- Email: [email protected]
Optimization of a measurement facility for radioactive waste free release by Monte Carlo simulation
Applied Radiation and Isotopes 87 (2014) 348–352
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Optimization of a measurement facility for radioactive waste free release by Monte Carlo simulation Jaroslav Solc a,n, Petr Kovar a, Jiri Suran a, Virginia Peyres b, Eduardo García-Toraño b a b
Czech Metrology Institute, Inspectorate for Ionizing Radiation, Radiova 1, 102 00 Prague 10, Czech Republic Laboratorio de Metrología de Radiaciones Ionizantes, CIEMAT, Avenida Complutense, 40, 28040 Madrid, Spain
H I G H L I G H T S
Monte Carlo simulations utilized for optimization of a novel free release facility. Cosmic radiation and chamber walls contribute most to the background. Hn(10) rate inside the optimized facility decreased to 33% of the outside value.
art ic l e i nf o
a b s t r a c t
Available online 12 November 2013
A novel free release measurement facility (FRMF) was developed within the joint research project “Metrology for Radioactive Waste Management” of the European Metrology Research Programme. Before and during FRMF design and construction, Monte Carlo calculations with MCNPX and PENELOPE codes were used to optimize the thickness of the shielding, the dimensions of the container, and the shape of detector collimators. Validation of the numerical models of the FRMF detectors and the results of the optimization are discussed in the paper. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Free release measurement Low activity measurement Monte Carlo simulation MCNPX PENELOPE
1. Introduction The free release measurement facility (FRMF) enables a verification of compliance with legal clearance levels for free release of radioactive waste. The design of such facilities involves selection of appropriate detectors, measurement electronics, material of shielded measurement chamber and mechanical hardware, as well as an operator console and associated software. The joint research project “Metrology for Radioactive Waste Management” (MetroRWM) of the European Metrology Research Programme (EMRP), which started in October 2011, includes within its research topics the development of new measurement methodologies and measurement devices for the assessment of radioactive waste. One of these systems is an improved facility for measurements related to the free release of radioactive waste, with traceability to national standards of EU member countries. Critical aspects of the new measurement facility are its modularity enabling to build the system dimensioned to match the supposed waste throughput, as well as improved spectrometric measurement and decreased minimum detectable activity. The measurement facility is fully transportable including the shielded chamber made of low-
n
Corresponding author. Fax: þ420 266 020 466. E-mail address: [email protected] (J. Solc).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.005
activity concrete blocks instead of environmentally unfriendly massive lead shielding (Suran et al., 2013). This paper describes the optimization of the FRMF main parameters by means of Monte Carlo simulations using the PENELOPE and MCNPX. The aim of the optimization was to achieve maximum decrease of the detector count rate caused by natural background radiation inside the FRMF and to find optimal container dimensions for achieving maximum throughput of measured material.
2. Materials and methods 2.1. Free release measurement facility The FRMF is a testing facility used for the measurement of the activity of waste released from nuclear sites into the environment. It consists of a walk-through shielded chamber with the inner width, length and height of 120 cm, 300 cm, and 200 cm, respectively, a conveyor for transportation of containers filled with the measured material, and four Interchangeable Detector Modules (IDM) High-Purity germanium (HPGe) detectors (ORTEC; Ge crystal volume 180 cm3; FWHM@1332 keV E2.0 keV; rel. efficiency 50%) with Stirling cycle cooler. Two detectors are located above the measured material, the other two below. The shielded chamber is made of blocks from a composite, lead-free building material
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349
2.2.3. Sources of natural background For the optimization of the shielded chamber parameters, a total of five sources of natural radiation were considered: (1) concrete floor of the hall where the FRMF was located, (2) air in the hall, (3) composite material of the chamber, (4) air inside the shielded chamber, (5) cosmic rays. Main characteristics of the sources are presented in Table 1. Mass or volume activities of the materials and cosmic ray fluence are typical average values only and may slightly differ from the real values depending on the location of the experimental site. Activity of air inside the chamber is an estimate based on the fact that a ventilation unit will flush the outer air inside the FRMF through aerosol filters in order to achieve a significant reduction of radon concentration. The calculated quantity was the count rate per hour per keV in seven energy windows in the energy range from 59 keV to 1.7 MeV, summed from all four IDM detectors (thereinafter “count rate”). This quantity was obtained from the simulation results stated in “number of detector counts per source particle” using particle yields and activities stated in Table 1. The energy windows of greater interest were 137Cs (662 keV) and 60Co (1.33 MeV) windows. Independent simulation was performed for each particle source from Table 1 and subsequently the simulated count rates were summed into the total count rate caused by all sources of natural background radiation (thereinafter “total count rate”). Variance reduction techniques and PTRAC file (Particle Track output) were utilized to increase the efficiency of the calculations. Other sources of radiation background like possible radionuclide contamination of lead collimators or radon decay products deposited on surfaces were neglected.
