European Project Metrology for Radioactive Waste Management

Transcription

1 European Project Metrology for Radioactive Waste Management Petr Kovar Czech Metrology Institute Okruzni , Brno, Czech republic Jiri Suran Czech Metrology Institute Okruzni , Brno, Czech Republic Jaroslav Solc Czech Metrology Institute Okruzni , Brno, Czech Republic Dirk Arnold Physikalisch-Technische Bundesanstalt Bundesallee , Braunschweig, Germany ABSTRACT The operation of nuclear facilities and their decommissioning, once their life cycle ends, must be done in a way to minimise environmental impacts. This can only be achieved by accurately measuring the radioactivity of radionuclides in waste and environmental samples, using standardised and traceable measurement methods. In 2011, within the European Metrology Research Programme (EMRP 2010), the joint research project Metrology for Radioactive Waste Management (MetroRWM) began. Thirteen European metrology institutes participate in the project with a total budget of over 4M. The project will finish in September During the project, the following items are being developed: standardised traceable measurement methods for solid radioactive waste for free release into the environment or acceptance to repositories, novel instruments and methods for in-situ measurements, gaseous effluent monitors/samplers for stored wastes as well as standard sources and reference materials. In addition, decay data for long-lived radionuclides are being improved. This presentation aims to illustrate the results achieved within the development of standardised traceable methods for measurement of solid materials and objects released from nuclear facilities into the environment

2 902.2 KEYWORDS Free release measurement; activity measurement; gamma-ray spectrometry; low-background shield; reference materials; Monte Carlo simulations. 1 INTRODUCTION One of the basic conditions for the release of solid materials and objects from nuclear facilities, either in operation or in the decommissioning stage, into the environment is the satisfaction of strict limits as required by national regulators regarding the content of radionuclides in the released material. Release limits are given in units of the mass activities of the radionuclides and are generally about Bq/kg. Legislation in this area varies in different countries; in some places the limits must be satisfied for each kilogram, while elsewhere averaging can be done over different weights of the measured material. [1] Reaching the limits for release is easy in laboratory conditions, but given the huge amount of measured material (tens to hundreds of thousands of tons in the decommissioning of a nuclear power plant) it is very difficult, given the required throughput of the measuring devices, and therefore the shortest measurement time is generally minutes for a single charge. The difficulty of measuring is further complicated in that there is almost always a mix of radionuclides in the released materials. This is why it is necessary to reduce the background in the measurement chamber to a minimum and use a gamma-ray spectrometer for measuring. This paper describes the construction of a free release measurement facility, developed within the project MetroRWM. This facility is based on germanium detectors and special shielding made from environmentally friendly material. Shielding capacities are demonstrated when measuring the spectra of gamma rays and reducing the dose rate inside the measuring chamber. This paper also describes reference materials developed for efficiency calibration of the facility. 2 MATERIALS AND METHODS 2.1 Free release measurement facility (FRMF) The FRMF was built as a facility for testing standardised traceable methods for free release measurement, developed within the framework of the project "MetroRWM". The internal dimensions of the chamber were optimized so that it was possible to place four coaxial HPGe detectors (FWHM 2.0 kev, rel. eff. 50%) with electric cooling, and to be able to measure the released materials in a standard container with dimensions of 40 x 80 x 120 cm and three measurement positions. The thickness of the floor, walls, and ceiling were optimized using a model created in MCNPX code, and these values were set as optimal: floor thickness 60 cm, wall thickness 40 cm, and ceiling thickness 50 cm. The thickness of the sliding door was determined as the maximum tolerable, due to the weight of the door and its shift, at 20 cm. Because of the lesser thickness of the door, the chamber floor was extended in the direction of the door outside the chamber so that the contribution of natural background radiation did not increase in the chamber. The FRMF is in Figure 1, and the inside of the measuring chamber is in Figure 2.

3 902.3 Figure 1: Free release measurement facility Figure 2: Inside the FRMF measuring chamber Building blocks (Fig. 3) were used as shield building material, made of a special lowbackground concrete with a density of 2.4 g/cm 3 and the following concentration of natural radionuclides: Ra-226 (1.0 ± 0.1), Bq/kg, Th-232 (0.7 ± 0.1) Bq/kg and K-40 (10.6 ± 0.5) Bq/kg. Due to the displacement of radon from the chamber, it was maintained at a low overpressure using filtered air from outside the chamber.

