Remediation strategy and capping construction for the mill tailings pond, Ningyo-toge Uranium Mine, Japan 2012.11.6 ～ 11.8 ENVIRONET Annual Forum, IAEA HQ, Vienna Hiroshi Saito Ningyo-toge Environmental Engineering Center, Japan Atomic Energy Agency

1. History and current status 2. Basic idea of remediation 3. Ongoing remediation

1. History and current status

1.1 Location and history Mill tailings ● Discovery of U outcrop Tottori pref. Okayama pref. Japan Sea Ningyo-toge　 Togo Mine Ningyo-toge Mine ● 　　Discovery of U outcrop 1950’s – 1960’s　 Exploration　　　　　　　　　　　　　　 1960’s – 1980’s　 Mining (gallery/open-pit), Milling 　　Mine-related activities terminated 　　　　 Environmental Remediation commenced Mill tailings

1.2 Related facilities 17 Waste Rock Yards Togo Mine Japan Sea Waste Storage Uranium Enrichment Demonstration Plant Waste Incinerating Facilities Katamo Refining and Conversion Facility Asabatake 17 Waste Rock Yards (incl. 2 remediation completed) Togo Mine Ore Test Facilities Kannokura Former Open-pit Mine Uranium Enrichment Engineering Facilities Tottori pref. Chojya Akawase Ningyo-toge Toge, Yotsugi Exhibition Hall Nakatsugo Okayama pref. Ningyo-toge Mine Water Treatment Facility Mill Tailings Pond Heap-leaching Facility Mine-related facilities Other facilities

1.3 Current status of Mill Tailings Pond History 1965　　Concrete dam built on riverbed Specifications 　　- Approved volume : approx.40,000m3 　　- Current volume : approx.87% filled 　　- Dose rate : < 1mSv/y 　　- Radionuclides’ concentration rate : 　　　　238U : 3.0Bq/g 　　　　226Ra : 16Bq/g Current status Stable Upstream vs Submerged Downstream Deposit tailing from former milling facility & mining waste from water treatment facility Impound water as a buffer reservoir Capping under construction (ONLY for Upstream) 　 Seepage collection pit Concrete dam 64m Downstream Mine water Mining waste 210m Upstream 鉱さいたい積場 Downstream Concrete dam Upstream

2. Basic idea of remediation

2.1 Necessity of remediation of the Mill Tailings Pond Concrete dam - Structural health diagnostics shows enough seismic-resistance strength - Enough space left for expected waste - The site is leased land from locals must be remediated before return - Large social impact in the event of outflow dam failure caused by earthquake - Presence is a cause for worry for locals 　　　 located upstream of water-source river However ….. Possible outflow Necessary and The highest priority

2.2 Remediation strategy Upstream Downstream Substitute facilities - Two-step strategy (Upstream --> Downstream) - Experience/data used for Downstream - Capping to reduce radon emanation & gamma radiation 　　　 　 minimize water infiltration use natural material only - Substitute facilities before Downstream remediation Upstream Confirmation of remediation effectiveness Site characterization Designing Capping Downstream Ultimate target Capping plus impermeable wall? 　これからの人形峠センターの事業は、 ・廃止措置および放射性廃棄物の処理処分を計画的かつ合理的に進めること、 ・鉱山施設の措置を適切に行い、長期的な管理を合理的に進めること、 　すなわち、「核燃料施設の廃止措置の魁」として、技術開発も含めて、廃止措置を適切に進めていくことと、安全最優先の風土を確立し、地域の皆様の信頼を得て、地域との共生を図っていくこと、を基本方針として取り組んでおります。 Site characterization Designing Monitoring No Water treatment Returning land Substitute facilities Mine water reservoir Mining waste pit

2.3 The merit of Two-step strategy Why Upstream first? - Not submerged - Considerably dried / stable Downstream could be used as a water reservoir even during remediation Mining waste Mine water Upstream Downstream submerged

3. Ongoing remediation

3.1 Area of remediation Legend Mill Tailings Pond (Upstream) Capping Ends of the steep capping slope Legend Mill Tailings Pond (Upstream) Capping Gabion work Soil improvement Uncovered area No tailing deposited (original ground-surface)

3.2 Specifications of Multi-layered Capping Material Purpose Vegetation (against erosion) Protection Grass seeds (only for slope) Decomposed granite soil Prevent erosion Green surface to blend in with the scenery Upper-filter Sand Prevent clogging of Drain-layer Drain Gravel Drain penetrated rainwater laterally Lower-filter Prevent outflow of swollen Bentonite Bentonite mixture Mixture of decomposed granite soil and bentonite (>17.2%） Lower dose-rate and Rn emanation Gradient modification Contour the structure of capping - Studied in radioactive waste disposal fields - Swelling property Contoured (5%) Drainage channel Drainage channel Mill tailings

(1) Bentonite mixture layer 3.3 Construction (1) Bentonite mixture layer Pump Water Hopper/Feeder (Decomposed granite soil) (Bentonite) Crusher/ Mixer Mixture of Bentonite and Decomposed granite soil Bentonite mixture layer compaction CONTROL Mixture rate (Bentonite > 17.2%) Grain size (max.<10mm) Thickness Hydraulic conductivity (< 1.0×10-9m/s ) Degree of compaction (> 95%)

(1) Bentonite mixture layer 3.3 Construction (1) Bentonite mixture layer Hopper/Feeder (Decomposed granite soil) Crusher/Mixer (150m3/d) Hopper/Feeder (Bentonite) Mixture of Bentonite and Decomposed granite soil Surface compactor (10t) Surface compactor (1t)

3.3 Construction (2) Capping Bentonite mixture Layer Lower-filter Layer Drain Layer Upper-filter Layer Vegetation Layer

Drain hole for Drain Layer 3.3 Construction (3) Drainage channel Expected capping surface 50cm Drainage channel Around the capping Drain hole for Drain Layer (every 0.5m)

(4) Gabion work / Mattress basket 3.3 Construction (4) Gabion work / Mattress basket Constructed at the end of the capping slope (51m length/ 5m high/ 4m width)) Inexpensive Flexible to topograpy Not impede groundwater flow

Gravel Compaction Pile Method 3.3 Construction (5) Soil improvement 1. 2. 3.～4. 5. 6. Casing Gravel Gravel pile Vibro- hummer Gravel Compaction Pile Method Set a casing under Viblohummer. Penetrate a casing down to the target depth. Put gravel into the casing. Pull out the casing to leave the gravel. Penetrate the casing again to tamp down the gravel. Repeat 3.～5.

Gravel piles after soil improvement 3.3 Construction (5) Soil improvement Groundwater flow Gravel-filled Casing Viblohummer Gravel piles after soil improvement

3.4 Monitoring for confirmation of effectiveness Drainage volume -- Confirmation of expected drainage ability (Reduce load to water treatment facility) Rn emanation rate / dose rate -- Radiation evaluation Settlement amount / rate -- Long-term stability Influence on drainage ability Underground temperature -- Freezing depth Influence on Drain Layer Drainage channel Mill tailings Surface water Penetrated rainwater Precipitation Freezing depth 1-year monitoring results reflected in specifications of Downstream remediation

Thank you for your attention and time Questions?