based on aggregate with a low internal content of radionuclides with a density of 2.4 g/cm3. The activity concentrations of 40K, 226 Ra and 228Th in the composite material were measured and reached (10.6 70.5) Bq/kg, (1.07 0.1) Bq/kg and (0.7 70.1) Bq/kg, respectively (k ¼1). Dose measurements performed after the FRMF construction showed that the ambient dose equivalent rate, Hn(10), decreased from (101 77) μSv/h outside to (33.572.4) μSv/h inside the chamber (k ¼1). 2.2. Monte Carlo simulations 2.2.1. Monte Carlo codes A complex model of the whole FRMF was created in the Monte Carlo code MCNPX (Fig. 1) and the calculations were done with the version v2.7e of the code (Pelowitz et al., 2011). A standalone model of the IDM detector was created in the PENELOPE code (Salvat et al., 2006) as well and used for optimization of the container dimensions. Results of all calculations were obtained with the statistical uncertainties below 3% (k ¼1). 2.2.2. Validation of the IDM detector models Both IDM detector models were validated by measurements with point sources located 25 cm above the cryostat front face and 25 cm aside to the cryostat side wall. Measured and calculated full-energy peak detection efficiencies were compared for photon energies of 122 keV, 662 keV, 898 keV, 1173 keV, and 1332 keV.
2.3. Optimization of shielded chamber thickness The inner dimensions of the FRMF were fixed because they depend on the dimensions of the IDM detectors, mechanical parts of the FRMF and dimensions of the container filled with the measured material. Therefore, the Monte Carlo calculations were focused on shielded chamber thickness only. 2.4. Optimization of collimator shape Four collimator shapes were studied: standard cone, enhanced cone with enlarged thickness of the conical part, pyramid, and enhanced pyramid with enlarged thickness of the pyramidal part (Fig. 2). The pyramidal collimators were taken into consideration because every detector measures count rate related to a square section area of the container. In addition, the simulations were performed for collimators at the standard position and shifted by 1.5 cm up or down. The collimators were made of lead. The study
Fig. 1. Visualization of the FRMF Monte Carlo model. 1—IDM detectors, 2— container with load, 3—FRMF walls, 4—FRMF door, 5—FRMF ceiling, 6—FRMF floor, 7—hall floor. Table 1 Characteristics of particle sources considered in the simulations. Natural background source
Source dimensions
Energy spectrum
Concrete floor of the hall
Volume, 15.6 9 0.3 m3
Photons from
238
U series þ 232Th series þ 40K
Air in the hall
Volume, 15.6 9 6 m3
Photons from
238
U series U series þ 232Th series þ 40K
U series
Composite material of the chamber
Whole volume of the composite material
Photons from
238
Air inside the chamber
Whole volume of air inside the shielded chamber Rectangle, 2 3.2 m2 (parallel particle beam perpendicular to chamber ceiling)
Photons from
238
Cosmic rays
EXPACS database (Sato, 2006) of spectra of photons, electrons, neutrons, protons and muons at Earth surface (Prague coordinates and elevation) originated from interactions caused by cosmic particles
Activity
Total yield
30 Bq/kg þ30 Bq/ kg þ300 Bq/kg 100 Bq/m3
162.6 s 1/kg for photons 179 s 1/m3 for photons 4.7 s 1/kg for photons
1.0 Bq/kg 0.7 Bq/kg 10.6 Bq/kg 1 Bq/m3
Particle distribution: 37% photons, 6% electrons, 37% neutrons, 1% protons, 19% muons
1.8 s 1/m3 for photons 4.92 min 1/ cm2 for all particles
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considered only such collimator modifications which resulted in no more than 1.5% decrease of the full-energy peak detection efficiency of photons emitted from the container load. 2.5. Optimization of the container dimensions The basic container dimensions were predetermined by the need to use standardized containers. Therefore, only rectangular parallelepiped containers with load of 120 cm in length, 80 cm or 100 cm in width and with various heights were retained for calculations. Optimization process focused on height and density of the load and it was performed with respect to the throughput of the detection system, defined as the count rate per activity concentration in the container. This quantity can be calculated by multiplying the detection efficiency by the total container load mass. The material inside the container was modelled as iron homogeneously distributed in the whole volume contaminated with a homogeneous photon emitter. The container bottom surface was kept at the constant distance of 15 cm from the lower detector pair. Densities and heights were considered between 0.5 g/cm3 and 3.0 g/cm3 and between 40 cm and 80 cm, respectively. The optimization was performed by calculating full-energy peak detection efficiencies for 122 keV, 662 keV and 1332 keV photons.
