May 15, 2007
Japan Nuclear Technology Inst ituteCaution 2007-1
Prevention of Control Rod Withdrawal Event at BWR while the Control Rod
Drive Is Operating in Non-return Mode

It was revealed that, during the fifth annual outage in 1999 of Shika Unit 1,
three control rods came off and caused the reactor to reach a state of criticality.
This incident took place as workers were preparing for the confirmation tests after
the reactor shutdown function reinforcement work, when they isolated thehydraulic control unit (HCU) while the CRD was operating in non-return mode,
which contributed to a gradual pressure increase in the CRD system.
It was also confirmed that similar control rod insertion or withdrawal events hadoccurred at other BWR plants when attempting to isolate and restore the HCU
while the CRD was operating in non-return mode.
Based on technical lessons learned from these cases, in order to ensure theprevention of such control rod withdrawal events during reactor shutdown, theJapan Nuclear Technology Institute decided to issue this Call for Attention,
requesting operators of any BWR with a hydraulically-driven CRD to report theirplans and implementation of recurrence prevention measures.
1.

Problems with Shika Unit 1 during the Event(1) Isolating many HCU modules while the CRD is operating in non-return
mode brings about the pressure increase in the CRD, which may cause somecontrol rods to come off inadvertently. In spite of this risk, the plant workers did
not take measures to prevent the rise of pressure in the CRD.
(2) Due to the lack of close communication with the HCU isolation crews, theoperation crews were not fully aware that the HCU was being isolated andtherefore did not take necessary actions such as monitoring the CRD hydraulicfluid flow rate and the CRD coolant head pressure.
Moreover, at that time, the “Reactor/CRD Coolant Header Pressure Difference
High/Low” alarm was deliberately cut off to prevent unnecessary and frequentactuations by the decreased pressure difference.
(3) The HCU isolation crews and the individuals that planned and confirmed theisolation were not sufficiently aware of the risks associated with the HCU
isolation procedure and the HCU isolation itself.

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2.

Recurrence Prevention Measures
With a few exceptions, control rod withdrawal events similar to the case inShika Unit 1 occurred at other BWR plants when attempting to isolate or restore
the HCU while the pressure in the CRD was high. As with the case of Shiga Unit 1,
inadequate inspection procedures and monitoring contributed to such events.
The electric power companies operating any BWR in Japan have individuallytaken measures to prevent recurrence. Nevertheless, to ensure that such measures
are fully implemented and to reinforce safety, we request the relevant companies toexamine the need for the following measures in view of the situation at their
nuclear power stations:(1) Place the CRD in return mode before isolating the HCU to prevent anexcessive increase of the CRD pressure.
(2) While the HCU is isolated, ensure that the reactor/CRD coolant header
pressure difference is monitored using indicators and alarms, and that the control
rod position is also monitored. In addition, clearly display the CRD operation
status (return mode, non-return mode or CRD pump not working) in the main
control room.
(3) Describe in procedure documents and the like the control procedures to befollowed during the HCU isolation and the procedures to be followed when arelevant alarm is activated. In addition, ensure that the operation crews and theHCU isolation crews are familiar with these procedures and educated about therisks associated with the HCU isolation.
(4) When isolating the HCU, ensure that close communication is kept between
the HCU isolation crews in the field and the operation crews in the main controlroom. The operation crews in the main control room should perform propermonitoring while being well aware of the activities in the field.
By June 15, each electric power company that owns any BWR with ahydraulically-driven CRD must send us (Japan Nuclear Technology Institute) areport on their review of their own recurrence prevention policy and plan for
implementing recurrence prevention measures.
The principle of heat generation in a nuclear power plant is nuclear fission. To
extract energy from nuclear fission in a stable and safe manner, it is essential toproperly control the criticality of the reactor. Each electric power company inJapan should learn from the recent reports on control rod withdrawal events, renew
their awareness of this basic requirement, and improve the training of theiroperation crews on operation procedures to be followed when responding to a
critical state that deviates from the normal control range-2-