Radiation Protection and On-Site Inspection of the Elevated Subway Construction with an Ir-192 Gamma-Source

Transcription

1 Radiation Protection and On-Site Inspection of the Elevated Subway Construction with an Ir-192 Gamma-Source Bernhard Redmer, Stefan Hohendorf, Falk Hille BAM Berlin

2 Abstract To proof the integrity of riveted steel structures of truss bridges, various non-destructive testing methods can be used. Together with the department Safety of Structures of the BAM Berlin a testing procedure was developed in which the gusset plates were examined radiographically at selected test positions of a bridge construction of the Berlin elevated subway line U1. The inspection was performed during the night break of train traffic. A special service locomotive was ordered to specific bridge positions to generate pre-calculated load pattern to the construction. The load distribution supported the opening of potential radial cracks in the riveted gusset plates. The gamma radiographic inspection was successfully chosen to search for possible fatigue cracks. A gamma source, Irimium-192, with an activity of approx GBq and different digital detectors, such as a digital detector array and imaging plates, were used. The bridge construction was located in regions with residential and commercial buildings. This had to be considered as well as the vehicle and pedestrian traffic to fulfil the radiation protection requirements. The article describes the preparation and performance of the on-site inspection with a gamma source from the technical perspective of radiation protection. In accordance with the German standard DIN part 1 and some basic mathematical calculations for the control area boundary were calculated and compared with locally measured dose rate values. The results were in good agreement with the measurements.

3 Outlines Motivation and Initial Situation Description of the on-site situation Dose calculations for on-site use and comparison Experiences of on-site examinations Summary References The cited map (in parts) is taken: data from OpenStreetMap published under ODbL

4 Motivation Riveted steel truss girder viaduct of the elevated metro line U1 in Berlin-Kreuzberg. Start of operation: 1902, forced break in operation between 1961 und 1995 for reasons of Berlins division. Verification of the structures safety against fatigue failure for the coming 30 years.

5 Preliminary proof testing of fatigue resistance Bridge load tests for identifying the structural system. Laboratory tests at a removed truss girder. Analytical assessment of fatigue resistance. Laboratory tests Determination of critical structural parts regarding fatigue failure critical are gusset plates, hidden by u-shaped girders, testing of radiographic methods for periodic inspections.

6 Concept for periodic inspections with radiography Ca. 400 gusset plates selection of particular critical gusset plates. Decisive selection criteria were: cross sectional weakening in the tensile section of the gusset plate because of structural design, number of rivets or rivet shank diameter, existent or assumed damaging of the gusset plate, detectible by visible deformation, hairline cracks in the coat of paint, corrosion and repair welding.

7 Initial situation from the perspective of test procedure RT is an applicable volume test procedure UT is not possible, due to a) the multi-layer rivet connection: air between the sheets metal and b) the coupling of the test probe is complicated by the application of corrosion-protection paint Wall thickness: mm (single wall), steel RT with Iridium-192 and film / image plates / digital detector array Security and observation of the Ir192-container Work carried out on scaffolding and platforms, in approx. 4 m height Initial situation from the perspective of radiation protection Intended test time: during the service break of U1: between 0:30 and 3:30 o clock; setting up at the test position before; Lively street life (even at night); Buildings: on both sides of the location of examination; lower level: trade, small firms; upper level: living area Discotheques: (unpredictable) public traffic (cars, pedestrians); taxis in stand-by (despite no stopping sign!) Road closure: application to the district office required

8 Situation at the location of examination Parkanlage Parkanlage

9 Situation at the location of examination View to the subway-stop Schlesisiches Tor View to the Oberbaumbrücke... vehicles- and individual traffic... setting up at the test position

10 Parameters for calculations Iridium 192, Activity: 2220 GBq (October 2013) Test sample: t=36 mm, Steel (HVL: 14 mm) Collimator: t=25 mm, Tungsten (HVL:2,5 mm) Boundary of the controlled area: 40 µsv/h Calculations acc. to DIN , Annex 1 Diagram gives the boundary of the controlled area for useful radiation 95 for 2,22*10 12 Bq: a I =95 m for test sample 2,6 HVL Fe : a II =0,45*95 m= 43 m for collimator 10 HVL W : a III =0,05 * 95 m= 4,7 m 2,2*10 12