3. Results and discussion
also shielded part of photons from the hall floor. The detectors were shielded by conical collimators. It can be also seen from Fig. 3 that the contribution from the floor in the hall rapidly decreases with increasing wall thickness, reaching approximately 10% of the total count rate for 30 cm wall thickness. In addition, air inside the FRMF contributes approximately by 0.1% to the total count rate. Sorption of radon decay products on chamber inner surface was not considered but concluding from simulation results it is assumed that this source of natural background is not significant even if the sorption increases the assumed volume activity of 1 Bq/m3 significantly. Based on these results it was decided that the chamber would have 40 cm thick side walls. 3.3. Door thickness The door of the shielded chamber cannot be as thick as the side walls (40 cm) due to the weight and difficulties with door manipulation. Therefore the simulations were performed for door thicknesses varying from 10 cm to 30 cm with the constant side walls, floor, and ceiling thickness of 40 cm, 60 cm, and 50 cm, respectively. It was found that for the 662 keV energy window, the Table 2 Results of the validation of the IDM detector models for a point source positioned at 25 cm above the cryostat front face. Full-energy peak detection efficiencies and the differences between measured and simulated values are given. Photon Measured full-energy peak energy (keV) detection efficiency
3.1. Validation Table 2 shows the result of validation of the IDM detector models with point sources located above the detector. The differences between measured and simulated full-energy peak detection efficiencies were within 1.5% for photon energies from 81 keV to 1332 keV. For a point source located aside of the detector, the difference between the measurement and both simulations were in almost all cases within 2%. The models were therefore considered as validated and suitable for the optimization of the FRMF parameters.
81.0 122.1 661.7 898.3 1173.2 1332.5
(4.347 0.06) 10 3 (4.847 0.06) 10 3 (1.15 70.02) 10 3 (8.787 0.12) 10 4 (6.937 0.10) 10 4 (6.157 0.09) 10 4
Difference (%) MCNPX/ measurement
PENELOPE/ measurement
0.34 7 1.39 0.717 1.32 1.42 7 1.45 0.337 1.42 0.067 1.51 0.53 7 1.43
0.357 1.40 0.687 1.33 1.43 7 1.50 0.047 1.47 0.127 1.59 0.75 7 1.51
3.2. Shielded chamber side walls thickness Fig. 3 shows the dependence of the total count rate in the 662 keV energy window on the thickness of the shielded chamber side walls. The total count rate decreases with increasing side wall thickness up to a certain constant value which is reached at a wall thickness of approximately 30–35 cm. At this thickness the cosmic rays become the dominant contributor to the total count rate which results in no additional decrease of the total count rate with increasing wall thickness. Very similar dependence with the plateau starting at around 30–35 cm is valid for 122 keV and 1332 keV energy windows as well Simulations were performed for a constant ceiling thickness of 50 cm, constant chamber floor thickness of 60 cm which already shielded most of photons from the hall floor, and for a constant door thickness of 20 cm which
Fig. 3. Contribution of different sources of natural background to the total count rate in 662 keV energy window in dependence on shielded chamber wall thickness. The lines represent a cubic spline interpolation to guide the eye.
Fig. 2. Visualization of the collimator models—(A) standard cone, (B) enhanced cone, (C) pyramid, (D) enhanced pyramid.
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summed contribution of hall floor and hall air to the total count rate is 17.5%, 6.5%, and 3.1% for door thicknesses of 10 cm, 20 cm, and 30 cm, respectively. Very similar contribution to the total count rate was obtained for the 1332 keV energy window as well. Based on these results, it was decided that the door thickness will be 20 cm. In addition, to decrease the contribution from the hall floor coming into the chamber through the door, it was proposed to build an extended floor made of low-activity blocks on the floor in front of both doors which can be also used as a table for manipulation of the containers. The length, width, and height of the extended floor were 130 cm, 200 cm, and 60 cm, respectively. The extended floor decreased the contribution of hall floor to the total count rate from 6.4% to 3.3% in 662 keV window, and from 5.7% to 3.7% in 1332 keV window. 3.4. Ceiling thickness Fig. 4 presents the dependence of count rate caused by cosmic rays on the ceiling thickness, for 40 cm thick chamber walls. The contribution of cosmic rays to the total count rate decreased from 52% (no ceiling) to 31% (60 cm thick) in case of 662 keV energy window, and from 62% (no ceiling) to 42% (60 cm thick) in case of 1332 keV energy window. Taking into account both the simulations and the mechanical properties of the shielding material, it was decided that the ceiling thickness of the FRMF will be 50 cm. Detailed description of MCNPX simulation of a germanium detector response to cosmic rays was published by Solc et al. (2012). 3.5. Detector collimators Fig. 5 shows the count rates in the 662 keV energy window caused by different components of the natural background with and without the presence of 5 cm thick collimators of the shape visualized in Fig. 2A. The total count rate with the attached collimators decreased to 56% and 62% in 662 keV and 1332 keV windows, respectively, compared to the total count rates without the collimators. Contribution of all background components decreased except for cosmic rays. This is caused by interactions of cosmic rays with lead resulting in an increased count rate. Results presented in Fig. 5 confirmed that the utilization of the collimators is highly recommended. That is why an additional optimization of the collimator shape and position with respect to the germanium crystal was performed. Only photons originating in the shielded chamber material were considered for this study.
