- Email: [email protected]
boron carbide composites as neutron shields
Polymer
ELSEVlER
Degradation
and Stability
50 (1995)
235-240
Elsevier Science Limited Printed in Northern Ireland 0141.3910/95/$09,50
0141-3910(95)00177-S
Ethylene-propylene diene rubber/low density polyethylene/boron carbide composites as neutron shields M. M. Abdel-Aziz,” S. E. Gwaily,” A. S. Makarious* & A. El-Sayed Abdob *National Center of Radiation Research and Technology, A.E.A., 3 Ahmed El-zumer, Gziro, Egypt bNuclear Research Center, A.E.A., Cairo, Egypt
8th Sector, Nasr City, PO Box 29,
(Received 4 July 1995; accepted 22 July 1995)
Composites of ethylene-propylene diene rubber and low density polyethylene were prepared with two different concentrations of boron carbide powder, namely, 47 and 57 wt%. The composites were investigated for their gamma and slow neutron radiation shielding properties. In addition, the temperature dependence of the electrical and thermal properties of the composites in the temperature range 30-150°C has been studied. The initial 1.5 cm thickness of the composite sample containing 57 wt% boron carbide sharply reduced the initial direct slow neutron flux by about 85%. The total macroscopic cross-section of this sample is 0.215 cm-‘.
1 INTRODUCTION
the USA) Radiation Shielding Information Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee; National Council on Radiation Protection and Measurements, Beheads, Maryland; Radiation Protection and Shielding Division of the American Nuclear Society, La Grange Park, Illinois; (in Europe) OECD Nuclear Energy Agency Data Bank, Gif-Sur-Yvette, France; and the European Shielding Information Services, Ispra, Italy. Boron carbide (B4C) is a widely used shielding material, which may be used alone, as a powder (density about 1.2 g cmp3), or in a hot-pressed (sintered) form with densities as high as 2.5 g cmp3. Borated plastics and wood-based particle board are also widely used in special neutron shielding applications. Hence, a trial was made to prepare a composite material consisting of an ethylenepropylene diene rubber (EPDM)/low density polyethylene (LDPE) blend with two different concentrations of boron carbide (B4C), to be tested as shielding materials for thermal neutrons. The compositions of these composites are given in Table 1. The electrical and thermal properties of these composites were also studied.
Many types of radiation, such as neutrons and x-ray or y-ray photons, cause ionization of the media with which they interact, through a complicated mechanism involving the emission of energetic secondary charged partic1es.l The ionizing ability of these types of radiation is the reason for the importance of studying shields. A shield is a physical entity interposed between a source of ionizing radiation and an object to be protected, such that the radiation level at the position of the object will be reduced. The object to be protected is most often a human being, but it can be anything that is sensitive to ionizing radiation. Significant textbooks and handbooks providing broad coverage on shielding are available.2~” In addition, many reports, monographs and journal articles’* have been published on various particular aspects of nuclear radiation shielding. Of particular significance as sources of such information are the following organizations: (in * To whom all correspondence
should be addressed. 235
236
M. M. Abdel-Aziz
Table 1. Composition
Ingredients 1. 2. 3. 4. 5. 6. 7.
(phr)
LDPE” EPDMb Boron carbide ZnO Stearic acid PbNP Bpd
of B,C loaded composites
SO 50 50 5 1 1 3
EPDM/LDPE
Sl
S*
50 50 100 (47 wt%) 5 1 1 3
50 150 (Zvt%) 5 1 1 3
Ingredients listed in order of addition during preparation. “From Dow Co., Spain. ‘Vistalone 5600, Belgium. ‘Antioxidant. dBenzoyl peroxide as curing agent.
2 EXPERIMENTAL 2.1 Sample preparation All rubber/plastic mixes were prepared as previously described.13 The EPDM/LDPE mixes were first prepared on a two roll rubber mill at 80°C for 10 min. The rest of the ingredients were then added, except for the curing agent (BP), which was added after cooling the recipe to about 50°C. The total duration of mixing was 30 min. Cylindrical shaped samples of 6 cm diameter and about 5 mm thickness were compression molded using a Carver hot press at 160°C and 60 kg/cm* pressure for 20 min. A group of these circular samples was arranged together to form an assembly about 6.6 cm thick for the attenuation measurements.
