As reported in NIT 138, on August 26, when a 3-ton relay device used during replacement of fuel in the Monju Prototype Fast Breeder Reactor (280 MWe) was being removed, it dropped back into the reactor vessel.
According to Monju's owner-operator, Japan Atomic Energy Agency (JAEA), the device was supplied by Toshiba.
The relay device was dropped when a "gripper" lost its grip (see diagram in NIT 138). "Grippers" are used to hold 15 other items of equipment besides this relay device. These other grippers are designed in a way that prevents rotation. The only gripper not so designed was the one that dropped the relay device. It seems reasonable to conclude, therefore, that the accident resulted from a design error. The sodium leak and fire accident that occurred at Monju fifteen years ago on December 8 resulted from a design error in the sheath of a thermocouple (see NIT 134). This piece of equipment was also supplied by Toshiba.
JAEA attempted to remove the relay device so that it could ascertain the damage caused by the impact of the fall, but an alarm went off indicating excess load. JAEA used the maximum force allowed, but it was unable to remove the device and abandoned the attempt on October 13.
The relay device is made up of tubes connected by eight pins. It appears that the problem is related to this structure.
JAEA inserted a small camera to investigate the problem. It announced on November 9 that its investigation had revealed that a gap in a section connecting the tubes had expanded by between 7.5mm and 9.5mm. (The gap was originally between 5mm and 7mm, but after the accident it was 14.5mm.) This suggests that as a result of the impact of the fall the connecting section was deformed in some way, or a pin was ejected, or a tube was distorted. It is most probable that this is the reason why the device cannot be removed.
JAEA says that it will continue to consider how to conduct observations of the outer surface of the device and that it will continue to assess in a comprehensive manner how to remove the device. However, the camera looks through an aperture from quite a distance from the relay device and the environment of argon gas and sodium vapor makes it difficult to obtain clear results from such observations.
Apparently JAEA is considering using heat to expand the opening through which the relay device is extracted, or using more force, but in either case there is a high possibility that the device will be damaged in the process. At this stage no foolproof method has been identified. As a last resort, it could be necessary to remove the whole device from the vessel head. However, in that case it will be necessary to prevent air from mixing with the sodium in the reactor. This will require designing and constructing a large new piece of equipment into which the 12-meter relay device and the equipment connecting it to the vessel head can all fit. Air coming into contact with the sodium, which is heated to above 200oC, would cause a fire or explosion. In any case, there is no doubt that resolving the problem will take a long time.
Monju was restarted in May after being shut down for fourteen and a half years. So far it has completed core confirmation tests at zero power. It is supposed to proceed to tests at 40 percent power and, after the results of those tests have been assessed, to increase to 100 percent power. However, the tests have stumbled at the first hurdle and there is no indication when the reactor will be able to restart.
Hideyuki Ban (CNIC Co-Director)
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