is necessary to select the component to which OLM is applicable.
Fig. 5 indicates the flow of selecting the components to which OLM is applicable considering thefollowing criteria:[Criteria for selecting the components to which OLM is applicable]-The component is located outside the containment vessel.
The isolation of the component will not lead to plant shutdown.
With the component being isolated, the rated output can be maintained.

Component Identification

Yes

Components subject to high radiation,high temperature/pressure environment(labor safety)No
Yes

Isolation of the component leads toplant shutdown.

No
Yes

Isolation of the component prevents
the rated output being maintained.

NoYes
The component is subject to LCO

OLM can be implemented evenunder the current situation.

OLM can be implemented after
amending Tech. Spec.

Difficult to introduce OLM

Fig. 5. Flow of Selecting the Components which OLM is applicable113

N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line Maintenance2) Identification of the isolation sphere and work load
In order to illustrate the screening results in Fig.5 in the Fig.6 system diagram, the isolation
sphere for the components subject to OLM is established according to the following procedure.
Then the components within the isolation sphere in Fig.6 are identified, and the maintenancetasks and individual task are determined.
[Method of setting the isolation sphere]-Consider the effects on the continued power generation, exposure doses, workers' safety andenvironment.
The boundary of the isolation sphere is set at the manual valve in principle.
Assume the implementation of functional tests of the components after OLM is implemented.
Isolation sphere of components
subject to LCO

OLM is not applicable.

Isolation sphere of components not
subject to LCO

Fig. 6. Example of Identification for the Isolation Sphere at OLM(Residual Heat Removal (RHR) System)3) Evaluation of risk levels when multiple components are made out of service
In planning OLM for the components subject to LCO, it is necessary to evaluate in advance therisk level when multiple components are made out of service simultaneously. In this regard, risk
matrices are prepared to show the incremental conditional core damage probabilities (ICCDP:Allowable outage time (AOT) is used.) according to the combinations of components.
Fig 7 shows an example of risk matrix which indicates the risk levels when two components arein AOT simultaneously. The risk levels are determined in comparison with the U.S. ICCDP
acceptance criteria of 5E-07.

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Backup battery charger

Tech. Spec. Article 82
Emergency DC power
AOT = 10 days
Battery charger B

Battery charger A

D/G-B startup air reservoir

D/G-A startup air reservoir

Tech. Spec. Article 81
Diesel generator fuel oil,rubricating oil and startup air
AOT = 48 hours
D/G-B fuel tank

D/G-A fuel tankTech. Spec. Article 79
D/G
AOT = 10 days
D/G-B

D/G-A

SWP-C

Tech. Spec. Article 74
SWP system
AOT = 10 days
SWP-B

SWP-A

CCW heat exchanger B

CCWP-D

Tech. Spec. Article 73
CCWP system
AOT = 10 days
CCWP-CCCW heat exchanger A

CCWP-B

CCWP-A

Tech. Spec. Article 72
Condensate tank
AOT = 7 days
Condensate water pit

TD-AFW pump

Tech. Spec. Article 71
MO-AFW pump system
AOT = 10 days
MO-AFW pump B

MO-AFW pump A

Tech. Spec. Article 64
LP C/V spray pump system
AOT = 10 days
C/V spray pump B

RHR pump B

C/V spray pump A

Tech. Spec. Article 52
ECCS
LP injection system
AOT = 10 days
RHR pump A

HP injection pump B

HP injection pump A

Tech. Spec. Article 52
ECCS
HP injection system
AOT = 10 days

N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line Maintenance

Maintenance of a single component
HP injection pump A
HP injection pump B
RHR pump A
RHR pump B
C/V spray pump A
C/V spray pump B
MO-AFW pump A
MO-AFW pump B
TD-AFW pump
Condensate water pit
CCWP-A
CCWP-B
CCW heat exchanger A
CCWP-C
CCWP-D
CCW heat exchanger B
SWP-A
SWP-B
SWP-C
D/G-A
ICCDP：5×10-7 or higher

D/G-B

Below ICCDP：5×10-7

D/G-A fuel tank

Below ICCDP：5×10-8

D/G-B fuel tank

The simultaneous shutdown of these components is subject to manual plant
shutdown according to the Tech. Spec.

D/G-A startup air reservoir
D/G-B startup air reservoir
Battery charger A
Battery charger B
Backup battery charger

Fig. 7. An Example of Risk Matrix

2.3. Procedure of OLM planning
In planning OLM, many relevant factors, including the reduction of exposure doses, risk
minimization, assurance of capable workers, avoidance of overlapped tasks and resource load leveling,should be considered. This study intends to establish an OLM planning approach which can assure thework load leveling while risks are minimized.
(Step 1) Definition of OLM program cycles- The OLM program cycle involves the operating cycle, OLM program cycles and OLM unit
periods as defined in Fig.8. The OLM unit period represents the term during which the samesystem configuration is maintained to perform each maintenance task.

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N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line Maintenance
Plant operating cycle
OLM program cycleOLM program cycle OLM program cycleOLM program cycle
OLM unit period

OLM unit period

Fig. 8. OLM Program Cycle(Step 2) Modeling of an OLM program cycle- Within the scope of combinations of two components allowed in the Fig.7 matrix, it is planned
to perform all the maintenance tasks for the components subject to LCO during a single OLM
program cycle. In the planning process, it is intended to reduce the maximum risk when twocomponents subjects to LCO are in AOT below the acceptable limit (Fig.9) [2].
- There are a number of combinations of components subject to LCO that are simultaneously
placed in AOT. Among these combinations, it is necessary to select ones that can minimizerisks.