-
c--i
100Cm T
et al.
2.2 Attenuation measurements A Pu-a-Be source (2.5 curie) was used as a radiation source. It was placed inside the duct of a collimator composed of a lead cylinder surrounded by a borated paraffin cylinder. The samples were placed at the inside end of the duct inside the collimator to prevent any scattering or background from the surroundings. A 7 cm thick piece of paraffin was placed between the source and the samples for the thermalization of fast neutron from the source. A schematic diagram of the experimental arrangement is shown in Fig. 1. As the sample thickness is about 6.6 cm, its effect on the attenuation of fast neutrons is very small, so it was decided to study the attenuation of both slow neutrons and y-rays. For this purpose a Harshow thermoluminescent (TLD) LiF-600 together with LiF-700 ribbons were used, respectively. The TLD ribbons have the dimensions 3.1 X 3.1 X 0.89 mm. The TLD LiF-600, which is composed almost entirely of LiF-6 isotope and about 4% LiF-7, is an efficient dosimeter for slow neutrons. On the other hand, TLD700 is an efficient y-ray dosimeter and is nearly transparent to thermal neutrons when they are detected together. The efficiency of the dosimeters to haC~ relative to LiF is 1.0 for both LiF-600 and LiF-700. By direct subtraction of the TL response of the LX-600 dosimeter, the net response obtained is due to slow neutrons. Before using the TLD dosimeters, a preirradiation annealing at 400°C for 1 h followed by 2 h at lOO”C, then a post-irradiation and pre-read annealing at 100°C for 10 min, have been carried out. A TLD readout instrument (Teledyne Isotopes 7100) was used.
50Cm7
2.3 Electrical properties
_
L
’ ;1berated
5
Pu---
Fig. 1. Experimental
7 cm length
The electrical conductivity of the composite samples was measured using a Multi Mega Ohm meter type MOM11 and a measuring cell 0DW2 (WTW Co., Germany) at various temperatures from 30 to 150°C.
paraffin
Lead
Paraftln(
amp+5
2.4 Thermal properties )
Eie(Source)
configuration.
The determination of the thermal properties, namely thermal conductivity, specific heat and
231
EPDM /LDPE /boron carbide composites as neutron shields
thermal diffusivity of the samples, in the temperature range 30-15O”C, was carried out using the flash method. The technique is described elsewhere.14
3 RESULTS AND DISCUSSION The radiation attenuation for y-rays and slow neutrons in EPDM/LDPE blend samples as well as those including B4C, as a function of their thickness, are illustrated in Figs 2 and 3, respectively. It can be seen from Fig. 2 that the dependence of TL response for y-rays on sample thickness is represented by straight lines. The y-ray linear attenuation coefficients (m) were calculated from the slope of the straight lines in Fig. 2. The relaxation lengths were obtained from the relation A = l/p. In addition, the half thickness values for y-rays were obtained from the relation x1/2 = 0.6931~. The linear attenuation coefficient, the relaxation length and the half thickness value for the different samples were calculated and are presented in Table 2. From this table, it can be seen that the addition of B,C to the matrix decreases its y-ray shielding efficiency and this decrease is proportional to the concentration of the B,C added. This result may be attributed to the absorption of the thermal neutrons in the shield material by the incorporated BX, which leads to the production of secondary captured y-photons. Thus, the captured y-dose rate at the shield surface is the
10 0
1
2
3
4
5
Fig. 3. The change of attenuation EPDM/LDPE/B,C composites as thickness.
