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

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

3. 1. History and current status

4. 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

5. 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

6. 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

7. 2. Basic idea of remediation

8. 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

9. 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

10. 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

11. 3. Ongoing remediation

12. 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)

13. 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

14. (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%)

15. (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)

16. 3.3 Construction (2) Capping Bentonite mixture Layer
Lower-filter Layer
Drain Layer
Upper-filter Layer
Vegetation Layer

17. 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)

18. (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

19. Gravel Compaction Pile Method
3.3 Construction
(5) Soil improvement ～
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.

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

21. 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

22. Thank you for your attention and time
Questions?