OLM unit period○：Implementation

OLM program cycle
Period①
Components subject to LCO①A-SIP

・・・

Period⑫・・・○
Isolation sphere (major components)②B-SIP③A-RHRP・・・○・・・⑲A-EDG○⑳B-EDG
Risk

Period②

3.65E-08○・・・・・・・・・・・・○・・・
Risk (ICCDP)
・・・

5.65E-08

・・・・・・

1.49E-08

Acceptance criteria
Risk・・・Period

OLM program cycle modeling process

Fig. 9. Modeling Image of OLM program cycle(Step 3) Deployment of OLM program cycles in an operating cycle- The OLM tasks planned during the period i are assigned to the periods i in the other OLM
program cycle in a distributed manner so that the OLM programs can be deployed throughout
an operating cycle. Accordingly, the risk during an OLM unit period can be reduced to below
the maximum risk, which was determined in the above Step 2.
- It is intended to deploy the OLM program cycles in an operating cycle so that the work loads
for the maintenance tasks, including those for the components not subject to LCO, can beleveled throughout the plant operating cycle. (Fig.10) [2]116N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line Maintenance

OLM unit period○：Implementation

Operational cycle
OLM program cycle ①① ②①A-SIPComponents subject to LCO②B-SIP○…

OLM program cycle ②…

⑫…

○

① ②…

○

…

⑫…

…

…

…
…

○

③A-RHRP…

・ in the・ OLM program cycles
Deploy the・・…
modeling ・process in an entire operating・ cycle・・・⑲A-EDG…

○⑳B-EDG○

Components not subject to LCO

21 A-CVP
○

○

○

…

…

…

…

…

Work
…load leveling should be…achieved …
throughout the operating cycle

・・・・・・

40 S/C○…

○・・・・・・…

…

Acceptance criteria
Risk・・・・・・・・・

PeriodTarget value
Work load・・・・・・・・・Period

Operating cycle modeling process

Fig. 10. Deployment Image of OLM Program Cycle

2.4. Evaluation of the validity of OLM plans
It is necessary to set the indices to be used for the evaluation of the validity of OLM. In order toevaluate if the work load leveling is achieved while minimizing risks, which is one of the factors to beconsidered in planning OLM, the two indices are set in this study, that is, the task volme index andrisk index.
TVI : Task Volume Index= ( TVmax − TVmin) / TVmax
TVmax : Maximum number of workers per dayTVmin : Minimum number of workers per day*The degree of work load leveling is represented by numerical values between 0 and 1. As thevalue gets closer to 0, the degree of work load leveling becomes high.
Risk index (⊿CDFPEAK, ⊿CDFAVG)⊿CDFPEAK: Maximum incremental core damage frequency (CDF)⊿CDFAVG: Average incremental CDF*To represent the ratio of risk increase when a component is made out of service for themaintenance work, the maximum and average incremental core damage frequencies areindicated in numerical values.

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for the On-Line Maintenance

3. Simulations to validate the OLM planning approach
Two types of simulation were performed for a PWR model plant by utilizing the approaches
shown in Section 2 to validate the effectiveness of the OLM planning approach.
1) Simulation of plant-wide work load leveling
We performed a simulation to estimate the degree how work loads can be leveled plant-wide byshifting the maintenance tasks conducted during the outage to OLM in a stepwise manner as shown inFig. 11.

Fig. 11. Simulated Task Volume Changes after Introducing OLM

The TVI was improved by extending the scope of OLM:
Before shifting maintenance tasks during outage to OLM :TVI＝0.533
After shifting maintenance tasks of components subject to LCO only to OLM : TVI＝0.402
After shifting maintenance tasks of components subject to LCO as well to OLM : TVI＝0.323
As shown in the above, it was quantitatively verified that the degree of work load leveling can be
improved by introducing OLM furthermore.
2) Risk assessment simulation for the systems subject to LCO
We established the OLM planning approach for the components constituting the systems shown
below, which are subject to LCO, and performed a risk assessment simulation.
- Residual heat removal system- Safety injection system- Containment spray system- Safety component area cooling system- Auxiliary feedwater system- Component cooling water system- Component cooling seawater system- Emergency diesel generators

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N. Chigusa, et al./A Study for the Practical Methods and Evaluations
for the On-Line MaintenanceIn performing the risk assessment simulation, we established the OLM plans assuming thatmaintenance tasks are implemented during an allowable limiting time (AOT).
The result of the risk assessment is shown in Fig.12. [1](⊿CDF)
1.00E-06
1.0E‑06⊿CDFAVG approx. 1.6E-07

8.00E-07
8.0E‑07
6.00E-07
6.0E‑07
4.00E-07
4.0E‑072.0E‑07
2.00E-07
0.0E+00
0.00E+00

OLM program cycle
Baseline CDF approx. 1.2E-07

Operating cycle(Period)

Fig. 12. Risk assessment

As a result, it was shown that ⊿CDFPEAK≒6.0E-07 and ⊿CDFAVG≒1.6E-07, both of which werelower than the U.S. acceptance criteria at ⊿CDF＜1.0E-05.
Considering the above simulation results, the effectiveness of the OLM planning approach wasverified.

4. Conclusion
1) We proposed an OLM planning approach which can achieve the work load leveling while
minimizing risks.
2) Regarding the PWR model plant, we established an OLM planning approach and performed
simulations to evaluate the validity of the planning approach. As a result the effectiveness of theOLM planning approach was verified.
3) There remain other factors, including the reduction of exposure doses that need to be evaluated inthe future. We will make a continuous effort to improve the OLM planning approach in consistent
with the status of Japan’s NPPs.

References[1] INPO AP-928, Work Management Process Description, Revision 1 (November 2003).
[2] Joint research program by Japanese PWR utilities, “Research on the planning approach for an
optimized maintenance program by introducing OLM” (September 2009), (in Japanese).

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