Natural radioactivity in waste from former non-uranium mining areas in Norway

Transcription

1 Natural radioactivity in waste from former non-uranium mining areas in Norway Bjørn Lind 1, Torbjørn Gäfvert 1 and Anne Liv Rudjord 1. 1 Norwegian radiation Protection Authority, P.O. Box 55, N-1332 Østerås, Norway. INTRODUCTION Mining activities normally generate large amounts of waste in the form of waste rock, mining water and waste from processing the ore. Since certain types of minerals and ores also contain elevated levels of natural radioactivity from the uranium and thorium decay series, 238 U, 232 Th and their associated daughter nuclides (often referred to as NORM), may be found in different types of wastes from the mining activities at different concentrations. This may give rise to problems such as elevated exposure to the public from external gamma radiation, radon and groundwater contamination. In this project a number of locations have been identified where mining activities have been carried out in the past, and where potential problems with elevated concentrations of natural radioactivity could exist. Three former non-uranium mining sites where the public may be exposed to radiation from piles of mining waste containing NORM have been studied. This paper will present results from an initial screening of dose rates and activity concentrations of uranium and thorium decay series nuclides in waste materials, such as waste rock and water in samples collected from these sites. The most northern site, Oterstranda is located in Gildeskål municipality in Nordland County. The mining activity was based on extraction of molybdenum minerals from the ores. An initial flotation process was performed at the site. Approximately ton of mining residuals were deposited on a beach in the close vicinity to the flotation plant. Several summer cottages are located in the area and the residuals from the mining activities are being used as a public beach for sunbathing and swimming during the summer by the local population. In Elsjø at the border between Nittedal and Nannestad municipality in Akershus County, several smaller mines and waste piles are located. The bedrock in the area consists of uranium rich alum shale. Zinc minerals have been extracted from ore from this site. The waste piles from former mining activities are located in a popular recreation area. In the southern part of Norway, in Kragerø municipality in Telemark County a third mining locality were investigated. Several mines and quarries are located at Lindvikskollen just outside Kragerø city. Iron, feldspar and titanium dioxide were previously extracted from ores in this area.

2 Figure 1. Sampling locations. MATERIALS AND METHODS To obtain maps of the ambient dose equivalent (H * (10)) rate at the piles of mining residues, dose rate measurements were performed based on predefined grid systems with 3-4 meters between the measuring points. A portable dose rate instrument, Thermo Eberline FH 40 GL- 10, with an external FHZ 672 E-10 organic scintillator probe was used for measuring the ambient dose equivalent rate. Measurements were performed at two different heights, 10 cm and 1 m above ground. Waste rock samples from selected localities were collected and measured for activity concentrations of uranium and thorium decay series radionuclides by use of high resolution gamma spectroscopy with HPGe-detectors in a low level background laboratory. 2-5 litres water samples were collected from streams running through or from some of the waste rock areas, filtered (1 micron) and measured for levels of 234 U, 235 U, 238 U and 226 Ra after radiochemical separation followed by alpha spectroscopy. RESULTS Table 1. Measured ambient dose equivalent rates (nsv/h) in areas with mining residuals. Sites/ Locality Dose rate 1 m above ground on waste pile, nsv/h Dose rate 1 m above ground close to waste pile, nsv/h Range Mean (n) Range Mean (n) Oterstranda: Waste rock area Flotation plant area Elsjø: Røros (15 ) (2) Sulitjelma (4 ) (3) Kongens gruve site (11) (5) Kongens gruve site (10) (5) Kongens gruve site (11) (5) Lindvikskollen: Site (9) Site 2 (anomaly in the 850 (at ground 850 (1) bedrock) level) Site (5) Site (5)

