Unrestricted Release of a Thorium-Contaminated Building

Transcription

1 Unrestricted Release of a Thorium-Contaminated Building A. Reichelt 1, K.-H. Lehmann 1, B. Sitte 1, W. Hauk 2 1 Technical Inspection Agency of Southern Germany (TÜV), Munich, Germany 2 State Office of Industrial Saftey and Medicine, Suhl, Germany ABSTRACT A building which was contaminated with thorium has been decontaminated in such a manner that it could be demolished without any restrictions. In order to reach this target, following work phases have been effected: determination of the radiological status of building and ground, calculation of the clearance levels on the basis of the 10 µsv-concept, determination of the extent of contamination of the neighbouring grounds, the decontamination of each floor followed by controlling measurements carried out by the responsible authority, repetition of the measurements in order to determine the extent of the contamination of the neighbouring grounds due to the decontamination activities. Since the radionuclide vector of contamination which penetrated the building structure is the same as the natural background of the building material it has been a great challenge for the evaluation method. The targets planned for the future are the clearing of the base-plate and of the surrounding ground. INTRODUCTION AND EARLY HISTORY The production of all thoriated gas mantles in the former German Democratic Republic (GDR) took place from the 1950-ties till 1985 in a facility in Thuringia, in the village of Schweina. During 30 years of production of gas mantles, radioactive thorium was used in several chemical forms (oxide, nitrate, hydroxide, oxalate). In this way the production building and the near vicinity have been partly contaminated to a high extent. After the Reunification of the two German states, the Technical Inspection Agency of Southern Germany (TÜV Süddeutschland) 1) has received the order from the responsible local authority, the Thuringian State Office for Occupational Safety and Hygiene (LAFAS) 2), to carry out radiological measurements and to determine the extent of the contamination of buildings and ground. Due to the high contamination which partly penetrates the whole building structures, the decision has been taken in 1998 to remediate the building for unrestricted release according to the 10 µsv-concept and subsequent pulling down the building. Therefore, for the first step mass- and surface-specific clearance levels for the unrestricted release have been determined using site specific parameters which have been confirmed by the LAFAS. The decontamination of the facility itself is carried out by AEA Technology 3) by order of the owner ( Treuhandliegenschaftsgesellschaft, TLG) 4). Beginning at the top of the building, each floor of the building has been decontaminated and cleared step by step. After the decontamination of a floor has been finished, TÜV Süddeutschland carried out radiation measurements to prove that clearance levels are not exceeded. Before starting with the actual remedial activities, the extent of the contamination of the neighbouring ground next to the object has been determined. The same measurements have been repeated after finishing the remedial activities in order to show that the ground has not been contaminated by these activities. 1) Landesamt für Arbeitsschutz und Arbeitsmedizin, Schleusinger Straße 30, D Suhl 2) TÜV Süddeutschland, Hauptabteilung Strahlenschutz u. Entsorgung, Westendstraße 199, D München 3) AEA Technology QSA GmbH, Gieselweg 1, D Braunschweig 4) TLG, Treuhand Liegenschaftsgesellschaft mbh, Steigerstraße 24, D Erfurt EXTENT OF CONTAMINATION In 1995 the extent of the contamination of the building and of the ground was investigated (1). Figure 1 shows the building. 1

2 Figure 1. Thorium contaminated gas-mantle production facility in Schweina (Thuringia) At each gable front of the building there is a two-floor-extension. Additionally, there is an open storage area and a small stable. The ground floor has 11 rooms, the first floor 15, the second floor 7 and the attic 4 rooms. Each room of the building has been investigated separately by means of several measurement methods (gamma spectrometry, dose rate measurements, Rn-220 measurements, measurements of surface contamination using surface contami-nation detectors). Numerous drillings in the building structure and in the ground, up to a depth of 5 m, have also been carried out. As a result of the survey measurements, the following rooms have been regarded as contaminated: Ground floor: room No. 1, 2, 4, 5 floor and walls room No. 3, 6, 7, 8, 9, 15 floor 1st floor: room No. 2, 3, 4, 5, 6 floor and walls room No. 8, 9, 10, 11, 12, 13, 14, 15 floor 2nd floor: room No.1, 2, 3, 4, 5, 6 floor Attic: not investigated All rooms were more or less radioactively contaminated except room No. 10 of the ground floor. Contamination of walls has been determined only in the ground floor and 1st floor. Table 1 shows the results of measured activity with depth of a wall of room No. 1 in the 1st floor. 2

