Measures for Reduction of Radiation Exposure at Higashidori Nuclear Power Station Nuclear Power Dept. Tohoku Electric Power Co.

Tohoku Electric Power Company Nuclear Power Plants Onagawa MWe Type Commercial Operation Unit-1(O-1) 524 BWR Unit-2(O-2) 825 BWR Unit-3(O-3) 825 BWR Higashidori MWe TypeCommercial Operation Unit-1(A-1) 1100BWR Electricity Supply area Onagawa NPS Higashidori NPS Tokyo

Exposure reduction measures at Higashidori Nuclear Power Station ◎ ◎ ◎ 保守・点検作業の省力化 作業性の改善 Improvement of equipment Crud reduction Cobalt reduction Material surface treatment Water chemistry control Anti-adhesion Radiation shielding Remote handling and automation Reliability improvement Labor-saving in maintenance and checking Operational improvement Adoption of hollow fiber membrane filters for condensate cleanup system Adoption of weather resistant steel and low-alloy steel for turbine system Enhanced measures of storage during construction and commissioning Oxygen injection into feed water system Adoption of low-cobalt stainless steel Alternative to stellite Adoption of low-cobalt Inconel Electropolishing Pre-filming of feedwater heater tubes Operation at extremely low iron concentration Reduction of CUW pump temperature Installation of permanent shielding

Radiation sources that control plant doses Replacement-type sources Radioactive ions in reactor water are incorporated in the oxidized film (oxidized scale) generated on hot portions of the reactor piping system. ・ PLR/ CUW piping and components Radioactive crud in reactor water is deposited at horizontal portions and other portions where water flow is stagnant or slow. ・ CRD flanges ・ Filters ・ Low-temperature pipe sections, such as those in the RHR system ・ Horizontal portions of PLR/CUW piping ・ Nozzle sleeves Deposition-type sources Replacement-type sources Oxidized film Reactor water Ion Crud Base metal

Measures to reduce crud (Clean plant action No. 1) ① Improvement of work environment ② Protection ③ Maintenance of inner surface cleanliness Thorough storage management and maintenance of cleanliness on inner/outer surfaces of system piping and equipment Prevention of carried-in dust by installing air guns and jet sprays at doorways

系統試験時 Purity control of test water Reduction of carried-in crud During system test Thorough storage management Measures to reduce crud (Clean plant action No. 2) ★ ★ ★ ★ During start-up test First cycle Primary system cleanup operation Condensate/feedwater purification operation Condensate/feedwater swing operation Cleanup of hot well Cleanup of residual heat removal system Control of water treatment system Suppression of reactor water activity concentration

Measures to reduce crud (Amount of crud generated in start-up test) 2.3 ～ ～ 270 Performance of existing plants for reference Higashidori unit ～ ～ Other BWR units Observed Target value 2.3 ～ ～ 270 Onagawa units 2 and ～ Amount of crud generated in condensedwater (in kg as Fe) 2.6 ～ Amount of carried-in crud In feedwater (in kg as Fe)

Water chemistry control (operation with extremely-low iron and high nickel concentrations) ニッケル鉄比制御極低鉄運転 Fe 配管 ①ﾌｪﾗｲﾄ化 Ni,Co ③ Co-60 溶出 ②放射化, 蓄積 配管 Ni,Co ①ﾓﾉｵｷｻｲﾄﾞ化 ③ Co-60 蓄積 せず、即溶出 ②放射化 Fe 配管への 放射能付着 疎な NiFe 2 O 4 Co-60 取り込まれる 密な NiFe 2 O 4 Co-60 取り込まれない ｸﾛﾑﾘｯﾁ 燃料表面 での挙動 Behavior on fuel surface Adhesion of radioactive material to piping Control of nickel/iron ratio Operation with extremely low iron concentration (1) Ferritizing (2)Activation and accumulation (3) Elution of Co-60 Piping (1) Mono-oxidization (2) Activation (3) Immediate elution of Co- 60 without accumulation Sparse NiFe 2 O 4 Incorporated Cr-rich Dense NiFe 2 O 4 Non incorporated

Water chemistry control (Progress of feed water iron concentration and reactor water nickel concentration)

Material surface treatment  Pre-filming of feedwater heater tubes 15214mm Average effective length 2780 Total number of heat transfer tubes 2110m / unit 2 Heat transfer area × 1.0mm SUS304TB-S equivalent Specifications of second high-pressure feedwater heater 15214mm × 1.0mm 【 Thermal treatment of heat transfer tubes is performed in a “hydrogen + steam” atmosphere. (conventionally, performed in a hydrogen atmosphere) Suppression of ion elution 【 Purpose 】 【 Steam oxidation treatment 】 Reactor vessel Condenser Turbine 復水 ろ過 装置 復水 脱塩 装置 Low-pressure feedwater heater Reactor recirculation pump Condensate filtration unit 復水 脱塩 装置 Condensate demineralizer Filtration demineralizer of reactor water cleanup system Second high-pressure feedwater heater First high-pressure feedwater heater (first to fourth) Material Outer diameter x thickness

Material surface treatment (Progress of Cr ion concentration in feedwater) Effective full power hours (until the end of the first cycle)

Material surface treatment (Progress of Co-60 activity concentration in reactor water) Effective full power hours (until the end of the first cycle)

Material surface treatment (Progress of Co ion concentration in reactor water) Effective full power hours

Effect of radiation exposure reduction measures (No. 1)  Dose rate on PLR piping Chemical decontamination performed Domestic BWRs Number of periodic inspections

Effect of radiation exposure reduction measures (No. 2) (Air dose rate in reactor containment vessel) A-1 First measurement O-3 First measurement Four days after reactor shutdown On the floor of recirculation pump motor Geometrically averaged dose rate: 0.01 mSv/h n=48 Area dose rate: 0.01 mSv Geometrically averaged dose rate: 0.03 mSv/h n=48 Area dose rate: 0.04 mSv

Summary  Surface oxidation of feedwater heater tubes was effective in suppressing carried-in Cr ions.  Suppression of carried-in Cr ions was effective in suppressing the increase of Co-60 activity concentration in the reactor water.  Dose rate on PLR piping was suppressed successfully.  Air dose rate in the dry well was suppressed successfully.  Total radiation exposure during periodic inspection was reduced successfully.

Collective Dose in BWR （ Average ）