Study on Radiation Shielding Performance of Reinforced Concrete Wall (2): Shielding Analysis

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1 20th International Conference on Structural Mechanics in Reactor Technology (SMiRT 20) Espoo, Finland, August 9-14, 2009 SMiRT 20-Division V, Paper 1865 Study on Radiation Shielding Performance of Reinforced Concrete Wall (2): Shielding Analysis Yoshinari Munakata a, Yoshiyuki Sato a, Takashi Maki a, Keiji Sekine a, Yoshinori Sakai b, Koji Oishi c, Kazuyuki Torii c a Japan Nuclear Fuel Limited, 4-108, Aza Okitsuke, Oaza Obuchi, Rokkasho-Mura, Kamikita-gun, Aomori-ken , Japan, yoshinari.munakata@jnfl.co.jp b Ohsaki Research Institute, Inc., Uchisaiwai-Cho, Chiyoda-ku, Tokyo -0011, Japan C Shimizu Corporation, Shibaura, Minato-ku, Tokyo , Japan Keywords: Concrete, Crack, Nuclear Fuel Facility, Radioactive waste, Shielding, MCNP5 1 ABSTRACT The estimation of the shielding abilities of cracked and uncracked concrete walls was assessed by a simulation using the three-dimensional Monte Carlo calculation code MCNP5 and its related nuclear data libraries MCPLIB04 and FSXLIB-J33. Simple and practical building model were used for calculations with low-level radioactive wastes. The source gamma rays and neutron spectra were obtained from the calculated spectra of vitrified radioactive waste. The crack width was varied from 0 to 10 mm for the assessment. Up to gamma rays were generated during the simulation. The calculated results were normalized with the strength data of practical radioactive wastes. The calculated gamma ray and neutron spectra were transformed to the dose rate by the flux-to-dose conversion factor based on ICRP Pub.74. The ratio of the two penetration rates was at most 10 when the crack width was 1 mm and the wall thickness was 1200 mm. It was concluded that the increase in the penetration rate of radiation through the concrete shield due to a crack width of up to 1 mm is not a serious problem even though its configuration is straight. 2 INTRODUCTION Concrete is widely used in nuclear facilities as an inexpensive shielding material due to its ability to shield both gamma ray and neutron. One of the disadvantages of the use of concrete as a shielding material is emerging cracks, mainly due to earthquakes. If a crack is sufficiently deep, the reduction of the shielding ability of concrete may become a matter of concern. Several benchmark experiments have been reported on the streaming problem, such as multi layered slit and narrow gap 1) 2). However, only few measurement of penetrating neutron and gamma ray through concrete crack have been reported. The reason is that it is very difficult to measure the change in the penetration rate of radiation through the thick concrete shield. The experimental set up of very narrow slit less than 1 mm systematically in concrete is also very difficult. In this study, the shielding abilities of cracked and uncracked concrete walls were assessed by simulation using a verified three-dimensional Monte Carlo calculation code system 3) 6). 3 CALCULATION METHOD The three-dimensional Monte Carlo code MCNP5 7) was employed for the analysis. The nuclear data libraries applied were MCPLIB04 8) for gamma rays and FSXLIB-J33 9) for neutrons. We used both a simple and practical building model of low-level radioactive wastes for the calculations. Gamma rays and neutrons were used as radiation sources, and their spectra (Fig. 1) were obtained from the calculated spectra of vitrified radioactive waste. The peak energy of neutron is around 2 MeV and that of gamma ray is around 1 MeV. The secondary gamma ray spectrum induced neutron was also obtained. They 1

2 are normalized to the source neutron and gamma ray. The intensity of the neutron and gamma ray was 1.3E+9 (n/s) and 2.81E+16 (n/s), respectively. 3.1 Simple modeling Figure 1. Used calculated neutron and gamma-ray spectra A cubic room having dimensions of 3 m 3 m 3 m and 1200-mm thick walls was used as a simple model. The source (volume: 1 m 3 ) was set at the center of the room. It was assumed that one of the walls had a straight crack extending from the floor to the ceiling. The crack width was varied from 0 to 10 mm. The cell tally was set outside the center of the crack. These are cubic cells having dimensions of 10 mm in the case of the simple model. The number of gamma rays generated during the simulation was up to The calculation was performed for around 2 months using 16 CPUs (frequency: 4 GHz or more). The obtained results were normalized with the strength data of practical radioactive wastes. The calculated gamma ray and neutron spectra were transformed to a dose rate by the flux-to-dose conversion factor based on ICRP Pub.74 10). 3.2 Practical modeling Figure 2. Calculation model for simple case The Monte Carlo calculations were also performed in the case of a practical building. A building model with dimensions of m m 5 m was used as a practical model. The radioactive wastes were filled in the drum, 566mm in diameter and 882mm high (capacity: 200 L). The thickness of the drum was 1.6mm and the 2

