Control rod stuck in Kashiwazaki Kariwa unit
19 October 2007
Tokyo Electric Power Company (Tepco) reported that a control rod cannot be removed from the reactor of unit 7 of its Kashiwazaki Kariwa nuclear power plant. The unit shut down automatically when an earthquake struck the plant on 16 July.A control rod is moved in or out of the central core of a nuclear reactor in order to control the neutron flux - increase or decrease the number of neutrons which will split further uranium atoms. This in turn affects the thermal power of the reactor, the amount of steam generated, and hence the electricity produced. They are usually combined into control rod assemblies and inserted into guide tubes within a nuclear fuel element.
In an emergency, the control rods are quickly inserted all the way into the fuel assembly to stop the fission reaction and shut down the reactor unit. All 205 of the 4-metre-long control rods at Kashiwazaki Kariwa 7's reactor were automatically inserted into the fuel as soon as the 6.8 magnitude earthquake hit, Tepco said.
Control rods stand vertically within a reactor core. In pressurised water reactors (PWRs), they are inserted from above, the control rod drive mechanisms being mounted on the reactor pressure vessel head. However, due to the necessity of a steam dryer above the core of a boiling water reactor (BWR), such as the Kashiwazaki Kariwa units, this design requires insertion of the control rods from underneath the core. BWRs require the hydraulic insertion of control rods in the event of an emergency shutdown, using water from a special tank that is under high nitrogen pressure.
Tepco said that in order to conduct in-core inspections, it had removed the lid of the reactor's pressure vessel and has started removing fuel assemblies and control rods from the reactor core to the fuel storage pools. So far, 106 control rods have been removed from the reactor. However, the company discovered that one of the control rods was jammed in the reactor core.
Company officials said that one reason for the rod becoming stuck could be that devices intended to prevent the rod from slipping remained locked. Another possibility is that the earthquake distorted the shape of the facility, preventing the rod from moving. Checking the exact cause, however, is likely to take some time as water that fills the reactor must first be drained before its interior can be examined.
At the time of the earthquake, three of the seven reactors at Kashiwazaki Kariwa - units 3, 4 and 7 - were in operation. Those reactors shut down safely as tremors began. Unit 2 was in the process of starting operation, and shut down automatically as well. Units 1, 5 and 6 were not operating as periodic inspections were being carried out.
The earthquake resulted in water being shaken from cooling pools of all the units and some of this drained away to be discharged to sea. In addition, many barrels of solid low-level radioactive waste were knocked over and an external electrical transformer failed and caught fire.
The discovery of the jammed control rod is likely to further delay the resumption of plant operations. All seven reactors at the plant remain offline while damage from the earthquake is assessed.