of slow neutrons in a function of their
dominant consideration in a shield formulation; this will be considered in another work. Figure 3 shows the TL response of slow neutrons as a function of the sample thickness. It can be seen that the initial 1.5 cm thickness of the borated samples sharply reduced the initial direct slow neutron flux by about 85%, followed by an exponential decrease in thickness. The sudden decrease of the thermal neutron flux may be due to their absorption in the B,C. The total cross-section of boron for neutrons is very high and has a value of 1200 barns at E, = 0.01 eV and decreases exponentially with increase of the neutron energy until it reaches 5 barns at E,= 100 eV.’ The exponential decrease of the slow neutron flux beyond the initial thickness ( - 1.5 cm) of the borated samples may be due to the attenuation of the generated thermalized neutrons in the sample. The 7 cm of paraffin between the source and the tested sample seems not to have thermalized all the fast neutrons from the source. Hence, the thermal neutrons in the shield sample arise from the thermalization of the
Sample
3
4
5
6
7
Thickness,cm Fig. 2. The change of attenuation of capture of y-rays in EPDM/LDPE/B,C composites as a function of their thickness.
7
Thickness, cm
Table 2. Gamma-attenuation characteristics EPDW/LDPE composites
2
6
___~. S” S, S,
Density
B,C
0.88 1.36 1.46
47 57
of B,C loaded
Xl/Z (cm) 0.152 0.145 0.1337
6.579 6.896 7.477
4.56 4.78 5.18
238
M. M. Abdel-Aziz
penetrated fast neutrons as well as thermal energy neutrons incident on the shield’s surface, The total macroscopic cross-section for the S, sample E = 0.215 cm-l and its relaxation length A = 4.65 cm, whereas the total macroscopic crosssection for the Sl sample Z: = 0.1385 cm-l and its relaxation length h = 7.22 cm. So we can say that from the shielding point of view, the slow neutron attenuation of the S, sample with 57 wt% of B,C is about 45% higher than the S, sample (47 wt%, B,C). The slow neutron distribution in the control sample (without B,C) is also shown in Fig. 3. It is clear that in this case the slow neutron flux shows no change up to a thickness of about 4 cm in the sample, as the decrease of the direct slow neutron flux is compensated by the new generated thermalized neutrons. Beyond this thickness the flux of slow neutrons decreases exponentially with increase of the sample thickness. Generally it may be concluded that most of the total macroscopic cross-section (E) for slow neutrons as well as the y-ray attenuation were in the first 1.5 cm layer of the borated samples. After that, the rate of attenuation decreased with increasing thickness of the borated samples. This y-rays is probably due to new secondary generated as a result of thermal neutron capture as well as new thermalized neutrons, which compensate for the loss of radiation in the initial layer.
0.1 r’ ”
”
20
40
”
” 60
”
” 80
”
” 100
”
”
”
”
120 140
et al.
4.5 0 Q 4 Go ;; . z 3,s 2 % * .* k:
3-
t 2.5 2~““““““‘““““““‘1~ 20
40
60
80
100
120
140
160
Temperature,‘C Fig. 5. Temperature EPDM/LDPE blend
dependence of specific heat for loaded with two different B,C contents.
Neutron and photon heating of radiation shielding material is of concern not only because of the introduction of thermal stresses but also because of other deteriorative effects on materials when the temperature rises. Hence, the effect of temperature on the electrical and thermal properties of the composite samples in the temperature range 30-150°C has been studied.
’
160
Temperature, ‘C Fig. 4. Temperature dependence of thermal conductivity for EPDM/LDPE blend loaded with two different B,C contents.
20
40
80
80 100 Temperature,‘C
120
140
150
Fig. 6. Temperature dependence of thermal diffusivity for EPDM/LDPE blend loaded with two different B,C contents.