3 Table 1. Specific activity in a wall of room No. 1, 1st floor depth [cm] material Ra-288 [Bq/kg] [Bq/kg] 0,0-1,5 tile I ,5-3,0 mortar I ,0-3,5 cardboard I ,5-5,0 tile II ,0-6,5 mortar II ,5-7,0 cardboard II ,0-10,0 mortar III ,0-24,2 paint Due to the age of the thorium contamination, there is an radioactive equilibrium between thorium and its decay products. As a result of chemical processing during production, different situations have been found (see table 2). Table 2. Specific activity in a floor of room No. 1, 1st floor depth [cm] material Th-232 [Bq/kg] Ra-288 [Bq/kg] [Bq/kg] Th-230 [Bq/kg] 0,0-2,5 tile, mortarl > 10 sewage pipe < The surface contamination was very inhomogenous. The values reached from 0,1 Bq/cm² up to 1311 Bq/cm² (layer depth 7 mm, all radionuclides of Th decay chain taken into account). The gamma dose rate varied between 160 nsv/h and 3430 nsv/h. The natural background inside the building was 140 nsv/h. The concentration of Rn-220 in the air of highly contaminated rooms varied between 170 Bq/m³ and 500 Bq/m³, near the sewage gullies up to 5000 Bq/m³. Outside the buildings, the ground contamination has been determined up to the loam layer which begins at a depth of 20 cm up to 80 cm. The ground contamination is very inhomogeneous and reaches values up to 8000 Bq/kg per radionuclide of the Th decay chain. At some places the contamination in the depth was higher than at the surface. The gamma dose rate varied between 200 nsv/h and 6000 nsv/h, the natural background was 143 nsv/h. CLEARANCE LEVELS Taking into account the relevant exposure pathways (external radiation, inhalation of aerosols and 220 Rn progenies, ingestion) mass- and surface-specific clearance levels for the unrestricted release have been determined using site specific parameters which have been confirmed by the LAFAS (2). The clearance levels for solid materials and for restricted release were taken from national recommendations of the German Radiation protection Commission (SSK) (3). In table 2 the clearance levels for unrestricted and restricted release are shown. If there are indications that radioactive equilibrium has not been reached, the clearance levels may not exceed by the radionuclide with the highest activity. 3

4 Object of release building: pulling down rubble: reuse solid material: reuse rubble and solid material: disposal metal scrap: recycling decon material: disposal Table 3. Clearance levels Clearance level per nuclide of decay chain Averaging mass/area Release Condition Method of determination 1 Bq/cm² whole wall, unrestricted actual assessment ceiling, floor 27 Bq/kg 300 kg unrestricted actual assessment 0,02 Bq/cm² 2) some 100 cm² unrestricted SSK recommendation 13 Bq/kg 300 kg 0,02 Bq/cm² 2) some 100 cm² restricted SSK recommendation 320 Bq/kg 300 kg 0,02 Bq/cm² 2) some 100 cm² restricted SSK recommendation 94 Bq/kg 300 kg 320 Bq/kg 300 kg restricted SSK recommendation CONTROL MEASUREMENTS Organisation of Measurements The task of the control measurements carried out by TÜV Süddeutschland has been to verify that the clearance levels given in table 3 are not exceeded after decontamination and after clearance measurements done by AEA Technology. After notification by AEA Technology, control measurements were performed floor by floor beginning at the top. Decontamination work of the next lower floor started after finishing control measurements of the floor above. Beginning at the top has been for reasons of structural engineering. Figure 2. Control measurements of rooms using collimated and uncollimated in situ gamma spectrometry Measuring techniques and strategies An advanced measurement methodology has been used: parallel collimated and uncollimated in situ gamma spectrometry (fig. 2) completed by gamma spectrometric analysis of samples. Surface contamination measurements and dose rate measurements have been used to identify inhomogeneous 4

5 contamination patterns which might be concentrated on small areas. Using the described measurement methodology, reliable detection of activity which has penetrated into the structure was possible in a most effective way with respect to the existing radionuclides. By uncollimated in situ gamma spectrometry a whole room or, in the case of big rooms, parts of them have been measured. Using this basic information of integral contamination, collimated in situ gamma spectrometry was performed at particular areas of the room. Number and areas of the measurements were chosen following the knowledge gained from the survey measurement campaign (1), the basic information from uncollimated in situ gamma spectrometry and screening by surface contamination and dose rate measurements. At selected areas for detailed characterisation of results, surface contamination measurements and sampling layer by layer have been performed. Additionally, dose rate measurements at all measuring positions have been performed. As the activity had migrated to different depths into the structures showing different distributions with depth (exponential, homogeneous) we determined the efficiency for the different distributions. The selection of the actual efficiency was based on an energy related analysis of the gamma spectrum. The natural nuclide vector of the building material is the same as the artificial nuclide vector due to gas mantle fabrication. Therefore, we determined the background for each material at known non-contaminated areas. In the case of in situ gamma spectrometry the correction due to background were done taking into account the depth of migration estimated from the different energies in gamma spectra and assuming a homogeneous distribution of natural contamination within the materials. RESULTS AEA Technology has carried out the decontamination work inside the building properly, so that all control measurements showed results below clearance levels. At small areas we found clearance levels which exceeded less than factor 2. After another decontamination of these small areas, the subsequent control measurements verified that clearance levels were not exceeded. Tables 4, 5, 6, and 7 show typical results from room No. 3 (attic). Table 4. Surface activity (uncollimated in situ gamma spectrometry) reference- contamination clearance area [m²] Ra-228 level exceeded? 231 0, ,38 30 no Table 5. Surface activity (collimated in situ gamma spectrometry) measuring reference contamination clearance object area [m²] Ra-228 level exceeded? wall 1 8,0 0, ,10 37 no floor 2,5 0, ,43 13 no floor 2,5 0, ,13 42 no floor 2,5 1,27 7 1,13 6 yes wall 2 7,4 1,08 9 0,98 8 yes 5