3 material was steel. These drums were set in the building and the number of the drums was 23,040. A view of the drums and a view of their horizontal and vertical layouts are shown in Figs. 3, 4, and 5, respectively [unit: cm] Drum Drum Palette Figure 3. Drums used in the calculation model [unit: cm] Drum and pallette Estimated concrete wall Figure 4. Horizontal layout of drums [unit: cm] Figure 5. Vertical layout of drums 3

4 The thickness of the concrete wall was 0 mm. The calculation conditions were the same as those used in the case of the simple model. The tally (diameter: 39 mm, thickness: 60 mm) were set every 0 mm along one of the walls, as shown in Fig. 6. The dimensions of the tally were the same as those of an ionization chamber. east-side concrete wall (thick= [cm]) ionization chambers (r =3.9, h=6.0 [cm]) Figure 6. Layout of tally Since rebars are arranged every 200 mm, cracks will emerge every 200 mm. The calculation model cracks in the wall is shown in Fig. 7. The width of each crack was 1 mm, and the cracks penetrate the concrete wall. [unit: cm] 20 Figure 7. Calculation model for cracks in a practical building 4 CALCULATED RESULTS 4.1 Simple modeling Calculated energy spectra Figures 8 and 9 show the spectra of neutrons and gamma rays that penetrated the 1200-mm-thick concrete wall, respectively. The crack width varied from 0 to 2.0 mm. The neutron spectra did not change remarkably, because the ratio of forward scattering is not significant. The neutrons were easily scattered by low-mass nuclides in the concrete and air in the crack. However, the gamma ray spectra changed as the crack width increased. When the crack width was 2.0 mm, the shape of the gamma ray spectrum approached that of the source gamma ray spectrum. From the result, it is assumed that gamma rays strongly influence the dose penetrating a crack in the concrete wall. 4

5 no crack straight w=0.05 cm straight w=0.10 cm straight w=0.15 cm straight w=0.20 cm Energy [MeV] Figure 8. Spectra of neutrons that penetrated the 1200-mm-thick concrete wall no crack straight w=0.05 cm straight w=0.10 cm straight w=0.15 cm straight w=0.20 cm Energy [MeV] Figure 9. Spectra of gamma rays that penetrated the 1200-mm-thick concrete wall Calculated results The calculated dose rates of neutrons, secondary gamma rays, and gamma rays for each crack width are listed in Table 1. Since the source intensity of the gamma rays was very high, the gamma ray dose penetrating the concrete wall was dominant. Table 1. Calculated dose rate for several crack widths (thickness of concrete wall: 1.2 m) unit: microsv/h!"#$%&'()*+&,--. /01*"23& 40$23)#"5&6#--#&"#5& 7#--#&"#5& 82*#9 : ;<=>?@:: A<=A?B:= C<D;?@:; C<DD?@:; :<D ;<=D?@:: A<;=?B:= ><=E?@:> ><=D?@:> = ;<;D?@:: A<;F?B:= A<FD?@:> A<FD?@:> =<D ;<E=?@:: A<;C?B:= =<CA?@:E =<CA?@:E ; ;<EA?@:: A<>;?B:= ><DG?@:E ><DG?@:E =: A<EE?@:: E<D:?B:= A<A:?@:D A<A:?@:D 5

6 The penetration rates of radiation through a cracked concrete shield to those of radiation through an uncracked concrete shield are shown in Fig. 10. The thickness of the concrete shield was 1200 mm. For gamma rays, ratios of approximately 10, 50, and 0 were obtained at crack widths of 1, 2, and 10 mm, respectively. On the contrary, the ratios of neutrons did not increase significantly with the crack width.!" &!" % )*+,-.)/!"0 )123443/!" /!"0!" $!"!!" "!" #! #$ " $ & ' (!"!$ :,;/440 Figure 10. Ratio of dose rate between cracked and uncracked concrete vs. concrete thickness It was experimentally verified that the maximum crack width should be less than 1 mm 11). The corresponding decrease in shielding ability will be 1/10. A parametric search of these ratios for the change in wall thickness has also been performed for a crack width of 1 mm. The results are shown in Fig. 11. Up to a wall thickness of 500 mm, the penetration rate of radiation through the cracked wall was almost the same as that of radiation through the uncracked wall. This was because the dose rate of gamma rays that penetrated the cracked wall was only slightly greater than that of gamma rays that penetrated the uncracked wall. The difference between the number of gamma rays penetrating the cracked and uncracked walls increased when the wall thickness was greater than 500 mm. However, the ratio was at most 10 when the wall thickness was 1200 mm.!"!"!" +!" *,-./01,23, ,895::523, ::523, ,-./01,23;</= ,895::523;</= ::523;</= !" )!" (!" '!" &!" %!" $!"!!" "!" #! " $" &" (" *"!""!$" >=<46,-??21@2/=-2?=<-AB234:7 Figure 11. Dose rate distribution in concrete wall 6