EPDMILDPElboron
carbide composites as neutron shields
The effect of temperature as well as the incorporation of B,C on the thermal properties, namely, thermal conductivity, specific heat and thermal diffusivity, of the tested samples, are shown in Figs 4-6, respectively. Figure 4 shows that the thermal conductivity (k) of the matrix sample (S,) slightly decreases with temperature over the range studied. Addition of B,C powder filler increased the conductivity, but the increase was not a simple arithmetic average of the thermal conductivities of the B,C ( - 120 w/m”C, at 100°C),lo and the matrix (0.2-0.26 w/m”C). At the two concentrations the rubber matrix apparently isolated the B,C particles and maintained appreciable thermal resistance. In addition, the thermal conductivity of the borated samples is almost independent of temperature over the range studied, increasing only slightly with temperature. Also, the conductivity increased with increasing B,C content. Roughly, the temperature dependence of thermal conductivities of the borated samples S, and S, can be interpreted as corresponding to a near-constant mean free path, whence the thermal conductivity follows the increase in specific heat (C,) with temperature (Fig. 5). It can be seen that, while the C, value of So exponentially decreased with temperature, the values of S, and S, slightly increased, following the same sample trend as thermal conductivity values. The thermal diffusivity value (a) of the matrix slightly increased with temperature (Fig. 6). The
0
100
rz
20
40
60
80
100 120 140 160
Temperahq ‘C Fig. 7. Temperature
for EPDM/LDPE
dependence of electrical conductivity blend loaded with two different B,C contents.
239
incorporation of 47 wt% of B,C raises the C, value of the matrix about 2.7 times, and it seems Increasing the independent of temperature. concentration of B,C to 57 wt% in the matrix decreased the C, value, but it is still higher than that of the matrix. The dependence of dc-conductivity of the composite samples on temperature is illustrated in Fig. 7. It can be seen that the electrical conductivity (u) of sample S,, steadily increased with the temperature. The incorporation of 47 wt% B,C (S,) caused an increase in CTof S,, of about five orders of magnitude and apparently its value changes slightly with temperature. On the other hand, increasing the concentration of B,C to 57 wt% (S,) resulted in no additional increase in 0. Hence, the appreciable improvement in the (T value of the borated samples makes them feasible as electromagnetic interface shields in addition to their function as slow neutron shields. Much importance is attributed to the ability of radiation shields to sustain an increased temperature without appreciable reduction of the operating reliability as well as their physical and chemical properties.
4 CONCLUSIONS An evaluation of EPDM and LDPE composites with two different concentrations of B,C (47 and 57 wt%) as radiation shields for slow neutrons and y-rays was carried out. The effect of temperature (30-150°C) on the electrical conductivity and the thermal properties-thermal conductivity, specific heat and thermal diffusivity-of the composite samples was investigated. The first 1.5 cm layer of the composite samples sharply reduced the initial direct slow neutron flux by about 85%, followed by a total macroscopic cross-section Z = 0.215 cm-’ for the composite containing 57 wt% B,C compared to X = 0.1385 cm-’ for the composite sample containing 47 wt% B,C. The incorporation of B,C into the EPDM/LDPE matrix improves its thermal properties. The electrical conductivity is raised by about five orders of magnitude. The thermal and electrical conductivities of the borated samples are nearly independent of temperature in the range studied.
240
M. M. Abdel-Aziz
REFERENCES
1. Chilton, A. B., Shultis, J. K. & Faw, R. E., Principles of Radiation Shielding. Prentice-Hall, Englewood Cliffs, NJ, 1984. 2. Fano, U., Spencer, L. V. & Berger, M. J., In Handbuch der Physik, Vol. 3812, ed. S. Flugge, p, 660. Springer-Verlag, Berlin, 1959. 3. Spencer, L. V., Chilton, A. B. & Eisenhauer, C. M., Structure shielding against fallout gamma rays from nuclear detonation, NBS Special Publication 570, United States Government Printing Office, Washington, DC, 1980. 4. Rockwell, T., III (ed.), Reactor Shielding Design Manual. Van Nostrand, Princeton, NJ, 1956. 5. Price, B. T., Horten, C. C. & Spinney, K. T., Radiation Shielding. Pergamon, Elmsford, NY, 1957. 6. Goldstein, H., Fundamental Aspects of Rector Shielding.
et al.
