NSUF 16-767: Xe Bubble Evolution in U3Si2: an in situ TEM Investigation

Gaseous fission products such as Xe and Kr form bubbles within nuclear fuel materials during operation. The fission gas bubbles compromise the fuel performance by causing gaseous swelling, degrading thermal conductivity, and eventually initiating fission gas release and fuel cracking. Fission gas behavior is therefore an important factor determining fuel performance. Fission gas bubbles form within grains (intragranular) as well as on grain boundaries (intergranular). The evolution mechanisms and influence on bulk properties differs between intragranular and intergranular bubbles. Thus, quantitative information such as the size distribution and number density of both types of bubbles is crucial to predicting the degradation of bulk fuel properties by these microstructural modifications. Because fission gas behavior is simultaneously driven by thermally-activated Xe diffusion and radiation-enhanced Xe diffusion, fuel temperature also plays an important role in controlling the size distribution and number density of various types of fission gas bubbles. As a result, previous experimental data focusing on low-temperature research reactor conditions are not sufficient for the establishment of advanced fuel performance codes to evaluate U3Si2 as an accident tolerant fuel (ATF) in LWRs. While in-pile neutron irradiation tests (ATF-1 campaign) and corresponding post-irradiation examination (PIE) microstructure characterization results will not be available till 2018, ion irradiation testing can work as an inexpensive alternative to study radiation effects in U3Si2. More important, in situ TEM Xe implantation investigations provide unique opportunities to not only examine the fission gas bubble morphology in U3Si2 but also capture those kinetic procedures such as bubble growth and coalescence. Hence, along with the ex situ high-energy Xe implantation experiment, this in situ study will produce valuable experimental references for the ongoing development of advanced fuel performance models to evaluate U3Si2 as an ATF in LWRs. Thus, the fission gas behavior models can be preliminarily optimized. Further fine optimization will be done one in-pile irradiation data produced by ATF-1 campaign are available. The U3Si2 used in this study was synthesized by arc melting a mixture of U and Si powders. The arc-melted sample can further be ball milled, cold pressed, and then sintered into pellets. As the sintering procedure induced secondary phases (UO2 and USi) and reduced grain size, the fission gas behavior in the sintered U3Si2 may differentiate from that in arc-melted U3Si2. Thus, both types of U3Si2 samples will be investigated in this study for better comparison. For each type of U3Si2 sample, four TEM thin foils will be prepared by FIB. The TEM foils typically are 50 nm thick. Those samples will be irradiated by 150 keV Xe ions up to 2.09E15 ions/cm2 at 300C, 450C, 600C, and 750C, respectively. The selection of the ion energy ensures that the majority of the Xe ions will be retained in the foils, while the target dose provides an average Xe concentration equivalent to 5% burnup. The four irradiation temperatures cover the typical LWR conditions of U3Si2 as well as the LOCA scenario. The Xe bubbles formed in irradiated TEM foils will be characterized by Fresnel edges in underfocus-overfocus TEM images. The size distribution and number density of both intragranular and intergranular Xe bubbles will be measured from the TEM images taken at different doses and irradiation temperatures. The results will provide precious experimental results for the model optimization of the advanced fuel performance code for U3Si2 at LWR conditions. The in situ TEM ion irradiation investigation takes one day for each sample. As there will be eight samples (2 fabrication techniques, 4 irradiation temperatures), we request eight (8) days of IVEM-Tandem facility time.

Additional Info

Field	Value
Abstract	Gaseous fission products such as Xe and Kr form bubbles within nuclear fuel materials during operation. The fission gas bubbles compromise the fuel performance by causing gaseous swelling, degrading thermal conductivity, and eventually initiating fission gas release and fuel cracking. Fission gas behavior is therefore an important factor determining fuel performance. Fission gas bubbles form within grains (intragranular) as well as on grain boundaries (intergranular). The evolution mechanisms and influence on bulk properties differs between intragranular and intergranular bubbles. Thus, quantitative information such as the size distribution and number density of both types of bubbles is crucial to predicting the degradation of bulk fuel properties by these microstructural modifications. Because fission gas behavior is simultaneously driven by thermally-activated Xe diffusion and radiation-enhanced Xe diffusion, fuel temperature also plays an important role in controlling the size distribution and number density of various types of fission gas bubbles. As a result, previous experimental data focusing on low-temperature research reactor conditions are not sufficient for the establishment of advanced fuel performance codes to evaluate U3Si2 as an accident tolerant fuel (ATF) in LWRs. While in-pile neutron irradiation tests (ATF-1 campaign) and corresponding post-irradiation examination (PIE) microstructure characterization results will not be available till 2018, ion irradiation testing can work as an inexpensive alternative to study radiation effects in U3Si2. More important, in situ TEM Xe implantation investigations provide unique opportunities to not only examine the fission gas bubble morphology in U3Si2 but also capture those kinetic procedures such as bubble growth and coalescence. Hence, along with the ex situ high-energy Xe implantation experiment, this in situ study will produce valuable experimental references for the ongoing development of advanced fuel performance models to evaluate U3Si2 as an ATF in LWRs. Thus, the fission gas behavior models can be preliminarily optimized. Further fine optimization will be done one in-pile irradiation data produced by ATF-1 campaign are available. The U3Si2 used in this study was synthesized by arc melting a mixture of U and Si powders. The arc-melted sample can further be ball milled, cold pressed, and then sintered into pellets. As the sintering procedure induced secondary phases (UO2 and USi) and reduced grain size, the fission gas behavior in the sintered U3Si2 may differentiate from that in arc-melted U3Si2. Thus, both types of U3Si2 samples will be investigated in this study for better comparison. For each type of U3Si2 sample, four TEM thin foils will be prepared by FIB. The TEM foils typically are 50 nm thick. Those samples will be irradiated by 150 keV Xe ions up to 2.09E15 ions/cm2 at 300C, 450C, 600C, and 750C, respectively. The selection of the ion energy ensures that the majority of the Xe ions will be retained in the foils, while the target dose provides an average Xe concentration equivalent to 5% burnup. The four irradiation temperatures cover the typical LWR conditions of U3Si2 as well as the LOCA scenario. The Xe bubbles formed in irradiated TEM foils will be characterized by Fresnel edges in underfocus-overfocus TEM images. The size distribution and number density of both intragranular and intergranular Xe bubbles will be measured from the TEM images taken at different doses and irradiation temperatures. The results will provide precious experimental results for the model optimization of the advanced fuel performance code for U3Si2 at LWR conditions. The in situ TEM ion irradiation investigation takes one day for each sample. As there will be eight samples (2 fabrication techniques, 4 irradiation temperatures), we request eight (8) days of IVEM-Tandem facility time.
Award Announced Date	2016-12-16T07:44:48.467
Awarded Institution	None
Facility	None
Facility Tech Lead	Wei-Ying Chen
Irradiation Facility	None
PI	Yinbin Miao
PI Email	[email protected]
Project Type	RTE
RTE Number	767