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Monte Carlo simulations of radioactive waste embedded into polymer
ARTICLE IN PRESS Radiation Physics and Chemistry 78 (2009) 800–805
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Monte Carlo simulations of radioactive waste embedded into polymer ¨ zdemir , Ali Usanmaz Tonguc- O Middle East Technical University, Department of Polymer Science and Technology, Ankara, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 February 2009 Accepted 28 April 2009
Radioactive waste is generated from the nuclear applications and it should properly be managed according to the regulations set by the regulatory authority. Poly(carbonate urethane) and poly (bisphenol a-co-epichlorohydrin) are radiation-resistant polymers and they are possible candidate materials that can be used in the radioactive waste management. In this study, maximum allowable waste activity that can be embedded into these polymers and dose rate distribution of the waste drum (containing waste and the polymer matrix) were found via Monte Carlo simulations. The change of mechanical properties of above-mentioned polymers was simulated and their variations within the waste drum were determined for 15, 30 and 300 years after embedding. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Poly(carbonate urethane) Poly(bisphenol a-co-epichlorohydrin) DGEBA Radioactive Waste MCNP
1. Introduction Radioactive wastes are inevitably generated from nuclear industry and nuclear applications; the generated radioactive waste should be managed in a safe manner. The nuclear industry generates considerable amount of low-, intermediate- and highlevel radioactive wastes. Different methods (such as evaporation, precipitation, ion exchange, adsorption on clay surface) are available to concentrate wastes for further treatment and common immobilization methods include embedding of radioactive wastes in cement, polymer or bitumen. Many researches have been carried out to investigate the possible use of polymers in nuclear industry and radioactive waste management (Sakr et al., 2003; Damian et al., 2001; Nicaise et al., 1986a, b; Baluch ¨ zdemir and Usanmaz, 2007, 2008, et al., 1977; Debre´ et al., 1997; O ¨ zdemir, 2008). Polymers are examples of materials that 2009; O could be used in the radioactive waste immobilization (Day et al., 1985). For example, polymers were used in radioactive waste management and they have high radiation stability (IAEA, 1988; ¨ zdemir, 2008). Epoxy resins have high Debre´ et al., 1997; O chemical and corrosion resistance, outstanding adhesion properties, low shrinkage upon cure (Fried, 2003) and high radiation resistance after curing (Gilfrich and Wilski, 1992), therefore epoxy/amine polymers were used as the immobilization matrix for radioactive waste management (Baluch et al., 1977; IAEA, 1988; Damian et al., 2001). The limitation for the use of epoxy is the possible interference reaction between waste constituents and
the hardener (IAEA, 1988). Polymers can also be used as a transport container for the radioactive waste (Bonin et al., 2004). Both poly(carbonate urethane) (PCU), copolymer of poly(1,6-hexyl-1,2-ethyl carbonate) diol, 4,40 -methylenebis (phenyl isocyanate) and 1,4-butanediol, and poly(bisphenol a-coepichlorohydrin), an epoxy pre-polymer resin with molecular formula of [C6H4-4-C(CH3)2C6H4-4-OCH2CH(OH)CH2O]n, are valuable commercial engineering polymers. To understand the possible use of PCU and PBEH in radioactive waste management as solidifying agents, the radiation stability of the PCU and PBEH was studied via Co-60 gamma irradiations at two different dose
Corresponding author. Tel.: +90 312 2958849; fax: +90 312 2958956.
¨ zdemir). E-mail address: [email protected] (T. O 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.04.025
Fig. 1. Tally cell locations in waste drum.
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
¨ zdemir and Usanmaz, 2007, 2008). rates of 1540 and 82.8 Gy/h (O The total dose of irradiation was up to 6.24 MGy for PCU and 1.1 MGy for PBEH. Degradation of both polymers was tested by means of mechanical and thermal tests. From the experimental observations, it has been shown that PCU and PBEH can be used for embedding radioactive waste from the radiation stability ¨ zdemir and Usanmaz, 2007, 2008) and due to the point of view (O limitation of possible interference reaction between waste constituents and the hardener; the PBEH was used without any hardener during that study. PCU was found to be resistible up to the total dose of 7500 kGy and PBEH was found to be resistible up
¨ zdemir and Usanmaz, 2007, 2008). to total dose of 1100 kGy (O Monte Carlo simulation for the waste embedded into polymer matrix has not been encountered in the literature. Repositories for disposal of radioactive waste include a multibarrier systems to isolate the waste from the biosphere. Multibarrier system typically comprises of natural geological formation and engineering barrier system. Engineering barrier systems may comprise a variety of components, such as the waste form, waste canisters, backfill, seals and plugs. The general purpose of engineering barrier systems are to prevent and/or delay the release of radionuclides from the waste to the repository host rock, at least during the first several hundred years after repository closure (NEA, 2003). Waste embedded into a polymer matrix can also be a part of an engineering barrier system and it would prevent and delay the release of radionuclides from the waste drum. In this study, based on previous experimental data of the total allowable dose, exceeding of which would result in total breakdown of mechanical properties, and setting the condition as an annular region at the center of waste drum not totally degraded at the end of 300 years, the maximum allowable waste activity than can be embedded in a waste drum was found for each of PCU and PBEH polymers. Dose rate distribution within the waste drum (containing waste and the polymer matrix) was simulated via Monte Carlo simulation for each polymer. The changes of mechanical properties of above-mentioned polymers were calculated and their variations within the waste drum were determined for 15, 30 and 300 years after embedding. Thermal degradation of the polymer matrices, which might arise due to the heat generation of waste, was not considered in this study and the radiation stability of polymers based on total dose was considered.
