NSUF 17-935: In situ ion irradiation of second phase particles in zirconium fuel cladding
Zirconium alloys are universally used as fuel cladding and support structures in Pressurised Water Reactors (PWRs), but suffer from aqueous corrosion in service that can limit the operating lifetime and the effective burnup of the uranium fuel. Premature cladding failure costs the nuclear industry many $bns each year, and generates additional high level waste that requires storage and eventual disposal. Understanding the corrosion kinetics is thus of great importance for undertaking more accurate, physically based lifetime predictions for cladding tubes, and providing information to teams designing new corrosion-resistant alloys.
One of the key outcomes of previous work in this field is the understanding that the distribution of alloying elements in the metal after exposure to high neutron fluxes in reactor is crucial to controlling the corrosion kinetics. These elements are initially found in second phase particles 50 – 500 nm in size, but these SPPs are damaged by neutron irradiation and release alloying elements to both the metal matrix and the oxide formed by corrosion. There are rather few techniques that can accurately study the concentrations of dilute elements between the fine SPPs, and in Oxford we are already using atom probe tomography to provide accurate solute concentrations in the matrix between the SPPs. Despite the importance of understanding the kinetic processes by which SPPs are damaged during irradiation, almost all the observations reported in the literature have been on cladding alloys after reactor exposure, and so are both expensive to undertake and offer observations on the damage processes after a very limited number of fluence and flux exposures. Some general conclusions can be drawn from published work from more than 4 decades: 1. That a neutron flux of over 1025 n/m2 is required to begin the amorphisation and subsequent dissolution process of the SPPs. There are unexplained changes in shape (faceting) of the remaining inner crystalline regions. 2. There is a strong temperature dependence of amorphisation. Above 450oC most SPPS remain crystalline. The balance between dynamic recovery and ballistic mechanisms for controlling amorphisation are not well understood at intermediate temperatures of most commercial interest. 3. That even without amorphisation, there is release of solute elements to the zirconium matrix – with Fe being depleted first from most kinds of SPP. 4. That in Nb containing alloys, release of Nb from the SPPs under irradiation is followed by the precipitation of extremely fine particles, assumed to be b-Nb. It is not known if this occurs during cooling from the irradiation temperature. In Sn-containing alloys, Zr5Sn3 precipitates may form at high temperatures. So there remain several key issues before a reliable model of SPP stability and its effect on matrix chemistry can be developed. In situ observations using heavy ions would help understand the key processes that occur at different temperatures and in different fluence regimes. Specifically, reliable data could be obtained on (a) the temperature range over which the fine scale precipitation occurs in irradiated Nb-containing alloys, and (b) the sequence of damage mechanisms occurs to release Fe into the metal matrix at different temperatures and fluences. While in situ ion beam irradiation of Zr alloys has been carried out in the IVEM previously, and more recently by high voltage electron irradiation in CEA Saclay, these experiments were to study the nucleation and growth of dislocation loops, not the stability of SPPs.
Additional Info
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| Abstract | Zirconium alloys are universally used as fuel cladding and support structures in Pressurised Water Reactors (PWRs), but suffer from aqueous corrosion in service that can limit the operating lifetime and the effective burnup of the uranium fuel. Premature cladding failure costs the nuclear industry many $bns each year, and generates additional high level waste that requires storage and eventual disposal. Understanding the corrosion kinetics is thus of great importance for undertaking more accurate, physically based lifetime predictions for cladding tubes, and providing information to teams designing new corrosion-resistant alloys. One of the key outcomes of previous work in this field is the understanding that the distribution of alloying elements in the metal after exposure to high neutron fluxes in reactor is crucial to controlling the corrosion kinetics. These elements are initially found in second phase particles 50 – 500 nm in size, but these SPPs are damaged by neutron irradiation and release alloying elements to both the metal matrix and the oxide formed by corrosion. There are rather few techniques that can accurately study the concentrations of dilute elements between the fine SPPs, and in Oxford we are already using atom probe tomography to provide accurate solute concentrations in the matrix between the SPPs. Despite the importance of understanding the kinetic processes by which SPPs are damaged during irradiation, almost all the observations reported in the literature have been on cladding alloys after reactor exposure, and so are both expensive to undertake and offer observations on the damage processes after a very limited number of fluence and flux exposures. Some general conclusions can be drawn from published work from more than 4 decades: 1. That a neutron flux of over 1025 n/m2 is required to begin the amorphisation and subsequent dissolution process of the SPPs. There are unexplained changes in shape (faceting) of the remaining inner crystalline regions. 2. There is a strong temperature dependence of amorphisation. Above 450oC most SPPS remain crystalline. The balance between dynamic recovery and ballistic mechanisms for controlling amorphisation are not well understood at intermediate temperatures of most commercial interest. 3. That even without amorphisation, there is release of solute elements to the zirconium matrix – with Fe being depleted first from most kinds of SPP. 4. That in Nb containing alloys, release of Nb from the SPPs under irradiation is followed by the precipitation of extremely fine particles, assumed to be b-Nb. It is not known if this occurs during cooling from the irradiation temperature. In Sn-containing alloys, Zr5Sn3 precipitates may form at high temperatures. So there remain several key issues before a reliable model of SPP stability and its effect on matrix chemistry can be developed. In situ observations using heavy ions would help understand the key processes that occur at different temperatures and in different fluence regimes. Specifically, reliable data could be obtained on (a) the temperature range over which the fine scale precipitation occurs in irradiated Nb-containing alloys, and (b) the sequence of damage mechanisms occurs to release Fe into the metal matrix at different temperatures and fluences. While in situ ion beam irradiation of Zr alloys has been carried out in the IVEM previously, and more recently by high voltage electron irradiation in CEA Saclay, these experiments were to study the nucleation and growth of dislocation loops, not the stability of SPPs. |
| Award Announced Date | 2017-04-26T10:13:26.833 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Wei-Ying Chen |
| Irradiation Facility | None |
| PI | Chris Grovenor |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 935 |