Discharged Radionuclides from the Slovenian Nuclear & Radiation Facilities and Their Actual Detection in the Environment

Transcription

1 Discharged Radionuclides from the Slovenian Nuclear & Radiation Facilities and Their Actual Detection in the Environment Milko J. Križman, Michel Cindro, Barbara Vokal Nemec Slovenian Nuclear Safety Administration Železna 16, SI1000 Ljubljana, Slovenia ABSTRACT All nuclear and radiation facilities discharge radioactivity to the environment. Some radionuclides that are released in higher amounts can easily be detected in the environmental media, even though they are dispersed and diluted. With monitoring network, it is not possible to identify all radionuclides that are discharged to the environment. Their detection depends mostly on the quantity and quality of environmental samples, the sensitivity of applied methods and the quality of the analytical equipment. The paper gives an overview of radionuclides discharged from Slovenian nuclear and radiation facilities and demonstrates which of these can actually be measured in the environment and reported to the competent authority. 1 INTRODUCTION The operation of nuclear and radiation facilities is inevitably related to discharging radioactive materials into the environment. The control of discharged radionuclides and the monitoring of their concentrations in the environment are important conditions of the operation licence. The monitoring of discharged radioactivity from a facility comprises sampling, preparation and the analyses of relatively high activity samples. Such samples can be effectively and easily measured, but require precautionary measures during preparation procedures to prevent the laboratory accessories and measuring equipment to become contaminated. On the other hand, the measurements of radioactivity in the environment, once it is dispersed and diluted to low activities, can be beyond the capabilities of standard laboratory measuring equipment. In such cases, the environmental concentrations are evaluated by the computer modelling of dispersion of gaseous and liquid effluents from the plant. The monitoring programme of environmental radioactivity at each facility depends on the types of discharges, transfer and exposure pathways and on the characteristics of the affected environment. Measuring radioactivity in the environment is one of the key elements for the assessment of public exposure. The estimated effective dose is finally compared to the authorised or statutory dose limits. The detection of radionuclides within the regular monitoring programme depends mostly on sampling methods, the quantity of samples, the sensitivity of applied methods and the quality and efficiency of the analytical equipment. Concentrations can be measured as low as detection limits. The routine monitoring of environmental radioactivity does not cover all radionuclides, present permanently (such as natural or globally dispersed ones) or

2 occasionally (in case of periodic discharges from facilities) in a particular environmental medium. The reasons for that are low concentrations, the limited sensitivity of counting equipment and last but not least, the costs of analyses and the importance of individual radionuclides for its exposure evaluation. The aim of this paper is to give an overview of the main radionuclides that are discharged from the Slovenian facilities and to demonstrate which of them are or can actually be measured in the environment and reported. 2 RADIONUCLIDES IN THE ENVIRONMENT Radionuclides in the environment are of natural and artificial origin [1]. Natural radionuclides originate from the Earth s crust (terrestrial radioactivity) and from cosmic radiation. Terrestrial radioactivity comprises radionuclides of 238 U, thorium 232 Th and 235 U decay chains and of potassium isotope 40 K. Beside these, some very long lived terrestrial radionuclides (with halflives years), such as 87 Rb, 138 La, 147 Sm, 176 Lu are also present in the environment. Cosmic rays permanently produce radionuclides such as 3 H, 7 Be, 14 C, 22 Na, and some others in the upper atmosphere. Natural radionuclides are more or less uniformly dispersed throughout the Earth s crust and atmosphere. Artificial radionuclides, appearing in the environment in Slovenia nowadays, mostly have their origin either in the past atmospheric nuclear weapon tests ( 3 H, 14 C, 90 Sr, 137 Cs) or in the Chernobyl accident ( 137 Cs, 90 