2. Switching components during operation

For systems with a standby component, periodical switches of components are made in order to keep the component operating times even.

The switching frequency is determined taking into account long-term plans of maintenance jobs to be performed during periodic inspections, items to be maintained such as through oiling etc. at comparatively short intervals, and matters to be considered from operating experiences at each plant

Since the switching operation will cause some disturbance to the system, it should be performed carefully.

2. Maintenance during operation

Plant component maintenance such as oiling (to be performed as a check item), armature brush replacement, periodic calibration of instruments and controls, replacement of burned out indicating lamps on control panels and ink container replacement for recorders are basically performed as maintenance jobs during periodic inspection.

And, maintenance and inspection of waste processing facilities are often performed during plant steady operation while generating relatively small amount of effluent, thus avoiding the period of the plant periodic inspection.

As instruments and controls belonging to major systems for the reactor, engineered safety features, turbine generator, and auxiliary components are intensively calibrated during plant shutdown, then calibration jobs during plant operation cover instruments and controllers of other auxiliary systems and waste processing facilities. Since the instruments and controllers are often components of control systems, careful work procedures including isolation are required to avoid disturbance of the operating systems when performing calibrations.

6. Efficiency Improvement of Periodic Inspection Work

Since overhaul inspections and nondestructive inspections of many components are performed to confirm integrity of a plant during periodic inspections, a long time is taken to conduct periodic inspection jobs. Therefore, various rationalizing measures have been studied from the standpoints

of reduction of dose rate which workers receive and improvement of availability factor.

The following matters for rationalizing periodic inspection works were studied in conjunction with activities for improvement and standardization of light water nuclear plants promoted by the Ministry of Economy, Trade and Industry,

• Improvement of plant layout

• Facility improvements to reduce the time needed for a periodic inspection

■ Improvements such as remote automation of facilities and devices for reduction of dose equivalent

1. Improvement of plant layout

Regarding plant layout, focus was placed from the basic design stage on securing spaces required for component overhaul inspections, securing lay- down spaces for disassembled parts, and securing utilities (power supplies, air, water, etc.) required for maintenance work, and plants that were easy to maintain were studied to develop second generation modified standard plants. The Fukushima Daini NPP Unit 2 and subsequent plants were built, reflecting those results on their design layouts.

2. Reduction of the time needed for the periodic inspection

A periodic inspection previously required almost three months, and with amendment of the Electricity Utilities Industry Law calling for rationalization of the electricity fee structure in 1995, the time needed for the periodic inspection was reduced from the standpoint of availability factor improvement which cut the time required for major work activities constituting the critical path of the periodic inspection jobs. In 2001, the 29-day periodic inspection was achieved through the following various component and work improvements and device developments.

1. Improvements of facilities, components, and tools (faster traveling speeds for the refueling machine and overhead traveling crane etc.)

2. Improvements of work practices (parallel work, replacement system etc.)

3. Review of work system (24-hour work)

4. Upgrading of work management

5. Review of tests and inspections (divided

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which they were constructed in accordance with the old Notice No. 501, it was necessary to establish a reasonable operation standard following the provisions of the US ASME Section XI, which allows continued plant operation when design assessment concludes that a safe continuous operation is possible even if there is degradation due to aging etc., From this background, the "Code for Nuclear Power Generation Facilities: Rules on Fitness-for- Service" was issued in 2000 by the Japan Society of Mechanical Engineers.

Currently, the Code provides a standard for fitness-for-service of operating NPPs in conjunction with the performance-based regulation of the National Government. For upgrading and rationalization of fitness-for-service management of facilities, and also measures addressing aging, the Rules on Fitness-for-Service were amended in 2002 and 2004 so as to include the provisions for inspection and evaluation on Class 1, 2, 3 components, Class MC components, support structures, and core internal components.

This section outlines the inspections according to the 2004 Edition of the Rules on Fitness-for-Service.