4 902.4 Figure 3: Building blocks for construction of the FRMF 2.2 Background Natural background sources To optimize the construction of the low-background chamber, the following sources of natural background were taken into consideration: Floor of the hall in which the measuring chamber is located. This is the main source of natural background inside the chamber, and the reason why the thickness of the shielding layer is the greatest (60 cm). Walls of the hall and air inside the hall. The influence of this component is largely eliminated by the chamber walls (thickness 40 cm). Air in the measuring chamber. The influence is almost completely eliminated by maintaining a weak overpressure of filtered air inside the chamber. Cosmic radiation. Cosmic radiation contains high-energy components, and has practically no influence on the background when measuring the spectrum of photons in an energy range up to 2 MeV. A slight effect was observed only when measuring of the dose equivalent rate Measured spectra Figure 4 shows the spectrum of gamma radiation measured by a germanium detector at the site before construction and inside the measuring chamber after its completion. Figure 5 shows a comparison with the background in various spectrometric laboratories, starting with a laboratory without shielding and ending with a laboratory in Gran Sasso (3.1 km underground). The figure shows that the background in the measuring chamber (FRMF) is lower than in a laboratory with classic 10 cm thick lead shielding (Standard laboratory).

5 902.5 Figure 4: Measured spectra of gamma rays (HPGe detector) Figure 5: Comparison of spectra measured in different laboratories

6 Reference materials To calibrate the detection efficiency for measuring in the free release measurement facility, the following new types of reference materials were developed, real or spiked: Gravel containing natural radionuclides of the uranium and thorium decay series and K-40. The specific activities of the radionuclides were set after establishing a radionuclide equilibrium by measuring on a germanium gamma-ray spectrometer which is part of the national standard of radionuclide activity. Specific activities were determined for radionuclides Ra-226, Th-228 and K-40 with a relative standard combined uncertainty of up to 5%. The reference material in the measuring container is shown in Figure 6. Figure 6: Gravel reference material in measuring container Steel pipes containing radionuclides Co-60 and Ag-110m. Specific activities of radionuclides were determined using a germanium spectrometer measuring the remains generated in the production of the pipes. The specific activities of radionuclides Co-60 and Ag-110m were determined with a standard combined uncertainty of up to 5% (see Figure 7). Figure 7: Reference material steel pipes

7 902.7 Point sources with the radionuclide Cs-137 in special cases placed inside petangue balls (imitation of steel pipes). The emitters were prepared by drops from a standard solution with traceability to the national standards of radionuclide activity (Figure 8). 3 RESULTS AND DISCUSSION 3.1 Count rate reduction Figure 8: Reference material petanque balls Figure 9 shows the reduction in the total count rate in the spectrum acquired using a coaxial germanium detector during the construction of the measuring chamber. The total count rate in the energy range of 20 kev to 2 MeV was reduced by approximately 30 times. This is a very good result for free release measurements. 3.2 Dose rate reduction Figure 9: Background count rate reduction Figure 10 shows the reduction of the ambient dose equivalent rate H*(10) approximately 3 times. This will play an important role in the possible use of shielding materials in the construction of residential rooms.

8 CONCLUSIONS Figure 10: Ambient dose equivalent rate reduction The use of special ecological shielding materials was successful in significantly reducing background when measuring low activities using a germanium gamma-ray spectrometer. This satisfied the basic requirement for equipment for measuring solid waste and objects released from nuclear power plants into the environment. Without a significant reduction of natural background during measuring, it would not be possible to fulfil the strict limits as given by national regulators. A significant advantage of special transportable shielding is also its easy assembly and disassembly as well as the possibility of using it in various locations. The shielding elements may be used to build an entire low-background laboratory, and given the significant (three-fold) reduction of the dose rate inside the shielding, the utilization of the shielding elements in the construction of residential rooms, such as bedrooms, is also possible for consideration. ACKNOWLEDGMENTS The European Metrology Research Programme is jointly funded by the EMRP, participating countries within EURAMET and the European Union. REFERENCES [1] F.J. Maringer et al, Metrology for radioactive waste management: Clearence levels and acceptance criteria legislation, requirements and standards, to be published in Applied Radiation and Isotopes