Sr). The longlived 129 I was measured recently over one of the regions of Slovenia, namely in precipitation and soils and in the Adriatic sea water and sea biota [2] within a research project of several years. Some artificial radionuclides ( 3 H, 14 C, 85 Kr, 129 I) are currently released from the fuel processing plants in France, Japan and United Kingdom as well as in nuclear reactors elsewhere and are also globally dispersed. The globally distributed radionuclides 3 H and 14 C are of double origin, artificial (reactor operation, reprocessing, nuclear weapon testing) and natural (reaction with cosmic rays). Global contamination with 239/240 Pu was not measured in Slovenia in the last two decades, while radionuclides 241 Am and 85 Kr were not measured in the environment at all. Artificial radioactivity discharged from nuclear and radiation facilities can be mostly found locally in their nearby surroundings [3, 4]. Power reactors are permanently discharging activation and fission products, including noble gases, aerosols, and iodine isotopes. The research reactors are continuously releasing noble gas 41 Ar during its operation. Uranium mines and ore processing plants are releasing considerable activity of natural radionuclides from uraniumradium decay series ( 238 U sec ). Radiopharmaceuticals ( 131 I, 99m Tc) are periodically discharged from clinics with nuclear medicine departments. In principle, all radionuclides in the environment can be identified by appropriate sampling, methods and techniques of measurement. In practice, most of radionuclides are not measured mostly due to expensive, time consuming analyses, or since the radioactive decay products or parent products have shorter half lives than measured radionuclides and secular equilibrium between them is assumed. There is no need for all radionuclides, discharged into the environment, to be analysed. Only a limited number of them contribute significantly to the total public radiation exposure. Radionuclides are monitored in all environmental media, such as in the nearground atmosphere, in precipitation, soil, surface waters and underground waters, in drinking water, the foodstuffs of vegetable and animal origin, feedingstuffs and sometimes also in bioindicators. Typical ranges for certain frequently monitored radionuclides in the environment of Slovenia outside nuclear facilities are given in Table 1.

3 All results of environmental radioactivity, measured in Slovenia, are entered in the data base named ROKO [5], established at the SNSA. The data base comprises the data of general radioactivity in the environment and of all facilities, discharging radioactivity into the environment, from the beginning of their operation. At the moment, there are about 220 thousand records stored altogether. Table 1. Concentration ranges of natural and artificial radionuclides, currently measured in the environment. Sample Radionuclide Range Exception Artificial Cs, 90 Sr H, 14 C, 85 Kr 3 H, 14 C, 85 Kr Air Natural [μbq/m 3 ] 238 U, 226 Ra, Pb, 7 Be 15x Rn, Rnprogeny Water [Bq/m 3 ] Food [Bq/kg] Radon 222 Rn & its shortlived progeny 5x x10 6 Natural, artificial H 0.8x x10 3 Natural, artificial Cs in wild food, game H 137 Cs in wild food, game 3 METHODS Discharges of radionuclides can be continuous, periodic or sporadic. They can be controlled or uncontrolled at source, and are planned or incidental. Therefore, environmental sampling has to be adequately arranged. For continuous discharging and for batch releases, the continuous sampling is the most relevant option. For radionuclides, discharged at regular intervals, the time of release shall be generally known in order to arrange an appropriate batch sampling. This is the case for 131 I discharges to the river waters, since they are usually based on a weekly schedule. In cases of incidental releases, the sampling is based on the available information about such events (information in advance for 137 Cs, Spain, overdue information for 131 I, Paks, Hungary). Depending on the radionuclide specificities, several analytical methods are used. Spectrometric methods are preferred since multiple radionuclides can be identified and measured with a single measurement. The most popular is gamma spectrometry, since no special preparation of the sample is required. Pure betaemitting radionuclides, causing considerable exposures or contamination of the environment ( 90 Sr, 3 H, 14 C), are measured separately, using radiochemical analyses, low level beta counting and scintillation counting. Alpha emitters are analysed after sample pretreatment and electrodeposition procedure by alphaspectrometry. The possibility of radionuclides detection at their low concentrations in the environment depends on: the quantity of discharged radionuclides and their concentrations in the environment,

