- Email: [email protected]
Radiation shielding for neutron guides
ARTICLE IN PRESS
Physica B 385–386 (2006) 1268–1270 www.elsevier.com/locate/physb
Radiation shielding for neutron guides T. Ersez, G. Braoudakis, J.C. Osborn Reactor Operations, ANSTO, PMB 1, Menai, NSW 2234, Australia
Abstract Models of the neutron guide shielding for the out of bunker guides on the thermal and cold neutron beam lines of the OPAL Reactor (ANSTO) were constructed using the Monte Carlo code MCNP 4B. The neutrons that were not reflected inside the guides but were absorbed by the supermirror (SM) layers were noted to be a significant source of gammas. Gammas also arise from neutrons absorbed by the B, Si, Na and K contained in the glass. The proposed shielding design has produced compact shielding assemblies. These arrangements are consistent with safety requirements, floor load limits, and cost constraints. To verify the design a prototype was assembled consisting of 120 mm thick Pb(96%)Sb(4%) walls resting on a concrete block. There was good agreement between experimental measurements and calculated dose rates for bulk shield regions. Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. Keywords: MCNP code; Neutron shielding assemblies; Neutron guides; Supermirror
1. Introduction In recent years there has been an increased use of supermirror (SM) coated neutron guides which transport neutrons from the source to the sample position over long distances by reflection from the inner surfaces of the guides. The contributions to the radiation dose along the guides mainly involve neutron capture reactions from the various components of the neutron guide assembly, e.g. neutron capture in the SM layers, borosilicate glass and aluminium windows covering the ends of guides. It is necessary to provide protective shielding against the resulting secondary gamma radiation. In this paper we will present modelling and design of compact radiation shielding for thermal (TG1) and cold (CG1) neutron guides at Australia’s new research reactor OPAL. The information given here can also be used in the design of radiation shielding for neutron guides at other facilities. The guides in the neutron beam facility comprise evacuated beam tubes made of borosilicate glass with polished surfaces, coated with special neutron reflecting materials of Ni–Ti SMs (Table 1). The coating allows an Corresponding author. Tel.: +61 2 9717 3476; fax: +61 2 9717 3200.
E-mail address: [email protected] (T. Ersez).
increase of the angle of total reflection when compared with a common Ni coated guide (m ¼ 1). This allows intense neutron beams to be transported to seven neutron scattering instruments, providing areas of low background in the guide hall. The material for the neutron guides’ vacuum housing consists of steel of 8 mm thickness and aluminium windows of 0.5 mm thickness over the ends of the neutron guides. 2. Method for computational modelling A model of the proposed guide shielding was constructed using MCNP 4B [1] and its effectiveness was estimated by the Monte Carlo method, considering neutrons and generated prompt g-radiation. The cross sections were based on ENDF/B-VI neutron data and MCPLIB02 photon data. Flux tallies were calculated on external surfaces of the model, with the surfaces being divided into segments. Conversion from radiation fluxes to doses is in correspondence with ANSI/ANS-6.1.1-1977 [2]. For TG1 the neutron source was assumed to be a Maxwellian (T ¼ 315 K) with a trajectory angle of 0.631 and a neutron loss flux of 7.2 105 neutrons cm2s1 [3]. In the case of CG1 the neutron source was also assumed to be a Maxwellian (T ¼ 20 K) but with a trajectory angle of 1.171 and a neutron loss flux of 1.5 106 neutrons cm2s1 [3].
