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概要

Irradiation-assisted stress corrosion cracking or IASCC is a well-known result of age-related degradation of baffle former bolts which are an integral compo- nent of PWR core internals. However, methods for analyzing the causes and assessing IASCC have yet to be established and are being studied. The baffle former bolts are components of the baffle structure and in order to maintain the integrity of the baffle structure, an effective approach for maintaining the bolts must be provided. This paper proposes an effective method of maintaining the functions of the baffle structure by improving the configuration and/or material of baffle former bolts. A relaxed configuration of the bolt neck reduces the stress and can ap- proximately halve the damage caused by IASCC. An improvement in the threshold value of materials' vulnerability to IASCC would not only extend the service life of all bolts but could also substantially retard IASCC of the bolts installed at upper and lower ends where flux is low. If the thresh- old fluence to IASCC could be made higher than approximately 5×1025n/m2 (E＞ 0.1MeV, 340℃), the bolts installed at lower and upper ends, which are conven- tionally replaced 30 years after plant start-up, would remain intact for 60 years after start-up. Hence, this approach is a very effective maintenance measure.

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