The ore has an average grade of approximately 15% U3O8. Ore will be mined using remote underground mining methods, following which it will be wet-ground and thickened underground, and pumped as a slurry to the surface. Here it will be placed into purpose-built containers before being transported over an 80 km long road to Key Lake, where it will be processed in the existing mill. This paper describes in general the underground ore handling system and in detail the development and the detailed design of the ore loading and transportation system (including the road), the testing of the custom-built containers, the filling and emptying these containers, and radiation protection and regulatory issues.
The radiation safety design criteria were chosen such that the operation would easily meet the new proposed dose limits of the Atomic Energy Control Board (AECB), keeping in mind the ALARA principle. In practice this means that the radiation doses will be below the new limits. The design of all facilities was selected to minimise radiation exposure and to prevent the build-up of radon progeny. During all stages of mining, ore handling and processing, the ore is fully contained. Radon emission is controlled using a combination of primary area ventilation with single pass air and secondary fume exhaust extraction from all vessels, sumps and equipment. The overall strategy has been to contain and isolate the ore from the workers using multiple barriers.
Gamma radiation is controlled by shielding such as thick steel walls in equipment, concrete walls and in some cases lead sheeting. However, shielding is not the primary radiation safety design component. The primary radiation safety features are the layout and the mining and ore handling systems. These features limit the time required for workers to be around ore radiation sources. Exposure time in ore handling areas is minimised by remote operation. The design criteria will give operation personnel sufficient flexibility to deal with unforeseen situations and still keep doses well below the new effective dose limits of 100 mSv per five years and 50 mSv per year.
Hazard and operability studies (HAZOPs) were carried out on all ore handling processes and mine ventilation system. The purpose of these studies was to postulate hazardous scenarios, identify the safeguards existing to mitigate the hazard, and make recommendations to the designers to reduce the severity or the likelihood of the hazard.
Project Status
The history, geology, underground exploration and the mining methods for this project were previously described in a paper at the Uranium Institute's Annual Symposium in 1997 (Ref 1). At that time Cameco had just received governmental approval to proceed with the project. Since then the development has progressed in all areas:
History of the Project
After eight years of systematic exploration the orebody at McArthur River was discovered in 1988 by SMDC, one of Cameco's predecessor companies. Core drilling from the surface outlined high grade mineralisation over a strike length of 1.7 km. By 1991, 36 of a total of 60 holes intersected uranium mineralisation at a depth of 500 to 600 m. At that point a resource of approximately 260 million lbs of U3O8 (100 000 tU) at an average grade of 5% U3O8 was estimated.
To understand the structure of the orebodies better it was decided to undertake an underground exploration programme. After receiving all governmental approvals, shaft sinking commenced in early 1993. A total of 300 m of horizontal development on the 530 m level followed, permitting diamond drilling along the strike length of the mineralised zone. This definition drilling increased the reserves and resources to 416 million lbs U3O8 (160 000 tU) at an average grade of 15%.
A second environmental impact statement (EIS) assessing the impacts of the construction, full underground production, ore transportation, and milling and tailings disposal at Key Lake, was submitted in late 1995. During 1996 public hearings were conducted and in May of 1997 approvals were received from both the provincial and federal governments. Licensing of various construction packages by the AECB and Saskatchewan Environment and Resource Management (SERM) is proceeding.
Geology and Mining
The mineralisation at McArthur River is associated with a major northeast trending fault, referred as the P2 Fault, and lies close to the unconformity which separates the unmetamorphosed sandstones of the Athabasca group from the older metamorphosed basement rock of the Canadian Shield beneath (see Figure 2). The main uranium mineral is pitchblende. At McArthur River the pitchblende is not associated with the cobalt-nickel-arsenosulphides normally found in other large high grade uranium deposits in Saskatchewan. Uranium mineralisation has been identified for 2 km along the P2 fault; the other 8 km of the fault have not yet been drilled.
Overriding criteria used to select the most appropriate mining methods are the wide range of ground conditions, including groundwater flow, and the high grade of the ore, exceeding 40% U3O8 in places. Therefore, non-entry mining methods were required, coupled with freezing techniques for groundwater control. Preferred options are raise boring, box-hole boring and remote box-hole stoping. Raise boring was selected for the initial mining method to extract the high grade ore from Pod 2.
