SLOVENIA

Zirovski Vrh Uranium Mine, Slovenia

Case History

Prepared by Zmago Logar

Pribram, Czech Republic, October 12 -14, 2004

ABSTRACT

 

1.         INTRODUCTION

 

The facilities of the Zirovski Vrh Uranium Mine are situated 45 km west from Ljubljana. Uranium mineralization in Groeden sandstone was discovered in 1960, the ore production started in 1982, yellow cake production started in 1984. Only 610,000 ton of ore with 0.7 kg of U per ton of ore was processed and 452 ton of yellow cake was produced during six years of the mill life. The production ceased in 1990 due to decision of the Slovenian Government, with no notice given to the operator.

 

The mine and its facilities lie on the north-eastern slopes of the Zirovski Vrh ridge (960 m). The region belongs to the Julian Alps foothills. The underground mine elevation is from 430 m (bottom of the river valley) to 580 m (P-1 adit). The mill was located in the valley of the Brebovscica river (415 m) and the mill tailings are located on the southern slope (530 – 560 m) of Crna Gora hill (611 m). A total of 3 307 000 tons of crude ore was won (633 000 tons of high graded ore, 206 000 tons of low graded ore and 2 468 000 tons of mine waste).

The mine is located in well-populated farming area. Radioactive impact is small, the yearly dose contribution swings from 0.3 to 0.4 mSv/a (Benedik 2002). Uranium and radium concentrations in nearby surface water flows are elevated.

 

Rudnik Zirovski Vrh, d.o.o. - public company for the close-out of uranium mine is a company owned by the state. Its basic purpose is the performance of permanent closeout of exploitation of Uranium Mine Zirovski vrh. After the submission of the last closing program that the Government of Slovenia adopted in the year 2001 on the basis of changes and amendments of the fundamental act of the year 2000, it is necessary to implement the project of the mine close-out in a five year period. The total costs of the project implementation will be 37.3 million EUR. The company will prepare a revision of the closing program due to delays in financing and implementation of existing closing program and new cognitions.

 

For the emission of contaminants via the water and air pathways the limits were set by the decision of the Ministry of Health of the Republic of Slovenia and are shown in the Table I (ZIRS 1996).

 

Non-radiological requirements for the discharge of contaminants in the water are defined by the environmental legislation concerning the conservation of water resources. Additionally, location permits define basic conditions for the operation of the mining and milling sites and also for their closeout. In the case of the waste piles P 1 and P 9 the location permit allows only a temporary storage of the mine waste. The requirements of the location permits are taken into consideration in the remediation plans of RZV.

 

Table I: Authorized limits issued for the closeout of the liabilities of the Zirovski Vrh Mine

 

 

Contaminant

Concentration

Annual load

Water discharges

 

 

 

Mine water (P-10)

Uranium

250 mg/m3

170 kg

 

Ra-226

60 Bq/m3

50 MBq

Mine waste pile Jazbec

Uranium

510 mg/m3

85 kg

 

Ra-226

40 Bq/m3

25 MBq

 

Th-230, Pb-210, Po-210 (total)

100 Bq/m3

 

Mill tailings Borst

Ra-226 (annual average)

60 Bq/m3

50 MBq

Airborne

 

 

 

Mine waste pile Jazbec

Rn-222

0.1 Bq/m2 s

 

Mill tailings Borst

Rn-222

0.74 Bq/m2 s

 

External radiation

Average over the first 1.25 m above the ground

200 nGy/h

 

RZV (total contribution)

Annual effective dose

0.3 mSv/a

 

 

 

 

2.         Underground Mine

 

2.1       Description

 

The underground uranium mine extends 2000 m in North-South direction, 150 m East-West and in height up to 180 m. About 60 km of underground mine openings were built mostly having a cross section to allow transport and production. The mine is divided into 4 horizons, the horisonts into 14 blocks of 200 m width. Uranium ore was mined by room-and-pillar method. 10 % of the total ore was won by top to bottom winning technology leaving extensive open mine space with height up to 15 m. 90 % of uranium were won from bottom up with cut-and-fill mining leaving a minimal open space.