0921-4526/$ - see front matter Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.028
ARTICLE IN PRESS T. Ersez et al. / Physica B 385–386 (2006) 1268–1270
1269
Table 1 Neutron guide characteristics Beam line
Size (h w), mm
Ni total thickness (mm)
Ti total thickness (mm)
TG1 (m ¼ 3) TG3 (m ¼ 3) CG1 (m ¼ 2) CG3 (m ¼ 2)
300 50 150 50 200 50 200 50
1.8596 1.8596 0.7088 0.7088
1.4570 1.4570 0.4578 0.4578
Typical critical angles (yc) of reflection from 58Ni for the thermal guides were considered to be 0.631 and for the cold guides to be 1.171. The neutrons at angles greater than yc are expected to have smaller path lengths through the SM, whereas the neutrons at angles less than yc are expected to be reflected. Therefore, the results obtained at these two neutron trajectory angles of 0.631 and 1.171 would be conservative. Dose rates were calculated at the external surface of the guide shielding in the region where there is a gap and regions next to the gap. The relative errors of the simulations were less than 5%, except when the result was close to zero (o0.1 mSv/h), for which relative errors were X20%. 3. Results and discussion The geometry of the TG1 guide model is shown in Fig. 1. The geometry of the CG1 guide model is very similar. The model includes the neutron guides, vacuum housing, shielding and concrete base. In the simulations performed, the main contributions to the dose levels arise from gammas produced from neutrons that were not reflected inside the guides but were absorbed by the Ni and Ti SM layers. Also, g-rays result from neutrons that were absorbed by the boron containing glass, which would contribute to the overall dose levels. The borosilicate glass in the guides contains 10B and when irradiated with low energy neutrons 4He (a particles) and 7 Li nuclei are emitted plus photons of energy 0.48 MeV. The energy of this g is rather low compared to g-radiation from neutron capture in Ni, with its strongest transition intensity of g-rays occurring at 9.0 MeV and for Ti it is 1.4 MeV [4]. However, there are two other strong transitions occurring for Ti at 6.4 and 6.8 MeV. As well, g-rays are produced by the other components in the borosilicate glass, such as Si, Na and K. Since the guide glass absorbs very little g-radiation and the boron is a g emitter, a relatively thick g absorber has to be further added (outside of the guide casing). In the models presented here Pb(96%)Sb(4%) was used. The small amount of Sb was included to strengthen the Pb shield so that a steel casing would not be required. If the neutron beam does not impinge on the Sb in the shield then the Sb should not activate. The guide shield consists of 420 mm length pieces with stepped joints, so as to allow for ease of removal to
Gap region
Neutron guide
Guide Shield
Pb(96%)Sb(4%)
Fig. 1. MCNP model showing the plan view of section of guide and shield with labyrinth type gap.
perform maintenance on guides. When the sections of guide shield are joined the shape of the gap between the mating pieces will be of a labyrinth type, i.e., two 901 bends with a 50 mm offset, so as to minimise radiation leakage. Adequate space has been allowed for the inside of the shield (clearance of 90–145 mm). The concrete base support extends 100 mm beyond guide shield sides and to the floor. This allows provision for installation of extra shielding along the neutron beam lines, if necessary, for example providing extra cover around the gap areas. The neutron dose rates in the region of the gap in the TG1 guide shielding (100 mm thickness) for gap sizes from 1 to 6 mm were determined to be 0.24 mSv h1. This was similar to the neutron dose rate (0.20 mSv h1) in the external surface regions where there was no gap in the guide shielding. The photon dose rates varied from 2.1 to 3.1 mSv h1 in the region of the gap. Manufacturing tolerances following cast moulding of the shield pieces is expected to result in gap sizes of p4 mm between adjacent pieces. Therefore, 100 mm thickness of Pb(96%)Sb(4%) material is appropriate to use for TG1 guide shielding (total dose rate in the region of the gap was 2.8 mSv h1 and in the near region where there is no gap, 2.2 mSv h1). For the guide shield design of CG1 the thickness of Pb(96%)Sb(4%) needed to be 120 mm, with total dose rate in the region of the gap being calculated as 1.3 mSv h1 and in the near region where there is no gap, 0.9 mSv h1. When the SM layers were not included in the simulations the total dose rate in the region of the gap was
ARTICLE IN PRESS 1270
T. Ersez et al. / Physica B 385–386 (2006) 1268–1270
