Implementation of the Borehole Disposal System for Disused Sealed Radioactive Sources in Ghana

Transcription

1 Implementation of the Borehole Disposal System for Disused Sealed Radioactive Sources in Ghana Eric T. Glover Radioactive Waste Management Centre Radiation Protection Institute Ghana Atomic Energy Commission Ghana

2 Outline Introduction Regulatory Framework Radioactive Waste Management The borehole disposal project Conclusion

3 Introduction The use of radioactive materials in Ghana began in the early 1950s in the then University College of Gold Coast (now the University of Ghana). Radioactive sources have since been in use in various sectors of the Ghanaian economy (medicine, agriculture, industry, research and teaching) Ghana Atomic Energy Commission (GAEC) was established by an Act of Parliament (Act 204) in 1963 which has been superseded by Act 588 in The main functions of the Commission include the promotion, development and peaceful application of nuclear and biotechnology techniques for the benefit of Ghana.

4 Legal And Regulatory Regime From 1993 to 2015 the Ghana Atomic Energy Commission (GAEC) carried out regulatory functions through the Radiation Protection Board (LI 1559 of 1993 ). There was conflict of interest since GAEC is also an operator The Nuclear Regulatory Authority Act 895 (2015) established the Nuclear Regulatory Authority (NRA) as the statutory regulatory authority in Ghana. The law specifies the key functions and activities that are very important for effective regulatory system, including notification authorization, inspection and enforcement.

5 Radioactive Waste Management The Centralized Waste Processing facility after the upgrade will have a office and laboratory operational areas for segregation of the waste materials, Conditioning unit disused sources treatment of liquid waste. The storage facility has an area for receipt of the waste, low dose storage area and a high dose storage area. Has physical protection systems 5

6 Implementation of the Borehole Disposal System- Site Geology Ghana Atomic Energy Commission site at Kwabenya The site underlain by the Dahomeyan and the Togo geological formations The Dahomeyan is the major bedrock and is composed of gneissic rocks and schist The Togo is characterized by quartzites and phyllitic rocks

7 Safety Assessment First iteration The first iteration safety assessment was done taking into account the inventory to be disposed and the preliminary site characteristics. data on the regional geology, hydrogeology and hydrochemical conditions Radionuclides considered are Co-60, Sr-90, Cs-137, Ra-226, Am- 241 and Cf-252 The AMBER software tool Unit inventory of 1 TBq per package 43 waste packages will be disposed, One borehole considered The total thickness of the disposal zone is 43.5 m. The total depth of the closure zone is 56.5m

8 Site Characterization Two exploratory boreholes ( 150m) drilled for site characterization Hydrochemical Data Groundwater sampling ph Major Cation Major Anion Isotopic Composition( O 18 H 2 ) Hydrogeological Data Hydraulic properties - (transmissivity, hydraulic conductivity, hydraulic gradient, storativity) Geological Data Nature and Rock distribution Fractures and faults Mineralogy

9 Boreh ole ID Laterit e Site Characterization-Geologic Log BH m 4-6m 6-12m Clay Quartzite Phyllite Mixed materials (gneissic & others) Schist Fracture Water Strike 12-13m m m 33-36m 50-51m 13-37m 38-50m BH m 2-4m 4-9m 19-52m 37-38m 47-55m m m 9-19m m m 31-41m 47-74m m 94-95m m 62-63m m

10 Site Characterization-Mineralogical Composition of the Rocks Granitic gneiss at depth 56m showing (a) quartz vein, undulose micro quartz grains (b) megacryst of microcline with tartan plaid and altered plagioclase. Epidote chlorite Schist showing (a) defined by preferred orientation of minerals (stretch quartz grains, plagioclase (b) epidote are characterized by the berlin blue interference colour whiles chlorite shows pale to leafy green colours A reducing environment below 40m, demonstrated by the fresh pyrite and iron rich minerals ( chlorite).