Addison-Wesley, Reading, MA, 1959. 7. Jaeger, T., Principles of Radiation Protection Engineering. MC Graw-Hill, New York, 1965. 8. Blizared, E. P. (ed.), Reactor Handbook, Vol. III, Part B: Shielding. Interscience, New York, 1962. 9. Schaeffer, M. M. (ed.), Reactor Shielding for Nuclear Engineers, TID-25951. National Technical Information Service, Springfield, VA, 1973. 10. Jaeger, R. G. (ed.), Engineering Compendium on Radiation Shielding, Vols l-3. Springer-Verlag, New York, 1968-1975. 11. Profio, A. E., Radiation Shielding and Dosimetry. Wiley, New York, 1979. 12. Abdel-Aziz, M. M., Badran, A. S., Abdel-Hakem, A. A., Helaly, F. M. & Moustafa, A. B., J. Appl. Polym. Sci., 32 (1991) 1073. 13. Abdel-Aziz, M. M., Abdel-Bary, E. M., Abou Zaid, M. M. & El Miligy, A. A., J. Elast. Plast., 2-4 (1992). 14. Gwaily, S. E., Nasr, G. M., Badaway, M. M. & Hassan, H. H., Polym. Deg. Stab., 47 (1995) 391.
ELSEVlER
Degradation
and Stability
50 (1995)
235-240
Elsevier Science Limited Printed in Northern Ireland 0141.3910/95/$09,50
0141-3910(95)00177-S
Ethylene-propylene diene rubber/low density polyethylene/boron carbide composites as neutron shields M. M. Abdel-Aziz,” S. E. Gwaily,” A. S. Makarious* & A. El-Sayed Abdob *National Center of Radiation Research and Technology, A.E.A., 3 Ahmed El-zumer, Gziro, Egypt bNuclear Research Center, A.E.A., Cairo, Egypt
8th Sector, Nasr City, PO Box 29,
(Received 4 July 1995; accepted 22 July 1995)
Composites of ethylene-propylene diene rubber and low density polyethylene were prepared with two different concentrations of boron carbide powder, namely, 47 and 57 wt%. The composites were investigated for their gamma and slow neutron radiation shielding properties. In addition, the temperature dependence of the electrical and thermal properties of the composites in the temperature range 30-150°C has been studied. The initial 1.5 cm thickness of the composite sample containing 57 wt% boron carbide sharply reduced the initial direct slow neutron flux by about 85%. The total macroscopic cross-section of this sample is 0.215 cm-‘.
1 INTRODUCTION
the USA) Radiation Shielding Information Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee; National Council on Radiation Protection and Measurements, Beheads, Maryland; Radiation Protection and Shielding Division of the American Nuclear Society, La Grange Park, Illinois; (in Europe) OECD Nuclear Energy Agency Data Bank, Gif-Sur-Yvette, France; and the European Shielding Information Services, Ispra, Italy. Boron carbide (B4C) is a widely used shielding material, which may be used alone, as a powder (density about 1.2 g cmp3), or in a hot-pressed (sintered) form with densities as high as 2.5 g cmp3. Borated plastics and wood-based particle board are also widely used in special neutron shielding applications. Hence, a trial was made to prepare a composite material consisting of an ethylenepropylene diene rubber (EPDM)/low density polyethylene (LDPE) blend with two different concentrations of boron carbide (B4C), to be tested as shielding materials for thermal neutrons. The compositions of these composites are given in Table 1. The electrical and thermal properties of these composites were also studied.
Many types of radiation, such as neutrons and x-ray or y-ray photons, cause ionization of the media with which they interact, through a complicated mechanism involving the emission of energetic secondary charged partic1es.l The ionizing ability of these types of radiation is the reason for the importance of studying shields. A shield is a physical entity interposed between a source of ionizing radiation and an object to be protected, such that the radiation level at the position of the object will be reduced. The object to be protected is most often a human being, but it can be anything that is sensitive to ionizing radiation. Significant textbooks and handbooks providing broad coverage on shielding are available.2~” In addition, many reports, monographs and journal articles’* have been published on various particular aspects of nuclear radiation shielding. Of particular significance as sources of such information are the following organizations: (in * To whom all correspondence
should be addressed. 235
236
M. M. Abdel-Aziz
Table 1. Composition
Ingredients 1. 2. 3. 4. 5. 6. 7.