3 Table 2. Activity concentrations in solid mining residues. Site/locality 226 Ra (Bq/kg) 228 Ra (Bq/kg) 238 U (Bq/kg) Range Mean (n) Range Mean (n) Range Mean (n) Elsjø Røros (5) (5) (5) Sulitjelma (3) (3) (3) Kongen (8) (8) (8) Lindvikskollen (5) (5) (5) Oterstranda (7) (7) (7) Table 3. Activity concentrations in water samples. Site/locality 234 U (mbq/l) 235 U (mbq/l) 238 U (mbq/l) 226 Ra (mbq/l) Elsjø Røros A 559 ± 40 17,1 ± 1,6 351 ± ± 4 Røros B 19.7 ± ± ± ± 3 Røros C 31.8 ± ± ± ± 0.7 Sulitjelma A 17.9 ± ± ± ± 4.5 Sulitjelma B 7.7 ± ± ± ± 4.8 Kongen D 3.6 ± ± ± ± 0.4 DISCUSSION AND CONCLUSIONS In the current national regulation, exemption limits are given for solid materials. These have not been intended for bulk volumes of NORM waste and are at the moment under revision. The existing exemption levels for 238 U, 232 Th and their daughter nuclides are given in Table 4. If several radionuclides are present, following summation rule has to be applied: Σ A k /A e,k 1 Where A k = activity for radionuclide k A e,k = exemption level for radionuclide k Table 4. Current exemption levels for solid waste in Norway (Ministry of Health and Care Services, 2004). Radionuclide Total activity (Bq) Activity conc. (Bq/g) Included daughter nuclides in equilibrium with parent nuclide 238 U Th and 234m Pa U-natural Th to 210 Po 226 Ra Rn to 210 Po 210 Pb Bi and 210 Po 210 Po Th-natural Ra to 212 Po 230 Th Ra Ac 228 Th Ra to 212 Po At two of the three sites that have been visited, residues from mining activities were found were the activity concentrations of 238 U, 232 Th or their associated progenies were above 1000 Bq/kg. For most bedrock types in Norway, except uranium rich alum shale and carbonatites in

4 certain areas, activity concentrations are normally well below 1000 Bq/kg for both uranium and thorium series radionuclides. Average activity concentrations found in normal soil in Norway are around 50 Bq/kg for both 238 U and 232 Th (UNSCEAR, 2000). At Elsjø, the ambient dose equivalent rate at the piles of mining waste were of similar magnitude as were found above exposed bedrock, but significantly higher compared with sites were the bedrock was covered with soil. At this location the mining activities have not generated waste with enhanced activity concentrations compared with the surrounding bedrock, but piles where dose rates are elevated compared to the surrounding area that did not exist before the mining activities started have been formed. Of 16 samples that were analysed, 10 exceeded the exemption limit for 238 U in equilibrium with daughter nuclides. Water samples were also collected at different locations at Elsjø and analysed with respect to uranium isotopes and 226 Ra (Table 3). The sample that was collected at one of the mine entrances showed higher uranium concentrations than the other samples. The level of uranium in this sample is however the same order of magnitude as normally can be found in wells drilled in bedrock (Mäkeläinen et al., 1999). At Oterstranda the activity concentration of mainly 238 U and associated daughter nuclides have been enhanced compared to the surrounding geological environment as a result of the mining activities. Some of the concentration process has taken place in a nearby flotation facility. After the residuals were deposited at the shore, a natural concentration process has also taken place, when seawater has separated mineral components with different densities. The disturbance of shore sediments by waves have removed lighter components, such as feldspar and quartz, leaving behind the heavier minerals with elevated concentrations of uranium. Considering the activity concentration, 4 of 7 samples were found to be above the exemption level for 238 U in equilibrium with daughter nuclides. At Lindvikskollen the residuals that were found consisted of unprocessed waste rock. Most samples that were analysed showed normal or moderately elevated levels of uranium and thorium radionuclides. Only 1 of 8 samples showed an activity concentration above the exemption level for 238 U in equilibrium with daughters. At one location, a natural anomaly in the bedrock was found where the dose rate close to the ground was measured to 850 nsv/h. A spectrum that was collected in situ with a portable 3x3 NaI-detector, indicated that the bedrock at this site (about 1 m 2 ) contained elevated levels of uranium. REFERENCES Ministry of Health and Care Services, Om strålevern og bruk av stråling: Lov 12. mai 2000 nr. 36 om strålevern og bruk av stråling (strålevernloven), forskrift 9. mai 2003 nr. 568 om anvendelsen av lov om strålevern og bruk av stråling på Svalbard og Jan Mayen, forskrift 21. november 2003 nr om strålevern og bruk av stråling (strålevernforskriften), forskrift 6. desember 1996 nr om systematisk helse-, miljø- og sikkerhetsarbeid i virksomheter (internkontrollforskriften). Oslo: Helsedepartementet, 2004 (In Norwegian). Mäkeläinen I, Salonen L, Huikuri P, Arvela H, Dose from drinking water in Finland. In: Nordic Society of Radiation Protection, proceedings of the 12 th ordinary meeting in Skagen, Denmark, aug Editors: Sörgaard-Hansen J and Damkjær A.

5 UNSCEAR, United Nations Scientific Committee on the Effects on Atomic Radiation. Sources and effects of ionizing radiation: United Nations report to the general assembly, with scientific annexes. Volume I: Sources. New York: United Nations, 2000.