- Email: [email protected]
Ion beam analysis of materials in the PBMR reactor
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1373–1377 www.elsevier.com/locate/nimb
Ion beam analysis of materials in the PBMR reactor Johan B. Malherbe *, E. Friedland, N.G. van der Berg Department of Physics, University of Pretoria, Lynnwood Road, Pretoria 0002, South Africa Received 18 September 2007 Available online 8 December 2007
Abstract South Africa is developing a new type of high temperature nuclear reactor, the so-called pebble bed modular reactor (PBMR). The planned reactor outlet temperature of this gas-cooled reactor is approximately 900 °C. This high temperature places some severe restrictions on materials, which can be used. The name of the reactor is derived from the form of the fuel elements, which are in the form of pebbles, each with a diameter of 60 mm. Each pebble is composed of several thousands of coated fuel particles. The coated particle consists of a nucleus of UO2 surrounded by several layers of different carbons and SiC. The diameter of the fuel particles is 0.92 mm. A brief review will be given of the advantages of this nuclear reactor, of the materials in the fuel elements and their analysis using ion beam techniques. Ó 2007 Elsevier B.V. All rights reserved. PACS: 28.41. i; 28.41.Bm; 28.50. k; 28.52.Fa; 61.72.Hh; 61.80. x; 61.80.Hg; 61.80.Jh; 61.72.Yx Keywords: Ion beam analysis; Nuclear fission reactor; Materials; RBS; Channeling
1. Introduction The present energy resource scare has prompted a renewed interest in nuclear fission power plants. Fig. 1 shows that in 2004 nuclear power represented only 6.2% of the world’s total annual energy consumption of 4.7 1020 J (=1.3 1014 kW h) [1]. The dominant source of energy is fossil fuel in its different forms. However, it follows from Table 1 that the fossil fuel reserves, with the exception of coal, are limited [1]. A recent study [2] of the oil reserves paints a very bleak picture. Although the reserves of the fission elements are also limited, it is expected that fission nuclear power plants will have to substitute the phasing out of fossil fuel power plants until alternative energy sources take over as the main suppliers. The attractiveness of using nuclear energy is further enhanced by the current debate about the reality and
*
Corresponding author. Tel.: +27 12 420 2896; fax: +27 12 362 5288. E-mail address: [email protected] (J.B. Malherbe).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.046
causes of global warming by the emission of carbon dioxide. Two key reasons for the unpopularity of fission nuclear power plants are nuclear waste management and the possibility of an uncontrollable explosive incident such as the Chernobyl accident. These and other factors have prompted the US Department of Energy’s Office of Nuclear Energy, Science and Technology to engaged governments, industry and the research community worldwide in a wide-ranging discussion on the development of next generation nuclear energy systems the so-called Generation IV power plants [3]. These reactors represent advances in sustainability, economics, safety, reliability and proliferation-resistance. An international task force is developing six Generation IV nuclear reactor technologies for deployment between 2010 and 2030 [4]. All of these operate at higher temperatures than today’s reactors. The pebble bed modular reactor (PBMR) is a relatively new design in nuclear power plants and could be the first successful Generation IV power plant. There are several designs of high temperature gas-cooled reactors pursued by researchers in different countries such as the USA, Rus-
1374
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
Fig. 1. A breakdown of the world energy consumption in 2004. In the pie chart Renewable denotes gas and liquid fuel obtained from plant material. Raw data taken from [1].
Table 1 The world’s proven fossil fuel resources (in terms of energy content) in 2004, which can be recovered profitably with current technology and energy production (consumption) in 2004 Resource
World reserves (1021 J)
World production (1021 J)
Lifetimeb (years)
Oil Natural gas Coala
6.852 6.569
0.1633 0.1078
42 61
26.32
0.1195
26.32
Table adapted from data in [1]. a The 2002 coal reserves are given. b Lifetime: World Reserves/World Production.
sia, Japan and The Netherlands. The PBMR programmes of China and South Africa are the most advanced. Both of them have expressed the intension of commissioning their reactors within the next decade. This paper describes some of the salient features of the South African design of the PBMR and shows how ion beams are employed in research on materials used in the reactor.
Fig. 2. Neutron fission cross-section of the uranium isotope [5].