As shown in Figure 3, in this method drifts will be established in barren waste rock below and above the orebody. These drifts are on the 530 and 640 m levels. A raise bore machine will drill a vertical pilot hole from the upper to the lower drift. Once this hole is complete the pilot drill bit is removed and replaced with a reaming head fitted with tungsten carbide cutters. The raise bore machine then pulls this rotating head up through the barren rock into the ore zone . The reaming head will initially be 2.4 m in diameter. Rotation of the reaming head and applied thrust from the raise bore machine produces rock chips typically the size of road gravel. These chips fall to the bottom of the raise and enter a transportable skid, described below. Reaming of a given raise continues to the top of the ore zone. At this stage the reaming head is lowered and removed. The hole created is then backfilled with a low strength concrete from the upper drift via the pilot hole.
Underground Ore Handling
The transportable skid, shown in Figure 4, is specifically designed to support the raise bore mining operation. It comprises two independent structures. The upper portion is a back mounted sliding gate and chute assembly, which is anchored in place with roof bolts after the pilot hole has penetrated the ore extraction chamber (the lower drift). The lower section is the main skid and consists of a heavy duty structural steel frame with a built in vibrating feeder, vibrating sizing screen, water spray and slurry pumping equipment.
An expandable seal between the raise borehole and the upper frame forms the first dust seal. A perimeter collection duct with several pick ups on strategically located points between the upper and lower section of the skid exhausts any dust, air or mist created during the spraying operation to a wet scrubber. The overall strategy of the operation is to contain and isolate the ore from the operators. The transportable skid is therefore designed for automatic operation with minimum operator attendance. Closed circuit TV cameras are installed on the skid and the operation can be monitored in the control room. The controls for the transportable skid are interlocked with the raise boring operation in the upper drift, which is also monitored in the control room.
Ore is mined 24 hours a day, although the production rate may be intermittent depending on the sequencing of the mining machines. Ore cut by the raisebore machine drops onto a vibrating feeder and moves to a screen. Figure 5 shows a simplified McArthur River process flowsheet. The sizing screen separates the ore at 20 mm. The plus 20 mm oversize material falls into a holding container which is transported by a remotely controlled scoop tram to the semi-autogenous (SAG) mill. The minus 20 mm material passes with the spray water through the screen and is pumped as a slurry to cyclones. Cyclone underflow drops directly into the ore surge tank. Cyclone overflow passes over a trash screen whose undersize is 100% minus 0.5 mm. The dilute slurry passing through the trash screen is then pumped to the underground ore thickeners. The trash screen oversize reports into the previously mentioned ore surge tank. The surge tank provides a buffer between mining and milling. The layout of the cyclones and the ore surge tank is shown in Figure 6.
Most of the grinding circuit feed comes from this ore surge tank. Design ore feed rate is 25 tonnes/hour. Grinding takes place in a single SAG mill operated in closed circuit with a 500 micron opening classifying screen, thereby assuring that the ore is ground to the size required for further pumping and processing stages. Figure 6 shows the arrangement of the grinding equipment. The grinding circuit operates as long as is necessary to grind the required daily tonnage of ore. Ground ore is pumped to the two underground thickeners (see Figure 7) where the ore is thickened to 50% solids (by weight). Underflow density for each thickener will be measured by instrumentation.
The thickener underflow is pumped to an air agitated tank which is located between the 620 and 640 m levels. This tank can hold 200 m³ of slurry or approximately 160 t of ore. This large tank is used to feed one of two positive displacement pumps. The tank is supported about 6 m above the floor of the pumping chamber (see Figure 8) to provide 5 m of net positive suction head to each pump. The slurry is hoisted to the surface by pumping through two slurry pipes. The pipes run through dedicated boreholes and into the ore storage facility on surface. Here they discharge into four large ore storage pachucas.
