Figure 5.2.4 Standard schedule for periodic Inspection

3. Major activities in the periodical inspection period

Major activities in the periodical inspection period are opening and checking the reactor vessel, checking the soundness of fuel and making replacements, inspecting SG tubes, opening and checking the turbine, verifying the integrities of components and materials (in-service inspections), and carrying out leak rate tests of the containment vessels, operability tests of safety protection circuits, and plant performance tests.

The sequence of major check and inspection activities of the primary systems in a typical periodical inspection period of a typical PWR plant is described below when no plant modification is implemented.

i ) Opening of reactor

The sequence to shut down an operating plant starts with the disconnection of the main generator from the grid, continues to the reduction of the temperature and the pressure of the reactor coolant system followed by subsequent degassing and oxidizing operations of the reactor

coolant After the reactor coolant.system is cooled down, and the containment air is purged by the ventilation system, the air lock of the containment is opened and operations in the containment are begun. After the missile shielding structures are removed, interconnected systems and devices such as seismic supports, electrical cables of the control rod drive mechanism and cooling fan connecting ducts are disconnected. Secondly, in- core thermocouple housings in contact with the reactor coolant are disassembled and stud bolts of the reactor vessel are loosened and removed. In parallel with the above activities, thimbles for in-core instrumentations are withdrawn. Subsequently, the reactor cavity is filled with water. The reactor vessel head is lifted, keeping pace with the increasing water level, and moved to a temporary lay-down space on the operating floor. When the reactor cavity is completely filled with water, control rod drive shafts are unlatched from the control rods and removed. The upper core internal assembly is lifted, but kept under water, and moved to a temporary lay-down

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location on the cavity floor.

The steps of the reactor opening are illustrated in Figure 5.2.5.

ii) Transfer and inspections of fuel

After the activities to open the reactor are completed, fuel assemblies in the reactor vessel are transferred to the spent fuel pit outside the containment using a fuel handling crane and a fuel transfer system. Spent fuel assemblies which are transferred to the pit are visually checked with an inspection unit installed in the pit (underwater TV inspection). Sipping inspections of fuel are also conducted, if necessary, to confirm the soundness. Burn-ups of spent fuel assemblies for which soundness has been confirmed by the inspections and are to be reloaded in the reactor are calculated, and the optimum loading pattern of these reused fuel assemblies mixed with new fue assemblies is established by pattern calculations to determine locations for the reused fuel assemblies. Based on the core loading pattern, items inserted into fuel assemblies are reshuffled and the fuel assemblies are loaded into the reactor core in the reverse order to the procedures for removing them. Fuel assemblies and items inserted into assemblies are visually confirmed to be in their proper locations with an underwater TV observation.

iii) Eddy current test of steam generator tubes

The number of heat transfer tubes of a SG is more than 3,000. For SGs with heat transfer tubes of nickel based Alloy 600 (Inconel ®600), inspections are made during every periodical inspection using the non-destr uctive eddy current test (ECT) method throughout the entire lengths of the tubes. For SGs with nickel based Alloy 690 (Inconel ®690) tubes, inspections are made only every two inspection periods. Reactor coolant in SG tubes needs to be drained out before the ECTs. When the water level in the reactor cavity is decreased once in the process of reactor opening, and the water in the SG tubes has drained out, the covers of manholes on the water chambers of SGs are opened and nozzles connecting the water chambers to the reactor coolant piping are closed with plugs to isolate the water chambers from the reactor coolant system. The nozzle plug design is a feature invented to rationalize

periodical inspections. Before this plug design was adopted, ECTs of SGs were interrupted every time when the reactor cavity water level was raised for the fuel removal or loading, and that resulted in long times and much effort for ECTs. The ECT equipment consists of two or four flaw testing coils (probes), and they are positioned at the inlets of the tubes by a robot in the first step and in the second step they are inserted into the tubes by a pusher. The robot and the pusher are remotely and/or automatically operated from an operating unit in a container house set outside of the containment. The soundness of the heat transfer tubes is verified based on the analysis of eddy current changes detected by the probes which move at high speeds in the tubes along their full length. A typical SG tube ECT system is illustrated in Figure 5.2.6.