4 the frequency of sampling (monthly or daily samples; continuous, composite or grab samples), the sensitivity of method and equipment (high efficiency, good resolution), good laboratory conditions (low background, shielding, radon elimination, active shielding with anticoincidence). The choice of the relevant method and toplevel instrumental equipment mostly affect the quality of measurements and extends the possibilities for identifying the radionuclides, present in the environment in low concentrations. According to the regulations on radioactivity monitoring, the required detection limit for a single radionuclide shall provide at least an exposure estimation of 1/30 of the dose constraint, approved for a particular nuclear or radiation facility. 4 RESULTS AND DISCUSSION According to the Slovenian legal classification, there are three nuclear installations in Slovenia: the Krško Nuclear Power Plant, the Research Reactor TRIGA and the Interim Low and Intermediate Radioactive Waste Storage, all of them in operation. The uranium mine at Žirovski Vrh is out of operation for twenty years but is the only radiation facility with permanent discharges of radioactive materials (with natural radionuclides) into the environment. It was in operation in the eighties, and was completely restored by this year. The results of environmental monitoring are available from the annual reports of operational monitoring, and from certain research studies. Results could be classified in several types, as regards the nature of discharge or presence of radionuclides in the environment: the results of good quality, usually related to high and steady discharges or steady natural production and a permanent presence in the environment (i.e. 3 H in waters, and 7 Be, 210 Pb, radon 222 Rn in air), the results of poor quality, near detection limits, related to low steady discharges and a permanent presence with low concentrations in the environment (i.e. Chernobyl 137 Cs and 90 Sr in air, water, food samples), the results that are more or less given in traces, with values near detection limit, related to occasional discharges or occasional appearance or presence in the environment (i.e. 58 Co in air, 60 Co in dry deposit, both in the surrounding of the NPP). An overview of the radionuclides that are definitively detected in the environment is presented in this section for all nuclear and radiation facilities in Slovenia. The results are summarised in Table Krško Nuclear Power Plant The Krško NPP is currently discharging activation and fission products, mostly 3 H, noble gases and 14 C [6]. The inventory of atmospheric and liquid discharges comprises altogether about thirty radionuclides. As it will be shown, only a small part of them can be identified in the environment within the existing environmental monitoring programme. 3 H liquid releases are about 20 TBq per year, so this radionuclide is monitored in the surface waters (Sava) and underground waters. 3 H in the Sava river appeared on average in concentrations of 3.3 kbq/m 3 superposed on 11.2 kbq/m 3 background 3 H level. The underground water, measured downstream the plant, reflects the tritium influence from the river

5 water, indicated with shortterm high values within the periods of major releases (29 kbq/m 3 in July 2007, 12 kbq/m 3 in July 2008). The atmospheric discharges of 14 C amounted on average to 0.1 TBq per year and are reflected in the levels well above the current background. Radionuclide 14 C in vegetation (plants) has not been routinely measured within the regular programme, but some research studies in the recent years showed up to 2030 % increase in plant leaves, above the basic value of 227 Bq/kgC. The ingestion of radionuclide 14 C is the major source of exposure to the population, and is of the order of magnitude 1 μsv/y. Radiocarbon is not measured in liquid effluents where it could also be present. Some radionuclides were very rarely detected in the environment, usually in traces. The discharges of radioactive aerosols were reflected in the detectable