0.3 mSv h1 and in the near region where there is no gap, 0.2 mSv h1. Thus, it is very important to include the SM material in radiation shielding models for neutron guides, omitting them will not take into account the produced hard gammas and can lead to an underestimate of the amount of shielding required. The SM layers are extremely thin, but the non-reflected neutrons can impinge on the SM close to the critical angle and have long path lengths in the layers. 4. Measurements and simulation of dose levels at external surfaces of the prototype guide shield A prototype guide shield was assembled consisting of two pieces of 120 mm thick Pb(96%)Sb(4%) walls resting on a concrete block. Dose rates resulting from a gradiation source of strength 12 Ci (444 GBq) of 192Ir were measured at the gap and near the gap on the external surface of the prototype guide shielding. The source comprised a 2 mm (diameter) 2 mm (length) cylindrical pellet contained in a 3 mm (diameter) 6 mm X0.5 mm (thickness) Ti capsule and the camera dimensions were 110 mm (diameter) 290 mm (length) with a depleted uranium shield. The pellet was located in the centre of the camera and 30 mm from the external surface of the camera’s window. The centre of the g-camera was lined up with the internal gap and placed as close as possible to the gap and not moved. Then measurements were made at the gap opening on the external surface and also, at external regions that lined up with the internal gap. The internal and external gap sizes between adjoining pieces were measured and found to vary from 0.05 to 3 mm.These gaps were modelled as measured and the midgap size was assumed to be half that of the total internal and external gap sizes. A model of the prototype guide shielding was constructed using MCNP 4B [1] and the calculated dose rates were compared with those obtained from the measurements so as to benchmark the effect of gaps and Pb(96%)Sb(4%) data in MCNP. In the simulations a point isotropic source was used and also a conical source (cone half-opening angle 33.691), simulating the angular range of the 40 mm 40 mm camera window, was considered. The source strength for the cone shape was adjusted, the fraction of the source used was 0.084 of 444 GBq. The neutron guide and its components were removed from the model so that it matched more closely the experimental set up. There was good agreement between the calculated and measured dose rates for gap sizes o0.9 mm.However, much higher dose rates were calculated in the regions around the gaps (X0.9 mm) than those measured. The differences could be due to in the calculations the source
having no holder and shielding around it, whereas in the gcamera the gammas emanate from the window in the direction of the gap. Also, discrepancies could arise from how close the centre of the pellet in the g-camera can be aligned with the centre of the gap. There could be a few millimeters of misalignment due to human error, since the mass of the device is 10 kg and alignment was carried out manually. Another possible cause for differences could be the fact that the pellet dimensions are comparable with the gap size. The simulations showed that moving the conical source 5 mm away from the internal gap the dose rate at the external surface region that is lined up with the internal gap reduces by a factor of about 10. 5. Conclusion The neutrons that were not reflected inside the guides but were absorbed by the Ni and Ti SM layers (covering the internal surfaces of the neutron guides) were noted to be a significant source of gammas due to the long path length in the layers for lost neutrons which impinge on the SM close to the critical angle. Also gammas arise from neutrons absorbed by the B, Si, Na and K contained in the glass. Modelling of the radiation shielding has produced compact shielding assemblies for neutron guides located outside of the reactor building. Also, there was good agreement obtained between experimental measurements and calculated dose rates for bulk shield regions in a prototype of the designed guide shielding assembly. Acknowledgements This work was supported by the ANSTO Neutron Beam Instruments Project. The authors would like to thank M. Deura, T. Noakes, T. Randall, P. Baxter, Dr. O. Kirstein and Dr S.J. Kennedy of the Bragg Institute (ANSTO) for meaningful discussions and their valuable suggestions to the design of the shielding assemblies. References [1] J.F. Briesmeister, (Ed.), ‘‘MCNPTM—A General Monte Carlo N-Particle Transport Code, Version 4B’’, LA-12625-M, LANL, Los Alamos, NM, USA, 1997. [2] ANS-6.1.1 Working Group, M.E. Battat (Chairman), ‘‘American National Standard Neutron and Gamma-Ray Flux-to-Dose Rate Factors’’, ANSI/ANS-6.1.1-1977 (N666), American Nuclear Society, LaGrange Park, Illinois, 1977. [3] Design of Secondary Shutter, Out-of-Pile Bunker Shielding and BeamStop, ANSTO Document Number: RRRP–6000–2CEIN–001–B, 4 April 2001. [4] M.A. Lone, R.A. Leavitt, D.A. Harrison, Atomic Data and Nuclear Data Tables 26 (1981) 511.