Fig. 5. The count rate in the 662 keV energy window caused by main components of the natural background with (light) and without (dark) attached 5 cm thick lead collimators.
The container was homogeneously filled with steel with density of 0.5 g/cm3 to take into account its shielding effect. Several results were concluded from the simulations. Initially, count rates in 662 keV window with the simple pyramid collimator increased by 6 to20% compared to a simple conical collimator. Although the pyramidal collimator sees exactly a square segment of the container, it is not a suitable shape because the solid angle of the pyramid is larger leading to higher background count rates. And more, count rates in 662 keV window with both enhanced cone and enhanced pyramid collimators were lower approximately by 4%, 1%, and 18% compared to a simple cone if the detector was not shifted and shifted outside or inside the collimator along its axis, respectively. The same tendency was observed for all considered photon energies. Both collimators result in similar count rates; however, due to much simpler shape and lower weight, the enhanced cone is the preferred one. And finally, the change of the collimator thickness from 5 cm to 3 cm results in approximately 25% increase of the background. Based on the results it was recommended to use either an enhanced conical collimator or a standard conical collimator where the germanium crystal would be positioned more deep inside the collimator. 3.6. Optimization of container dimensions Fig. 6 presents the results obtained for 80 cm wide and 120 cm long container. The only significant dependence of count rate per activity concentration on container height is noticeable for a load with a density of 0.5 g/cm3 where the count rate increases between 40 cm and 80 cm height by 8% and 13% for 662 keV and 1332 keV photons, respectively. For other studied energies and load densities, the count rates were kept constant. The situation for 100 cm wide container is very similar. The largest difference was found to be a 2–8% increase in the count rate in all studied cases compared to 80 cm wide container. It can be concluded that the throughput does not change significantly with the container dimensions studied.
4. Conclusions Fig. 4. Decrease of the count rate caused by cosmic rays only in the 662 keV and 1332 keV windows as a function of the ceiling thickness. The lines represent a cubic spline interpolation to guide the eye.
Two models of an IDM HPGe detector, utilized in the novel free release measurement facility, were created in MCNPX and PENELOPE codes and validated by measurements. An MCNPX detector
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detection efficiencies for special cases of material and activity distributions in the measured container load.
Acknowledgement This joint research project is financially supported by the European Commission in the frame of the European Metrology Research and Development Programme EMRP 〈http://www.emrpon line.eu〉 undertaken by several EU Member States under the Article 169 initiative, JRP contract identifier EMRP ENV09 MetroRWM.
References Fig. 6. Count rate per activity concentration as a function of a container height and a density of iron load for 122 keV, 662 keV and 1332 keV photons. The lines represent a cubic spline interpolation to guide the eye.
model was extended to include a model of the whole FRMF. It was subsequently used for the optimization of the shielded chamber thickness and collimator shape. The PENELOPE detector model was utilized for optimization of container dimensions. Results of the simulations were considered during the FRMF construction. In the future, the FRMF model will be used for calculation of
Pelowitz, D.B., Durkee, J.W., Elson J.S., Fensin, M.L, Hendricks, J.S., et al., 2011. MCNPX™ 2.7.E Extensions. Salvat, F., Fernández-Varea, J.M., Sempau, J., 2006. PENELOPE-2006: a code system for Monte Carlo simulation of electron and photon transport NEA 6222. OECD. Sato, T., 2006. EXPACS: Excel-based Program for Calculating Atmospheric Cosmicray Spectrum, User's Manual, Japan Atomic Energy Agency. Available online: 〈http://phits.jaea.go.jp/expacs/〉. Solc, J., Kovar, P., Dryak, P., 2012. MCNPX Simulation of Influence of Cosmic Rays on Low-activity Spectrometric Measurements. http://dx.doi.org/10.1016/j.rad physchem.2012.12.034. Suran, J., Kovar, P., Solc, J., Skala, S., Torano, E.-G., Peyres, V., 2013. EMRP JRP ENV09 Workpackage 1 Presentation, 1st JRP ENV09 Workshop, Paris 2013. Available online: 〈http://www.radwaste-emrp.eu〉.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Optimization of a measurement facility for radioactive waste free release by Monte Carlo simulation Jaroslav Solc a,n, Petr Kovar a, Jiri Suran a, Virginia Peyres b, Eduardo García-Toraño b a b
Czech Metrology Institute, Inspectorate for Ionizing Radiation, Radiova 1, 102 00 Prague 10, Czech Republic Laboratorio de Metrología de Radiaciones Ionizantes, CIEMAT, Avenida Complutense, 40, 28040 Madrid, Spain
H I G H L I G H T S
Monte Carlo simulations utilized for optimization of a novel free release facility. Cosmic radiation and chamber walls contribute most to the background. Hn(10) rate inside the optimized facility decreased to 33% of the outside value.