5.0E+03 1.0E+06 log (DR)
t=0
Dose rate (Gy/h)
4.0E+03
1.0E+04
experimental A
1.0E+02
B
3.0E+03
C
1.0E+00 0
10
20 x. r (cm)
30
2.0E+03 t=0 t = 15 y t = 30 y t = 300 y
1.0E+03
0.0E+00 0
5
10
15 x. r (cm)
20
25
801
30
Fig. 2. (a) Change of dose rate with the distance from the center in the waste drum embedded with PCU. (b) Semi-logarithmic plot of dose rate with experimental dose rate.
30 3000 1000 500 200 100 50
20 C
B
10
Y
A 0
-10
-20
-30 -30
-20
-10
0 X
10
20
Fig. 3. Contour plot of the dose rate distribution (Gy/h) of waste drum (Z ¼ 0).
30
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
802
2. Materials and methods
3. Results and discussions
Radioactive waste with spherical and cylindrical geometries embedded into polymer matrix was simulated. For cylindrical geometry, radioactive waste was taken as a Cs-137 source with the radius of 0.5 cm and height of 0.5 cm and located at the center of the waste drum. For spherical geometry, radioactive waste was also taken as a Cs-137 source with radius values of 5, 10, 15 and 20 cm. The radius of waste drum was taken as 28.5 cm with height of 83 cm. Monte Carlo simulations were performed to determine the maximum allowable waste activity to be embedded into waste drum on the basis of an annular region at the center of waste drum not totally degraded at the end of 300 years control period. Based on the previous experimental data of total allowable dose, the 23 cm radius was selected as the boundary for the total allowable dose that is the total dose at 23 cm from the center of drum would not exceed the maximum allowable dose for each polymeric material at the end of 300 years control period. Since, waste embedded into polymer matrix is a part of engineering barrier system and the purpose of engineering barrier systems is to prevent and/or delay the release of radionuclides from the waste to the host environment, by setting the 23 cm boundary, a 5.5 cm thick layer would provide a layer that still prevents the release of the radionuclides to the outer environment. Change of mechanical properties via total dose at the dose rate ¨ zdemir of 1540 Gy/h for PCU and PBEH is given in the literature (O and Usanmaz, 2007, 2008), based on these experimental data the change of the mechanical properties of polymers in the x-direction (radial) and z-direction was determined for different periods of 15, 30 and 300 years after embedding. MCNP is a general-purpose Monte Carlo N-particle input that can be used for neutron, photon, electron or coupled neutron/ photon/electron transport. For photons, the input takes account of incoherent and coherent scattering, the possibility of fluorescent emission after photoelectric absorption, absorption in pair production with local emission of annihilation radiation, bremsstrahlung and a continuous slowing down model is used for electron transport that includes positrons, k X-rays and bremsstrahlung (Briesmeister, 1997). In the MCNP simulations, both photons and photon-induced electrons were transported and F4 type tallies were used for determination of volume-averaged dose rates in 1 cm diameter spheres along the x- and z-axes. The dose rate per photon emitted from the source in the x (radial) direction was found via MCNP and the corresponding maximum activity was calculated based on the total allowable dose for each polymer in order not to exceed the total allowable dose at 23 cm away from the center in the x- (radial) direction. The cells used for the F4 type tallies are shown in Fig. 1. The dose rate distributions for the waste drum were determined for both of PCU and PBEH polymers at the initial period of embedding of radioactive waste. Moreover, dose rate distributions along the radius for different periods of 15, 30 and 300 years after embedding were determined. The history of the simulations was selected as 0.5 107. Mesh tally feature was used to obtain contour plot of dose rate distribution. The active institutional control period is accepted as 300 years for low and intermediate short-lived radioactive waste category disposal facilities (Debre´ et al., 1997). The total dose to the immobilization matrix during the active institutional control period can be calculated via Eq. (1), where TD is total dose (kGy), DRo is initial dose rate (kGy/year), t is period (year) and k is decay constant.