7 4.2 Practical modeling To improve the precision of the calculated result, the calculation was performed using only gamma rays since the influence of neutrons and the secondary gamma ray dose penetrating the concrete wall is negligibly small. For the practical model, the wall thickness was 0 mm and cracks were formed every 200 mm. The calculated result of dose rate distribution behind the concrete wall is shown in Fig. 12. For the practical model, the ratio of the dose rate varied from 1.3 to 5.0, with an average of The reduction in shielding ability was at most 1/5 for a 0-mm thick concrete wall. This result did not contradict that obtained for the simple model radius of ionization chamber = 8 cm no crack (nps=1.44x10 11 ) crack [width=0.1cm] (nps=2.72x10 11 ) Distance on east-side concrete wall [cm] Figure 12. Gamma ray dose rate distribution behind the wall 5 DISCUSSION The analyses have been performed using a straight crack model; however, the configuration of cracks in concrete is typically not straight, because the concrete does not contain uniform compositions of aggregates and sand. An additional analysis was performed using the saw-tooth crack model, and the obtained results were compared with those of the straight model. Figure 13 shows the results obtained using a saw-tooth crack having a length and height of mm and 40 mm 12), respectively. The thickness of the concrete wall was 1200 mm, and the dimensions of the tally were 10 mm. saw - tooth crack model Crack (width) Concrete height length Expanded horizontal section Figure 13. Saw-tooth model (length: mm, height: 40 mm) 7

8 Table 2 shows the calculated results of the ratio of dose rate behind the concrete wall to that of an uncracked wall. The calculated ratios of dose rates in the straight model to those in the saw-tooth model are also listed in Table 2. The dose rate of gamma rays, which was dominant behind the concrete wall, reduced by approximately 1/500 as compared to that in the straight crack model. Gamma rays are easily blocked due to the multiple scattering in the saw-tooth crack. In this study, all estimations of the shielding ability were performed using the straight crack model. The results obtained in this study are more than sufficient for human safety. Table 2. Calculated ratio of dose rates in the straight model to those in the saw-tooth model!"#$%&' (")&'*+%,-.+//+ 0+//+1%+, ($%+2.3$1/&*" :; 9<=9=65= (+>?$&&$31/&*"4 96@A 965= :65B C+$2&D($%+2.3$E(+>?$&&$3F 9=6=7 6 CONCLUSION The penetration rate of radiation through a cracked concrete shield was compared with that of radiation through an uncracked concrete shield in order to estimate the decrease in shielding ability by using the Monte Carlo code MCNP5. When the crack width was 1 mm, which is a practical value, and the wall thickness was less than 500 mm, there was almost no difference between the two penetration rates. It was found that the difference in the penetration rate between cracked and uncracked wall increased with the wall thickness. The ratio of the two penetration rates was at most 10 when the wall thickness was 1200 mm. It was concluded that the increase in the penetration rate of radiation through the concrete shield due to crack formation is not a serious problem since the configuration of a practical crack in concrete is random and never uniformly straight. Acknowledgements. The authors would like to express their sincere thanks to emeritus professor Takashi Nakamura of Tohoku University for his stimulating and helpful discussions. REFERENCES 1) Hiroshi NAKASHIMA, Shunichi TANAKA and Hiroshi MAEKAWA, Experiments and Calculations Of 1.4 MeV Neutron Streaming through Multi-Layered Slit Assembly, Journal of NUCLEAR SCIENCE and TECHNOLOGY, 24[8], pp (August 1987). 2) Seijl MORl and Yasushi SEKl, Systematic Analysis of Decay Gamma-Ray Streaming through Narrow Gap in Fusion Experimental Reactor, Journal of NUCLEAR SCIENCE and TECHNOLOGY, 27[6], pp (June 1990). 3) D.J.Whalen, D.E.Hollowell and J.S.Hendricks, MCNP:Photon Benchmark Problems, LA-12196, ) J.D.Court, R.C. Brockhoff and J.S.Hendricks, Lawrence Livermore Pulsed Sphere Benchmark Analysis of MCNP ENDF/B-VI, LA-12885, ) F.B.Brown, R.D.Mosteller and A.Sood, Verification of MCNP5, LA-UR , ) D.A.Torres, R.D. Mosteller and J.E.Sweezy, Comparison of MCNP5 and Experimental Results on Neutron Shielding Effects for Materials, LA-UR , ) X-5 Monte Carlo Team, MCNP: A General Monte Carlo N-Particle Transport Code, Version 5, LA-UR , LANL (2003). 8) RSICC, CCC-710/MCNP: Data Libraries for MCNP5 (2003). 9) K. Kosako, N. Yamano, T. Fukahori, K. Shibata and A. Hasegawa: The Libraries FSXLIB and MATXSLIB Based on JENDL-3.3, JAERI-Data/Code (2003). 10) Conversion Coefficients for use in Radiological Protection against External Radiation, ICRP Publication 74, Annals of the ICRP 26(3/4), Pergamon Press, Oxford. 11) Yoshinari Munakata, et al., Study on Radiation Shielding Performance of Reinforced Concrete Wall (1) Loading Test of RC wall and Modeling of Concrete Cracks, This Conference 12) B.Bujadham, A.Fujiyoshi and K.Maekawa, Crack Surface Asperity on Stress Transfer Mechanism, Proceedings of the Japan Concrete Institute, Vol.11, No.2, 1989, pp