environmental concentrations exclusively during the outages of the NPP. Radioactive particles were collected on several sticky plates, distributed around the plant at the distances of approximately 500 metres. An occasional appearance of 60 Co and 58 Co (of some Bq/m 2 ) was observed for some years a decade ago. Within the period of outage works in the autumn 2007, the activation product 58 Co was detected also in the nearground atmosphere, at the level of 1 3 μbq/m 3. The radionuclide 58 Co was found in air filters at four sampling locations at the distances of 23 km from the plant [6]. Some single results of 131 I in air appeared in the annual reports in the last decade but one. They were of the same order of magnitude as detection limits and were not correlated with plant atmospheric releases. It is not likely that they reflect the real appearance of 131 I in air, since the single results were not confirmed neither with levels obtained at other locations, sampled simultaneously, nor with measurements performed by the NPP staff on the plant site. Activation and fission products released with liquid effluents were trapped on sediments. Some single results on 60 Co, 58 Co, and 54 Mn were recorded as results below the MDA (minimum detectable activity), but not confirmed by a definitive detection and identification. They were not in correlation with liquid discharges. 4.2 Research Reactor TRIGA The research reactor TRIGA MARK II (0.25 MW) was put in operation in During the full power operation, it evenly discharges noble gas 41 Ar at the rate of 250 kbq/s or totally around 1 TBq/y [7]. The release of this radionuclide is controlled in the discharge vent. It is measured indirectly with automatic doserate meter, qualitatively showing a continuous release rate. There is no other contamination in the environment due to reactor releases. The radiochemical laboratory, related to the reactor facility for neutron activation, has some minor radioactive discharges to the environment. In the effluents, only 241 Am and 60 Co were found at the concentrations of about 1 Bq/m 3 (2009). 4.3 Low and Intermediate Radioactive Waste Storage The environment of the LILW storage, located at the TRIGA reactor site, has been controlled through its own monitoring programme. Liquid releases are collected in a sump and the water is controlled before discharging to the environment. The radionuclides 60 Co, 137 Cs and 241 Am were analysed in the waste water in concentrations of some Bq/m 3, having no measurable effect on the environment. In the recent years, radon 222 Rn, emanating from the radium sources, was the main polluter. After the restoration of the facility and the conditioning of the radium sources, the radon releases were greatly reduced [8]. Radon releases are not measurable, radon concentrations at the site fence were estimated with the model at about Bq/m 3 above the background level. Measurements of external

6 radiation in the very vicinity of the facility, within the storage entrance, showed apparently elevated doserate levels in the past, but recently optimised distribution of the radioactive waste items in the storage practically eliminated additional external radiation fields. 4.4 Former Uranium Mine at Žirovski vrh After a short period of operation ( ) and a long period of restoration ( ), the remediation of the former uranium mining and milling complex at Žirovski Vrh was completed this year. The essential differences compared to the previously mentioned facilities are (i) that natural radionuclides were discharged and (ii) that after restoration these radionuclides are still going to be released into the environment, of course in significantly smaller amounts. During the operation, the radioactive contamination in all environmental media was observed, such as in air (particles and radon), surface and underground water, sediments, fish, soil, and also in some local food. Table 2. Radionuclide concentrations in the local environment resulting from radioactive discharges in Slovenia. Facility Radionuclides Range of concentrations Remarks Krško NPP air noble gases iodine aerosols ( 58 Co) 3 H, 14 C 13 μbq/m 3 during an outage surface water underground water dry deposition vegetation Žirovski vrh uranium mine air surface water underground water sediments Reactor TRIGA Radwaste storage 3 H kbq/ m 3 monthly averages 3 H kbq/ m 3 grab sampling 58 Co, 60 Co 15 Bq/m 2 during an outage 14 C pmc* 238 U, 226 Ra 140 μbq/m 3 including background 210 Pb mbq/m 3 including background 222 Rn 2035 Bq/m 3 including background 37 Bq/m 3 estimated mine contribution 238 U Bq/m Ra 540 Bq/m 3 monthly averages, 210 Pb, 210 Po 530 Bq/m 3 including background 238 U 226 Ra Bq/m Bq/m 3 including background 238 U, 226 Ra Bq/kg including background 41 Ar 222 Rn 15 Bq/m 3 including background 0.23 Bq/m 3 estimated RWS contribution * percent modern carbon (100 pmc=227 Bq/kgC)

