Environmental Engineering Reference
In-Depth Information
Based on the data and experiments, it was decided that a guideline would
be published with detailed interpretation. Japan published a guideline on
management activities such as inspection of the baffl e former bolts of actual
power plants in 2002. This guideline will be revised in the future to refl ect
up-to-date knowledge obtained through international collaboration.
In Korea in 2007, defects were found in the control rod guide tubes made
of Inconel X-750 in the internals of a nuclear reactor. The control rod guide
tubes were replaced with CW 316 stainless steel tubes. Since then, no cracks
have been found in the baffl e former bolts (Hwang
et al
., 2010 ).
In the United States, the Westinghouse Owners Group (WOG) has
inspected cracks in baffl e former bolts from PWRs in other countries. They
have also provided information on activities planned for Westinghouse
power plants with potential cracking. The WOG has clearly demonstrated
that destruction of minor bolts would not have a serious impact on safety
because a number of baffl e former bolts would still support the structure.
The WOG activities are as follows:
Development of analytical methods and acceptance criteria for bolt
analysis.
Performance of risk-informed evaluations.
The Nuclear Energy Institute (NEI) which consists of the Materials Technical
Advisory Group (MTAG) of the United States is formed of energy company
representatives. The MTAG has received support from EPRI to prepare a
guideline for In-Service Inspection (ISI) of RVI equipment which has signif-
icant impact on continued and safe operation of power plants. In preparing
an inspection guideline, the damage to equipment inside the nuclear reactor
by inspection, fatigue, abrasion and corrosion were considered. Recently,
Westinghouse and AREVA have published a report using screening based
on various signifi cant damage mechanisms as a part of EPRI MRP on reac-
tor internals.
7.4.3 Degradation in steam generators
Mechanisms
The fi rst crack found in a steam generator tube was in a hot leg side tube
of a steam generator in the Obrigheim plant in 1971. This was the fi rst
reported incidence of PWSCC at this site (Shah, 1992). As of 1994, at least
61 power plants had experienced PWSCC in the tubes; 32 plants experi-
enced U-bend PWSCC and 5 plants experienced PWSCC in the denting
zone. In most cases, it occurred in hot leg side tubes, but in some it occurred
in cold leg side tubes. The cracks were found in 1-10 EFPY (effective full
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