The maximum allowable activity of the Cs-137 source with cylindrical geometry that can be embedded into PCU-filled waste drum was found as 1.58 1013 Bq based on the condition that an
TD ¼
Z
Strain at break
2.0 t=0
1.6
t = 15 y
1.2
t = 30 y
0.8
t = 300 y
0.4 0.0
Tensile Strength (MPa)
0
5
10
15
20
25
30
10
15
20
25
30
10
15 x, r (cm)
20
25
30
44 39 34
t=0
29
t = 15 y
24
t = 30 y
19
t = 300 y
14 0
5
Toughness (J/cm3)
50 40 t=0
30
t = 15 y
20
t = 30 y
10
t = 300 y
0 0
5
z (cm)
Fig. 4. Change of the mechanical properties of PCU along the radius depending on time period after embedding.
40
40
30
30
20
20
10
t=0
10
t=0
t = 15 y
t = 15 y
t = 30 y
t = 30 y
t = 300 y
t = 300 y
0
0 0
1 Strain at break
2
0.0
0.2 0.4 Strain at break
300
DRo ekt dt 0
(1)
Fig. 5. Change of mechanical properties in along z-axis (X ¼ 0, Y ¼ 0) (a) PCU (b) PBEH.
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
annular region at the center of waste drum would not be totally degraded at the end of 300 years. The total allowable dose would be the total dose about 23 cm from the center at the end of 300 years. Change of dose rate from the center of drum, point P in Fig. 1, along the x-axis is given in Fig. 2a for different time periods of initial and 15, 30, 300 years after embedding. Semi-logarithmic plot of dose rate versus radius is given in Fig. 2b. The points A and B in Fig. 2b show the dose rates of 1540 and 82.8 Gy/h, at which
the experimental data were available. The point C in Fig. 2b shows the radius and the initial dose rate at which the total dose would be equal to total allowable dose of 7500 kGy after 300 years. The contour plot of dose rate on xy coordinate is given in Fig. 3 for waste embedded into PCU matrix. The dose rates at the isodose circles of A, B and C in Fig. 3 correspond to the dose rate shown as points A, B and C in Fig. 2b, respectively. Tensile strength, strain and toughness change with total dose, the change of these mechanical properties with total dose was ¨ zdemir and Usanmaz, 2007). The total experimentally found (O dose after time periods of 15, 30 and 300 years was calculated and the graphs of mechanical properties versus radius are given in Fig. 4. Since PCU can resist up to 7500 kGy total dose, doses greater than 7500 kGy would lead a strain value of zero for PCU. Change of strain from the center of drum in the z-axis for PCU is given in Fig. 5a and points where total dose greater than 7500 kGy would lead strain value of zero. The maximum activity of the Cs-137 source with cylindrical geometry that can be embedded into PBEH-filled waste drum was found as 2.11 1012 Bq based on the assumption that an annular region at the center of waste drum would not be totally degraded at the end of 300 years. Change of dose rate from the center of drum along the x (radial) axis is given in Fig. 6a for different time periods of initial and 15, 30 and 300 years after embedding. Semi-logarithmic plot of dose rate versus radius is given in Fig. 6b. The points A and B show the dose rates of 1540 and 82.8 Gy/h, at which the experimental data were available, respectively. The point C in Fig. 6b shows the radius and the initial dose rate at which the total dose would be equal to the total allowable dose of 1100 kGy after 300 years. The contour plot of dose rate on xy coordinate is given in Fig. 7 for waste embedded into PBEH matrix. The dose rates at the
5.0E+02 t=0
log (DR)
1.0E+04
Dose rate (Gy/h)
4.0E+02
A
experimental B
1.0E+02
C 1.0E+00
3.0E+02
0
10
20
30
x. r (cm)
2.0E+02 t=0 t = 15 y
1.0E+02
t = 30 y t = 300 y
0.0E+00 0
5
10
15 x, r (cm)
20
25
803
30
Fig. 6. (a) Change of dose rate with the distance from the center in the waste drum embedded with PBEH. (b) Semi-logarithmic plot of dose rate with experimental dose rate.
30 300 200 100 50 40 10
C
20
10 B
Y
A
0
-10
-20
-30 -30
-20
-10
0 X
10
20
Fig. 7. Contour plot of the dose rate distribution (Gy/h) of waste drum (Z ¼ 0).
30
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
804
0.5
Table 1 Maximum allowable activity of radioactive waste with spherical geometry to be embedded into PCU for different radius values of spherical geometry.
Strain at Break
0.4
t=0 t = 15 y
0.3
t = 30 y t = 300 y
0.2
r (cm)
Max. allowable activity (Bq)
5 10 15 20
1.51E+13 1.39E+13 1.26E+13 1.04E+13
0.1 0.0 0
5
10
15
20
25
Tensile Strength (MPa)
63.5 53.5 43.5 t=0
33.5
r (cm)
Max. allowable activity (Bq)
5 10 15
2.09E+12 2.03E+12 1.92E+12
t = 15 y t = 30 y
23.5
t = 300 y
13.5 3.5 0
5
10
15
20
25
20
Toughness (J/cm3)
Table 2 Maximum allowable activity of radioactive waste with spherical geometry to be embedded into PBEH for different radius values of spherical geometry.
