Maintenance to repair facilities and components after they have failed is called corrective maintenance, and this type of maintenance is applied to components the failure of which would not compromise plant reliability and continued operation and for which maintenance can be performed even during operation.

(2) Concept of preventive maintenance

Maintenance of nuclear power plants can be roughly divided into daily checks and periodic inspections (an inclusive term for the periodic inspections and licensee’s periodic inspections and periodic checks provided in Section 4.2.2). Daily checks include those on operational conditions of major components, periodic operational checks of engineered safety features, etc., and checks and maintenance of components accessible during operation.

On the other hand, during a periodic inspection, the power plant is shutdown according to a schedule and the facilities are comprehensively inspected and maintained based on various data such as operation data obtained from daily maintenance tasks and the facility inspection program with its long-term perspective, to ensure stable operation of the power plant up to the next periodic inspection.

That is, the outage period of the plant is also an opportunity to take various improvement measures for reliability improvement

The basic principle of maintenance of NPPs in Japan is preventive maintenance to prevent their facility malfunctions by replacing and upgrading their components before a malfunction occurs.

The following is taken into consideration in planning and implementing such preventive maintenance measures.

1. Daily checks

As malfunction symptoms due to aging degradation latent in various components and parts will manifest themselves during operation in many cases, various operation parameters (pressure, temperature, flow rate, vibration, etc.) are monitored and recorded (see Section 4.2.5 for daily checks).

2. Periodic inspections

(a) Overhaul inspections

In order to recognize conditions of aging degradation of mechanical components, it is

effective to disassemble them and inspect their parts by visual examination etc. An overhaul inspection cycle for each component should be established so that it is always possible to efficiently grasp inspection status of the overall facility.

2. In-service inspections

In order to grasp aging degradation status of components and piping in a planned manner, in­service inspection programs are established, and various nondestructive inspections are conducted (see Section 4.2.4 for in-service inspections).

2. Calibrations and characteristic tests

By calibrating and performing characteristic tests, etc. on electric instrumentation and control components (process monitoring instruments etc. composed of electronic components), their aging degradation status is grasped.

2. Functional tests

In order to confirm that the system or component functions intended at the basic and detailed design stages of the plant are still maintained after the plant has been put into operation, their functional tests are performed.

3. Survey of other plants’ operational information etc.

The issues concerning aging degradation and for ensuring reliability can be better understood by studying and evaluating operating information from domestic and overseas plants in addition to that of the plant concerned, and the required measures for this are implemented in a planned manner during periodic inspection.

2. Periodic Inspections and Licensee’s Periodic Inspections

Safety and reliability of a NPP are ensured by a safe design based on the principle of defense in depth at the design and construction stages of the power plant, strict quality control, and maintenance management at its operation stage, and among them, periodic inspections and periodic checks of the power plant at its operation stage play very important roles for ensuring its safety.

Based on past inspections and operating experiences, the systems for periodic inspections and periodic checks were studied and established by the government hand-in-hand with the private

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NSRA, Japan