 

The underground mine is connected to the surface by shafts and adits, which were used for ventilation, material supply and transport. Adits P-1 and P-36 are used mainly for ventilation purposes. The ventilation station P-1 is located in the southeastern part of the underground mine; the ventilation station P-36 is located in the Northwest. The ventilation of the underground mine is realized by depression induced by the ventilation stations at these adits, removal of the mine air while fresh air flows in via the ventilation shafts, and adits situated over the whole mine area. The deepest adit P-10 is used for mine water discharge. The main mine access is through adit P-11.

 

The Zirovski Vrh deposit is one of numerous uranium occurrences in the Groeden Sandstone of Permian age found in a zone extending from the Southern Alps in northern Italy, through southern Austria and central Slovenia, into Hungary. The ore-hosting rocks are grey Sandstone and conglomerates (grey series) of the lower part of the Groeden Sandstone of Middle Permian age. The Groeden Sandstone rests unconformable on black shale of Permo carboniferous age and is overlain by upper Permian Bellerophon dolomite. During the Alpine Orogeny the entire sequence was folded, heavily sheared, cut by north-south and northwest-southeast faults. The complex geology leads to geomechanical stability problems with the mine workings. The rock is relatively friable making extensive rock bolting and support necessary.

 

In the northwestern part the uranium bearing structures of Zirovski Vrh are overthrust by the Jazbec scale (Groeden rocks). It is estimated that this thrust represents an impermeable barrier with a hydraulic conductivity of 10-9 m/s. The overlying layers of sandstone and conglomerates have a high hydraulic conductivity RZV 9/00.

 

Water inflow into the mine mainly occurs on the fractures and cracks in sandstone and conglomerates along discontinuities and fault zones. Particularly the conglomerates present an important aquifer (3rd horizon). The Northwest part of the mine is more permeable because of the cracks. The layers with ore deposit are less permeable.

 

Ground water entering the mine becomes contaminated due to passing trough ore-enriched zones. Another source of mine water contamination are exploration boreholes in the ore horizons. Some mine workings (blind tunnels) cannot drain and get flooded. The water in these tunnels have uranium concentrations exceeding 6 mg/l.

During the ore production the inflow of groundwater in the upper parts of the mine decreased. The highest inflow of water is presently found on the deepest mine levels. According to (IBE 2000n) the connection between water inflow and precipitation is evident in the higher parts of the mine, while in deeper parts a long delay can be observed. The total discharge from the mine ranges between 15 and 30 l/s.

 

The yearly average of the uranium concentration in the mine water discharged through the adit P-10 was between 250 and 350 μgU/l in the time period 1992 to 2000. The radium concentration (Ra-226) was between 30 and 100 Bq/m3 during the same time. A decrease of uranium and radium concentration in the mine water was observed after uranium production ceased. Iron concentration in the discharged mine water is below the detection limit. The sulphate concentration is very low compared to the discharge observed in other mines.

 

2.2       Objectives of the Mine Remediation

 

UNI 4/99 defines the objectives regarding the remediation of the mine as follows:

Reduction of radon emission from the underground openings,

Long term stability of underground mine facilities to prevent the inflow of surface waters to the mine resulting from the collapse of mine openings,

Permanent discharge of mine water through adit P-10 meeting the regulated limits for uranium,

Permanent discharge of uncontaminated groundwater through P-11 and P-9,

Protection of underground and surface water (within the regulated limits),

Reclamation of the surface area in the surrounding of shafts, raises and adits,

Prevention of unauthorised access to underground openings.

 

2.3.      Work done in the mine

 

For a better collection of the inflowing groundwater and to avoid the contact of groundwater with the ore deposit, drainage boreholes of a length between 150 and 170 m were drilled in the deepest part of the mine in 1993, the uranium concentration in the inflowing groundwater decreased at the same time. There are measurements of uranium concentration in the drift H-7 for the time before and after drilling of the drainage boreholes. In the concentration of uranium which is affected by drainage boreholes is compared to uranium concentration of H-8 where no drainage boreholes exist.

 

The adit P-10 was completely remediated over a length of 520 metres (out of its total length of 1200 metres) between 1996 and 1997. Further mine facilities on the surface, e.g. the crushing plant, the chemical plant and transport bridges, were torn down or handed over after decontamination to users outside RZV.