15
and tabulated in Table 2, based on the same condition that the total dose to be 1100 kGy at the 23 cm from the center at the end of 300 years. The results have shown that the waste with spherical geometry (with radius values of 5, 10, 15, 20 cm) has lower allowable activity than the allowable activity found for the waste with cylindrical geometry (radius 0.5 cm and height 0.5 cm) both for the PCU and PBEH cases.
t=0
4. Conclusions
t = 15 y
10
t = 30 y t = 300 y
5 0 0
5
10
15 x, r (cm)
20
25
Fig. 8. Change of the mechanical properties of PBEH along the radius depending on time period after embedding.
iso-dose circles of A, B and C in Fig. 7 correspond to the dose rate shown as points A, B and C in Fig. 6b, respectively. Tensile strength, strain and toughness change with total dose, the change of these mechanical properties with total dose ¨ zdemir and Usanmaz, 2008). The was experimentally found (O total doses after different periods of 15,30 and 300 years after embedding were calculated. The change of mechanical properties of PBEH is given in Fig. 8. The PBEH can resist up to 1100 kGy total dose, that is, points where total dose greater than 1100 kGy would lead tensile strength value of zero. Change of strain from the center of drum in the z-axis for PBEH is given in Fig. 5b and points at which total dose greater than 1100 kGy would lead tensile strength value of zero. Since the geometry, position and type of the radioactive waste in the drum are the parameters that would affect dose rate distribution and thus the total dose delivered to the embedding matrix, simulations for radioactive waste with spherical geometry with different radius values of 5, 10, 15 and 20 cm were also carried out. The maximum allowable activity for radioactive waste with spherical geometry that can be embedded into PCU was found and tabulated in Table 1, based on the same condition that the total dose to be 7500 kGy at the 23 cm from the center at the end of 300 years. The maximum allowable activity for radioactive waste with spherical geometry that can be embedded into PBEH was found
The maximum allowable activity for radioactive waste with cylindrical geometry that can be embedded into the center of the waste drum was found as 1.58 1013 Bq for PCU and 2.11 1012 Bq for PBEH and in these cases the polymer matrices would have a layer, not totally degraded, between waste and the outer environment at the end of 300-year control period. PCU can accomodate higher radioactive waste activity compared with PBEH. Mechanical properties would totally be deteriorated up to 23 cm at the end of 300 years in the x-, y- and z-directions. However, it should be noted that the geometry, position and type of the radioactive waste in the drum would certainly affect the dose rate distribution, and thus the total dose delivered to the polymer matrix. References Baluch, M.H., Arab, A.K., Toblert, R.N., Hackett, R.M., 1977. Long term chemical stability of epichlorohydrin/bisphenol resin polymer concrete. Cement and Concrete Research 7, 637–642. Bonin, H.W., Walker, M.W., Bui, V.T., 2004. Application of polymers for the longterm storage and disposal of low- and intermediate-level radioactive waste. Nuclear Technology 145, 82–101. Briesmeister, J.F., (Ed.), Monte Carlo N-Particle Transport I˙nput, LA-12625-M, Version 4B March 1997. Damian, C., Espuche, E., Escoubes, M., 2001. Influence of three ageing types (thermal oxidation. radiochemical and hydrolytic ageing) on the structure and gas transport properties of epoxy–amine networks. Polymer Degradation and Stability 72, 447–458. Day, D.H., Hughes, A.E., Leake, J.W., Marples, JA.C., Marsh, G.P., Rae, J., Wade, B.O., 1985. The management of radioactive wastes. Reports on Progress in Physics 48, 101–169. Debre´, O., Nsouli, B., Thomas, J.–P., Stevenson, I., Colombini, D., Romero, M–A., 1997. Gamma irradiation-induced modifications of polymers found in nuclear waste embedding processes Part I: the epoxy/amine resin. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 131, 313–320. Fried, J.R., 2003. Polymer science and technology. Prentice Hall, Upper Saddle River, NJ. Gilfrich, H.P., Wilski, H., 1992. The radiation resistance of thermoset plastics-V. Epoxy plastics. Radiation Physics and Chemistry 39, 401–405.
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
IAEA, International Atomic Energy Agency, 1988. Immobilization of low and intermediate level radioactive wastes in polymer. IAEA, Vienna (Technical Report Series–289). NEA, A., 2003. Engineered barrier systems and the safety of deep geological repositories. OECD, Paris. Nicaise, E., Gandini, A., Cheradame, H., 1986a. Behavior of composites used in the confinement of nuclear wastes 1- water absorption of polymer matrices. British Polymer Journal 18, 247–250. Nicaise, E., Gandini, A., Cheradame, H., 1986b. Behaviour of composites used in the confinement of nuclear wastes 2- water absorption in particle-filled epoxide networks. British Polymer Journal 18, 251–255. ¨ Ozdemir, T., Usanmaz, A., 2007. Degradation of poly(carbonate urethane) by gamma irradiation. Radiation Physics and Chemistry 76, 1069–1074.