Physica B 385–386 (2006) 1268–1270 www.elsevier.com/locate/physb
Radiation shielding for neutron guides T. Ersez, G. Braoudakis, J.C. Osborn Reactor Operations, ANSTO, PMB 1, Menai, NSW 2234, Australia
Abstract Models of the neutron guide shielding for the out of bunker guides on the thermal and cold neutron beam lines of the OPAL Reactor (ANSTO) were constructed using the Monte Carlo code MCNP 4B. The neutrons that were not reflected inside the guides but were absorbed by the supermirror (SM) layers were noted to be a significant source of gammas. Gammas also arise from neutrons absorbed by the B, Si, Na and K contained in the glass. The proposed shielding design has produced compact shielding assemblies. These arrangements are consistent with safety requirements, floor load limits, and cost constraints. To verify the design a prototype was assembled consisting of 120 mm thick Pb(96%)Sb(4%) walls resting on a concrete block. There was good agreement between experimental measurements and calculated dose rates for bulk shield regions. Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. Keywords: MCNP code; Neutron shielding assemblies; Neutron guides; Supermirror
1. Introduction In recent years there has been an increased use of supermirror (SM) coated neutron guides which transport neutrons from the source to the sample position over long distances by reflection from the inner surfaces of the guides. The contributions to the radiation dose along the guides mainly involve neutron capture reactions from the various components of the neutron guide assembly, e.g. neutron capture in the SM layers, borosilicate glass and aluminium windows covering the ends of guides. It is necessary to provide protective shielding against the resulting secondary gamma radiation. In this paper we will present modelling and design of compact radiation shielding for thermal (TG1) and cold (CG1) neutron guides at Australia’s new research reactor OPAL. The information given here can also be used in the design of radiation shielding for neutron guides at other facilities. The guides in the neutron beam facility comprise evacuated beam tubes made of borosilicate glass with polished surfaces, coated with special neutron reflecting materials of Ni–Ti SMs (Table 1). The coating allows an Corresponding author. Tel.: +61 2 9717 3476; fax: +61 2 9717 3200.
E-mail address: [email protected] (T. Ersez).
increase of the angle of total reflection when compared with a common Ni coated guide (m ¼ 1). This allows intense neutron beams to be transported to seven neutron scattering instruments, providing areas of low background in the guide hall. The material for the neutron guides’ vacuum housing consists of steel of 8 mm thickness and aluminium windows of 0.5 mm thickness over the ends of the neutron guides. 2. Method for computational modelling A model of the proposed guide shielding was constructed using MCNP 4B [1] and its effectiveness was estimated by the Monte Carlo method, considering neutrons and generated prompt g-radiation. The cross sections were based on ENDF/B-VI neutron data and MCPLIB02 photon data. Flux tallies were calculated on external surfaces of the model, with the surfaces being divided into segments. Conversion from radiation fluxes to doses is in correspondence with ANSI/ANS-6.1.1-1977 [2]. For TG1 the neutron source was assumed to be a Maxwellian (T ¼ 315 K) with a trajectory angle of 0.631 and a neutron loss flux of 7.2 105 neutrons cm2s1 [3]. In the case of CG1 the neutron source was also assumed to be a Maxwellian (T ¼ 20 K) but with a trajectory angle of 1.171 and a neutron loss flux of 1.5 106 neutrons cm2s1 [3].