235
U. From
neutron energy dependence of the fission reaction crosssection of the 235U nucleus [5] is shown in Fig. 2. Although the reaction cross-section increases significantly for thermal neutrons, there are several peaks in the reaction cross-section for higher energy neutrons. The PBMR design utilizes the neutron absorption in these peaks as an inherent safety self regulation mechanism. The width of the reaction crosssection peak is an inherent self-regulating mechanism. If the temperature of the fuel increases the parasitic neutron absorption increases and causes a decrease in the reactivity of the core and subsequently the rate of the fission. The enormously long radioactivity of the fission products and especially of the actinides for a conventional light water reactor [6] is depicted in Fig. 3. Consequently, the safe storage of the nuclear waste is a problem, which has been and is being investigated in depth by various groups. The PBMR design handles this problem by building protective containment layers around the small UO2 kernels. The structure of the PBMR fuel element spheres is shown in Fig. 4. This structure of the fuel pebbles known as the TRISO particle consists of a round UO2 kernel of approx-
2. South African PBMR The South African PBMR is based on an original German prototype, which successfully operated for about 21 years in Ju¨lich, Germany. It is direct cycle reactor with a planned reactor outlet temperature of about 900 °C. The coolant, which also drives the power generating turbines (Brayton cycle), must be a chemical inert gas such as helium or nitrogen or carbon dioxide, with helium being the preferred choice because of its superior heat transfer capacity, its chemical inertness and very low neutron capture cross-section. It is, however, much more expensive than nitrogen and carbon dioxide and difficult to retain in a vessel. Graphite is used as moderator material. The
Fig. 3. The time dependence of the total radioactivity of nuclear reactor waste from a light water reactor. From [6].
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
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riers for the fission products and to be barriers for chemically reactive species used in the manufacturing processes. The reasons for adding a SiC layer in the TRISO particle are to provide mechanical support and structural rigidity to the coated particle and act as a containment layer for the radioactive waste. In practice this means that SiC acts as a diffusion barrier for the radioactive waste elements. Furthermore, the chemically inert SiC layer also acts a barrier for leaching from the outside during long term storage after its removal from the Spent Fuel Storage Tank. 3. Ion beam analysis of materials used in the PBMR reactor
Fig. 4. An SEM image of a cross-section of the TRISO particle, showing the UO2 kernel with the four surrounding layers. OPyC denotes the outer pyrolytic carbon layer and IPyC the inner pyrolytic carbon layer.
imately 0.5 mm diameter. This kernel is surrounded by four layers making the final diameter of the TRISO particle approximately 0.92 mm. The inner layer consisting of a porous graphite buffer layer absorbs gaseous fission products and accommodates thermal expansion and swelling of the UO2 kernel [8]. A pyrolytic carbon layer is grown on top of this. Due to the low magnification in Fig. 4, the distinction between the buffer and inner pyrolytic carbon layer (IPyC) is not visible. Next a SiC layer is grown, with the final outer layer consisting again of pyrolytic carbon (OPyC). Approximately 15,000 of these TRISO particles are compressed into a sphere (also called a fuel pebble) of about a tennis ball diameter using a graphite bonder. The total amount of uranium in the fuel sphere is about 9 g. This small amount of uranium leads to low concentrations of radioactive waste. Furthermore, the absorption of neutrons in the resonance peaks of 238U leads to the production of Plutonium. However, due to the long residence time of the fuel pebbles in the core the quality of the Plutonium in terms of creating a reliable nuclear explosive is very poor. Consequently, the probability of spent fuel being stolen by groups to use in a ‘‘dirty” bomb becomes very small. The fuel spheres moves slowly through the reactor. A schematic chart of the path and the reactor is shown on the webpage of PBMR [7]. This process is repeated several times until the fissionable 235U concentration left in the fuel sphere has decreased below its designed concentration. The sphere is then stored in a tank below the reactor until the initial high radioactivity has decreased to acceptable levels for handling and external storage. The main functions of pyrolytic carbon layers are to protect the SiC layer and to act as additional diffusion bar-
