The effect of silicon on palladium migration in pyrolytic carbon and graphite
- Authors: Downey, Justin Michael
- Date: 2021-12
- Subjects: Port Elizabeth (South Africa) , Eastern Cape (South Africa) , South Africa
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/53665 , vital:45684
- Description: The pebble-bed reactor (PBR) is a graphite-moderated, gas-cooled high temperature nuclear reactor design and it is one of six nuclear reactor concepts included in the Generation IV initiative. Pebble-bed reactors use spherical fuel elements (called pebbles) which consist of pyrolytic graphite (which acts as the moderator) and contain thousands of micro-fuel particles called tristructural isotropic (TRISO) particles. These TRISO fuel particles consist of a fissile material (such as U235 in oxide or carbide form) surrounded by a carbon buffer layer, a pyrolytic carbon (PyC) layer, a silicon carbide (SiC) ceramic layer for structural integrity and metallic fission product containment, and an outer PyC layer. The PBR is claimed to be a passively safe design. The commercial development of the first pebble bed reactor was that of the German AVR reactor (German: Arbeitsgemeinschaft Versuchsreaktor) developed during the 1960s. The AVR design was later updated and marketed by a company called HTR. In 1999 the South African electricity company ESKOM obtained the right to access the HTR engineering database that included details of the Siemens/Interatom HTR-Module design. ESKOM worked with HTR on a new design and dubbed it the pebble bed modular reactor (PBMR). The PBMR Co. Ltd. was formed in 1999 and mandated to license and build PBMR reactors. One of the safety considerations which emerged from research is that of metal fission product release from the TRISO fuel particles. Ag110m is a radioactive metallic fission product found to have been released from intact TRISO particles. The release of this Ag isotope is of particular concern because it is highly gamma active and has a half-life of approximately 250 days, resulting in unsafe environments for maintenance workers of PBRs. During the past four decades, many different mechanisms for Ag transport in SiC and release from TRISO particles have been proposed. A promising more recent mechanism suggests that the metallic fission product palladium (Pd) plays a significant role in the transport and release of Ag from intact TRISO particles. In this mechanism Ag transport in irradiated TRISO particle fuel takes place in the presence of the fission product Pd. The Pd reacts with the SiC layer and penetrates the SiC layer along grain boundaries to form a silicide layer which provides a rapid diffusion path for Ag in the SiC. The presence of thin silicide layers in irradiated TRISO particles was subsequently confirmed. , Thesis (PhD) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics, 2021
- Full Text: false
- Date Issued: 2021-12
- Authors: Downey, Justin Michael
- Date: 2021-12
- Subjects: Port Elizabeth (South Africa) , Eastern Cape (South Africa) , South Africa
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/53665 , vital:45684
- Description: The pebble-bed reactor (PBR) is a graphite-moderated, gas-cooled high temperature nuclear reactor design and it is one of six nuclear reactor concepts included in the Generation IV initiative. Pebble-bed reactors use spherical fuel elements (called pebbles) which consist of pyrolytic graphite (which acts as the moderator) and contain thousands of micro-fuel particles called tristructural isotropic (TRISO) particles. These TRISO fuel particles consist of a fissile material (such as U235 in oxide or carbide form) surrounded by a carbon buffer layer, a pyrolytic carbon (PyC) layer, a silicon carbide (SiC) ceramic layer for structural integrity and metallic fission product containment, and an outer PyC layer. The PBR is claimed to be a passively safe design. The commercial development of the first pebble bed reactor was that of the German AVR reactor (German: Arbeitsgemeinschaft Versuchsreaktor) developed during the 1960s. The AVR design was later updated and marketed by a company called HTR. In 1999 the South African electricity company ESKOM obtained the right to access the HTR engineering database that included details of the Siemens/Interatom HTR-Module design. ESKOM worked with HTR on a new design and dubbed it the pebble bed modular reactor (PBMR). The PBMR Co. Ltd. was formed in 1999 and mandated to license and build PBMR reactors. One of the safety considerations which emerged from research is that of metal fission product release from the TRISO fuel particles. Ag110m is a radioactive metallic fission product found to have been released from intact TRISO particles. The release of this Ag isotope is of particular concern because it is highly gamma active and has a half-life of approximately 250 days, resulting in unsafe environments for maintenance workers of PBRs. During the past four decades, many different mechanisms for Ag transport in SiC and release from TRISO particles have been proposed. A promising more recent mechanism suggests that the metallic fission product palladium (Pd) plays a significant role in the transport and release of Ag from intact TRISO particles. In this mechanism Ag transport in irradiated TRISO particle fuel takes place in the presence of the fission product Pd. The Pd reacts with the SiC layer and penetrates the SiC layer along grain boundaries to form a silicide layer which provides a rapid diffusion path for Ag in the SiC. The presence of thin silicide layers in irradiated TRISO particles was subsequently confirmed. , Thesis (PhD) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics, 2021