 

3.         Waste Rock Piles

 

3.1       Description of the Mine Waste Rock Piles

 

The waste rock piles P-1 (70 000 ton) and P-9 (143 000 ton) are situated close to the adits P-1 and P-9 respectively. There are no water sources in the area of waste pile P-1. From the adit P-9 about 0,5 l/s uncontaminated mine water is discharged and passes the waste pile P-9. The uranium concentration of this water is about 400 mg/l.

 

Jazbec is the main waste rock pile of RZV (1,5 million ton) situated in the Jazbec valley right below the entrance to adit P-11. The slope of the waste pile is 1:2 with 3 m wide berms placed at each 12 m of height. At the present the highest point of the waste pile is 493 m a.s.l. On the supporting dam formed by mine waste on the eastern pile face a road leads to the former crusher site. Judged by the measured water levels in the mine pile, this dam is less permeable than the stored material. There is no seepage water observed on the pile face.

In addition to waste rock Red Mud from the uranium ore processing was disposed off on the waste pile. The red mud is a precipitate of the uranium leaching solution generated by neutralisation with lime. The main components are calcium, gypsum, iron hydroxide, uranium (0,5 Bq /g), Ra-226 (0,2 Bq/g) and Th-230 (62 Bq/g). Th-230 has a 8 times higher enrichment than the Ra-226 concentration in the tailings. The material was stored alternately in layers: on 80 cm of mine waste 20 cm of red mud follows. A 5 m thick zone on the aerial side of the pile consists only of coarse mine waste. Demolition debris from the remediation of the milling site was deposited on the waste pile as well.

 

The concentrations of the radionuclides Ra-226 and uranium in the waste rock are about 0,5 Bq/g. The sulphate and iron concentration of the waste rock is low. Acid mine drainage in the pile is unlikely to develop. The mine waste material is a very water-permeable material, that consists of a quartz conglomerate, sandstone and aleurit. The red mud is a less-permeable material consisting of an aleurite clay fraction. The uranium content of the waste rock is the main source of contamination of the mine waste pile seepage.

The Hotavlje fault, with some wet moor areas on the surface cut the Jazbec valley. At the bottom of the Jazbec valley are dark grey limestone outcrops IBE 2000n showing characteristic karstic properties. Along the Brebovščica river numerous springs exist which are fed by karstic groundwater. There is a 329 m long concrete channel at the bottom of the waste pile. In the upstream section (152 m) the channel has a diameter of 1 m and in the downstream section (177 m) of 3 m. This channel collects the water from three creeks in the upstream valleys as well as the drainage water from the pile. This water is discharged into the Brebovščica river. Before construction of the waste pile a drainage system was build at the bottom of the valley. Pipes connected to the concrete channel dewatered small springs. At present the drainage channel is in good condition, however, the drainage pipes are mostly in a poor condition. Currently additional horizontal drainage boreholes are drilled from a large diameter well in the upper part of the waste pile (near P-11) to reduce groundwater inflow to the pile from the hinterland.

The groundwater is polluted because of the infiltration of meteoric water percolating through the uncovered waste pile and leaching the red mud. Various sampling points on and around the waste pile allow the evaluation of the groundwater. The water at the bottom of the waste pile contains up to 5000 μg/l uranium. Downstream the pile in the alluvial terrace of the Brebovščica river no groundwater is found (borehole BS-20) while concentrations of about 150 μg/l uranium were measured in the deeper karst aquifer. Downstream, the Brebovščica river springs are probably fed by water from the karstic limestone layer (RZV 9-2000).

The water discharged from the drainage channel at the bottom of the waste pile has annual average uranium concentrations of 250 to 550 μUg/l (1991 to 1999). In the same period the sulphate concentrations were between 200 and 300 mgU/l, which is about 10 times higher than in the mine water discharge.