This paper deals with materials research on mainly the materials contained in the fuel pebbles. As was mentioned above, the operating temperature of the South African PBMR reactor is about 900 °C. This means that the temperatures inside the TRISO particles are even higher. Very little research has been done on materials at these high temperatures and under the radiation conditions in a nuclear reactor. One reason for this is the lack of test stations in research nuclear reactors, which can attain such high temperatures. Radiation damage caused by the neutron flux in a nuclear reactor is of practical and economical consequences for the operation of a reactor. Materials in a nuclear reactor are subjected to a complex neutron spectrum, which depends on the reactor’s design and operational mode. In the PBMR reactor the neutron energies span a range from less than 0.001 eV to more than 10 MeV with a maximum differential flux inside fuel kernels at 0.1 eV. The fast neutron flux (E > 100 keV), which is mainly responsible for radiation damage, is roughly 30% of the total neutron flux and reaches a maximum of approximately 7 1013 n cm 2 s 1 in the central core regions. The ion beam channeling technique has in recent years become a workhorse to study radiation damage in crystalline materials and their annealing behaviour [9]. The technique has a number of advantages for these kinds of investigations. They include the ease of operation and analysis in a well-designed experimental set-up. In principle it is also possible to identify the main type of defects. It is well known that extensive radiation damage can cause material failure in a nuclear reactor. In the PBMR reactor graphite is used as reflector and moderator. The anisotropic nature of the crystalline structure of graphite leads to anisotropic effects of radiation damage. Anisotropic radiation-induced swelling [10] of the graphite moderator blocks can cause mechanical transfer problems. Later we shall discuss the effects of radiation-induced and radiation-enhanced diffusion in the PBMR reactor. A range of defects is created by neutron bombardment. Let us consider SiC as an example material for the kind of damage, which can be created. Assuming an average displacement energy of 30 eV for SiC, the minimum neutron energy to create a Frenkel pair directly in a head-on
1376
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
collision is 180 eV for carbon and 420 eV for silicon atoms. However, point defects can also be produced indirectly by thermal neutrons via nuclear reactions. Neutron capture reactions, which have relatively large cross-sections at thermal energies, leave the compound nuclei 13C, 14C, 29Si, 30Si and 31Si in highly excited states. The de-excitation by cradiation transfers recoil energies of a few hundred eV to these nuclei, which is sufficient to produce a couple of Frenkel pairs. In addition 14C and 31Si are unstable and produce impurity atoms 14N and 31P by b-decay. Contributions by (n,p) and (n,a) reactions can probably be neglected as their Q-values for all stable C and Si isotopes are negative. Only the high energy tail of the neutron spectrum is involved and the production of defects and impurity atoms due to these reactions will be comparatively small. The produced hydrogen and helium atoms tend to agglomerate at previously created vacancy clusters and might eventually grow into relatively large gas bubbles. The embrittlement observed in some materials used for the construction of reactor pressure vessels is probably due to this mechanism. Interstitial atoms and vacancies in SiC are highly mobile at temperatures above 200 °C and will in most cases either recombine or migrate to the surface, although some will form relatively stable point defect clusters. The formation of stacking fault tetrahedrons by vacancies is of special importance, as their production rate seems to increase at high temperatures. However, most damage is expected from atomic collisions with fast neutrons, which can produce large collision cascades. In a dense collision cascade the interstitial atoms tend to move to the periphery, creating a vacancy-rich core, which usually collapses to form a rather stable extended defect. However, in relatively open collision cascades, which typically form in fast neutron collisions with light target atoms, a large fraction of the displaced atoms will recombine with vacancies and the final defect density is significantly less than estimated by standard computer codes using binary collision approximations. Molecular dynamic calculations could in principle determine the final damage distribution, but the enormous computing power needed to obtain statistically significant results limits severely its applicability. Simulation of neutron radiation damage by energetic ions presents itself as a cost effective and time saving possibility and has therefore been studied in many laboratories. However, in applying this method one has to be aware of the fundamental differences of interactions by neutrons and ions with target atoms. The high operating temperature of the PBMR reactor is a big advantage to minimise radiation damage. In Fig. 5 aparticle channeling spectra are shown of 360 keV Ag+ ions implanted into 6H-SiC. In (a) the implantation was done at room temperature and a comparison between the channeled spectrum and the random direction spectrum clearly shows that the single crystal SiC substrate was amorphised from the surface to the implanted depth. In Fig. 5(b) the SiC substrate was at 600 °C during implantation. In this
Fig. 5. Random and a-particle channeling spectra of single crystal 6H-SiC implanted with 360 keV Ag+ ions to a fluence of 2 1016 Ag+ cm 2. In (a) the SiC was at room temperature during implantation while in (b) it was at 600 °C.