805
¨ zdemir, T., 2008. Gamma irradiation degradation/modification of 5-ethylidene 2O norbornene (ENB)-based ethylene propylene diene rubber (EPDM) depending on ENB content of EPDM and type/content of peroxides used in vulcanization. Radiation Physics and Chemistry 77, 787–793. ¨ Ozdemir, T., Usanmaz, A., 2008. Degradation of poly(bisphenol a-co-epichlorohydrin) by gamma irradiation. Radiation Physics and Chemistry 77, 799–805. ¨ Ozdemir, T., Usanmaz, A., 2009. Use of poly(methyl methacrylate) in radioactive waste management: I, radiation stability and degradation. Progress in Nuclear Energy 51, 240–245. Sakr, K., Sayed, M.S., Hafez, M.B., 2003. Immobilization of radioactive waste in mixture of cement clay and polymer. Journal of Radioanalytical and Nuclear Chemistry 256, 179–184.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Monte Carlo simulations of radioactive waste embedded into polymer ¨ zdemir , Ali Usanmaz Tonguc- O Middle East Technical University, Department of Polymer Science and Technology, Ankara, Turkey
a r t i c l e in f o
a b s t r a c t
Article history: Received 10 February 2009 Accepted 28 April 2009
Radioactive waste is generated from the nuclear applications and it should properly be managed according to the regulations set by the regulatory authority. Poly(carbonate urethane) and poly (bisphenol a-co-epichlorohydrin) are radiation-resistant polymers and they are possible candidate materials that can be used in the radioactive waste management. In this study, maximum allowable waste activity that can be embedded into these polymers and dose rate distribution of the waste drum (containing waste and the polymer matrix) were found via Monte Carlo simulations. The change of mechanical properties of above-mentioned polymers was simulated and their variations within the waste drum were determined for 15, 30 and 300 years after embedding. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Poly(carbonate urethane) Poly(bisphenol a-co-epichlorohydrin) DGEBA Radioactive Waste MCNP
1. Introduction Radioactive wastes are inevitably generated from nuclear industry and nuclear applications; the generated radioactive waste should be managed in a safe manner. The nuclear industry generates considerable amount of low-, intermediate- and highlevel radioactive wastes. Different methods (such as evaporation, precipitation, ion exchange, adsorption on clay surface) are available to concentrate wastes for further treatment and common immobilization methods include embedding of radioactive wastes in cement, polymer or bitumen. Many researches have been carried out to investigate the possible use of polymers in nuclear industry and radioactive waste management (Sakr et al., 2003; Damian et al., 2001; Nicaise et al., 1986a, b; Baluch ¨ zdemir and Usanmaz, 2007, 2008, et al., 1977; Debre´ et al., 1997; O ¨ zdemir, 2008). Polymers are examples of materials that 2009; O could be used in the radioactive waste immobilization (Day et al., 1985). For example, polymers were used in radioactive waste management and they have high radiation stability (IAEA, 1988; ¨ zdemir, 2008). Epoxy resins have high Debre´ et al., 1997; O chemical and corrosion resistance, outstanding adhesion properties, low shrinkage upon cure (Fried, 2003) and high radiation resistance after curing (Gilfrich and Wilski, 1992), therefore epoxy/amine polymers were used as the immobilization matrix for radioactive waste management (Baluch et al., 1977; IAEA, 1988; Damian et al., 2001). The limitation for the use of epoxy is the possible interference reaction between waste constituents and
the hardener (IAEA, 1988). Polymers can also be used as a transport container for the radioactive waste (Bonin et al., 2004). Both poly(carbonate urethane) (PCU), copolymer of poly(1,6-hexyl-1,2-ethyl carbonate) diol, 4,40 -methylenebis (phenyl isocyanate) and 1,4-butanediol, and poly(bisphenol a-coepichlorohydrin), an epoxy pre-polymer resin with molecular formula of [C6H4-4-C(CH3)2C6H4-4-OCH2CH(OH)CH2O]n, are valuable commercial engineering polymers. To understand the possible use of PCU and PBEH in radioactive waste management as solidifying agents, the radiation stability of the PCU and PBEH was studied via Co-60 gamma irradiations at two different dose
Corresponding author. Tel.: +90 312 2958849; fax: +90 312 2958956.
¨ zdemir). E-mail address: [email protected] (T. O 0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.04.025
Fig. 1. Tally cell locations in waste drum.
