Dose Rate Reduction Methods at Shimane Nuclear Power Station Tadashi Kanaoka1, Hiroyasu Kajitani1,...

Dose Rate Reduction Methods at Shimane Nuclear Power

Station

Tadashi Kanaoka1, Hiroyasu Kajitani1, Tsuneo Sato1, and Naoshi Usui2

1The Chugoku Electric Power Company, INC2Hitachi-GE Nuclear Energy, Ltd

Copyright©　 The Chugoku Electric Power Co.,Inc. All rights reserved. 2

0.The location of　 Shimane NPS and Head office　Shimane　 NPS

Head offie


1.Introduction

Shimane Nuclear Power Station Unit 1 continues operating smoothly since commercial operation started in 1974.

An increasing trend of the dose rate has been observed since the hydrogen injection was applied at 21st operating cycle (1998) and the dose reduction became the urgent problem.

unit1

unit2


2(1).Radiation exposure of the outage

0.92 0.90

1.58

2.39

3.00

0.75

1.24

0.0

1.0

2.0

3.0

4.0

21 22 23 24 25 26 27

Number of outage

To

tal

rad

iati

on

ex

po

sure

of

ou

tag

e w

ork

s (p

erso

n・S

v)

Figure 1 Radiation exposure at Shimane Nuclear Power Station Unit 1 after hydrogen injection

Highest level among the total radiation exposure of the outage works in recent Japanese BWR plants.


2(2).The dose rate of the PLR piping

Figure 2 PLR dose rate trend at Unit 1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

22-1

22-2 23 24 25 26

27-1

27-2 28

Number of outage

PL

R p

ipin

g do

se r

ate

(mS

v/h)

.

HWC

Before chemicaldecon.

After chemicaldecon.

?

Atmospheric dose rate in the pressure containment vessel are increasing since the hydrogen injection start　


3.Dose rate reduction committee

3WG were settled under the committee.Maintenance staff and headquarter persons were added to the committee .

Superintendent (Station)

Dose rate reduction committeeChief examiner:Deputy

SuperintendentManagers of each section of

stationManagers of headquarter

Chemistry control WG Equipment management WG Radiation control WG

Headquarter staff

General Manager(Nuclear dept.)

ReportReport

Technical support Hitachi-GE Nuclear Energy

Figure 3 Structure of dose rate reduction committee


4.Examination items of dose rate reduction committeeShort term target is to lessen the dose rate of 28th outage at Unit 1. Middle and long term items aim for permanent dose rate reduction.

ItemChemistry control

WGEquipment

management WGRadiation control WG

Objectives

Dose rate reduction in PCV

Improvement of work methods and work environment

Improvement of radiation control

Matters

NWC pre-oxidation operation,CF bypass operation,Hi-F Coat, etc

Enlargement of chemical decontamination area ,etc

Piping dose rate prediction method,Temporary shielding,Survey of good practice, etcExtension to middle

and long term

Figure 4 Examination items at dose rate reduction committee

Middle and long

term matters

Zinc injection,Improvement of water chemistry control, etc

Adjustment of execution timing of inspection to reduce dose rateChemical decontamination to Valve and PLR-pump impeller , etc

Establishment of piping dose rate prediction method Improvement of mounting method of temporary shielding ,etc


5(1).Applied countermeasures until 28th outage

Hi-F Coat was applied after chemical decontamination at the 27th outage. NWC pre-oxidation operation of 90 days was conducted at the beginning

of the 28th operating cycle to form a fine oxide structure under NWC conditions.

Pump

CoolerIon exchange column

Catalyst column

Chemical injection system

Filter

Surge tank（ with

heater）

Pump

Target（ PLR piping etc.）

　 Figure 5 Outline of Hi-F Coat treatment equipments


5(2).Results of countermeasures(1) dose rate

Figure 6-1 PLR dose rate trend at Unit 1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

22-1

22-2 23 24 25 26

27-1

27-2 28

Number of outage

PLR

pip

ing

dose

rat

e (m

Sv/h

)

HWC

The PLR dose rate has decreased to the same level before beginning of HWC.　


5(2).Results of countermeasures(2) dose rate

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

22nd Afterdecon.

23rd 27th Afterdecon.

28th

Number of outage

Dos

e ra

te o

f PL

R

（mSv

/h）

Higher dose rateby rebound

Lower dose rateby countermeasuresto suppress rebound

Figure 6-2 Dose rate change of PLR piping after the 27th outage

Dose rate of PLR piping was about 0.5 mSv/h at the 28th outage at Unit 1 and became much lower than that at the 27th outage


5(2).Results of countermeasures(3) total radiation exposure

1710

0

500

1,000

1,500

2,0005/

24 6/7

6/21 7/5

7/20 8/2

8/16

8/30

9/13

9/27

End ・・・

End

Tot

al r

adia

tion

expo

sure

（per

son･m

Sv) Outage work（Prediction）

Outage work（Result)

Reduced to 70 %

Outage shorteningof about 40 days

Figure 7 Trend of total radiation exposure during the 28th outage (Outage works)

Total radiation exposure could be suppressed to about 70% of the planned value. Term of the outage could be cut for about 40 days compared to the original plan


6.Future activities(1) On-line monitor

Experimental equipments to gather basic data such as energy distribution of gamma ray and atmospheric dose rate in the PCV.

Figure 8 Appearance of experimental equipments


6.Future activities(2) Zinc injection

Zinc injection is a permanent countermeasure to reduce dose rate to meet the company need to continue hydrogen injection.

DZO

FW pump

FW piping

Figure 9 Image　 of　 zinc injection　 system


7.Conclusion

■We have decreased the dose to the same level 　 before beginning of HWC.

■We will evaluate the contribution and durability of Hi-F 　 Coat etc..

■We will research the Zinc injection etc. as further 　countermeasures.

Dose Rate Reduction Methods at Shimane Nuclear Power Station Tadashi Kanaoka1, Hiroyasu Kajitani1,...

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