art ic l e i nf o
a b s t r a c t
Available online 12 November 2013
A novel free release measurement facility (FRMF) was developed within the joint research project “Metrology for Radioactive Waste Management” of the European Metrology Research Programme. Before and during FRMF design and construction, Monte Carlo calculations with MCNPX and PENELOPE codes were used to optimize the thickness of the shielding, the dimensions of the container, and the shape of detector collimators. Validation of the numerical models of the FRMF detectors and the results of the optimization are discussed in the paper. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Free release measurement Low activity measurement Monte Carlo simulation MCNPX PENELOPE
1. Introduction The free release measurement facility (FRMF) enables a verification of compliance with legal clearance levels for free release of radioactive waste. The design of such facilities involves selection of appropriate detectors, measurement electronics, material of shielded measurement chamber and mechanical hardware, as well as an operator console and associated software. The joint research project “Metrology for Radioactive Waste Management” (MetroRWM) of the European Metrology Research Programme (EMRP), which started in October 2011, includes within its research topics the development of new measurement methodologies and measurement devices for the assessment of radioactive waste. One of these systems is an improved facility for measurements related to the free release of radioactive waste, with traceability to national standards of EU member countries. Critical aspects of the new measurement facility are its modularity enabling to build the system dimensioned to match the supposed waste throughput, as well as improved spectrometric measurement and decreased minimum detectable activity. The measurement facility is fully transportable including the shielded chamber made of low-
n
Corresponding author. Fax: þ420 266 020 466. E-mail address: [email protected] (J. Solc).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.11.005
activity concrete blocks instead of environmentally unfriendly massive lead shielding (Suran et al., 2013). This paper describes the optimization of the FRMF main parameters by means of Monte Carlo simulations using the PENELOPE and MCNPX. The aim of the optimization was to achieve maximum decrease of the detector count rate caused by natural background radiation inside the FRMF and to find optimal container dimensions for achieving maximum throughput of measured material.
2. Materials and methods 2.1. Free release measurement facility The FRMF is a testing facility used for the measurement of the activity of waste released from nuclear sites into the environment. It consists of a walk-through shielded chamber with the inner width, length and height of 120 cm, 300 cm, and 200 cm, respectively, a conveyor for transportation of containers filled with the measured material, and four Interchangeable Detector Modules (IDM) High-Purity germanium (HPGe) detectors (ORTEC; Ge crystal volume 180 cm3; FWHM@1332 keV E2.0 keV; rel. efficiency 50%) with Stirling cycle cooler. Two detectors are located above the measured material, the other two below. The shielded chamber is made of blocks from a composite, lead-free building material
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349
2.2.3. Sources of natural background For the optimization of the shielded chamber parameters, a total of five sources of natural radiation were considered: (1) concrete floor of the hall where the FRMF was located, (2) air in the hall, (3) composite material of the chamber, (4) air inside the shielded chamber, (5) cosmic rays. Main characteristics of the sources are presented in Table 1. Mass or volume activities of the materials and cosmic ray fluence are typical average values only and may slightly differ from the real values depending on the location of the experimental site. Activity of air inside the chamber is an estimate based on the fact that a ventilation unit will flush the outer air inside the FRMF through aerosol filters in order to achieve a significant reduction of radon concentration. The calculated quantity was the count rate per hour per keV in seven energy windows in the energy range from 59 keV to 1.7 MeV, summed from all four IDM detectors (thereinafter “count rate”). This quantity was obtained from the simulation results stated in “number of detector counts per source particle” using particle yields and activities stated in Table 1. The energy windows of greater interest were 137Cs (662 keV) and 60Co (1.33 MeV) windows. Independent simulation was performed for each particle source from Table 1 and subsequently the simulated count rates were summed into the total count rate caused by all sources of natural background radiation (thereinafter “total count rate”). Variance reduction techniques and PTRAC file (Particle Track output) were utilized to increase the efficiency of the calculations. Other sources of radiation background like possible radionuclide contamination of lead collimators or radon decay products deposited on surfaces were neglected.