7 The following longlived and shortlived radionuclides were monitored: 238 U sec, 226 Ra, 230 Th, 210 Pb, 210 Po; radon 222 Rn, and its shortlived decay products 218 Po, 214 Pb, 214 Bi, 214 Po. The highest contribution to the public exposure still comes from the inhalation of radon decay products in the settled nearby environment [9]. Dissolved uranium in the Brebovščica recipient stream still occasionally reached 200 Bq/m 3 of 238 U, while the uncontaminated stream has only 5 Bq/m Ra content in sediments is evidently still enhanced (100 Bq/kg) to up to twice the background value. The radon contribution from the mining and milling facilities to environmental levels decreased after restoration from 79 Bq/m 3 to not more than 34 Bq/m 3. The average background concentrations of 222 Rn in this region are around 25 Bq/m 3. During operation, the increase of radioactivity in the environmental media was measurable, since concentrations were distinctly higher than corresponding background levels. After restoration, only radon concentrations and radon shortlived decay products outdoors, uranium in surface water, and radium in sediments have been measurable with sufficient quality, since they can be clearly distinguished from the background. 4.5 General Environment The monitoring of artificial radioactivity in general environment on the territory of Slovenia started in 1961, and was established and financed by the former Yugoslav Federal Commission for Nuclear Energy. Firstly, total beta activity in air, precipitation and water samples was predominantly measured, with the determination of 90 Sr activity in precipitation, soil, and foodstuffs. At the end of the sixties, the introduction of gammaspectrometry enabled also 137 Cs determination. The levels of both longlived radionuclides in that period were much higher than nowadays (some orders of magnitude), since atmospheric nuclear weapon tests continued until The Chernobyl accident in 1986 contaminated the territory with 137 Cs, on average five times as much as all previous weapon tests, while 90 Sr levels were only slightly increased. In the first weeks after the accident, the contamination with iodine 131 I was of the prevailing concern. Daily air concentrations of 131 I raised up over 30 Bq/m 3 taking into account solid and gaseous iodine. It was the only time when 134 Cs was globally dispersed, with the activities of a little less than a half of 137 Cs values. Several short lived radionuclides (with halflives of 210 days) were identified in air and precipitation, namely 99 Mo, 132 Te, and 136 Cs, since the transport time was much shorter as in the case of a distant nuclear weapon test. Both longlived fission radionuclides, 90 Sr and 137 Cs, are still the main contributors to population exposure from manmade radioactivity in the environment. The regular measurements of 3 H were introduced in the monitoring programme in the nineties (precipitation, drinking water). The radionuclide 14 C has been not measured at all [10]. The existing monitoring network is also a convenient and sensitive tool to detect and identify incidental releases to the atmosphere. The most apparent such case was the 137 Cs source that had been melted in the steel plant in Algeceiras, Spain, in May An increased level of radiocaesium air concentration was recorded in Ljubljana in the first days of June 1998, with the maximum daily concentration of mbq/m 3 and monthly average of 0.1 mbq/m 3. Another incidental release was recorded in the second decade of April, 2003, when 131 I escaped during the outage from the Hungarian nuclear power plant at Paks. The operational monitoring network of the Krško NPP recorded 131 I daily concentrations in air (100 μbq/m 3 ) and in precipitation (25 Bq/m 3 ) as a monthly average at Krško at the distance of 300 km from Paks. Radioactive discharges from hospitals are not regularly monitored in Slovenia. The annual amounts of discharged radioactivity are estimated from the data of purchased and administered activity. Short lived radionuclide 131 I was regularly measured in the Sava river