 

3.2       Objectives of Mine Waste Remediation (Jazbec)

 

The specific aims of the technical measures for the final arrangement of the mine waste pile are as follows:

Limitation of the radon exhalation,

Limitation of leaching of contaminants and erosion protection of the mine waste pile by covering,

Limitation of the effect of surface and ground water from hinterland on the mine waste (erosion, wetting of waste),

Geo-mechanical stability of mine waste pile, erosion resistance by reshaping of mine waste pile,

Prevention of erosion and excessive wetting of the cover by means of a drainage system,

Relocation of the waste rock piles at P-1 and P-9 to the mine waste disposal Jazbec,

Prevention of excessive dusting.

 

4.         Mill Tailings Disposal Boršt

 

4.1       Mill Tailings Description

 

From the beginning of the operation till the end of uranium production at RUZV 600 000 ton of hydro-metallurgical tailings had been generated and deposited on the tailings pile Boršt. The volume of the deposited material is approximately 375 000 m3. The tailings pile has an area of 4.11 ha. The storage site is about 2 km away from the former uranium processing plant. There is an access road at the southern and western periphery of the pile. To allow access to the tailings ground with heavy machinery, regularly arranged roads were constructed on the tailings pile (at every 5 m of elevation) using 73 000 t of mine waste rock. The tailings are deposited on a natural slope.

During deposition of the tailings the following principles were followed concerning the environment controls, water management and geotechnical stability of the tailings:

Diversion of surface waters out of the tailings pile to prevent contact with tailings,

Capturing of ground water and springs and their linking to the drainage system,

Sealing of the bottom of the tailings pile using clay material,

Capturing of (contaminated) surface and seepage water from the tailings and discharge into the retention pond,

20o embankments slopes with horizontal berms.

 

Covering of the tailings surface after reaching the planned elevation with material that was previously removed from the base of the tailings pile and vegetation with grass. The general slope of the tailings pile at the northern part is approximately 20°, the inclination between berms is 24°- 27°. The slope was covered with 25 cm of top soil and vegetated with grass. The top plateau of tailings pile is partly covered with waste rock from the construction of the drainage tunnel below the tailings pile. The immediate covering was done to reduce the radon exhalation.

 

The tailings material is a relatively coarse ground sandy-aleuritic residue of the uranium ore processing with grain size under 0.5 mm, but more than 50% greater than 0.125 mm. The average contents of uranium is 0.9 Bq/g or 80 g/t U3O8 and 8.6 Bq /g of Ra-226. It mainly consists of SiO2, calcite and sulphate salts. The tailings were deposited on site with a water content of 20 – 24 %. Presently, the tailings are in a solid state. According to the planning documents there is no danger of liquefaction. However, during deposition liquefaction of the material was observed.

 

Before the placement of tailings started, the ground on the base of the pile was prepared using the material removed during the site preparation works. Drains and pipes were laid on the surface and covered with a layer of clay. Water form 2 springs subsequently covered with tailings was captured and drained in 2 PVC pipes. The water discharge from these springs is sampled at the sampling point SDIJ outside of the tailings pile.

 

The aim of the sealing layer at the bottom of the pile was to lead the groundwater from upstream of the pile to the receiving Todraščica brook and separate it from the tailings seepage. Shortly after the placement of tailings started it was found that the ground water drained below the tailings pile was chemically and radioactively contaminated. A gradual increase of the concentrations was observed.

 

The tailings were deposited in layers and at every 4-5 m elevation in series of nearly horizontal drains (slightly inclined to the surface) were placed. These drains are approx. 10 m long and drain only the face of the pile. The drained water is collected in surface water channels on the berms. Together with the precipitation water, the drainage water flows to the retention pond before it is released into the Todraščica brook.

 

The retention basin has a total volume of 4100 m3, which allows to retain the centennial maximum precipitation for one hour. It is divided into two compartments enabling its function without hindrance even in case of cleaning or maintenance. The bottom of the reservoir is reinforced and protected by a watertight asphalt layer. The reservoir is vegetated with reed. Sediments from the reservoir are regularly removed and stored on the Boršt tailings pile.

In case of rainfall, the surface run-off water from the hinterland is diverted by ditches outside the tailings pile area to a torrent gorge on the western site of the pile. During dry weather these ditches are empty.