based on aggregate with a low internal content of radionuclides with a density of 2.4 g/cm3. The activity concentrations of 40K, 226 Ra and 228Th in the composite material were measured and reached (10.6 70.5) Bq/kg, (1.07 0.1) Bq/kg and (0.7 70.1) Bq/kg, respectively (k ¼1). Dose measurements performed after the FRMF construction showed that the ambient dose equivalent rate, Hn(10), decreased from (101 77) μSv/h outside to (33.572.4) μSv/h inside the chamber (k ¼1). 2.2. Monte Carlo simulations 2.2.1. Monte Carlo codes A complex model of the whole FRMF was created in the Monte Carlo code MCNPX (Fig. 1) and the calculations were done with the version v2.7e of the code (Pelowitz et al., 2011). A standalone model of the IDM detector was created in the PENELOPE code (Salvat et al., 2006) as well and used for optimization of the container dimensions. Results of all calculations were obtained with the statistical uncertainties below 3% (k ¼1). 2.2.2. Validation of the IDM detector models Both IDM detector models were validated by measurements with point sources located 25 cm above the cryostat front face and 25 cm aside to the cryostat side wall. Measured and calculated full-energy peak detection efficiencies were compared for photon energies of 122 keV, 662 keV, 898 keV, 1173 keV, and 1332 keV.
2.3. Optimization of shielded chamber thickness The inner dimensions of the FRMF were fixed because they depend on the dimensions of the IDM detectors, mechanical parts of the FRMF and dimensions of the container filled with the measured material. Therefore, the Monte Carlo calculations were focused on shielded chamber thickness only. 2.4. Optimization of collimator shape Four collimator shapes were studied: standard cone, enhanced cone with enlarged thickness of the conical part, pyramid, and enhanced pyramid with enlarged thickness of the pyramidal part (Fig. 2). The pyramidal collimators were taken into consideration because every detector measures count rate related to a square section area of the container. In addition, the simulations were performed for collimators at the standard position and shifted by 1.5 cm up or down. The collimators were made of lead. The study
Fig. 1. Visualization of the FRMF Monte Carlo model. 1—IDM detectors, 2— container with load, 3—FRMF walls, 4—FRMF door, 5—FRMF ceiling, 6—FRMF floor, 7—hall floor. Table 1 Characteristics of particle sources considered in the simulations. Natural background source
Source dimensions
Energy spectrum
Concrete floor of the hall
Volume, 15.6 9 0.3 m3
Photons from
238
U series þ 232Th series þ 40K
Air in the hall
Volume, 15.6 9 6 m3
Photons from
238
U series U series þ 232Th series þ 40K
U series
Composite material of the chamber
Whole volume of the composite material
Photons from
238
Air inside the chamber
Whole volume of air inside the shielded chamber Rectangle, 2 3.2 m2 (parallel particle beam perpendicular to chamber ceiling)
Photons from
238
Cosmic rays
EXPACS database (Sato, 2006) of spectra of photons, electrons, neutrons, protons and muons at Earth surface (Prague coordinates and elevation) originated from interactions caused by cosmic particles
Activity
Total yield
30 Bq/kg þ30 Bq/ kg þ300 Bq/kg 100 Bq/m3
162.6 s 1/kg for photons 179 s 1/m3 for photons 4.7 s 1/kg for photons
1.0 Bq/kg 0.7 Bq/kg 10.6 Bq/kg 1 Bq/m3
Particle distribution: 37% photons, 6% electrons, 37% neutrons, 1% protons, 19% muons
1.8 s 1/m3 for photons 4.92 min 1/ cm2 for all particles
350
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considered only such collimator modifications which resulted in no more than 1.5% decrease of the full-energy peak detection efficiency of photons emitted from the container load. 2.5. Optimization of the container dimensions The basic container dimensions were predetermined by the need to use standardized containers. Therefore, only rectangular parallelepiped containers with load of 120 cm in length, 80 cm or 100 cm in width and with various heights were retained for calculations. Optimization process focused on height and density of the load and it was performed with respect to the throughput of the detection system, defined as the count rate per activity concentration in the container. This quantity can be calculated by multiplying the detection efficiency by the total container load mass. The material inside the container was modelled as iron homogeneously distributed in the whole volume contaminated with a homogeneous photon emitter. The container bottom surface was kept at the constant distance of 15 cm from the lower detector pair. Densities and heights were considered between 0.5 g/cm3 and 3.0 g/cm3 and between 40 cm and 80 cm, respectively. The optimization was performed by calculating full-energy peak detection efficiencies for 122 keV, 662 keV and 1332 keV photons.