case the SiC remains crystalline with only limited dechanneling taking place due to the implanted silver atoms. During initial orientation and analysis of virgin single crystals they were typically irradiated by the analyzing beam with a fluence of 1017 He+ cm 2 at room temperature, which creates approximately 0.1 displacements per atom (dpa) in the sub-surface region. Although a 10% lattice disorder should be easily detected by channeling spectroscopy, absolutely no damage was observed. Obviously the point defect annealing rate at 300 K is more than sufficient to prevent any measurable defect accumulation at a flux of approximately 1013 He+ cm 2 s 1. This is no surprise as the first annealing stage in SiC occurs at 80 K and interstitial atoms are highly mobile at room temperature. It was mentioned that a big advantage of the PBMR technology is that the radioactive waste products are contained within the TRISO particle and there is no leakage
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
from the outside into the particle. The reason being that pyrolytic carbon is impervious to water and the chemically inert SiC layer acts as an effective diffusion barrier to most fission product elements. Although diffusion depends primarily on the diffusant/substrate combination there are other factors which can significantly increase the diffusion kinetics for a specific diffusant/substrate system. The existence of polycrystalline boundaries, micro- or nano-pipes, cracks, extended defects such as dislocations usually increase the rate of diffusion at relatively low temperatures. The diffusion process occurs via a defect process. Consequently radiation damage can induce diffusion in systems, which normally will not exhibit significant diffusion. Naturally radiation damage usually leads to enhanced diffusion in a system. The previous remarks clearly indicate that the microstructure and the ambient conditions (e.g. neutron flux and neutron energy distribution) of the substrate can significantly change the diffusion kinetics from that of a single crystal substrate. This means experimental measurements are the only viable way to really determine diffusion behaviour of the fission products in the layers of the TRISO particle. Because the protective layers of the PBMR fuel particle consist of layers of carbon and SiC for which the atomic masses are all much lighter than that of the fission waste products, RBS is an excellent analytical technique to study the diffusion of fission products in carbon and SiC substrates. For such studies care must be taken that the microstructure of the substrates corresponds closely to those layers in the TRISO particle. Another traditionally ion beam technique also well suited for such diffusion studies is SIMS. The low detection limit of SIMS is an excellent candidate to investigate deviations from Fickian diffusion by comparing low and high dose implantations. The diffu-
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sion of different isotopes can also be investigated using SIMS. 4. Summary A brief review was given of the salient features of the South African pebble bed modular reactor. The design of this nuclear fission reactor is very different to the conventional and common water-cooled reactors. The main strengths of PBMR reactor are its high operating temperature, its self-regulatory behaviour and the containment of the radioactive fission waste within the fuel particles making it possible to have a direct cycle system where the coolant gas also drives the power-generating turbines. It is pointed out that the high operating temperature of the PBMR reactor has beneficial and detrimental aspects for the materials used in the reactor. Ion beam methods together with other techniques such as electron microscopy are being used to investigate the effects of the high temperature and radiation on the materials in the reactor. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
http://www.eia.doe.gov/emeu/aer/inter.html, 26 June 2007. R.A. Kerr, Science 316 (2007) 351. http://nuclear.energy.gov/genIV/neGenIV1.html, 26 June 2007. http://www.uic.com.au/nip77.html, 26 June 2007. W.M. Stacey, Nuclear Reactor Physics, Wiley–VCH Verlag, Weinheim, 2004, p. 7. J. Xu, C. Chan, IAEA Bull. 31 (1989). http://www.pbmr.com/index.asp?Content=103&ShowCat=True, 26 June 2007. R.P. Wichner, W.P. Barthold, Evaluation of MHTGR Fuel Reliability, NUREG/CR-5810, ORNL/TM-12014, R1, R7, R8, 1992. E. Friedland, Crit. Rev. Solid State Mater. Sci. 26 (2001) 87. H.O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerenes, Noyes Publications, 1993 (Chapter 7).
