Resolution of Generic Safety Issues: Item B-48: BWR Control Rod Drive Mechanical Failures ( NUREG-0933, Main Report with Supplements 1–34 )
Surface cracks have been discovered in control rod drive internal parts at some operating BWR plants. Although only observed to be localized in nature, this cracking, if propagated, could potentially affect the capability of the control rod drive to perform its design function. Cracking has been identified in the following control rod drive internal parts: the collet retaining tube (CRT), the poison tube, and the index tube.
The extent of the cracking observed at operating BWRs to date has not been severe enough to impair the ability of the control rod drives to operate in their required capacity. As an interim precautionary measure, additional technical specification requirements were established for operating reactors which require shutdown within 48 hours of detecting an inoperable control rod, unless investigation demonstrates that the cause of the failure was not due to a separated collet tube. Another technical specification that has long been in effect for operating reactors requires exercising fully or partially withdrawn rods once every week as part of the corrosion prevention program. GE is working with utilities to replace old-style surface-hardened collets with new nonsurface-hardened collets. This issue is documented in NUREG-0471.3
This issue has been RESOLVED and the evaluation and corrective measures are documented in NUREG-0479.222