3. Results and discussion
also shielded part of photons from the hall floor. The detectors were shielded by conical collimators. It can be also seen from Fig. 3 that the contribution from the floor in the hall rapidly decreases with increasing wall thickness, reaching approximately 10% of the total count rate for 30 cm wall thickness. In addition, air inside the FRMF contributes approximately by 0.1% to the total count rate. Sorption of radon decay products on chamber inner surface was not considered but concluding from simulation results it is assumed that this source of natural background is not significant even if the sorption increases the assumed volume activity of 1 Bq/m3 significantly. Based on these results it was decided that the chamber would have 40 cm thick side walls. 3.3. Door thickness The door of the shielded chamber cannot be as thick as the side walls (40 cm) due to the weight and difficulties with door manipulation. Therefore the simulations were performed for door thicknesses varying from 10 cm to 30 cm with the constant side walls, floor, and ceiling thickness of 40 cm, 60 cm, and 50 cm, respectively. It was found that for the 662 keV energy window, the Table 2 Results of the validation of the IDM detector models for a point source positioned at 25 cm above the cryostat front face. Full-energy peak detection efficiencies and the differences between measured and simulated values are given. Photon Measured full-energy peak energy (keV) detection efficiency
3.1. Validation Table 2 shows the result of validation of the IDM detector models with point sources located above the detector. The differences between measured and simulated full-energy peak detection efficiencies were within 1.5% for photon energies from 81 keV to 1332 keV. For a point source located aside of the detector, the difference between the measurement and both simulations were in almost all cases within 2%. The models were therefore considered as validated and suitable for the optimization of the FRMF parameters.
81.0 122.1 661.7 898.3 1173.2 1332.5
(4.347 0.06) 10 3 (4.847 0.06) 10 3 (1.15 70.02) 10 3 (8.787 0.12) 10 4 (6.937 0.10) 10 4 (6.157 0.09) 10 4
Difference (%) MCNPX/ measurement
PENELOPE/ measurement
0.34 7 1.39 0.717 1.32 1.42 7 1.45 0.337 1.42 0.067 1.51 0.53 7 1.43
0.357 1.40 0.687 1.33 1.43 7 1.50 0.047 1.47 0.127 1.59 0.75 7 1.51
3.2. Shielded chamber side walls thickness Fig. 3 shows the dependence of the total count rate in the 662 keV energy window on the thickness of the shielded chamber side walls. The total count rate decreases with increasing side wall thickness up to a certain constant value which is reached at a wall thickness of approximately 30–35 cm. At this thickness the cosmic rays become the dominant contributor to the total count rate which results in no additional decrease of the total count rate with increasing wall thickness. Very similar dependence with the plateau starting at around 30–35 cm is valid for 122 keV and 1332 keV energy windows as well Simulations were performed for a constant ceiling thickness of 50 cm, constant chamber floor thickness of 60 cm which already shielded most of photons from the hall floor, and for a constant door thickness of 20 cm which
Fig. 3. Contribution of different sources of natural background to the total count rate in 662 keV energy window in dependence on shielded chamber wall thickness. The lines represent a cubic spline interpolation to guide the eye.
Fig. 2. Visualization of the collimator models—(A) standard cone, (B) enhanced cone, (C) pyramid, (D) enhanced pyramid.
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summed contribution of hall floor and hall air to the total count rate is 17.5%, 6.5%, and 3.1% for door thicknesses of 10 cm, 20 cm, and 30 cm, respectively. Very similar contribution to the total count rate was obtained for the 1332 keV energy window as well. Based on these results, it was decided that the door thickness will be 20 cm. In addition, to decrease the contribution from the hall floor coming into the chamber through the door, it was proposed to build an extended floor made of low-activity blocks on the floor in front of both doors which can be also used as a table for manipulation of the containers. The length, width, and height of the extended floor were 130 cm, 200 cm, and 60 cm, respectively. The extended floor decreased the contribution of hall floor to the total count rate from 6.4% to 3.3% in 662 keV window, and from 5.7% to 3.7% in 1332 keV window. 3.4. Ceiling thickness Fig. 4 presents the dependence of count rate caused by cosmic rays on the ceiling thickness, for 40 cm thick chamber walls. The contribution of cosmic rays to the total count rate decreased from 52% (no ceiling) to 31% (60 cm thick) in case of 662 keV energy window, and from 62% (no ceiling) to 42% (60 cm thick) in case of 1332 keV energy window. Taking into account both the simulations and the mechanical properties of the shielding material, it was decided that the ceiling thickness of the FRMF will be 50 cm. Detailed description of MCNPX simulation of a germanium detector response to cosmic rays was published by Solc et al. (2012). 3.5. Detector collimators Fig. 5 shows the count rates in the 662 keV energy window caused by different components of the natural background with and without the presence of 5 cm thick collimators of the shape visualized in Fig. 2A. The total count rate with the attached collimators decreased to 56% and 62% in 662 keV and 1332 keV windows, respectively, compared to the total count rates without the collimators. Contribution of all background components decreased except for cosmic rays. This is caused by interactions of cosmic rays with lead resulting in an increased count rate. Results presented in Fig. 5 confirmed that the utilization of the collimators is highly recommended. That is why an additional optimization of the collimator shape and position with respect to the germanium crystal was performed. Only photons originating in the shielded chamber material were considered for this study.