(1) Plant inspections according to the Rules on Fitness-for-Service (2004 Edition)

The inspections according to the Rules on Fitness- for-Service include individual inspections of core internal components to address aging degradation events typified by stress corrosion cracking (SCC) and standard inspections corresponding to the ISI of components, such as vessels, pipes, pumps and valves, which were previously conducted according to JEAC-4205 "In-Service-Inspection of Components of Light Water Nuclear Power Plants."

1. Standard inspections

The standard inspections adopt a concept of fixed point sampling for component inspection. Moreover, the objects to be inspected cover almost all core internal components.

1. Inspection requirements for Class 1 components

Class 1 components are pressure parts of equipment etc. which constitute the reactor coolant pressure boundary and their support structures.. They are required to undergo nondestructive tests (visual tests (VTs), ultrasonic

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tests (UTs) etc.) and leak tests.

2. Inspection requirements for Class 2 components.

Class 2 components are those which belong to the direct systems, reactor emergency shutdown system and the cooling systems required for reactor shutdown among engineered safety features. They are required to undergo nondestructive tests and leak tests. Moreover, the components belonging to the turbine system and the containment boundary are required to undergo leak tests.

3. Inspection requirements for Class 3 components

Class 3 components are those belonging to the indirect systems of engineered safety features and spent fuel storage facility and its cooling system. They are required to undergo nondestructive tests.

4. Inspection requirements for Class MC components

Class MC components include the reactor containment vessel (made of steel). They are required to undergo nondestructive tests and leak tests.

5. Inspection requirements for support structures

VTs are required for the support structures of Class 1,2 and 3 components.

6. Inspection requirements for core internals

VTs are required for core internal components,

2. Individual inspections

Individual inspections have individual inspection requirements for the following components including some of the core internal components and Class 1 components.

1. Individual inspections of Class 1 components

1. Neutron measurement housing

2. Control rod drive housing

2. Individual inspections of core internal components

1. Shroud support

2. Shroud

3. Upper grid

4. Jet pumps

5. In-core spray piping and sparger

The individual inspections in the Rules on Fitness- for-Service were established according to the inspection requirements in consideration of specific

aging events for each component in the "Guideline for BWR Core Internal Inspection and Assessment" established by the Thermal and Nuclear Power Engineering Society, and they are generally equivalent to the Guideline. The aging events specified here are mainly due to SCC of austenitic stainless steel and high-nickel (Ni) alloy weld metal, and areas to be inspected are limited to welded joints for many components. In addition, as the concept of fixed point sampling is not adopted for the individual inspections, unlike for the standard inspections, all plural components of the same type are fundamentally subject to inspection.

Table 4.3.1 summarizes requirements for the individual inspection provided in the 2004 Edition of the Rules on Fitness-for-Service.

(2) Upgrading of inspection technology

Since 2001, many cracks due to SCC have been discovered in the core shrouds made of low-carbon stainless steel (SUS316L or SUS304L). Similarly, SCC has also been discovered at the welds of recirculation system piping made of low-carbon stainless steel. The inspection technology using a phased array type ultrasonic flaw detector which has not been used previously for ISIs because it was not standardized has played an active role in discovering and sizing these cracks and this detector has been used to perform a wide range of high- accuracy inspections. Recently, inspection accuracy of this inspection technology was confirmed through research sponsored by the National Government, and it is being standardized as a high-accuracy measurement technique of crack depths and lengths.

In order to inspect core internal components during periodic inspections, various kinds of remote inspection devices have been developed and applied. A small underwater vehicle was developed for visual inspection of core internal components; this device moves underwater to access and inspect almost any location in core.. The shroud UT vehicle attaches itself onto the internal and external surfaces of the shroud by suction, and it is used for UT on circumferential weld lines with the phased array method. The vehicle releases and applies suction to move along. A shroud support UT vehicle has been developed for UTs on weld lines below the baffle plate, and it has been put to actual use.

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