11 BDS Scoping Tool - Specific Data Parameter Value Justification Site Hydrogeology Hydraulic Conductivity 4.49 m/y Highest value from the drilling report Hydraulic Gradient 0.034m/m As calculated in the drilling report Water-Filled Porosity 0.49 Value for Schist taken from literature on the high side to be conservative; range Site Geochemistry ph 6.68 Value obtained from analysis of the Groundwater from the site Eh -281mV This parameter has not been determined; value for fresh reducing high ph was used Total Inorganic Carbon 42.52mg/l Same comments as on the choice of Eh Chloride mg/l Highest value obtained from analysis of the Groundwater from the site Sulphate mg/l Highest value obtained from analysis of the Groundwater from the site

12 Borehole Design Disposal Depth Calculations Layer on top of each container= 750mm 13 of such layers x 750mm = 9750mm 2 X 603mm Disp. Containers= 1206mm 3 X 375mm Disp. Containers= 1125mm 8 X 199mm Disp. Containers= 1592mm Thickness of plug = 500mm BOREHOLE DESIGN Total length of disposal zone= 14173mm = 14.17m Depth of entire borehole= 130m m = m Closure Zone Depth=130m Eight 199mm Disposal Containers for all cat. 3 to 5 DSRS and Chinese Cat. 2 Co-60 Disposal Zone length=14.17m Three 375 mm Disposal Containers for Gamma cell Two 603 mm Disposal Containers for Hungarian Cat.2 Co-60 Thickness of plug = 500mm Not to scale

13 BDS Scoping Tool - Output Indicative Failure Times Backfill cement starts to significantly degrade at Backfill cement has completely degraded after Disposal container fails after Containment barrier cement starts to significantly degrade at Containment barrier cement has completely degraded after Capsule fails after Plume arrives at well after [y] [y] [y] (failure is caused by general corrosion) [y] [y] [y] (failure is caused by general corrosion) [y]

14 Results from BDC Scoping Tool The capsule fails after years The plume arrives at a well 100m away from the disposal borehole after years with peak dose of 4E-4 Sv/y The scoping tool considers groundwater pathway as an advective transport with no sorption. This implies that, the peak dose that a receptor will receive via ingestion of contaminated water or inhalation of gas for all cases is below the dose constraint of 0.3mSv/y. The results from the scoping tool suggest that the capsule and the disposal container will provide enough containment for the disposal system being considered at the proposed site.

15 Post Closure Safety Assessment -Site Specific Data Radionuclide inventory Depth of disposal zone Near-field degradation times (taken from BDS Scoping Tool) Number of capsules: 13 large capsules Co-60: 3.28E13 Bq Sr-90: 1.37E10 Bq Cs-137: 7.81E10 Bq Ra-226: 7.03E9 Bq Am-241: 3.70E7Bq P-32,Ca-45, Fe-59, Sr-89, In-113m, I-131, Ir-192 were screened out due to their short half life and low activity From 130m to 145 m below ground level Start of backfill cement degradation: 755 yrs End of backfill cement degradation: yrs Failure time for disposal container: yrs Start of barrier cement degradation: yrs End of barrier cement degradation: yrs Failure time for capsule: yrs

16 Post Closure Safety Assessment Site-specific Scenarios The scenarios identified in the IAEA Generic Safety Assessment for the borehole disposal concept (IAEA 2013) were reviewed modified to produce Ghana site-specific scenarios Design Scenario as per GSA with radionuclides released in the liquid phase into the saturated disposal zones Defect Scenario due to either defective manufacturing of waste packages (e.g. welding defects) or defective implementation in the Borehole Disposal System Well Scenario well for drinking and irrigation water located at downstream 100m from the disposal borehole Type Geosphere is set to, Anaerobic Porous with reducing conditions based on geochemical results from the site

17 D_Tot_Chain (Sv y-1) Post Closure Safety Assessment - Results Design Scenario E-08 1E-09 1E-10 1E-11 1E-12 1E-13 D_Tot_Chain[Farmer] Co60 Sr90 Cs137 Ra226 Am241 Cf252 Design Scenario Anaerobic Porous medium Total dose resulting from all the radionuclides and their chains is 2E-8Sv/y 1E-14 1E Time (Years) The maximum accumulated dose for all the scenarios simulated is below the dose constraint of 0.3 msv/yr.

18 D_Tot_Chain (Sv y-1) Post Closure Safety Assessment - Results Defect Scenario Anaerobic 1E-07 1E-08 1E-09 D_Tot_Chain[Farmer] Porous medium Welding Defect in one waste container and one waste capsule. 1E-10 1E-11 Co60 Sr90 The faulty capsule is in the faulty container. 1E-12 1E-13 1E-14 1E-15 Cs137 Ra226 Am241 Cf252 The total peak dose is below the acceptance dose of 0.3mSv/y. 1E Time (Years)

19 Ongoing Tasks Development a comprehensive site characterization report Integration of all the data into Site specific safety assessment Development of safety Case and environmental impact assessment

20 Thanks for Your Attention