Fig. 5. The count rate in the 662 keV energy window caused by main components of the natural background with (light) and without (dark) attached 5 cm thick lead collimators.
The container was homogeneously filled with steel with density of 0.5 g/cm3 to take into account its shielding effect. Several results were concluded from the simulations. Initially, count rates in 662 keV window with the simple pyramid collimator increased by 6 to20% compared to a simple conical collimator. Although the pyramidal collimator sees exactly a square segment of the container, it is not a suitable shape because the solid angle of the pyramid is larger leading to higher background count rates. And more, count rates in 662 keV window with both enhanced cone and enhanced pyramid collimators were lower approximately by 4%, 1%, and 18% compared to a simple cone if the detector was not shifted and shifted outside or inside the collimator along its axis, respectively. The same tendency was observed for all considered photon energies. Both collimators result in similar count rates; however, due to much simpler shape and lower weight, the enhanced cone is the preferred one. And finally, the change of the collimator thickness from 5 cm to 3 cm results in approximately 25% increase of the background. Based on the results it was recommended to use either an enhanced conical collimator or a standard conical collimator where the germanium crystal would be positioned more deep inside the collimator. 3.6. Optimization of container dimensions Fig. 6 presents the results obtained for 80 cm wide and 120 cm long container. The only significant dependence of count rate per activity concentration on container height is noticeable for a load with a density of 0.5 g/cm3 where the count rate increases between 40 cm and 80 cm height by 8% and 13% for 662 keV and 1332 keV photons, respectively. For other studied energies and load densities, the count rates were kept constant. The situation for 100 cm wide container is very similar. The largest difference was found to be a 2–8% increase in the count rate in all studied cases compared to 80 cm wide container. It can be concluded that the throughput does not change significantly with the container dimensions studied.
4. Conclusions Fig. 4. Decrease of the count rate caused by cosmic rays only in the 662 keV and 1332 keV windows as a function of the ceiling thickness. The lines represent a cubic spline interpolation to guide the eye.
Two models of an IDM HPGe detector, utilized in the novel free release measurement facility, were created in MCNPX and PENELOPE codes and validated by measurements. An MCNPX detector
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detection efficiencies for special cases of material and activity distributions in the measured container load.
Acknowledgement This joint research project is financially supported by the European Commission in the frame of the European Metrology Research and Development Programme EMRP 〈http://www.emrpon line.eu〉 undertaken by several EU Member States under the Article 169 initiative, JRP contract identifier EMRP ENV09 MetroRWM.
References Fig. 6. Count rate per activity concentration as a function of a container height and a density of iron load for 122 keV, 662 keV and 1332 keV photons. The lines represent a cubic spline interpolation to guide the eye.
model was extended to include a model of the whole FRMF. It was subsequently used for the optimization of the shielded chamber thickness and collimator shape. The PENELOPE detector model was utilized for optimization of container dimensions. Results of the simulations were considered during the FRMF construction. In the future, the FRMF model will be used for calculation of
Pelowitz, D.B., Durkee, J.W., Elson J.S., Fensin, M.L, Hendricks, J.S., et al., 2011. MCNPX™ 2.7.E Extensions. Salvat, F., Fernández-Varea, J.M., Sempau, J., 2006. PENELOPE-2006: a code system for Monte Carlo simulation of electron and photon transport NEA 6222. OECD. Sato, T., 2006. EXPACS: Excel-based Program for Calculating Atmospheric Cosmicray Spectrum, User's Manual, Japan Atomic Energy Agency. Available online: 〈http://phits.jaea.go.jp/expacs/〉. Solc, J., Kovar, P., Dryak, P., 2012. MCNPX Simulation of Influence of Cosmic Rays on Low-activity Spectrometric Measurements. http://dx.doi.org/10.1016/j.rad physchem.2012.12.034. Suran, J., Kovar, P., Solc, J., Skala, S., Torano, E.-G., Peyres, V., 2013. EMRP JRP ENV09 Workpackage 1 Presentation, 1st JRP ENV09 Workshop, Paris 2013. Available online: 〈http://www.radwaste-emrp.eu〉.