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1373–1377 www.elsevier.com/locate/nimb
Ion beam analysis of materials in the PBMR reactor Johan B. Malherbe *, E. Friedland, N.G. van der Berg Department of Physics, University of Pretoria, Lynnwood Road, Pretoria 0002, South Africa Received 18 September 2007 Available online 8 December 2007
Abstract South Africa is developing a new type of high temperature nuclear reactor, the so-called pebble bed modular reactor (PBMR). The planned reactor outlet temperature of this gas-cooled reactor is approximately 900 °C. This high temperature places some severe restrictions on materials, which can be used. The name of the reactor is derived from the form of the fuel elements, which are in the form of pebbles, each with a diameter of 60 mm. Each pebble is composed of several thousands of coated fuel particles. The coated particle consists of a nucleus of UO2 surrounded by several layers of different carbons and SiC. The diameter of the fuel particles is 0.92 mm. A brief review will be given of the advantages of this nuclear reactor, of the materials in the fuel elements and their analysis using ion beam techniques. Ó 2007 Elsevier B.V. All rights reserved. PACS: 28.41. i; 28.41.Bm; 28.50. k; 28.52.Fa; 61.72.Hh; 61.80. x; 61.80.Hg; 61.80.Jh; 61.72.Yx Keywords: Ion beam analysis; Nuclear fission reactor; Materials; RBS; Channeling
1. Introduction The present energy resource scare has prompted a renewed interest in nuclear fission power plants. Fig. 1 shows that in 2004 nuclear power represented only 6.2% of the world’s total annual energy consumption of 4.7 1020 J (=1.3 1014 kW h) [1]. The dominant source of energy is fossil fuel in its different forms. However, it follows from Table 1 that the fossil fuel reserves, with the exception of coal, are limited [1]. A recent study [2] of the oil reserves paints a very bleak picture. Although the reserves of the fission elements are also limited, it is expected that fission nuclear power plants will have to substitute the phasing out of fossil fuel power plants until alternative energy sources take over as the main suppliers. The attractiveness of using nuclear energy is further enhanced by the current debate about the reality and
*
Corresponding author. Tel.: +27 12 420 2896; fax: +27 12 362 5288. E-mail address: [email protected] (J.B. Malherbe).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.046
causes of global warming by the emission of carbon dioxide. Two key reasons for the unpopularity of fission nuclear power plants are nuclear waste management and the possibility of an uncontrollable explosive incident such as the Chernobyl accident. These and other factors have prompted the US Department of Energy’s Office of Nuclear Energy, Science and Technology to engaged governments, industry and the research community worldwide in a wide-ranging discussion on the development of next generation nuclear energy systems the so-called Generation IV power plants [3]. These reactors represent advances in sustainability, economics, safety, reliability and proliferation-resistance. An international task force is developing six Generation IV nuclear reactor technologies for deployment between 2010 and 2030 [4]. All of these operate at higher temperatures than today’s reactors. The pebble bed modular reactor (PBMR) is a relatively new design in nuclear power plants and could be the first successful Generation IV power plant. There are several designs of high temperature gas-cooled reactors pursued by researchers in different countries such as the USA, Rus-
1374
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
Fig. 1. A breakdown of the world energy consumption in 2004. In the pie chart Renewable denotes gas and liquid fuel obtained from plant material. Raw data taken from [1].
Table 1 The world’s proven fossil fuel resources (in terms of energy content) in 2004, which can be recovered profitably with current technology and energy production (consumption) in 2004 Resource
World reserves (1021 J)
World production (1021 J)
Lifetimeb (years)
Oil Natural gas Coala
6.852 6.569
0.1633 0.1078
42 61
26.32
0.1195
26.32
Table adapted from data in [1]. a The 2002 coal reserves are given. b Lifetime: World Reserves/World Production.
sia, Japan and The Netherlands. The PBMR programmes of China and South Africa are the most advanced. Both of them have expressed the intension of commissioning their reactors within the next decade. This paper describes some of the salient features of the South African design of the PBMR and shows how ion beams are employed in research on materials used in the reactor.
Fig. 2. Neutron fission cross-section of the uranium isotope [5].