11 Calculations Radiation protection acc. to education/training guide of the DGZfP * A with : F * H K Ir192 a K, N A 2220GBq, H Ir192 K msv * m² h * GBq 40 µsv h Controlled area (CA)- demarcation at useful radiation attenuated radiation through the sample attenuated radiation through the collimator F F F N N N 1 5, a KI = 97 m a KII = 40 m a KIII = 3 m

12 Demarcation at the location of examination... Parkanlage Fernbedienung Parkanlage Demarcation of the controlled area Workers for the oberservation/monitoring Test site, radiation direction

13 Calculations Application of the passage dose (pedestrian, drivers) Radiation source Ir-192, A= 2220 GBq (October 2013) Dose rate constant: Equation Ir192* A* H v * a with Consideration in direction of the radiation: 0 msv * m² h * GBq Ir192 vehicle Distance test site - roadway: Velocity in average: Dose per passage: a 0 = 5 m v = 30 km/h = m/h H = 8,0 µsv pedestrian distance test site (middle) pedestrian way: a 0 = 15 m Velocity in average: v = 3 km/h = m/h Dose per passage: H = 27 µsv 1. Application of the passage dose must be allowed by the responsible regional authorities. 2. Demarcation could be optimized.

14 Necessary demarcation at the location of examination... Parkanlage Fernbedienung Parkanlage Demarcation of the controlled area Workers for observation/monitoring Test site, radiation direction Free traffic (Pedestrians, vehicle) higher number of trained employees (restricted areas, areas to be supervised) Use of dose rate meter or alarm monitor Technical means (walkie-talkies) Safety through resolute appearance (demarcation, to redirect individual traffic, information etc.) Supervision of the equipment (remote control, container, test site)

15 Structure xy South Axis xy upper position Direction of view: south north Direction of radiation: south sorth Rivet 2: Volume and linear indication under the rivet head: scratch or suspicion of crack?? Iridium 192; DDA: Vidisco (145µm) SDD=80 cm; 20s/f: 320 sec Image is highpass-filtered (EnhanceDetails).

16 Structure xy South Axis xy upper position Direction of view: south north Direction of radiation: south north Rivet 1: Crack in the gusset plate connection at the vertical strut Iridium 192; DDA: Vidisco (145µm) SDD=80 cm; 20s/f: 320 sec Image is highpass-filtered (EnhanceDetails).

17 Measurements of the dose rate, on site 42 µsv/h Parkanlage 80 µsv/h Fernbedienung 81 µsv/h Parkanlage 50 µsv/h Boundary of controlled area Measuring point Location of examination, Direction of radiation

18 Comparison of the calculations and measured values Demarcation at Acc. to DIN Acc. to equation Measured value, on-site Useful radiation 95 m 97 m --- Attenuated radiation (test object) 43 m 40 m (m) µsv/h µsv/h Attenuated radiation (the collimator) 4,7 m 3 m (m) 11 42µSv/h 1. Distance values between standard/equation: The factors a II and a III are rounded up. Regarding to the DIN the determined distance values are greater. conservative approach 1. Attenuated radiation behind the collimator: The measured distance value is a factor 2-3 greater than the calculated values. Possible cause: Scatter radiation through the test object. Corrective actions: additional shielding with lead balls in sacks at the collimator or lead plates behind the test object

19 Modified demarcation at the location of examination µsv/h Parkanlage 80 µsv/h Fernbedienung 81 µsv/h Parkanlage 50 µsv/h Boundary of controlled area Measuring point Location of examination, Direction of radiation Individual and vehicle traffic

20 Summary Based on DIN , annex 1, the determination of the control area delimitation gives a good overview for the practice. The DIN is a practical tool for on-site use. The distance values obtained from the DIN are "conservative" values and are consistent with the "mathematical calculations". For the attenuated radiation the distance values behind the collimator are greater by a factor of 2-3 than the calculated distance values. The application of passage dose is an effective way to minimize the restrictions on individual and vehicle traffic. The implementation of measures for radiation protection requires always personnel and technical resources depending on the situation. Sometimes, the "use of resources" shall to expand greatly.. Increasing efforts are required for security and monitoring of the control area with complicated local conditions. The employees must be trained und instructed in the handling of the technical equipment. They need knowledge of about the specific situation on-site and skills to react in case of unexpected occurrences.