Press Release Back Number(2007)

Registration of Control Rod Withdrawal Events in NUCIA (Notice)

March 19, 2007
Chubu Electric Power Co., Inc.

In the aftermath of the incident that occurred at Shika Nuclear Power Station Reactor No. 1, operated by Hokuriku Electric Power Company, Chubu Electric has registered details about the control rod withdrawal event we experienced at Hamaoka Nuclear Power Station Reactor No. 3 in May 1991 in NUCIA (*), for the purpose of making the information available over the Internet.


The incident in question did not result in a critical chain reaction and was controlled by the reactor's emergency shutdown functions. A full investigation was carried out to discover the cause and countermeasures were established accordingly.


(*) NUCIA stands for the NUClear Information Archives and is an Internet website operated by the Japan Nuclear Technology Institute. NUCIA provides information on accidents, malfunctions and other minor problems occurring at nuclear power station facilities, together with information related to reliability, to allow such information to be publicly shared.


Attachment: Outline of Registered Information in NUCIA



< Attachment >

Outline of Registered Information in NUCIA


1. Introduction

In relation to the incident (*1) that occurred at Shika Nuclear Power Station Reactor No.1, operated by Hokuriku Electric Power Company, Chubu Electric has registered details concerning the control rod withdrawal event experienced in the past in NUCIA (*2), for the purpose of making the information available.

An outline of the incident is as follows.


2. Registered Contents

Name of power station:

Hamaoka Nuclear Power Station Reactor No.3


Name of event:

Control rod withdrawal event, which occurred during periodic inspection


Date and time of occurrence:

5:58 PM, May 31, 1991


Location of event:

Control rod drive mechanism of instrumentation and control system facilities


Situation of plant at time of occurrence:

Fuel was being replaced (during periodic inspection)


Situation at time of occurrence:

At 5:58 PM on May 31, 1991, the "control rod drift" alarm in the central control room went off, and it was found that one of the control rods had drifted (*3). Because the event occurred at the site while the valves of the control rod drive system water pressure control unit were being restored, after the end of the check of the reactor protection system set values (*4), the central control room immediately confirmed that the reactor was in a subcritical condition, and instructed the operators at the site to stop operating the valves concerned and to return them to their initial condition. Subsequently, while assessing the operation panel in the central control room, it was found that a total of three control rods, including the control rod concerned, were in a withdrawn condition.

Even while the three control rods were in a withdrawn condition, the scram function, which caused the reactor to undergo an emergency shutdown, was maintained. In addition, the workers were not exposed to radiation and there was no leakage of radiation to the outside of the reactor.


Cause of the incident:

When the cooling water differential pressure in the control rod drive system was high (non-return operation (*5) condition), the isolation cancellation operation of the valves was performed. It is considered that because the valve on the insertion side (valve 101) was fully closed, and the valve on the withdrawal side (valve 102) was open at this time, a force was applied to the control rod withdrawal side by the cooling water pressure, causing the control rods to be withdrawn.


Measures to prevent reoccurrence:

At the time, in light of the incident, we established a document called "Safety Measures in the Event of a Stoppage of the Reactor," which summarized the safety measures to be adopted in the event that the reactor stops, and incorporated work precautions into the procedure, as well as a safety measure into the facilities, with a view toward preventing reoccurrence.


  1. (1)Enactment of "Safety Measures in the Event of a Stoppage of the Reactor"
    Aiming at strengthening the safety measures used in the event of a stoppage of the reactor, we enacted "Safety Measures in the Event of a Stoppage of the Reactor," including the incident concerned. (Enacted on January 23, 1992.)
  2. (2)Incorporation of work precautions into the procedure
    As precautions for the procedure pertaining to the operation of the control rod drive system facilities, we added precautions to be observed during isolation, and cancellation of control rod drive system water pressure control unit isolation.
  3. (3) Incorporation of a safety measure into the facility
    We divided the "Cooling water differential pressure" alarm into "Cooling water differential pressure high" and "Cooling water pressure low" alarms.

*1: The incident that was confirmed at Shika Nuclear Power Station Reactor No.1 is as follows: According to a public announcement made by the Hokuriku Electric Power Company, while Reactor No.1 was stopped due to the fifth periodic inspection, valves related to the control rods were operated, and as a result, three control rods were unexpectedly withdrawn, causing the reactor to go into critical condition.


*2: NUCIA stands for the NUClear Information Archives and is an Internet website operated by the Japan Nuclear Technology Institute. NUCIA provides information on accidents, malfunctions and other minor problems occurring at nuclear power station facilities, together with information related to reliability, to allow such information to be publicly shared.


*3: A drift condition is a condition in which the position of a control rod cannot be correctly confirmed.


*4: The check of the reactor protection system set values involves inputting pseudo signals to each detection element such as pressure, water level, and temperature, which are used for operating safety and protection system facilities, and then checking the operation values. These operations include inserting the control rods in an emergency, starting the emergency core cooling system, starting the emergency diesel generator, and isolating the reactor storage vessel. Before performing this check, all of the control rods are fixed in the insertion position so as to prevent them from actually moving.


*5: The operating condition of the control rod drive system in which part of the cooling water of the control rod drive system is fed to the reactor via the water feed pipe is called a return operation, and the operating condition in which the total volume of the system water of the control rod drive system is used as the cooling water for the control rod drive system is called a non-return operation.


Control rod position



Outline diagram of control rod drive system