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
¨ zdemir and Usanmaz, 2007, 2008). rates of 1540 and 82.8 Gy/h (O The total dose of irradiation was up to 6.24 MGy for PCU and 1.1 MGy for PBEH. Degradation of both polymers was tested by means of mechanical and thermal tests. From the experimental observations, it has been shown that PCU and PBEH can be used for embedding radioactive waste from the radiation stability ¨ zdemir and Usanmaz, 2007, 2008) and due to the point of view (O limitation of possible interference reaction between waste constituents and the hardener; the PBEH was used without any hardener during that study. PCU was found to be resistible up to the total dose of 7500 kGy and PBEH was found to be resistible up
¨ zdemir and Usanmaz, 2007, 2008). to total dose of 1100 kGy (O Monte Carlo simulation for the waste embedded into polymer matrix has not been encountered in the literature. Repositories for disposal of radioactive waste include a multibarrier systems to isolate the waste from the biosphere. Multibarrier system typically comprises of natural geological formation and engineering barrier system. Engineering barrier systems may comprise a variety of components, such as the waste form, waste canisters, backfill, seals and plugs. The general purpose of engineering barrier systems are to prevent and/or delay the release of radionuclides from the waste to the repository host rock, at least during the first several hundred years after repository closure (NEA, 2003). Waste embedded into a polymer matrix can also be a part of an engineering barrier system and it would prevent and delay the release of radionuclides from the waste drum. In this study, based on previous experimental data of the total allowable dose, exceeding of which would result in total breakdown of mechanical properties, and setting the condition as an annular region at the center of waste drum not totally degraded at the end of 300 years, the maximum allowable waste activity than can be embedded in a waste drum was found for each of PCU and PBEH polymers. Dose rate distribution within the waste drum (containing waste and the polymer matrix) was simulated via Monte Carlo simulation for each polymer. The changes of mechanical properties of above-mentioned polymers were calculated and their variations within the waste drum were determined for 15, 30 and 300 years after embedding. Thermal degradation of the polymer matrices, which might arise due to the heat generation of waste, was not considered in this study and the radiation stability of polymers based on total dose was considered.
5.0E+03 1.0E+06 log (DR)
t=0
Dose rate (Gy/h)
4.0E+03
1.0E+04
experimental A
1.0E+02
B
3.0E+03
C
1.0E+00 0
10
20 x. r (cm)
30
2.0E+03 t=0 t = 15 y t = 30 y t = 300 y
1.0E+03
0.0E+00 0
5
10
15 x. r (cm)
20
25
801
30
Fig. 2. (a) Change of dose rate with the distance from the center in the waste drum embedded with PCU. (b) Semi-logarithmic plot of dose rate with experimental dose rate.
30 3000 1000 500 200 100 50
20 C
B
10
Y
A 0
-10
-20
-30 -30
-20
-10
0 X
10
20
Fig. 3. Contour plot of the dose rate distribution (Gy/h) of waste drum (Z ¼ 0).
30
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
802
2. Materials and methods
3. Results and discussions
Radioactive waste with spherical and cylindrical geometries embedded into polymer matrix was simulated. For cylindrical geometry, radioactive waste was taken as a Cs-137 source with the radius of 0.5 cm and height of 0.5 cm and located at the center of the waste drum. For spherical geometry, radioactive waste was also taken as a Cs-137 source with radius values of 5, 10, 15 and 20 cm. The radius of waste drum was taken as 28.5 cm with height of 83 cm. Monte Carlo simulations were performed to determine the maximum allowable waste activity to be embedded into waste drum on the basis of an annular region at the center of waste drum not totally degraded at the end of 300 years control period. Based on the previous experimental data of total allowable dose, the 23 cm radius was selected as the boundary for the total allowable dose that is the total dose at 23 cm from the center of drum would not exceed the maximum allowable dose for each polymeric material at the end of 300 years control period. Since, waste embedded into polymer matrix is a part of engineering barrier system and the purpose of engineering barrier systems is to prevent and/or delay the release of radionuclides from the waste to the host environment, by setting the 23 cm boundary, a 5.5 cm thick layer would provide a layer that still prevents the release of the radionuclides to the outer environment. Change of mechanical properties via total dose at the dose rate ¨ zdemir of 1540 Gy/h for PCU and PBEH is given in the literature (O and Usanmaz, 2007, 2008), based on these experimental data the change of the mechanical properties of polymers in the x-direction (radial) and z-direction was determined for different periods of 15, 30 and 300 years after embedding. MCNP is a general-purpose Monte Carlo N-particle input that can be used for neutron, photon, electron or coupled neutron/ photon/electron transport. For photons, the input takes account of incoherent and coherent scattering, the possibility of fluorescent emission after photoelectric absorption, absorption in pair production with local emission of annihilation radiation, bremsstrahlung and a continuous slowing down model is used for electron transport that includes positrons, k X-rays and bremsstrahlung (Briesmeister, 1997). In the MCNP simulations, both photons and photon-induced electrons were transported and F4 type tallies were used for determination of volume-averaged dose rates in 1 cm diameter spheres along the x- and z-axes. The dose rate per photon emitted from the source in the x (radial) direction was found via MCNP and the corresponding maximum activity was calculated based on the total allowable dose for each polymer in order not to exceed the total allowable dose at 23 cm away from the center in the x- (radial) direction. The cells used for the F4 type tallies are shown in Fig. 1. The dose rate distributions for the waste drum were determined for both of PCU and PBEH polymers at the initial period of embedding of radioactive waste. Moreover, dose rate distributions along the radius for different periods of 15, 30 and 300 years after embedding were determined. The history of the simulations was selected as 0.5 107. Mesh tally feature was used to obtain contour plot of dose rate distribution. The active institutional control period is accepted as 300 years for low and intermediate short-lived radioactive waste category disposal facilities (Debre´ et al., 1997). The total dose to the immobilization matrix during the active institutional control period can be calculated via Eq. (1), where TD is total dose (kGy), DRo is initial dose rate (kGy/year), t is period (year) and k is decay constant.