(phr)
LDPE” EPDMb Boron carbide ZnO Stearic acid PbNP Bpd
of B,C loaded composites
SO 50 50 5 1 1 3
EPDM/LDPE
Sl
S*
50 50 100 (47 wt%) 5 1 1 3
50 150 (Zvt%) 5 1 1 3
Ingredients listed in order of addition during preparation. “From Dow Co., Spain. ‘Vistalone 5600, Belgium. ‘Antioxidant. dBenzoyl peroxide as curing agent.
2 EXPERIMENTAL 2.1 Sample preparation All rubber/plastic mixes were prepared as previously described.13 The EPDM/LDPE mixes were first prepared on a two roll rubber mill at 80°C for 10 min. The rest of the ingredients were then added, except for the curing agent (BP), which was added after cooling the recipe to about 50°C. The total duration of mixing was 30 min. Cylindrical shaped samples of 6 cm diameter and about 5 mm thickness were compression molded using a Carver hot press at 160°C and 60 kg/cm* pressure for 20 min. A group of these circular samples was arranged together to form an assembly about 6.6 cm thick for the attenuation measurements.
-
c--i
100Cm T
et al.
2.2 Attenuation measurements A Pu-a-Be source (2.5 curie) was used as a radiation source. It was placed inside the duct of a collimator composed of a lead cylinder surrounded by a borated paraffin cylinder. The samples were placed at the inside end of the duct inside the collimator to prevent any scattering or background from the surroundings. A 7 cm thick piece of paraffin was placed between the source and the samples for the thermalization of fast neutron from the source. A schematic diagram of the experimental arrangement is shown in Fig. 1. As the sample thickness is about 6.6 cm, its effect on the attenuation of fast neutrons is very small, so it was decided to study the attenuation of both slow neutrons and y-rays. For this purpose a Harshow thermoluminescent (TLD) LiF-600 together with LiF-700 ribbons were used, respectively. The TLD ribbons have the dimensions 3.1 X 3.1 X 0.89 mm. The TLD LiF-600, which is composed almost entirely of LiF-6 isotope and about 4% LiF-7, is an efficient dosimeter for slow neutrons. On the other hand, TLD700 is an efficient y-ray dosimeter and is nearly transparent to thermal neutrons when they are detected together. The efficiency of the dosimeters to haC~ relative to LiF is 1.0 for both LiF-600 and LiF-700. By direct subtraction of the TL response of the LX-600 dosimeter, the net response obtained is due to slow neutrons. Before using the TLD dosimeters, a preirradiation annealing at 400°C for 1 h followed by 2 h at lOO”C, then a post-irradiation and pre-read annealing at 100°C for 10 min, have been carried out. A TLD readout instrument (Teledyne Isotopes 7100) was used.
50Cm7
2.3 Electrical properties
_
L
’ ;1berated
5
Pu---
Fig. 1. Experimental
7 cm length
The electrical conductivity of the composite samples was measured using a Multi Mega Ohm meter type MOM11 and a measuring cell 0DW2 (WTW Co., Germany) at various temperatures from 30 to 150°C.
paraffin
Lead
Paraftln(
amp+5
2.4 Thermal properties )
Eie(Source)
configuration.