0921-4526/$ - see front matter Crown Copyright r 2006 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.028
ARTICLE IN PRESS T. Ersez et al. / Physica B 385–386 (2006) 1268–1270
1269
Table 1 Neutron guide characteristics Beam line
Size (h w), mm
Ni total thickness (mm)
Ti total thickness (mm)
TG1 (m ¼ 3) TG3 (m ¼ 3) CG1 (m ¼ 2) CG3 (m ¼ 2)
300 50 150 50 200 50 200 50
1.8596 1.8596 0.7088 0.7088
1.4570 1.4570 0.4578 0.4578
Typical critical angles (yc) of reflection from 58Ni for the thermal guides were considered to be 0.631 and for the cold guides to be 1.171. The neutrons at angles greater than yc are expected to have smaller path lengths through the SM, whereas the neutrons at angles less than yc are expected to be reflected. Therefore, the results obtained at these two neutron trajectory angles of 0.631 and 1.171 would be conservative. Dose rates were calculated at the external surface of the guide shielding in the region where there is a gap and regions next to the gap. The relative errors of the simulations were less than 5%, except when the result was close to zero (o0.1 mSv/h), for which relative errors were X20%. 3. Results and discussion The geometry of the TG1 guide model is shown in Fig. 1. The geometry of the CG1 guide model is very similar. The model includes the neutron guides, vacuum housing, shielding and concrete base. In the simulations performed, the main contributions to the dose levels arise from gammas produced from neutrons that were not reflected inside the guides but were absorbed by the Ni and Ti SM layers. Also, g-rays result from neutrons that were absorbed by the boron containing glass, which would contribute to the overall dose levels. The borosilicate glass in the guides contains 10B and when irradiated with low energy neutrons 4He (a particles) and 7 Li nuclei are emitted plus photons of energy 0.48 MeV. The energy of this g is rather low compared to g-radiation from neutron capture in Ni, with its strongest transition intensity of g-rays occurring at 9.0 MeV and for Ti it is 1.4 MeV [4]. However, there are two other strong transitions occurring for Ti at 6.4 and 6.8 MeV. As well, g-rays are produced by the other components in the borosilicate glass, such as Si, Na and K. Since the guide glass absorbs very little g-radiation and the boron is a g emitter, a relatively thick g absorber has to be further added (outside of the guide casing). In the models presented here Pb(96%)Sb(4%) was used. The small amount of Sb was included to strengthen the Pb shield so that a steel casing would not be required. If the neutron beam does not impinge on the Sb in the shield then the Sb should not activate. The guide shield consists of 420 mm length pieces with stepped joints, so as to allow for ease of removal to
Gap region
Neutron guide
Guide Shield
Pb(96%)Sb(4%)
Fig. 1. MCNP model showing the plan view of section of guide and shield with labyrinth type gap.
perform maintenance on guides. When the sections of guide shield are joined the shape of the gap between the mating pieces will be of a labyrinth type, i.e., two 901 bends with a 50 mm offset, so as to minimise radiation leakage. Adequate space has been allowed for the inside of the shield (clearance of 90–145 mm). The concrete base support extends 100 mm beyond guide shield sides and to the floor. This allows provision for installation of extra shielding along the neutron beam lines, if necessary, for example providing extra cover around the gap areas. The neutron dose rates in the region of the gap in the TG1 guide shielding (100 mm thickness) for gap sizes from 1 to 6 mm were determined to be 0.24 mSv h1. This was similar to the neutron dose rate (0.20 mSv h1) in the external surface regions where there was no gap in the guide shielding. The photon dose rates varied from 2.1 to 3.1 mSv h1 in the region of the gap. Manufacturing tolerances following cast moulding of the shield pieces is expected to result in gap sizes of p4 mm between adjacent pieces. Therefore, 100 mm thickness of Pb(96%)Sb(4%) material is appropriate to use for TG1 guide shielding (total dose rate in the region of the gap was 2.8 mSv h1 and in the near region where there is no gap, 2.2 mSv h1). For the guide shield design of CG1 the thickness of Pb(96%)Sb(4%) needed to be 120 mm, with total dose rate in the region of the gap being calculated as 1.3 mSv h1 and in the near region where there is no gap, 0.9 mSv h1. When the SM layers were not included in the simulations the total dose rate in the region of the gap was
ARTICLE IN PRESS 1270
T. Ersez et al. / Physica B 385–386 (2006) 1268–1270
