- Title
- Corrosion and hydrogen resistant modified zirlo surfaces for nuclear fuel cladding
- Creator
- Ngongo, Sinoyolo
- Subject
- Port Elizabeth (South Africa)
- Subject
- Eastern Cape (South Africa)
- Subject
- South Africa
- Date Issued
- 2021-12
- Date
- 2021-12
- Type
- Doctoral theses
- Type
- text
- Identifier
- http://hdl.handle.net/10948/54077
- Identifier
- vital:46252
- Description
- Zirconium (Zr) alloys are used as fuel cladding material in all modern water cooled commercial nuclear reactors. Zr alloys have many desirable properties for the nuclear reactor core environment, however, it also has a weakness in that overheated zirconium alloy fuel rods react with hot steam resulting in the release of hydrogen. High temperatures increase the oxidation rate and hence the hydrogen production rate which in turn increase the risk of a hydrogen gas explosion such as what is surmised to have happened in the 2011 Fukushima Daiichi nuclear plant incident. The some of the hydrogen produced will also diffuse into the zirconium alloy and form zirconium hydrides which will weaken the original zirconium alloy material. The development of accident tolerant fuel became a major focus area after the Fukushima event in March 2011 with the U.S. congress directing the U.S. Department of Energy to develop fuel with enhanced accident tolerance at high temperatures. The main objective in the case of zirconium alloy cladding has been the modification of cladding surface layers by coating or doping in order to reduce the oxidation rate and hydrogen generation rate by the reaction of steam with zirconium alloy cladding at high temperatures. This thesis focuses on two surface modifications to decrease the surface corrosion rate of ZIRLOTM (the Zr alloy used in this study) and reduce hydrogen uptake by ZIRLO. The first modification involves an oxidation-resistant nano-crystalline diamond (NCD) film deposited on ZIRLO. The NCD layer was deposited in a microwave plasma-enhanced linear antenna chemical vapor deposition (MW-LA-PECVD) system after immersion of the ZIRLO tubes in a water-based solution containing nanodiamonds. ZIRLO tube samples were coated with 200 nm and 500 nm thick NCD layers. The 200 nm thick NCD coated ZIRLO was kept as control sample whereas the 500 nm thick NCD coated ZIRLO was exposed to steam oxidation at 360 °C for 90 days. The results revealed that the NCD exfoliated in some areas leaving the ZIRLO exposed to the steam. In the areas where the diamond did not exfoliate, the diamond was still attached to the ZIRLO with an amorphous carbon layer present between the NCD and the zirconium oxide layer.The transmission electron microscopy (TEM) results of this study indicate the presence a sp3 – hybridised diamond phase for the NCD layer as well as an intermediate amorphous carbon layer between the NCD and ZrO2 layers. Analyses of the area where the NCD layer had exfoliated showed no evidence of an NCD layer. It is possible that the amorphous carbon layer between the diamond and the zirconium oxide is responsible for weak interfacial bonding leading to partial exfoliation during oxidation in steam 360 °C for 90 days. In the second modification, the effectiveness of a Cr doped ZrO2 layer to reduce the migration rate of hydrogen in ZrO2 was investigated. The ZIRLO tube used for this part of the study did not have a thin oxide passivation surface layer. Two different procedures were used to create chromium doped zirconium oxide surface layers on ZIRLO. The ZIRLO surface was coated with chromium using cathodic arc deposition followed compression plasma flow (CPF) treatment to facilitate intermixing of the Cr and the ZIRLO. In procedure 1, the surface modification was achieved through the incorporation of chromium into the zirconium surface layer using a compression plasma flow (CPF) technique, followed by the oxidation of the chromium doped zirconium. The oxidation process was conducted at 500 °C for 6 hours. In procedure 2, the chromium was incorporated into the zirconium oxide layer again using the CPF technique. Hydrogen desorption was measured from pure ZIRLO and CPF modified samples using a special gas reaction controller system. The key finding of the results is that the chromium doped (incorporated) zirconium oxide layer on ZIRLO sample exhibited the lowest hydrogen desorption rate of all the samples analysed. This indicates that H in the Cr doped zirconium oxide layer had the lowest mobility of all the samples investigated. This is consistent with the theoretical predictions that the doping of ZrO2 by Cr reduces the solubility of hydrogen in ZrO2.
- Description
- Thesis (PhD) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics, 2021
- Format
- computer
- Format
- online resource
- Format
- application/pdf
- Format
- 1 online resource (180 pages)
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
- Rights
- All Rights Reserved
- Rights
- Open Access
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