The maximum allowable activity of the Cs-137 source with cylindrical geometry that can be embedded into PCU-filled waste drum was found as 1.58 1013 Bq based on the condition that an
TD ¼
Z
Strain at break
2.0 t=0
1.6
t = 15 y
1.2
t = 30 y
0.8
t = 300 y
0.4 0.0
Tensile Strength (MPa)
0
5
10
15
20
25
30
10
15
20
25
30
10
15 x, r (cm)
20
25
30
44 39 34
t=0
29
t = 15 y
24
t = 30 y
19
t = 300 y
14 0
5
Toughness (J/cm3)
50 40 t=0
30
t = 15 y
20
t = 30 y
10
t = 300 y
0 0
5
z (cm)
Fig. 4. Change of the mechanical properties of PCU along the radius depending on time period after embedding.
40
40
30
30
20
20
10
t=0
10
t=0
t = 15 y
t = 15 y
t = 30 y
t = 30 y
t = 300 y
t = 300 y
0
0 0
1 Strain at break
2
0.0
0.2 0.4 Strain at break
300
DRo ekt dt 0
(1)
Fig. 5. Change of mechanical properties in along z-axis (X ¼ 0, Y ¼ 0) (a) PCU (b) PBEH.
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
annular region at the center of waste drum would not be totally degraded at the end of 300 years. The total allowable dose would be the total dose about 23 cm from the center at the end of 300 years. Change of dose rate from the center of drum, point P in Fig. 1, along the x-axis is given in Fig. 2a for different time periods of initial and 15, 30, 300 years after embedding. Semi-logarithmic plot of dose rate versus radius is given in Fig. 2b. The points A and B in Fig. 2b show the dose rates of 1540 and 82.8 Gy/h, at which
the experimental data were available. The point C in Fig. 2b shows the radius and the initial dose rate at which the total dose would be equal to total allowable dose of 7500 kGy after 300 years. The contour plot of dose rate on xy coordinate is given in Fig. 3 for waste embedded into PCU matrix. The dose rates at the isodose circles of A, B and C in Fig. 3 correspond to the dose rate shown as points A, B and C in Fig. 2b, respectively. Tensile strength, strain and toughness change with total dose, the change of these mechanical properties with total dose was ¨ zdemir and Usanmaz, 2007). The total experimentally found (O dose after time periods of 15, 30 and 300 years was calculated and the graphs of mechanical properties versus radius are given in Fig. 4. Since PCU can resist up to 7500 kGy total dose, doses greater than 7500 kGy would lead a strain value of zero for PCU. Change of strain from the center of drum in the z-axis for PCU is given in Fig. 5a and points where total dose greater than 7500 kGy would lead strain value of zero. The maximum activity of the Cs-137 source with cylindrical geometry that can be embedded into PBEH-filled waste drum was found as 2.11 1012 Bq based on the assumption that an annular region at the center of waste drum would not be totally degraded at the end of 300 years. Change of dose rate from the center of drum along the x (radial) axis is given in Fig. 6a for different time periods of initial and 15, 30 and 300 years after embedding. Semi-logarithmic plot of dose rate versus radius is given in Fig. 6b. The points A and B show the dose rates of 1540 and 82.8 Gy/h, at which the experimental data were available, respectively. The point C in Fig. 6b shows the radius and the initial dose rate at which the total dose would be equal to the total allowable dose of 1100 kGy after 300 years. The contour plot of dose rate on xy coordinate is given in Fig. 7 for waste embedded into PBEH matrix. The dose rates at the
5.0E+02 t=0
log (DR)
1.0E+04
Dose rate (Gy/h)
4.0E+02
A
experimental B
1.0E+02
C 1.0E+00
3.0E+02
0
10
20
30
x. r (cm)
2.0E+02 t=0 t = 15 y
1.0E+02
t = 30 y t = 300 y
0.0E+00 0
5
10
15 x, r (cm)
20
25
803
30
Fig. 6. (a) Change of dose rate with the distance from the center in the waste drum embedded with PBEH. (b) Semi-logarithmic plot of dose rate with experimental dose rate.
30 300 200 100 50 40 10
C
20
10 B
Y
A
0
-10
-20
-30 -30
-20
-10
0 X
10
20
Fig. 7. Contour plot of the dose rate distribution (Gy/h) of waste drum (Z ¼ 0).
30
ARTICLE IN PRESS ¨ zdemir, A. Usanmaz / Radiation Physics and Chemistry 78 (2009) 800–805 T. O
804
0.5
Table 1 Maximum allowable activity of radioactive waste with spherical geometry to be embedded into PCU for different radius values of spherical geometry.