The determination of the thermal properties, namely thermal conductivity, specific heat and
231
EPDM /LDPE /boron carbide composites as neutron shields
thermal diffusivity of the samples, in the temperature range 30-15O”C, was carried out using the flash method. The technique is described elsewhere.14
3 RESULTS AND DISCUSSION The radiation attenuation for y-rays and slow neutrons in EPDM/LDPE blend samples as well as those including B4C, as a function of their thickness, are illustrated in Figs 2 and 3, respectively. It can be seen from Fig. 2 that the dependence of TL response for y-rays on sample thickness is represented by straight lines. The y-ray linear attenuation coefficients (m) were calculated from the slope of the straight lines in Fig. 2. The relaxation lengths were obtained from the relation A = l/p. In addition, the half thickness values for y-rays were obtained from the relation x1/2 = 0.6931~. The linear attenuation coefficient, the relaxation length and the half thickness value for the different samples were calculated and are presented in Table 2. From this table, it can be seen that the addition of B,C to the matrix decreases its y-ray shielding efficiency and this decrease is proportional to the concentration of the B,C added. This result may be attributed to the absorption of the thermal neutrons in the shield material by the incorporated BX, which leads to the production of secondary captured y-photons. Thus, the captured y-dose rate at the shield surface is the
10 0
1
2
3
4
5
Fig. 3. The change of attenuation EPDM/LDPE/B,C composites as thickness.
of slow neutrons in a function of their
dominant consideration in a shield formulation; this will be considered in another work. Figure 3 shows the TL response of slow neutrons as a function of the sample thickness. It can be seen that the initial 1.5 cm thickness of the borated samples sharply reduced the initial direct slow neutron flux by about 85%, followed by an exponential decrease in thickness. The sudden decrease of the thermal neutron flux may be due to their absorption in the B,C. The total cross-section of boron for neutrons is very high and has a value of 1200 barns at E, = 0.01 eV and decreases exponentially with increase of the neutron energy until it reaches 5 barns at E,= 100 eV.’ The exponential decrease of the slow neutron flux beyond the initial thickness ( - 1.5 cm) of the borated samples may be due to the attenuation of the generated thermalized neutrons in the sample. The 7 cm of paraffin between the source and the tested sample seems not to have thermalized all the fast neutrons from the source. Hence, the thermal neutrons in the shield sample arise from the thermalization of the
Sample
3
4
5
6
7
Thickness,cm Fig. 2. The change of attenuation of capture of y-rays in EPDM/LDPE/B,C composites as a function of their thickness.
7
Thickness, cm
Table 2. Gamma-attenuation characteristics EPDW/LDPE composites
2
6
___~. S” S, S,
Density
B,C
0.88 1.36 1.46
47 57
of B,C loaded
Xl/Z (cm) 0.152 0.145 0.1337
6.579 6.896 7.477
4.56 4.78 5.18
238
M. M. Abdel-Aziz
penetrated fast neutrons as well as thermal energy neutrons incident on the shield’s surface, The total macroscopic cross-section for the S, sample E = 0.215 cm-l and its relaxation length A = 4.65 cm, whereas the total macroscopic crosssection for the Sl sample Z: = 0.1385 cm-l and its relaxation length h = 7.22 cm. So we can say that from the shielding point of view, the slow neutron attenuation of the S, sample with 57 wt% of B,C is about 45% higher than the S, sample (47 wt%, B,C). The slow neutron distribution in the control sample (without B,C) is also shown in Fig. 3. It is clear that in this case the slow neutron flux shows no change up to a thickness of about 4 cm in the sample, as the decrease of the direct slow neutron flux is compensated by the new generated thermalized neutrons. Beyond this thickness the flux of slow neutrons decreases exponentially with increase of the sample thickness. Generally it may be concluded that most of the total macroscopic cross-section (E) for slow neutrons as well as the y-ray attenuation were in the first 1.5 cm layer of the borated samples. After that, the rate of attenuation decreased with increasing thickness of the borated samples. This y-rays is probably due to new secondary generated as a result of thermal neutron capture as well as new thermalized neutrons, which compensate for the loss of radiation in the initial layer.
0.1 r’ ”
”
20
40
”
” 60
”
” 80
”
” 100
”
”
”
”
120 140
et al.
4.5 0 Q 4 Go ;; . z 3,s 2 % * .* k:
3-
t 2.5 2~““““““‘““““““‘1~ 20
40
60
80
100
120
140
160
Temperature,‘C Fig. 5. Temperature EPDM/LDPE blend
dependence of specific heat for loaded with two different B,C contents.