0.3 mSv h1 and in the near region where there is no gap, 0.2 mSv h1. Thus, it is very important to include the SM material in radiation shielding models for neutron guides, omitting them will not take into account the produced hard gammas and can lead to an underestimate of the amount of shielding required. The SM layers are extremely thin, but the non-reflected neutrons can impinge on the SM close to the critical angle and have long path lengths in the layers. 4. Measurements and simulation of dose levels at external surfaces of the prototype guide shield A prototype guide shield was assembled consisting of two pieces of 120 mm thick Pb(96%)Sb(4%) walls resting on a concrete block. Dose rates resulting from a gradiation source of strength 12 Ci (444 GBq) of 192Ir were measured at the gap and near the gap on the external surface of the prototype guide shielding. The source comprised a 2 mm (diameter) 2 mm (length) cylindrical pellet contained in a 3 mm (diameter) 6 mm X0.5 mm (thickness) Ti capsule and the camera dimensions were 110 mm (diameter) 290 mm (length) with a depleted uranium shield. The pellet was located in the centre of the camera and 30 mm from the external surface of the camera’s window. The centre of the g-camera was lined up with the internal gap and placed as close as possible to the gap and not moved. Then measurements were made at the gap opening on the external surface and also, at external regions that lined up with the internal gap. The internal and external gap sizes between adjoining pieces were measured and found to vary from 0.05 to 3 mm.These gaps were modelled as measured and the midgap size was assumed to be half that of the total internal and external gap sizes. A model of the prototype guide shielding was constructed using MCNP 4B [1] and the calculated dose rates were compared with those obtained from the measurements so as to benchmark the effect of gaps and Pb(96%)Sb(4%) data in MCNP. In the simulations a point isotropic source was used and also a conical source (cone half-opening angle 33.691), simulating the angular range of the 40 mm 40 mm camera window, was considered. The source strength for the cone shape was adjusted, the fraction of the source used was 0.084 of 444 GBq. The neutron guide and its components were removed from the model so that it matched more closely the experimental set up. There was good agreement between the calculated and measured dose rates for gap sizes o0.9 mm.However, much higher dose rates were calculated in the regions around the gaps (X0.9 mm) than those measured. The differences could be due to in the calculations the source
having no holder and shielding around it, whereas in the gcamera the gammas emanate from the window in the direction of the gap. Also, discrepancies could arise from how close the centre of the pellet in the g-camera can be aligned with the centre of the gap. There could be a few millimeters of misalignment due to human error, since the mass of the device is 10 kg and alignment was carried out manually. Another possible cause for differences could be the fact that the pellet dimensions are comparable with the gap size. The simulations showed that moving the conical source 5 mm away from the internal gap the dose rate at the external surface region that is lined up with the internal gap reduces by a factor of about 10. 5. Conclusion The neutrons that were not reflected inside the guides but were absorbed by the Ni and Ti SM layers (covering the internal surfaces of the neutron guides) were noted to be a significant source of gammas due to the long path length in the layers for lost neutrons which impinge on the SM close to the critical angle. Also gammas arise from neutrons absorbed by the B, Si, Na and K contained in the glass. Modelling of the radiation shielding has produced compact shielding assemblies for neutron guides located outside of the reactor building. Also, there was good agreement obtained between experimental measurements and calculated dose rates for bulk shield regions in a prototype of the designed guide shielding assembly. Acknowledgements This work was supported by the ANSTO Neutron Beam Instruments Project. The authors would like to thank M. Deura, T. Noakes, T. Randall, P. Baxter, Dr. O. Kirstein and Dr S.J. Kennedy of the Bragg Institute (ANSTO) for meaningful discussions and their valuable suggestions to the design of the shielding assemblies. References [1] J.F. Briesmeister, (Ed.), ‘‘MCNPTM—A General Monte Carlo N-Particle Transport Code, Version 4B’’, LA-12625-M, LANL, Los Alamos, NM, USA, 1997. [2] ANS-6.1.1 Working Group, M.E. Battat (Chairman), ‘‘American National Standard Neutron and Gamma-Ray Flux-to-Dose Rate Factors’’, ANSI/ANS-6.1.1-1977 (N666), American Nuclear Society, LaGrange Park, Illinois, 1977. [3] Design of Secondary Shutter, Out-of-Pile Bunker Shielding and BeamStop, ANSTO Document Number: RRRP–6000–2CEIN–001–B, 4 April 2001. [4] M.A. Lone, R.A. Leavitt, D.A. Harrison, Atomic Data and Nuclear Data Tables 26 (1981) 511.