Strain at Break
0.4
t=0 t = 15 y
0.3
t = 30 y t = 300 y
0.2
r (cm)
Max. allowable activity (Bq)
5 10 15 20
1.51E+13 1.39E+13 1.26E+13 1.04E+13
0.1 0.0 0
5
10
15
20
25
Tensile Strength (MPa)
63.5 53.5 43.5 t=0
33.5
r (cm)
Max. allowable activity (Bq)
5 10 15
2.09E+12 2.03E+12 1.92E+12
t = 15 y t = 30 y
23.5
t = 300 y
13.5 3.5 0
5
10
15
20
25
20
Toughness (J/cm3)
Table 2 Maximum allowable activity of radioactive waste with spherical geometry to be embedded into PBEH for different radius values of spherical geometry.
15
and tabulated in Table 2, based on the same condition that the total dose to be 1100 kGy at the 23 cm from the center at the end of 300 years. The results have shown that the waste with spherical geometry (with radius values of 5, 10, 15, 20 cm) has lower allowable activity than the allowable activity found for the waste with cylindrical geometry (radius 0.5 cm and height 0.5 cm) both for the PCU and PBEH cases.
t=0
4. Conclusions
t = 15 y
10
t = 30 y t = 300 y
5 0 0
5
10
15 x, r (cm)
20
25
Fig. 8. Change of the mechanical properties of PBEH along the radius depending on time period after embedding.
iso-dose circles of A, B and C in Fig. 7 correspond to the dose rate shown as points A, B and C in Fig. 6b, respectively. Tensile strength, strain and toughness change with total dose, the change of these mechanical properties with total dose ¨ zdemir and Usanmaz, 2008). The was experimentally found (O total doses after different periods of 15,30 and 300 years after embedding were calculated. The change of mechanical properties of PBEH is given in Fig. 8. The PBEH can resist up to 1100 kGy total dose, that is, points where total dose greater than 1100 kGy would lead tensile strength value of zero. Change of strain from the center of drum in the z-axis for PBEH is given in Fig. 5b and points at which total dose greater than 1100 kGy would lead tensile strength value of zero. Since the geometry, position and type of the radioactive waste in the drum are the parameters that would affect dose rate distribution and thus the total dose delivered to the embedding matrix, simulations for radioactive waste with spherical geometry with different radius values of 5, 10, 15 and 20 cm were also carried out. The maximum allowable activity for radioactive waste with spherical geometry that can be embedded into PCU was found and tabulated in Table 1, based on the same condition that the total dose to be 7500 kGy at the 23 cm from the center at the end of 300 years. The maximum allowable activity for radioactive waste with spherical geometry that can be embedded into PBEH was found
The maximum allowable activity for radioactive waste with cylindrical geometry that can be embedded into the center of the waste drum was found as 1.58 1013 Bq for PCU and 2.11 1012 Bq for PBEH and in these cases the polymer matrices would have a layer, not totally degraded, between waste and the outer environment at the end of 300-year control period. PCU can accomodate higher radioactive waste activity compared with PBEH. Mechanical properties would totally be deteriorated up to 23 cm at the end of 300 years in the x-, y- and z-directions. However, it should be noted that the geometry, position and type of the radioactive waste in the drum would certainly affect the dose rate distribution, and thus the total dose delivered to the polymer matrix. References Baluch, M.H., Arab, A.K., Toblert, R.N., Hackett, R.M., 1977. Long term chemical stability of epichlorohydrin/bisphenol resin polymer concrete. Cement and Concrete Research 7, 637–642. Bonin, H.W., Walker, M.W., Bui, V.T., 2004. Application of polymers for the longterm storage and disposal of low- and intermediate-level radioactive waste. Nuclear Technology 145, 82–101. Briesmeister, J.F., (Ed.), Monte Carlo N-Particle Transport I˙nput, LA-12625-M, Version 4B March 1997. Damian, C., Espuche, E., Escoubes, M., 2001. Influence of three ageing types (thermal oxidation. radiochemical and hydrolytic ageing) on the structure and gas transport properties of epoxy–amine networks. Polymer Degradation and Stability 72, 447–458. Day, D.H., Hughes, A.E., Leake, J.W., Marples, JA.C., Marsh, G.P., Rae, J., Wade, B.O., 1985. The management of radioactive wastes. Reports on Progress in Physics 48, 101–169. Debre´, O., Nsouli, B., Thomas, J.–P., Stevenson, I., Colombini, D., Romero, M–A., 1997. Gamma irradiation-induced modifications of polymers found in nuclear waste embedding processes Part I: the epoxy/amine resin. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 131, 313–320. Fried, J.R., 2003. Polymer science and technology. Prentice Hall, Upper Saddle River, NJ. Gilfrich, H.P., Wilski, H., 1992. The radiation resistance of thermoset plastics-V. Epoxy plastics. Radiation Physics and Chemistry 39, 401–405.
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