verified as to their soundness and they are reassembled in the reverse order to the opening and disassembling steps. When the turbine is completely reassembled, preparations for test runs of the turbine unit, such as system cleaning and oil flushing, are conducted. Finally, the turbine unit is given a test run coordinated with the primary system, and functions of the appurtenance systems are verified. When no problems are found in the verification tests of the steam turbine unit, its periodical inspection and periodical utility inspection are completed, and the unit is ready to be used for the plant power operation.

2. Reactor coolant pumps

Periodical utility inspections of reactor coolant pumps (a three-loop plant has three pumps) are executed on a planned basis (for example, one pump in each periodical inspection period). In periodical inspections of a reactor coolant pump, the shaft seals are overhauled and parts of the seals are adjusted or replaced as needed.

More specifically, the three seals used in series for a pump are finely crafted assemblies. The No.l seal is a non-contact and controlled leakage type. The seal surfaces (face plates), O-rings and slipper seals are checked and cleaned, and replaced if necessary. The No.2 seal is a backup seal and the No.3 seal is a stop seal. Sliding surfaces of carbon stationary plates and super­alloy rotary plates are checked, and expendable parts of the seals such as O-rings are replaced. The seals are disassembled in the order of No.3, No.2 and No.l, or in the order of their elevations, and reassembled in the reverse order to the disassembling order, after being checked and repaired. All these tasks are conducted under the condition that the reactor coolant level is lowered below the level of the lowest seal (the No.l).

3. Maintenance during plant normal operation

Preventive maintenance of a NPP during the plant normal operations, i.e. monitoring of the conditions and operability of various components and systems composing the plant with routine checks, detecting signs of their failures in early stages and taking necessary actions in time, is essential to improve

the plant availability.

1. Checks during plant normal operation

Most failures of normally operated components and systems can be detected at early stages by monitoring the operating parameters on the main control boards and checking plant equipment during routine patrols. Operators and maintenance personnel routinely walk every day through wide areas of the reactor auxiliary building, the turbine building, and the containment to check the statuses of operating components and systems.

Components and systems not normally operated, including those used as standby items under the plant normal operating condition, are verified as to their functionality, interlocks, alarming set points and so on by periodical check activities. Engineered safeguard systems and systems needed to shut down the plant safely are periodically tested regarding their proper functioning during the plant normal operation.

2. Maintenance dining plant normal operation

If a failure or a sign of failure of a component or a system is detected by checks during the plant normal operation, the feasibility of maintenance work is first studied. When the study ensures that the component or system can be temporarily placed out of service without adversely affecting the safe operation of the plant and the safety of maintenance personnel, the affected component or system is restored to its proper conditions by appropriate actions including repairs, without stopping the normal operation of the plant. The affected component or system is temporarily isolated from other parts of the plant for repair work, as necessary

4. Rationalization of Plant Features Related to Periodical Inspections

A lot of components and systems are overhauled or tested by non-destructive methods in the periodical inspections. The duration of the period for inspecting activities gets longer as the numbers of activities increase, resulting in an increase of cumulative dosages of workers and the reduction of plant availabilities. Various measures have been investigated and applied to rationalize periodical inspection activities of plants. Keeping pace with

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NSRA, Japan