 

Due to the high radium contents in tailings, both the uncovered top surface area and the covered embankments are a considerable radon source; the exhalation was evaluated to approximately 5 Bq/(m2.s) (in the range of 1 and 10 Bq/(m2.s)). Surface waters, water in tailings as well and underneath the tailings are contaminated by uranium and soluble inorganic materials (NH4+, SO42-, Cl-). Ammonia and total inorganic materials are above the regulated discharge limits.

 

The geological foundation of the Boršt tailings pile consists of karstic sediments, yellow, red and grey clay, alternating with aleuritic layers with tuff and tuffaceous inclusions. A large number of sub-vertical faults disrupt the geological base.

 

After heavy rainfalls in November 1990 a crack with 20 - 30 cm of vertical movement appeared in the road on top of the tailings due to land sliding. The extent and depth of the sliding area have been established by surveying, drilling and borehole inclination measurements. The landslide occurred on the contact of the karstic and tuffaceous rocks. The evaluation of the borehole survey confirmed that the tailings pile was placed on an old landslide (paleoslide). The landslide underlies the major part of the Boršt tailings pile and involves a large part of geological foundation. The general movement of the slide was towards North. The average thickness of the slide is 50 m.

 

Tailings moved by landslide

   328,579 m3

Masses of ground moved by landslide

2,593,175 m3

Total volume of the landslide

2,921,754 m3

 

The main effect on the movement of the landslide was achieved by the construction of the drainage tunnel in 1995. The tunnel starts in the sliding face crossing the main sliding plane at about 214 m from the entrance of the tunnel. At the end of the tunnel two drainage wings were excavated where a drainage screen is connected to 21 boreholes drilled from the surface. The distance between the boreholes is about 10 m. The inflow into the drainage tunnel is in average 1.3 l/s (40 000 – 60 000 m3/a). 40 % of the inflow comes from the drainage boreholes while 60 % enter through the tunnel face. The construction of the tunnel has a considerable effect on the local groundwater regime. Most of the water levels measured in the piezometers in the tailings pile and in the close vicinity of the site decreased, however, no correlation to the distance from the drainage tunnel was found. After construction of the tunnel the discharge of the captured springs and measured at SDIJ, rapidly decreased (to about 20 %).

 

4.2       Objectives of Mill Tailings Remediation (Boršt)

 

The remedial objectives concerning the tailings storage are as follows:

Provision of permanent landslide stability by drainage channel, drainage screen and cover construction,

Drainage curtain will be designed to facilitate tailings consolidation,

Provision of the tailings stability and enhancement of erosion resistance by reshaping of the tailings pile,

Prevention of the radon exhalation, the leakage of hazardous contaminants into water streams and the erosion of tailings by covering,

Protection of the tailings pile from the surface and ground water from the hinterland (erosion, increased infiltration through the cover and the tailings),

Prevention of the cover and the tailings from erosion by construction of a drainage system,

Prevention of excessive dust formation.

 

5.         Investment costs

 

By the submission of the last closing program estimate of closeout costs was made both per fixed and per current prices (in EUR and SIT). In the text further we use the presentation of investment costs and estimated fund sources of the project in fix prices in EUR. Presentation per fixed prices includes prices as of December 2000. Presentation per fixed prices was prepared also in EUR. The exchange rate valid on 30 June 2000 was considered in the translation, i.e. EUR 1 = SIT 205.3542 (IBE 2000n).

 

Closing out of the Uranium Ore Mine Zirovski vrh has been in progress since 1990. Costs utilized to the end of the year 2000 were estimated on the basis of the actual utilization to the time when this novelty to the program and plan of fund utilization for the year 2000 were drawn up an was 29.4 million EUR. Costs utilized in the year 2000 were about 2 million EUR.

 

According to the novelty to the program that the Government of the Republic of Slovenia adopted in the year 2001 on the basis of changes and amendments of the fundamental act of the year 2000, future costs of the project implementation shall amount to 37.3 million EUR (included VAT) in the period of five years. As the project practically started in the beginning of the year 2002 we estimate that it will be finished by the end of the year 2006.