On average, a positive displacement pump is expected to hoist 54 t/hr of ore solids. Therefore, with a 10% U3O8 ore grade, a pump would be required to operate from 4 to 6 hours per day. With a 30% U3O8 ore grade, a pump would be required to operate for only 2 hours per day. The design flow rate is determined by the pipe diameter and flow velocity. Hence the slurry pipes have a 93 mm inner diameter and the flowrate is 73 m³/hr. This combination of flowrate and pipe size generates an in-line velocity of 3 m/s, well above the deposition velocity. In the event of an unexpected or emergency shut-down of the pumps, the slurry hoisting lines must be drained back to the thickener underflow slurry tank. In this case the slurry will pass through pressure reducing orifices. Figure 9 indicates the relative positioning of the underground ore handling circuits in the mine.
Surface Storage and Load-out Facility
Ore slurry from underground is stored in four 650 m³ air agitated tanks (pachucas) on the surface. The tanks provide sufficient surge capacity between the mining operation at McArthur River and the milling operation at Key Lake to smooth out ore mining rates and ensure a steady supply of ore to the Key Lake mill. They also allow for blending of ore to grades required for shipment. An on-stream analyser ahead of a four-way distributor measures the ore grade in the slurry stream. Depending on the indicated ore grade the distributor directs the ore to one of the four pachucas.
From the storage pachucas the ore is pumped via an ore mix tank to a 12 m diameter load-out thickener, which provides a consistently high solids content in the ore slurry, thereby minimising shipment of water to Key Lake. The thickener also serves a secondary function, which is to handle waste solids from other process circuits on the surface and underground. These solids include sludge from the primary and secondary water treatment plant, underground recycle water bleed, container wash, etc.
The underflow of the thickener is pumped 24 hours a day to the container feed tank in the slurry load-out area. The desired solids content in the underflow is at least 50% solids. Overflow from the thickener reports to the primary water treatment plant. The container feed tank is agitated to keep the slurry in suspension. From the container feed tank the ore slurry is circulated through a pipe header to four batch tanks and back to the container feed tank. When the truck-mounted ore transport containers are in location to be filled, the ore slurry is diverted from the circulation loop to the batch tanks. The batch tanks have a capacity of 5 m³ each and will overflow if excess slurry is pumped into them. This assures that the ore transport containers are not overfilled. When all four batch tanks are full the slurry loop returns to circulation mode and the transport containers on the truck can be filled.
Ore Container
The containers transporting the ore from McArthur River to Key Lake are classified by the IAEA as an Industrial Package Type II (IP-II) with a combined surface contamination limit of 4 Bq /cm² for gamma, beta and low toxicity alpha emitters; the external radiation level at 3 m from the unshielded material must not exceed 10 mSv/hr. The IAEA surface contamination limits are based, in order of importance, on the increased risk of radiation exposure to the workers, the public and the environment, in accordance with the ALARA principles.
Figure 10 shows the general design of the container. The design, manufacturing, testing, operation, maintenance and inspection of the containers are required to be subject to quality insurance programmes in accordance with the IAEA Regulations for the Safe Transport of Radioactive Material (1990 Edition).
The container vessel has a simple geometry. It consists of a cylindrical centre section with ends of standard semi-elliptical formed heads. A protective outer frame surrounds the vessel. The container is designed to hold 5.2 m³ of slurry when 95% filled. A full container weighs 14.5 t.
The container wall is fabricated from 16 mm thick carbon steel using a low alloy steel. This material was selected because of its strength, and its ductile-to-brittle transition temperature of minus 50°C. The container vessel is enclosed within 4 mm thick carbon steel plate fixed to the protective outer frame. The combined thickness of 20 mm of steel will attenuate gamma radiation from the ore to less than 0.1 mSv/h at a distance of 2 m from the vertical plane of the side of the truck, with all containers filled by a 50% solids slurry at grade 30% U3O8 . All cavities between the frame and the vessel are filled with Zonolite industrial pearlite loose fill insulation. Tests with an earlier prototype container insulated with Zonolite showed that the slurry does not freeze when subjected to the most extreme weather conditions (e.g. minus 40°C for 24 hours).
The frame has been designed to provide support and protection for the vessel for safe transport and handling. In addition, it has been equipped with forklift guides for easy handling during container maintenance.
The single container opening is sealed by a 8 inch (35 cm) full port ball valve. This valve is opened and closed by a double acting pneumatic actuator mounted on the container. A check valve is also installed below the ball valve, mounted in the container on the same flange as the ball valve. The check valve is of rubber elastomer construction. The rubber is formed and vulcanised such that the check valve returns to its closed position whenever it is not being forced opened. For the filling and emptying process, the fill pipe or unload pipe which is inserted into the container will be forced through the rubber check valve. When the pipe is retracted, the valve will again close. The check valve creates a second means of container sealing in addition to the ball valve.
