Kyushu Electric Power informs the cause and counter measure about the increase of flow ofRCP seal of the Genkai NPS Unit 4,The Genkai NPS Unit 4, a 1180Mwe PWR under a periodical inspection, decided tosuspend the restart preparation and examine around the RCP No.2 seal since it has beenfound that the flow rate, which protect the leakage of high pressure primary coolant waterand is called return flow of No.2 seal, is larger than usual.
There is no primary coolant leakage outside the affected system. Also, there is noradioactive impact on the environment.
It was pressed on 3rd May 2018.
As a result of the inspection, it was estimated that the seal surface of No. 2 seal part withmovable function opened and No.2 seal water return flow increased due to the following
reasons.
1. By filling the purge water line of the primary coolant pump and No. 2 seal water returnline with full water for storage, a large force was added to the No. 2 seal portion, and theO-ring related to the movable function of No. 2 seal portion bites into the gap.
2. Therefore, due to the restraint of the O-ring, the No. 2 seal portion stuck and mobility
worsened, and the seal surface opened.
Based on the research result, we will perform following counter measures.
For the primary coolant pump, we will replace the No. 2 seal and the No. 3 seal, which
are the integral assembly.

When storing the purge water line of the primary coolant pump and the No. 2 seal water
return line, we will change the method for storage to ensure that the gas phase issecured and not to store the full water.

Kyushu Electric Power Company continuously conducts every work steadily and attentively,
ensuring safety as No.1 priority.
【O-ring related to the functional movement of No.2 seal】
It is the ring made of lubber and is located in the gap between outside and inside of the 2cylindrical shaped metal(Translated by JANSI)Genkai Nuclear Power Plant Unit 4 Reactor Coolant Pump

Electric Motor

Seal water injection
Main
rotor

Seal parts

Outflowof RCS
Inflow of RCS

System Outline

Reactor Coolant Pump Outline

The system diagram of RCP seal waterStandpipe

Purge waterhead tank

No.3Seal

No.3 seal return line
No.2 seal return line
VolumeControl tankPurge water line

O-ring
To RCS

No.1
Seal

Outside Containment
VesselSeal Water return line
Inside Containment
VesselNo.2
SealStandpipe

Pump
rotor

Charging pump
Sea water injection line
(Seal injection)
Sealed water is injected at even higher
pressure so that the primary coolant ofhigh pressure does not leak out of thebearing

Containment
Vesselcoolant drain tank

The situation in which the seal surface of No.2 became open
①Regarding the purge water line of the primary coolant pump and No. 2 seal leak off line,
the temperature of the storage water has risen, the pressure of the No.2 seal increased,and large pressure occurred to No.2 seal since by the increase of the temperature ofstorage water due to air temperature rise after fully filling occurred (gas phase part isslight) by carefully performing the air venting.
②Due to the large force, since the O-ring bites into the gap, the binding force of the O-ringbecame large.
③ As a result, the No. 2 seal portion stuck and the seal surface opened due to the pressure
rise of the primary coolant system.

②O-ring bites into the gap

PumprotorO-ring① Larger pressure than normal condition

PurgewaterNo2. Seal part
Operate up anddowninnormalcondition③No. 2 seal portion stuck andthe seal surface opened