- Full Text: false
- Date Issued: 2021-12
Investigation of the microstructure of nuclear grade matrix graphite
- Authors: Downey, Justin Michael
- Date: 2009
- Subjects: Graphite -- South Africa , Microstructure , Nanostructured materials -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10540 , http://hdl.handle.net/10948/944 , Graphite -- South Africa , Microstructure , Nanostructured materials -- South Africa
- Description: This dissertation focuses on the investigation of the microstructures of two nuclear grade matrix graphites. These graphites were intended for use in the core components of a high temperature test reactor (HTTR) of the pebble bed modular reactor (PBMR) design. The graphites were provided in the form of fuel spheres and a reflector block. The techniques used in the analysis of the materials include fracturing, etching, scanning electron microscopy (SEM), nano-indentation, x-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructures of the materials were characterized successfully. The fuel sphere material consisted of a high concentration of curved graphite flakes and grains in contact with turbostratic matrix graphite. The well graphitized flakes and grains were polycrystalline in nature. Delamination cracks were prevalent in the graphite crystallites. There was no significant difference in the microstructures of the center, interior and surface regions of the fuel sphere material. No evidence of amorphous carbon or resin residues was found. The reflector material consisted of a low concentration of graphite crystallites embedded within turbostratic matrix graphite. Delamination cracks were observed within the graphite crystallites, and many cavities were present in the material. TEM observation also revealed the presence of diamond crystallites. It was concluded that the fuel sphere graphite was most probably suitable for use as is, provided that the material also possessed other required properties for use in a HTTR. The reflector material however was considered to be unsuitable for use in a HTTR. It was thus suggested that the reflector material could be made more suitable by sufficient graphitization of the turbostratic graphite which formed the bulk of the material.
- Full Text:
- Date Issued: 2009
- Authors: Downey, Justin Michael
- Date: 2009
- Subjects: Graphite -- South Africa , Microstructure , Nanostructured materials -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10540 , http://hdl.handle.net/10948/944 , Graphite -- South Africa , Microstructure , Nanostructured materials -- South Africa
- Description: This dissertation focuses on the investigation of the microstructures of two nuclear grade matrix graphites. These graphites were intended for use in the core components of a high temperature test reactor (HTTR) of the pebble bed modular reactor (PBMR) design. The graphites were provided in the form of fuel spheres and a reflector block. The techniques used in the analysis of the materials include fracturing, etching, scanning electron microscopy (SEM), nano-indentation, x-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructures of the materials were characterized successfully. The fuel sphere material consisted of a high concentration of curved graphite flakes and grains in contact with turbostratic matrix graphite. The well graphitized flakes and grains were polycrystalline in nature. Delamination cracks were prevalent in the graphite crystallites. There was no significant difference in the microstructures of the center, interior and surface regions of the fuel sphere material. No evidence of amorphous carbon or resin residues was found. The reflector material consisted of a low concentration of graphite crystallites embedded within turbostratic matrix graphite. Delamination cracks were observed within the graphite crystallites, and many cavities were present in the material. TEM observation also revealed the presence of diamond crystallites. It was concluded that the fuel sphere graphite was most probably suitable for use as is, provided that the material also possessed other required properties for use in a HTTR. The reflector material however was considered to be unsuitable for use in a HTTR. It was thus suggested that the reflector material could be made more suitable by sufficient graphitization of the turbostratic graphite which formed the bulk of the material.
- Full Text:
- Date Issued: 2009
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