Neutron and photon heating of radiation shielding material is of concern not only because of the introduction of thermal stresses but also because of other deteriorative effects on materials when the temperature rises. Hence, the effect of temperature on the electrical and thermal properties of the composite samples in the temperature range 30-150°C has been studied.
’
160
Temperature, ‘C Fig. 4. Temperature dependence of thermal conductivity for EPDM/LDPE blend loaded with two different B,C contents.
20
40
80
80 100 Temperature,‘C
120
140
150
Fig. 6. Temperature dependence of thermal diffusivity for EPDM/LDPE blend loaded with two different B,C contents.
EPDMILDPElboron
carbide composites as neutron shields
The effect of temperature as well as the incorporation of B,C on the thermal properties, namely, thermal conductivity, specific heat and thermal diffusivity, of the tested samples, are shown in Figs 4-6, respectively. Figure 4 shows that the thermal conductivity (k) of the matrix sample (S,) slightly decreases with temperature over the range studied. Addition of B,C powder filler increased the conductivity, but the increase was not a simple arithmetic average of the thermal conductivities of the B,C ( - 120 w/m”C, at 100°C),lo and the matrix (0.2-0.26 w/m”C). At the two concentrations the rubber matrix apparently isolated the B,C particles and maintained appreciable thermal resistance. In addition, the thermal conductivity of the borated samples is almost independent of temperature over the range studied, increasing only slightly with temperature. Also, the conductivity increased with increasing B,C content. Roughly, the temperature dependence of thermal conductivities of the borated samples S, and S, can be interpreted as corresponding to a near-constant mean free path, whence the thermal conductivity follows the increase in specific heat (C,) with temperature (Fig. 5). It can be seen that, while the C, value of So exponentially decreased with temperature, the values of S, and S, slightly increased, following the same sample trend as thermal conductivity values. The thermal diffusivity value (a) of the matrix slightly increased with temperature (Fig. 6). The
0
100
rz
20
40
60
80
100 120 140 160
Temperahq ‘C Fig. 7. Temperature
for EPDM/LDPE
dependence of electrical conductivity blend loaded with two different B,C contents.
239
incorporation of 47 wt% of B,C raises the C, value of the matrix about 2.7 times, and it seems Increasing the independent of temperature. concentration of B,C to 57 wt% in the matrix decreased the C, value, but it is still higher than that of the matrix. The dependence of dc-conductivity of the composite samples on temperature is illustrated in Fig. 7. It can be seen that the electrical conductivity (u) of sample S,, steadily increased with the temperature. The incorporation of 47 wt% B,C (S,) caused an increase in CTof S,, of about five orders of magnitude and apparently its value changes slightly with temperature. On the other hand, increasing the concentration of B,C to 57 wt% (S,) resulted in no additional increase in 0. Hence, the appreciable improvement in the (T value of the borated samples makes them feasible as electromagnetic interface shields in addition to their function as slow neutron shields. Much importance is attributed to the ability of radiation shields to sustain an increased temperature without appreciable reduction of the operating reliability as well as their physical and chemical properties.
4 CONCLUSIONS An evaluation of EPDM and LDPE composites with two different concentrations of B,C (47 and 57 wt%) as radiation shields for slow neutrons and y-rays was carried out. The effect of temperature (30-150°C) on the electrical conductivity and the thermal properties-thermal conductivity, specific heat and thermal diffusivity-of the composite samples was investigated. The first 1.5 cm layer of the composite samples sharply reduced the initial direct slow neutron flux by about 85%, followed by a total macroscopic cross-section Z = 0.215 cm-’ for the composite containing 57 wt% B,C compared to X = 0.1385 cm-’ for the composite sample containing 47 wt% B,C. The incorporation of B,C into the EPDM/LDPE matrix improves its thermal properties. The electrical conductivity is raised by about five orders of magnitude. The thermal and electrical conductivities of the borated samples are nearly independent of temperature in the range studied.
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