The top of the container frame is entirely enclosed by a drip pan which slopes from the perimeter of the frame down to a smooth transition into the ball valve. The drip pan is designed to divert potential drips from the filling or emptying pipe back into the container. This is achieved by directing wash water onto the drip pan. With the ball valve open, the wash water flows back into the container and collects in the check valve. A separate vacuum pipe is inserted into the wash water collecting in the check valve and this potentially dirty water is removed. After washing, the ball valve is closed.
This container configuration is the second generation design, succeeding the first prototype. It has been tested together with the loading and unloading devices and the drip pan. In addition paragraph 519 of the above-mentioned IAEA Regulations requires a drop test and a stacking test of the container. These tests can be avoided if other methods are applied, which include tests on scale models, reference to previous satisfactory demonstrations of a sufficiently similar nature and calculations or reasoned argument.
Transport Truck
Figure 11 illustrates the transport truck. The trailers for the ore slurry containers are of "B-Train" type, designed within the Saskatchewan gross vehicle weight restriction limits of 81 000 kg. Each trailer has three axles, as does the tractor, so the entire unit has nine axles with a load distribution of 6000 kg on the steer, 18 000 kg on the two drive axles and 28 500 kg on each of the two tridem axle groups. The overall length of the haul truck is 26.7 m and the width is 2.74 m.
Each trailer is fitted with a set of bolsters on the centre line of each container. The bolsters are of steel construction and are used to support the containers and the mechanical locking devices which engage the containers. The bolsters and the locking devices are located to maintain an accurate and even weight distribution.
Trailers are also equipped with control lines for the container ball valves. An air line is installed along both units. Branch tees are mounted at each container to operate the container ball valves. The tees are equipped with a quick coupling and shut off valve; a quick coupling is also mounted on the main air line for easy operator access. This quick coupling is used to connect plant air to the air lines. Similarly, a control signal cable runs along the trailers to each container with a quick coupling mounted near the front of the front trailer. Again, the control coupling is mounted for easy operator access such that the plant control signal cable can be easily attached to monitor the position of the ball valves.
To assure the safety of the transport process, each unit is equipped with ABS brakes, central tyre inflation, and additional fluorescent markers and lights. Standard equipment such as splash guards, bumpers and fenders are also fitted. Stone protection is also installed ahead of the landing gear and ahead of the running gear across the width of the trailer. On board load sensing equipment is installed in each unit.
The tractor units used to pull the B-Trains are standard highway tractors. Each truck is fully equipped with emergency equipment such as road flares, temporary road yield signs, fire extinguisher and shovel. Drivers will be fully qualified with Class 1A licences (Class 1 with air brakes endorsement).
Finally, operation of the B-Train design has been computer simulated to determine the dynamic handling capabilities. Road User Research International Inc. was contracted to determine the unit's performance with respect to rearward amplification, load transfer ratio, high speed steady state and dynamic off-tracking, low speed off-tracking, friction demand and braking stability. The simulation found that the current design performs beyond all requirements in all categories.
Emergency Recovery Vehicle
The initial response to a transport accident, including retrieving ore slurry containers which have become disengaged from the haul truck, is to mobilise the emergency recovery vehicle. Depending on conditions, the emergency recovery vehicle may be accompanied by a mobile crane. The emergency recovery vehicle would proceed to the accident to provide first aid, control any possible container leaks, clean up possible spills and recover the containers from the haul truck, or recover containers which may have separated from the haul truck.
The emergency recovery vehicle is a standard 10 t, four wheel drive truck with a fifth wheel mount. The back of the truck is compartmentalised to store the necessary spill response and first aid equipment. The truck will pull a trailer with an enclosed section large enough to fully contain a damaged container. The enclosure is fabricated from carbon steel plate complete with a removable lid. The trailer is similar in design to one trailer in the B-Train transport units.
