Restart information of the Ikata nuclear power station unit 3
July 25, 2016
Shikoku Electric Power Company

Ikata Unit 3: Investigation of No.3 Seal of RCP-3B
It was detected that the flow rate*1 increased in the No.3 seal leakoff line of the Reactor
Coolant Pump 3B (RCP-3B) of the Ikata Unit 3 (PWR, 890Mw) during the RCP test
operation.
Operators tried to adjust seating condition of the No.3 seal without success. At 9:20am,
July 17, the station decided to replace the affected seal with spare one.
The No.1 and 2 seals of RCP-3B are working in a good condition.
There is no radioactive impact to the environment due to this event.
(Press release on July 17, 2016)
The station investigated the No.3 seal and discovered that the seal ring slightly tilted, which
prevented it from moving smoothly.
On July 12, the station performed a pressure test of the containment vessel. During thetest, high pressure worked on No.3 seal and caused the O-ring*2 to stretch and fill gaps
around the seal, which supposedly produced strong friction. The seal ring was likely to getstuck because of this friction.
This is supposed to be a reason that the seal ring tilted when the RCP-3B started up,
causing the seat surface uncovered and allowing seal leakoff flow rate to increase.
The station is going to replace the RCP-3B No.2 and No.3 seals with spare components. At
the same time, seals of the RCP-3A and 3C will be also replaced to ensure a similar trouble
will not happen.*3
This event is not in the category of events which require reporting to the government
according to the law or circular notice.
*1No.3 seal leakoff rate
The RCP shaft is sealed by triple sealing system of No.1, 2 and 3. While the primarycoolant is sealed by No.2 seal, No.3 seal serves to control pure water to clean theNo.2 sealing part. The flow rate at the No.3 seal outlet is called the No.3 seal leakoffrate.
The No.3 leakoff water travels in the dedicated piping until reaching the tank within thecontainment vessel.
*2
O-ring is a ring-shaped sealing part which is circular in cross-section*3
No.2 and No.3 seals are replaced altogether because they connect each otherAttachment 1

Schematic of the RCP seals

Attachment 2

Mechanism of pressure reversely working on No.3 seal during a CV
pressure test

Attachment 3

Mechanism of seat surface to open
(Translated by JANSI)This notice has been published on the Shikoku’s website. For the original text, please go to:

http://www.yonden.co.jp/press/re1607/data/pr005.pdf

Attachment 1

Schematic of the Reactor Coolant Pump seals of Ikata Unit3
RCP sealing area
Affected area

Containment Vessel

The seat surface of No.3 seal opens andpurge water flows into the seal leakoff area,
which increases the flow rateNo.3 seal leakoff

SGNo.3sealNo.2 seal leakoff

Purge water*

RVPressurizer

No.2
seal

Seal water (going out)

No.1
seal
Seal water (coming in)*Purge water is pure water to clean sealing components

RCP main shaft

Attachment 2

Mechanism of pressure reversely working on No.3 seal during a CV pressure test

Purge water
Head tank: Atmospheric pressure in the CV: Separated from the CV atmosphere

3V-CS-263B

FI

Standpipe

OpenFI

No.3 seal leakoff line
3V-CS-258B

No.2 seal leakoff line
No.3
SealVolumeControl tankPurge water

Seal water (going out)
No.2SealMM
3V-CS-214StandpipeM

Outside CV

No.1
Seal

Inside CV

To Reactor
Coolant System

Filling pump

Ｓ

Seal water(coming in)

CV coolant
drain tank

3L CV-169

 During a CV pressure test*, pressure around No.3 seal outlet becomes the same asatmospheric pressure in the CV, which is higher than usual (318kPa). On the other
hand, pressure in the space between No.2 and No.3 seals remains unchanged(approx. 100kPa) due to sealing and valves both up- and downstream being shut. Asa result, pressure works in the opposite direction around inlet/outlet of No.3 seal.
*CV pressure test is a test to pressurize the inside of CV and ensure the CV is as strong as designed.

Attachment 3

Mechanism of the seat surface to open
Mechanismof friction
increasecausedby byO-ringof ofNo.3
seal
Mechanismof friction
increasecausedO-ringNo.3
seal (assumption)(assumption)
CV test pressure
CV test pressure

CVCV atmosphere
atmospherepressurepressure
O-ringO-ringUpwardUpwardforce
force

Purge water
Purge waterpressurepressure

Purge water
Purgewaterpressurepressure

Downwar
Downward
dforceforce

Spring force
Spring force

CV PressureTest

NormalDue
DuetotoCV
CVpressurepressuretest,test,strong
strongdownward
force
works
on
theO-ring.
downward force works on Additional
the O-ring.friction
isAdditional
generatedfriction
from thepushed
is generatedO-ring.
fromthe pushed O-ring.

Behavior of No.3 seal (assumption)

O-ring friction(large)O-ring friction (small)approx. 200mm

approx. 200mm

Seal ring
Seal ring

After CVpressure test

Seat surface
Seal runner

When RCP
starts up

Seat surface
Seal runner The seal runner moves down when RCP starts up. Uneven friction generated
around the affected O-ring is considered to have prevented the seal ring fromfollowing the movement of the seal runner when it went down. The seal ringtilted and the seat surface was let open.