Prevention of Control Rod Withdrawal Event at BWR while the Control Rod Drive Is Operating in Non-return Mode
Summary:
It was recently revealed that multiple control rod withdrawal events had occurred at shut-down BWR plants, due to increased pressure in the control rod drive (CRD) system when a number of hydraulic control units (HCU) were isolated while the CRD was operating in non-return mode. In some of the reported cases, this caused the reactor to reach a state of criticality.
Electric power companies have individually taken measures to prevent recurrence of such events. Nevertheless, to ensure that such measures are fully implemented and to reinforce safety, we requested each electric power company that owns any BWR with a hydraulically-driven CRD to plan and report recurrence prevention measures with due attention to operation management during HCU isolation.
In response to the recent reporting of these events, we requested all electric power companies, including the owners of PWRs, to ensure they provide thorough training on operations and procedures to be followed in the case of a criticality event.
Refer to the main text "Caution 2007-1"
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