Total closeout costs (incl. VAT) amount to 68,7 million EUR for the whole period between the years 1990 and 2007. The investment costs were estimated by the following presumptions:

Costs for redundant employees and unforeseeable costs were not planned,

After the closeout a 5-year surveillance over the influences on the environment was planned. Costs of surveillance were estimated annually 127 thousand EUR (total 633 thousand EUR) and were not incorporated in the total closeout costs,

Costs of implementation of RZV liquidation procedure have not been specially foreseen.

However, total costs of closeout works required from 1 January 2002 to the final closeout amount to 37.3 million EUR they are presented in detail in the table II (RZV 2002).

 

Table II: Summary of the required costs of the project

 

Kind of costs/project

TOTAL

2002-2006

Year

2002

2003

2004

2005

2006

Ore exploitation

16.438

3.660

5.883

3.837

1.451

1.607

Concentrate production

613

132

0

0

0

481

Tailings remediation

6.829

0

185

3.352

1.843

1.448

Other

8.863

1.797

1.936

1.814

1.714

1.602

Total

32.742

5.457

8.136

9.003

5.008

5.183

VAT

4.558

 

 

 

 

 

Grand total

37.300

 

 

 

 

 

 

Explanation;

Ore exploitation = the costs for mine close-out works,

Concentrate production = the costs for concentrate production site close out works (main works has already been done),

Tailing remediation = the costs for remediation of the mine waste pile Jazbec and mill tailings Boršt works.

The VAT amount is stated under the tables, but is not included in the closeout costs. The amount of VAT is only informative, since VAT is calculated according to the value of closeout works that are related to the plant for ore exploitation, plant for the production of uranium ore concentrate and disposal sites, and it is not calculated for the value of other works referring mainly to the costs of employees in RZV (salaries, change in employment, etc.). In accordance with the Act VAT is not added to the investment value.

 

The costs of other works include:

Costs of surveillance over influences on the environment (surveillance that should be performed at the time of closeout and at least 5 years after the closeout),

Costs of safety at work,

Costs of assurance of conditions (these costs are related to the operation of the investor at the time when the mine has not been finally closed out yet; these are cots of the laboratory, costs of operation of technological assemblies and costs of maintenance that are estimated in an amount),

Fixed liabilities  (include costs of material and services that are related to the operation of RZV, contribution to the land plot utilization, hire contracts and others and costs of training),

labour costs (the calculation includes labors costs of 36 employees),

Indemnification (indemnification that should be paid to land owners in case of possible intervention in their land),

Purchase of the equipment needed.

 

6.         Conclusion

 

The underground mine remediation will be finished at the end of 2005 in the scope of estimated costs. The main entrance will be finally closed in the first half of 2006. The work at the mine waste pile Jazbec will start in the middle of 2005 and will be finalized in 2007. The funds foreseen in the program will satisfy the needs. The funds we foresaw for the mill tailings remediation in the program are too small. New informative design has required considerable more funds for the tailings site remediation. The start of the remediation is planned for 2006 and the work will be finished in 2008.

 

References

 

Benedik L. et al., The Report on the Environment Radioactivity Measurements of the Uranium Mine Zirovski Vrh, Institute of Josef Stefan or Institute for Work Safety of the RS, Ljubljana, Volumes 1990–2002

IBE (2000n) Novelty to the Programme of Close-out of Uranium Exploitation in RUZV, IBE, Ljubljana, November 2000

Gantar I., Zirovski Vrh Uranium Mine – Geological Description, RZV Gorenja vas, September 2000

UNI (4-1999) Supplementary Mining Project – Permanent Close-out of Uranium Exploitation of Uranium Ore and Prevention of the Mining Consequences in the RUZV, University in Ljubljana, Ljubljana, April 1999

ZIRS (1999) Proceeding approval Reg. no. 531-4/231/76-34/L14, Ministry of Health, Ljubljana, 1996

RZV (2002) Evaluation of the Implementation of the Novelty to the Programme of Close-out of Uranium Exploitation in RUZV, RZV, April 2002

WIS (2001) Evaluation of the Technical and Economic Measures Planned in Relation to the Close-out of the Uranium Ore Mine Zirovski Vrh, Slovenia, WISMUT Chemnitz, June 2001