4. Reassembling of reactor vessels, water pressure testing of reactor coolant system and leakage rate testing of containment

After it is made certain all fuel is being properly loaded in the reactor core, the reactor vessel and its attachments are reassembled in the reverse order of its opening procedures. In- core thermocouple housings and vessel stud bolts are fastened and a leak test of the reactor coolant system is performed. The testing efforts are focused on the parts which were opened or disassembled for the refueling operations and the periodical inspections. After that all penetrations and the air lock of the containment are closed and leakage rate of the containment is tested. The containment pressure is raised to a specified value for this and it is maintained at this pressure for a specified period to check that the leakage rate is below a specified rate. Instead of such an overall leakage testing, called Class A testing, the leakage rate of the containment may be checked with a local leakage rate testing method (Class B and Class C tests).

5. Preparations for reactor startup

The leakage rate testing of the containment is the final step of the refueling operations and the checking of the plant equipment, and it is followed by activities for starting up the plant. The configurations of all systems are aligned for the plant startup. Performance tests of system

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CRDU Cooling Duct

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Missile Shield

Cavity Seal Ring Force Down Plate

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3. CRDM Cooling Duct Renova I

4. Electric Circuit Disconnection

5. Cavity Sea! Ring Force Down Plate Reaoval

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R/V Head Lift up Preparation

Figure 5.2.5 The steps of the reactor opening (1/2)

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Procedure ®

Procedure ®

Procedure®

.R/V Seal Plate

Figure 5.2.5 The steps of the reactor opening (2/2)

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before final testing at the full load. When the full load test is completed and the plant is verified to be operated steadily under its full load, the periodical inspection of the plant is completed.

Inside reactor containment

E CT probe

Robot (MR-lll)

TV camera

Inside loop compartment

Heat transfer tubes

Water chamber

Interface unit (No.land No.2)

Headphone

Monitoring/ communication relay box

Fiber optic transmission unit

Robot relay box

(doubling as air supply panel)

Pusher relay box

(doubling as air supply panel)

Pusher

(EWS)

Pusher power supply

Fiber optic cable

Monitoring/comm unication unit

Flaw testing computer

Main computer

Cabinet

Power supply(440V)

Fiber optic transmission unit

Container house

Interface unit

Outside reactor containment

Figure 5.2.6 Example of SG Tube ECT Unit

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4. In-service inspections

Non destructive inspections of the following equipment are required by law.

■ Class 1 components composing the reactor coolant pressure boundary including the reactor vessel

• Class C component—reactor containment vessel

• Class 2 components consisting of vessels, pumps, piping and valves of engineered safeguard systems and emergency reactor shutdown systems

• Class 3 components consisting of vessels, pumps, piping and valves of indirect supporting systems for the safety important systems

• Support structures for safety important equipment

• Core internals

Inspections of the above components are systematically planned and executed in the periods of periodical inspections of a plant to complete the inspections of necessary components in a 10-year

cycle. The inspections are conducted as part of periodical utility inspections. Details of items and methods of inspections are defined in the JSME S NAI-2002 “Maintenance Rules” published by the Japan Society of Mechanical Engineers.

Inspections described in the rules include surface checks such as visual checks and liquid penetrant tests (PT), non-destructive tests including volumetric tests represented by ultrasonic tests and system leakage tests. ECTs applied to SG tubes are referred to in the rules as a sort of volumetric test for in-service inspection.

Lower core internals in a reactor vessel are removed at least once every 10 years, to allow close inspection of welds in the reactor vessel. The reactor vessel is inspected from inside using ultrasonic flaw testing equipment; an A-UT machine is shown in Figure 5.2.7. Remote operation of the equipment and recording and evaluation of flaw data are all centralized to control panels and data processing units in container houses temporarily

Figure 5.2.7 Example of R/V ultrasonic testing unit

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