235
U. From
neutron energy dependence of the fission reaction crosssection of the 235U nucleus [5] is shown in Fig. 2. Although the reaction cross-section increases significantly for thermal neutrons, there are several peaks in the reaction cross-section for higher energy neutrons. The PBMR design utilizes the neutron absorption in these peaks as an inherent safety self regulation mechanism. The width of the reaction crosssection peak is an inherent self-regulating mechanism. If the temperature of the fuel increases the parasitic neutron absorption increases and causes a decrease in the reactivity of the core and subsequently the rate of the fission. The enormously long radioactivity of the fission products and especially of the actinides for a conventional light water reactor [6] is depicted in Fig. 3. Consequently, the safe storage of the nuclear waste is a problem, which has been and is being investigated in depth by various groups. The PBMR design handles this problem by building protective containment layers around the small UO2 kernels. The structure of the PBMR fuel element spheres is shown in Fig. 4. This structure of the fuel pebbles known as the TRISO particle consists of a round UO2 kernel of approx-
2. South African PBMR The South African PBMR is based on an original German prototype, which successfully operated for about 21 years in Ju¨lich, Germany. It is direct cycle reactor with a planned reactor outlet temperature of about 900 °C. The coolant, which also drives the power generating turbines (Brayton cycle), must be a chemical inert gas such as helium or nitrogen or carbon dioxide, with helium being the preferred choice because of its superior heat transfer capacity, its chemical inertness and very low neutron capture cross-section. It is, however, much more expensive than nitrogen and carbon dioxide and difficult to retain in a vessel. Graphite is used as moderator material. The
Fig. 3. The time dependence of the total radioactivity of nuclear reactor waste from a light water reactor. From [6].
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
1375
riers for the fission products and to be barriers for chemically reactive species used in the manufacturing processes. The reasons for adding a SiC layer in the TRISO particle are to provide mechanical support and structural rigidity to the coated particle and act as a containment layer for the radioactive waste. In practice this means that SiC acts as a diffusion barrier for the radioactive waste elements. Furthermore, the chemically inert SiC layer also acts a barrier for leaching from the outside during long term storage after its removal from the Spent Fuel Storage Tank. 3. Ion beam analysis of materials used in the PBMR reactor
Fig. 4. An SEM image of a cross-section of the TRISO particle, showing the UO2 kernel with the four surrounding layers. OPyC denotes the outer pyrolytic carbon layer and IPyC the inner pyrolytic carbon layer.
imately 0.5 mm diameter. This kernel is surrounded by four layers making the final diameter of the TRISO particle approximately 0.92 mm. The inner layer consisting of a porous graphite buffer layer absorbs gaseous fission products and accommodates thermal expansion and swelling of the UO2 kernel [8]. A pyrolytic carbon layer is grown on top of this. Due to the low magnification in Fig. 4, the distinction between the buffer and inner pyrolytic carbon layer (IPyC) is not visible. Next a SiC layer is grown, with the final outer layer consisting again of pyrolytic carbon (OPyC). Approximately 15,000 of these TRISO particles are compressed into a sphere (also called a fuel pebble) of about a tennis ball diameter using a graphite bonder. The total amount of uranium in the fuel sphere is about 9 g. This small amount of uranium leads to low concentrations of radioactive waste. Furthermore, the absorption of neutrons in the resonance peaks of 238U leads to the production of Plutonium. However, due to the long residence time of the fuel pebbles in the core the quality of the Plutonium in terms of creating a reliable nuclear explosive is very poor. Consequently, the probability of spent fuel being stolen by groups to use in a ‘‘dirty” bomb becomes very small. The fuel spheres moves slowly through the reactor. A schematic chart of the path and the reactor is shown on the webpage of PBMR [7]. This process is repeated several times until the fissionable 235U concentration left in the fuel sphere has decreased below its designed concentration. The sphere is then stored in a tank below the reactor until the initial high radioactivity has decreased to acceptable levels for handling and external storage. The main functions of pyrolytic carbon layers are to protect the SiC layer and to act as additional diffusion bar-
