concern may be measured during the plant normal operation to accurately evaluate the potential generation and/or progression of degradation phenomena.

2. Inspections of piping

Since access to piping inner surfaces for the direct detection of high cycle thermal fatigue cracking is difficult, ultrasonic non-destructive testing from outside the piping is usually used. Various research and developmental studies have been performed to improve the speed and the accuracy of detecting failures with the technique. Phased array inspection is an example of an advanced method. Ultrasonic wave sending devices arranged as arrays on films are used to control the timing of waves, and to improve the accuracy and the speed of the testing.

Inspection techniques for the sizing of cracks, for example the TOFB (Time of Flight Diffraction) method, have been developed and actually applied to plant inspections.

3. Evaluations of piping wall thinning

Thinning of the piping walls due to erosion or corrosion have often been seen in piping segments such as elbows and downstream piping of orifices where flow velocities are high and unbalanced, under certain temperature conditions. Since actual occurrences of piping wall thinning are largely affected by apparently slight differences of internal structures in piping parts, it is important to grasp tendencies of wall thinning by actually measuring wall thicknesses, for the effective management of the issue.

After the secondary system piping rupture incident occurred at the Mihama No.3 unit in August 2004, JSME established 'Technical Codes for the Management of Piping Thinning in PWR Nuclear Power Generating Plants-2006 ”

Detailed requirements for the selection of piping segments for piping thinning management and management practices are shown in the codes which are now the basis of piping wall thinning management of PWR plants in Japan.

3. Maintenance of Alloy 600

Primary water (PW) SCC of Alloy 600 components in contact with PWR coolant and composing reactor coolant pressure boundaries, has often occurred

in domestic and overseas plants. Hence, control of the issue is an urgent necessity. The following maintenance techniques have been developed and applied to actual plants.

1. inspection techniques

For the Alloy 600 used for PWR plant components other than SG tubes, such as nozzles and safe ends of reactor vessels and pressurizers, the soundness of weld joints of these parts needs to be verified by inspections. Ultrasonic testing probes with good sensitivity for fine PWSCC flaws and equipment to remotely and automatically scan the piping surfaces with the probes have been developed and used in inspections of actual plants.

Efforts are being continued to further enhance inspection techniques by improving accuracy and speed for the detection of fine cracks, and by making probes which are able to be used in narrow spaces within components.

2. )Techniques to relieve stresses

SCC of mechanical components is known to occur only under certain co-existing conditions of stresses, materials and environmental factors. Hence, if stresses in a component are relieved, SCC of the component will be prevented from occurring, or if cracks exist, their progression will be retarded.

Since the nozzles and the safe ends are close to welds, high tensile stresses are generated and left in the inner surfaces of these parts, to constitute or contribute to one of the conditions for PWSCC. An addition of compressive residual stresses to parts with residual tensile stresses will reduce the stresses or even change them to compressive stresses, and will eventually prevent the generation of PWSCC in the parts.

Some techniques used for the purpose of relieving stresses include peening of inner surfaces of components using a water jet to add compressive residual stresses, and rapid laser heating of outer surfaces of components to generate residual compressive stresses in the inner surfaces after cooling them down.

3. Techniques for repairing and replacing

Repairing and replacing techniques are needed when mechanical parts are found to be badly damaged by inspections, or when the repair or replacement of parts is determined as

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(2)Electrical and Instrumentation Equipment

Degradation of insulation on cables and devices used in NPPs represents aging issues related to electrical and instrumentation equipment. The soundness of cable insulations has been seriously investigated by various tests including acceleration tests with considerations to repair or replace such items.

Most electrical and instrumentation equipment of NPPs has been analogue. In non-nuclear industries however, most has been digitalized because of rapid progress in digital techniques. When an analogue device fails now, it is difficult to procure the same device as the analog equipment is not fabricated any more. Older plants have often been obliged to partially replace their old analog devices with digital ones for that reason; other additional reasons for replacement include reliability enhancement of equipment and operability improvement of plant systems.

Some aged plants have been upgraded through renewals of all the control room equipment to the latest digital devices, reflecting new human machine interface techniques. Workloads on operators are largely reduced and the numbers of maintenance activities for plant systems are reduced as well with the renewals of whole systems for plant upgrading.

References

1. Sano. T and Hakata. T, “Operating Performances of PWR Nuclear Power Plants”, Journal of Japan Electric Society, Vol.101, No.4, pp. 15-24, April, 1981

2. “Operation and Maintenance of PWR Plants - Nuclear Reactor Lecture Series”, pp.107-117, Thermal Power Engineering society, May, 1979

3. “Maintenance Management Rules for NPP (JEAC4209-2003)”, Japan Electric Association

4. “ Maintenance Standards (JSME S NA1-2002)”, The Japan Society of Mechanical Engineers

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