Upgrading of evaluation technology

according to the directives of the Nuclear and Industrial Safety Agency, continuous operation is also now possible if the plant integrity after a certain period of operation time is proved by the structural integrity assessment assuming the crack conditions at that time using the evaluation method provided in the chapter on assessment of the Rules on Fitness- for-Service.

NSRA, Japan

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2. Maintenance Technology against Aging

(1) Maintenance technology for core internal components

With aging of NPPs, in order to continue plant stable operation, it has become increasingly important to apply maintenance technology. Recent significant technologies for aging-related maintenance of core internal components of BWR plants are introduced here.

One of the aging-related major events to be considered for core internal components is SCC. SCC will occur when conditions for three factors overlap: materials, environment and stresses. Preventive maintenance technologies to mitigate these factors for each structure in a reactor, so as not to easily produce SCC, and repair technologies when SCC has occurred have been applied. Moreover, material technologies which replace core internal components including the core shroud and jet pump with those of better resistance to SCC have also been applied.

i) Residual stress mitigation technology

Recent reports have also found SCC in core shrouds etc. made of 304L and 316L low-carbon content stainless steels, which were considered to have low susceptibility to SCC. The cause for this SCC is considered as follows: transgranular SCC occurred in the surface-hardened layer due to machining etc., and then progressed from there as intergranular SCC. A mitigation measure to change tensile residual stress on the material surface due to welding etc. to compressive stress by peening the parts, has been applied as SCC preventive maintenance technology.

Peening methods which have been applied to actual plants include laser peening, shot peening, and water-jet peening. Laser peening forms a compressive stress layer on the material surface by impulsive force of plasma produced by the pulsed laser beam irradiated on the material surface underwater (Figure 4.3.1). It has the advantages that it can be applied without giving significant external force or vibrations to the structures subject to the peening, and it can be done in very narrow places easily. On the other hand, water-jet peening forms a compressive stress layer on the material surface by impact pressure produced when

cavitation due to the water jet ejected from a nozzle collapses (Figure 4.3.2).

Under laser irradiation

Compression

After laser irradiation

[Source] : "Repair and preventive maintenance method of BWR core shroud", Japan Society of Maintenology Vol. 3, No. 3 (2004)

Figure 4.3.1 Principle of laser peening

[Source] : "Repair and preventive maintenance method of BWR core shroud", Japan Society of Maintenology Vol. 3, No. 3 (2004)

Figure 4.3.2 Principle of water jet peening

2. Improvement of water quality environment

As preventive maintenance technologies from an environmental standpoint to improve reactor water quality and lower the corrosion potential of structure surfaces, there are such established technologies as hydrogen injection and noble metal addition. Moreover, the new technology titanium oxide injection has also been developed.

2. Repair technologies

1. Repair method to remove cracks.

As a method to completely remove SCC, electric discharge machining (EDM) and grinding processing have been applied.

EDM generates an electric discharge

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