This paper deals with materials research on mainly the materials contained in the fuel pebbles. As was mentioned above, the operating temperature of the South African PBMR reactor is about 900 °C. This means that the temperatures inside the TRISO particles are even higher. Very little research has been done on materials at these high temperatures and under the radiation conditions in a nuclear reactor. One reason for this is the lack of test stations in research nuclear reactors, which can attain such high temperatures. Radiation damage caused by the neutron flux in a nuclear reactor is of practical and economical consequences for the operation of a reactor. Materials in a nuclear reactor are subjected to a complex neutron spectrum, which depends on the reactor’s design and operational mode. In the PBMR reactor the neutron energies span a range from less than 0.001 eV to more than 10 MeV with a maximum differential flux inside fuel kernels at 0.1 eV. The fast neutron flux (E > 100 keV), which is mainly responsible for radiation damage, is roughly 30% of the total neutron flux and reaches a maximum of approximately 7 1013 n cm 2 s 1 in the central core regions. The ion beam channeling technique has in recent years become a workhorse to study radiation damage in crystalline materials and their annealing behaviour [9]. The technique has a number of advantages for these kinds of investigations. They include the ease of operation and analysis in a well-designed experimental set-up. In principle it is also possible to identify the main type of defects. It is well known that extensive radiation damage can cause material failure in a nuclear reactor. In the PBMR reactor graphite is used as reflector and moderator. The anisotropic nature of the crystalline structure of graphite leads to anisotropic effects of radiation damage. Anisotropic radiation-induced swelling [10] of the graphite moderator blocks can cause mechanical transfer problems. Later we shall discuss the effects of radiation-induced and radiation-enhanced diffusion in the PBMR reactor. A range of defects is created by neutron bombardment. Let us consider SiC as an example material for the kind of damage, which can be created. Assuming an average displacement energy of 30 eV for SiC, the minimum neutron energy to create a Frenkel pair directly in a head-on
1376
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
collision is 180 eV for carbon and 420 eV for silicon atoms. However, point defects can also be produced indirectly by thermal neutrons via nuclear reactions. Neutron capture reactions, which have relatively large cross-sections at thermal energies, leave the compound nuclei 13C, 14C, 29Si, 30Si and 31Si in highly excited states. The de-excitation by cradiation transfers recoil energies of a few hundred eV to these nuclei, which is sufficient to produce a couple of Frenkel pairs. In addition 14C and 31Si are unstable and produce impurity atoms 14N and 31P by b-decay. Contributions by (n,p) and (n,a) reactions can probably be neglected as their Q-values for all stable C and Si isotopes are negative. Only the high energy tail of the neutron spectrum is involved and the production of defects and impurity atoms due to these reactions will be comparatively small. The produced hydrogen and helium atoms tend to agglomerate at previously created vacancy clusters and might eventually grow into relatively large gas bubbles. The embrittlement observed in some materials used for the construction of reactor pressure vessels is probably due to this mechanism. Interstitial atoms and vacancies in SiC are highly mobile at temperatures above 200 °C and will in most cases either recombine or migrate to the surface, although some will form relatively stable point defect clusters. The formation of stacking fault tetrahedrons by vacancies is of special importance, as their production rate seems to increase at high temperatures. However, most damage is expected from atomic collisions with fast neutrons, which can produce large collision cascades. In a dense collision cascade the interstitial atoms tend to move to the periphery, creating a vacancy-rich core, which usually collapses to form a rather stable extended defect. However, in relatively open collision cascades, which typically form in fast neutron collisions with light target atoms, a large fraction of the displaced atoms will recombine with vacancies and the final defect density is significantly less than estimated by standard computer codes using binary collision approximations. Molecular dynamic calculations could in principle determine the final damage distribution, but the enormous computing power needed to obtain statistically significant results limits severely its applicability. Simulation of neutron radiation damage by energetic ions presents itself as a cost effective and time saving possibility and has therefore been studied in many laboratories. However, in applying this method one has to be aware of the fundamental differences of interactions by neutrons and ions with target atoms. The high operating temperature of the PBMR reactor is a big advantage to minimise radiation damage. In Fig. 5 aparticle channeling spectra are shown of 360 keV Ag+ ions implanted into 6H-SiC. In (a) the implantation was done at room temperature and a comparison between the channeled spectrum and the random direction spectrum clearly shows that the single crystal SiC substrate was amorphised from the surface to the implanted depth. In Fig. 5(b) the SiC substrate was at 600 °C during implantation. In this
Fig. 5. Random and a-particle channeling spectra of single crystal 6H-SiC implanted with 360 keV Ag+ ions to a fluence of 2 1016 Ag+ cm 2. In (a) the SiC was at room temperature during implantation while in (b) it was at 600 °C.