The recovery vehicle is equipped with a winch to pull containers which may have become detached from the haul trucks. The winch has a pulling capacity of 25 t and an extended reach of 100 m. A mobile crane, with a trapezoidal boom with a maximum extended length of 38 m ±4.5 m, and a self contained hydraulic pack, is available as required for retrieving containers. The crane is capable of 360° rotation with a nominal lifting capacity of 45 t. The crane provides capacity to hoist a 14.5 t (filled) container at any point on its planar rotation at a horizontal distance of 7.62 m from the pivot point.
The Road
As seen in Figure 11, the road between McArthur River and Key Lake is approximately 80 km long. It is designed to handle large trucks as well as general traffic. The road is connected to the southern part of the province via the Key Lake road. Road construction was completed by the end of July 1998 and the road is now used to transport equipment, material and other supplies to the McArthur River site. It is an all-weather dirt road with an 8.5 m top, 8 m clearing of ditches and a 4:1 slope on the ditches. A design speed of 100 km/hour was chosen to provide a margin of safety over the posted maximum speed of 80 km/hour. Significant features of the road are two bridges over a creek and a river. Each bridge is approximately 40 m long. Most of the road parallels a high voltage power line which was built in 1989 to supply the northern mines with electrical power. As part of the design the bridges and numerous culverts were sized to comply with the current federal/provincial guidelines for fish habitat protection.
The Receiving Station at Key Lake
The new automated ore receiving and storage facility is being constructed just northwest of the existing grinding plant. The receiving and storage facility receives the B-Train trucks, each carrying four ore containers, in the single drive-through truck bay. Approximately half of the building is dedicated to the truck bay. The remainder of the building houses ore slurry unloading equipment and the ore storage pachucas.
After arrival and truck positioning, the truck driver exits the cab and, from the local control room, starts the emptying cycle. The slurry unloading system platforms descend over the containers on the truck. The slurry is then drawn out of the containers by applying a vacuum. Slurry removed from the containers is collected in slurry receivers, from which the slurry is pumped into the ore storage pachucas.
Once the containers have been emptied, a platform mounted frame slides the emptying equipment away from above the container. The wash/dry/scan equipment, mounted to the same slide frame, moves into position above the container. This wash/dry/scan equipment includes a hood that encloses wash water jets, hot air drying ducts and a vacuum pipe. With the hood in position, wash water is sprayed onto the container drip pan. Wash water is heated in a propane fired water heater. The wash water collected above the check valve is evacuated through a vacuum pipe to a dedicated dirty container wash receiver.
Following the wash cycle, hot air is introduced into the hood to dry the washed area. To confirm that the exterior surface of the container is free of any particles, the area of the stainless steel drip pan subject to contamination during the emptying process is radiometrically scanned. Upon passing the scan process, the containers are ready for trucking back to McArthur River. After the unloading operation is successfully completed, the unloading platforms are raised from atop the containers. The driver enters the cab and departs for a northward journey to the ore slurry loading facility at McArthur River.
There are four ore storage pachucas in the ore receiving area. As described above, McArthur River ore slurry is pumped from the slurry receivers into one of the four ore storage pachucas. The ore slurry is kept in suspension by injecting compressed air into the bottom of the pachucas. Off gas from the ore storage pachucas is power vented through an exhaust fan, via a header, to the atmosphere. The high grade McArthur River ore is blended with special waste to obtain an average grade of approximately 4% U3O8 before it is treated. A 4% ore can be handled in the Key Lake mill without major modification to radiation protection systems.
This special waste — uneconomic low grade material from earlier days of the Key Lake mining operation — is reclaimed from a nearby stockpile and fed to the existing grinding circuit. Existing classification screens are used to control the grind size of the special waste. The ground special waste slurry is fed to the existing neutral thickener. Thickener underflow is pumped to the existing neutral thickener underflow tank, which serves as the ore blending tank. McArthur River ore slurry is withdrawn from the ore storage pachucas and pumped to the neutral thickener underflow tank. The rate of ore slurry delivery is controlled by an in-stream uranium analyser mounted on the neutral thickener underflow tank. The control strategy maintains the grade of the blended ore mix of McArthur River ore and special waste at 4% U3O8.
The blended ore slurry is then pumped 1700 m, using existing pumps, from the grinding facility to the mill site for processing. Figure 12 shows a simplified flowsheet for the ore receiving, storage and blending processes at Key Lake.