8 after the confluence with the Ljubljanica river into which liquid effluents are currently discharged. Considerable levels of 131 I are usually measured also in the Savinja river, a recipient surface water for nuclear medicine department in Celje. In both cases the levels of some 10 Bq/m 3 frequently appeared. The rivers Drava and Mura, flowing from Austria, showed only traces of 131 I (less than 1 Bq/m 3 ) in the last decade [11], which is much lower than in previous years. Radioactive effluents in the nuclear departments of Austrian hospitals are aging in waste tanks before being released. 5 CONCLUSIONS Up until now, a limited number of artificial radionuclides, mostly gamma emitters, have been regularly monitored in the environmental media. They were easily identified with approximate concentration levels of the order of magnitudes of 1 μbq/m 3 in air, 0.01 Bq/kg in fresh solid materials (foodstuffs and feedingstuffs), 1 Bq/m 3 in water (with exception of 3 H with the levels of 1 kbq/m 3 ), and 10 Bq/kg in soils. In the atmosphere, there are at least 36 radionuclides of natural origin. Less than a third of them are monitored regularly, taking into account all existing monitoring programmes in Slovenia [11]. Only two longlived artificial radionuclides are measured in the environment and evaluated regularly ( 137 Cs, 90 Sr). The highest exposures of the public come from soil contamination with 137 Cs (external radiation) and from 90 Sr content in foodstuffs (internal contamination due to food consumption). Radionuclides, requiring special sampling, expensive as well as lengthy preparation of sample (transuranic alpha emitters, 129 I, 14 C) or needing a delicate measuring equipment, are also not monitored at all. Radionuclides, present in the environment in very small concentrations that are below the levels quoted above, and radionuclides with small dose conversion factors (except 3 H) are not measured. The current requirement in EC (Commission recommendations 473/2000/Euratom) for the general monitoring does not require such quality standards as are currently achieved by the Slovenian monitoring [12]. Even though measurements are quite precise, the evaluations of results can be occasionally poor, not taking into account some other important parameters, such as river flowrate, the number of population using a particular water source or the quantity of milk collected within a particular diary network, and the consumption of particular foods. The overview can be used to further refine operational monitoring programmes, so that the impact on the environment can be assessed better. REFERENCES [1] UNSCEAR, Sources and Effects of Ionizing Radiation, UN, New York, UNSCEAR 2000, Vol. I: Sources; (2002) [2] A. Osterc, V. Stibilj, 127 I and 129 I/ 127 I isotopic ratio in marine alga Fucus virsoides from the North Adriatic Sea, J. Environ. Radioact., 99, /2008/ [3] Environment Agency, FSA, NIEA, SEPA, Radioactivity in food and the environment, 2008, Lowestoft, /Suffolk, UK/, 2009

9 [4] BAG, OFSP, UFSP, SFOPH, Environmental radioactivity and radiation exposure in Switzerland in 2003, Fribourg, 2004 [5] URSJV, ROKORadioaktivnost v okolju, [6] JSI, Annual reports on radioactivity measurements in the surroundings of the Krško Nuclear Power plant (annual reports for 1983,..2009), Jožef Stefan Institute, Ljubljana, 1984,.2010 (in Slovene) [7] JSI, Radioactivity measurements in the vicinity of the TRIGA Research Reactor (annual reports for 1988, 2009, Jožef Stefan Institute, Ljubljana, 1989,.2010 (in Slovene) [8] ARWM, Radioactivity measurements of the LILW Storage Facility (annual reports for 2000,..2009), Agency for Radioactive Waste Management, Ljubljana, 2001,.2010 (in Slovene) [9] JSI, Radioactivity in the environment of the Žirovski Vrh Uranium Mine (annual reports for 1985,.2009), Jožef Stefan Institute, Ljubljana, 1986,..2010, (in Slovene) [10] IOS, Environmental radioactivity in Slovenia (annual reports for 1961,..2010), Institute of Occupational Safety, Ljubljana, 1962, 2010 (in Slovene) [11] M. J. Križman, Radionuclide concentrations in air data estimation for Slovenia, Report No. URSJV/DP085/2005, Slovenian Nuclear Safety Administration,,Ljubljana, 2005, (in Slovene) [12] EC, Commission recommendation 2000/473/Euratom on the application of Article 36 of the Euratom Treaty, Brussels, 2000