case the SiC remains crystalline with only limited dechanneling taking place due to the implanted silver atoms. During initial orientation and analysis of virgin single crystals they were typically irradiated by the analyzing beam with a fluence of 1017 He+ cm 2 at room temperature, which creates approximately 0.1 displacements per atom (dpa) in the sub-surface region. Although a 10% lattice disorder should be easily detected by channeling spectroscopy, absolutely no damage was observed. Obviously the point defect annealing rate at 300 K is more than sufficient to prevent any measurable defect accumulation at a flux of approximately 1013 He+ cm 2 s 1. This is no surprise as the first annealing stage in SiC occurs at 80 K and interstitial atoms are highly mobile at room temperature. It was mentioned that a big advantage of the PBMR technology is that the radioactive waste products are contained within the TRISO particle and there is no leakage
J.B. Malherbe et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1373–1377
from the outside into the particle. The reason being that pyrolytic carbon is impervious to water and the chemically inert SiC layer acts as an effective diffusion barrier to most fission product elements. Although diffusion depends primarily on the diffusant/substrate combination there are other factors which can significantly increase the diffusion kinetics for a specific diffusant/substrate system. The existence of polycrystalline boundaries, micro- or nano-pipes, cracks, extended defects such as dislocations usually increase the rate of diffusion at relatively low temperatures. The diffusion process occurs via a defect process. Consequently radiation damage can induce diffusion in systems, which normally will not exhibit significant diffusion. Naturally radiation damage usually leads to enhanced diffusion in a system. The previous remarks clearly indicate that the microstructure and the ambient conditions (e.g. neutron flux and neutron energy distribution) of the substrate can significantly change the diffusion kinetics from that of a single crystal substrate. This means experimental measurements are the only viable way to really determine diffusion behaviour of the fission products in the layers of the TRISO particle. Because the protective layers of the PBMR fuel particle consist of layers of carbon and SiC for which the atomic masses are all much lighter than that of the fission waste products, RBS is an excellent analytical technique to study the diffusion of fission products in carbon and SiC substrates. For such studies care must be taken that the microstructure of the substrates corresponds closely to those layers in the TRISO particle. Another traditionally ion beam technique also well suited for such diffusion studies is SIMS. The low detection limit of SIMS is an excellent candidate to investigate deviations from Fickian diffusion by comparing low and high dose implantations. The diffu-
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sion of different isotopes can also be investigated using SIMS. 4. Summary A brief review was given of the salient features of the South African pebble bed modular reactor. The design of this nuclear fission reactor is very different to the conventional and common water-cooled reactors. The main strengths of PBMR reactor are its high operating temperature, its self-regulatory behaviour and the containment of the radioactive fission waste within the fuel particles making it possible to have a direct cycle system where the coolant gas also drives the power-generating turbines. It is pointed out that the high operating temperature of the PBMR reactor has beneficial and detrimental aspects for the materials used in the reactor. Ion beam methods together with other techniques such as electron microscopy are being used to investigate the effects of the high temperature and radiation on the materials in the reactor. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
http://www.eia.doe.gov/emeu/aer/inter.html, 26 June 2007. R.A. Kerr, Science 316 (2007) 351. http://nuclear.energy.gov/genIV/neGenIV1.html, 26 June 2007. http://www.uic.com.au/nip77.html, 26 June 2007. W.M. Stacey, Nuclear Reactor Physics, Wiley–VCH Verlag, Weinheim, 2004, p. 7. J. Xu, C. Chan, IAEA Bull. 31 (1989). http://www.pbmr.com/index.asp?Content=103&ShowCat=True, 26 June 2007. R.P. Wichner, W.P. Barthold, Evaluation of MHTGR Fuel Reliability, NUREG/CR-5810, ORNL/TM-12014, R1, R7, R8, 1992. E. Friedland, Crit. Rev. Solid State Mater. Sci. 26 (2001) 87. H.O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerenes, Noyes Publications, 1993 (Chapter 7).