6 Table 6. Surface activity (analysis of samples. - measurement results of separate layers) material measuring range of area of contamination object depth [cm] sample [m²] Ra σ error wood floor 0-2, ,07 5 0,07 8 grit floor 2,5-5, ,80 3 0,84 6 mortar wall ,97 3 0,98 6 Table 7. Surface activity (analysis of samples. - sum of separate layers) measuring sampling reference contamination clearence object depth [cm] area [m²] Ra-228 level exceeded? floor 5,0 2,5 0,87 3 0,91 6 no wall 2 1 7,4 0,97 3 0,98 6 no CONTAMINATION OF NEIGHBOURING GROUND Before starting with the actual remedial activities by means of collimated in-situ gamma spectrometry the contamination bearing radionuclides Ra-228 and have been determined on the neighbouring ground. About 150 measuring positions have been selected within a 10 m area around the site and have been marked afterwards. The results of the measurements show a contamination of Th-232 and its decay products in the whole surveyed area up to 100 Bq/kg per radionuclide. At some places the contamination reaches values up to 400 Bq/kg, maximum value have been 600 Bq/kg. After finishing the decontamination of the building these measurements have been repeated at 1/3 of the marked positions showing the same results taking into account unavoidable variations. That means that there have not been any impacts from decontamination work to neighbourhood. Figure 3. Determination of contami-nation of neighbouring ground using collimated in situ gamma spectrometry Figure 3 shows equipment used for outside measurements. 6

7 Figure 4 shows the results of the measurements performed before starting decontamination of the building Y X Koordinatenursprung 0 Figure 4: Measured specific soil activity (Ra-228) of neighbouring ground FUTURE ACTIVITIES The next step will be pulling down the building. Since clearance measurements have been performed and positively finished at the building this will be done without any restriction with respect to radiation protection. However, the dust limits of the conventional standards may not be exceeded. External exposure will be minimised by taking away hot spots, shielding and access restrictions. In the year 2000 it is planned to demolish the foundation of the building under radiation protection conditions. For the release of the site to a green meadow, appropriate clearance limits have to be determined. 7

8 LITERATURE 1. Reichelt, A.; Rauh, H.-J.: Kugel- und Rollenfabrik Schweina GmbH i. L.: Ermittlung des Kontaminationsumfanges und des radiologischen Status. Untersuchungsbericht erstellt vom TÜV Bayern Sachsen, März Reichelt, A.: Sanierung des ehemaligen Fertigungsgebäudes der Leuchtstoffwerke Bad Liebenstein (TLG Objekt Nr ), Bahnhofstr. 18, Schweina. Ermittlung der Freigabewerte. TÜV Süddeutschland, November Freigabe von Materialien, Gebäuden und Bodenflächen mit geringfügiger Radioaktivität aus anzeige- oder genehmigungspflichtigem Umgang, Empfehlung der Strahlenschutzkommission, verabschiedet in der 151. Sitzung am 12. Februar 1998; BAnz Nr. 193 vom , S bis Reichelt, A.: Sanierung des ehemaligen Fertigungsgebäudes der Leuchtstoffwerke Bad Liebenstein (TLG Objekt Nr ), Bahnhofstr. 18, Schweina. Radiologischer Status der angrenzenden Grundstücke vor den Sanierungsmaßnahmen (Meßzeitraum: ). TÜV Süddeutschland, Februar Reichelt, A.: Sanierung des ehemaligen Fertigungsgebäudes der Leuchtstoffwerke Bad Liebenstein (TLG Objekt Nr ), Bahnhofstr. 18, Schweina. Radiologischer Status der angrenzenden Grundstücke nach den Sanierungsmaßnahmen (Meßzeitraum: ). TÜV Süddeutschland, August Reichelt, A.: Sanierung des ehemaligen Fertigungsgebäudes der Leuchtstoffwerke Bad Liebenstein (TLG Objekt Nr ), Bahnhofstr. 18, Schweina. Kontrollmessungen im Sanierungsabschnitt Gebäude, Radiologischer Status der angrenzenden Grundstücke vor und nach den Sanierungsmaßnahmen, Abschlußbericht, August