Pellet Cladding Mechanical Interactions of Ceramic Claddings Fuels Under Light Water Reactor Conditions

Ceramic materials such as silicon carbide (SiC) are promising candidate materials for nuclear fuel cladding and are of interest as part of a potential accident tolerant fuel design due to its high temperature strength, dimensional stability under irradiation, corrosion resistance, and lower neutron absorption cross-section. It also offers drastically lower hydrogen generation in loss of coolant accidents such as that experienced at Fukushima. With the implementation of SiC material properties to the fuel performance code, FRAPCON, performances of the SiC-clad fuel are compared with the conventional Zircaloy-clad fuel. Due to negligible creep and high stiffness, SiC-clad fuel allows gap closure at higher burnup and insignificant cladding dimensional change. However, severe degradation of SiC thermal conductivity with neutron irradiation will lead to higher fuel temperature with larger fission gas release. High stiffness of SiC has a drawback of accumulating large interfacial pressure upon pellet-cladding mechanical interactions (PCMI). This large stress will eventually reach the flexural strength of SiC, causing failure of SiC cladding instantly in a brittle manner instead of the graceful failure of ductile metallic cladding. The large interfacial pressure causes phenomena that were previously of only marginal significance and thus ignored (such as creep of the fuel) to now have an important role in PCMI. Consideration of the fuel pellet creep and elastic deformation in PCMI models in FRAPCON provide for an improved understanding of the magnitude of accumulated interfacial pressure. Outward swelling of the pellet is retarded by the inward irradiation-induced creep, which then reduces the rate of interfacial pressure buildup. Effect of PCMI can also be reduced and by increasing gap width and cladding thickness. However, increasing gap width and cladding thickness also increases the overall thermal resistance which leads to higher fuel temperature and larger fission gas release. An optimum design is sought considering both thermal and mechanical models of this ceramic cladding with UO2 and advanced high density fuels

Effects of temperature and irradiation damage on fracture around nanoindents

Indentation based fracture toughness measurements remain one of the fastest and most convenient ways of measuring fracture toughness and are widely used even though there are known inaccuracies with the methodologies used. In this work we use single crystal and monocrystalline silicon carbide to study the effects of temperature and irradiation damage on crack propagation and morphologies Please click Additional Files below to see the full abstract

High-temperature fracture test using chevron-notched tungsten microcantilevers

The combination of focused-ion beam (FIB) based sample preparation and nanoindentation allows fracture tests to be conducted at the micro-scale. Micro-fracture tests are of great interest to the nuclear materials community, as it allows direct measurements of fracture toughness within thin ion-irradiated layers and significantly reduces the volume of radioactive samples required as compared to working with neutron irradiated samples. The main drawback of existing micro-fracture tests is its limitation to brittle materials, as only linear-elastic fracture mechanics (LEFM) solutions have been developed so far. Tungsten-tantalum (W-Ta) alloy, is the primary candidate material for the plasma facing components of a future fusion reactor divertor, however is a semi-brittle material. LEFM solutions neglect any local plastic deformation that contribute to the blunting of the crack tip, therefore underestimate the true fracture toughness. Elastic-plastic fracture mechanics (EPFM) is necessarily to quantitatively analyse the complete fracture process, this greatly complicates both sample manufacture and experimental analysis. This research introduces a novel chevron-notch design to the W-Ta micro-cantilevers to promote stable crack growth which is a requisite for the EPFM approach. Cantilevers, manufactured using FIB machining, were loaded via a cyclic method, using a G200 Nanoindenter to monitor the stiffness in each cycle. By monitoring the decrease in stiffness of the cantilever through the cycles, crack length can be measured. Given detailed information of the crack length and the cantilever geometry, the complete fracture process of the semi-brittle W-Ta alloy can be quantitatively analysed. Initial results showed the fracture toughness of W-1%Ta alloy at room temperature is 2.7 MPa·m0.5, showing no significant R-curve behaviour before the onset of unstable fracture. This revealed no crack tip blunting occurred when tested at room temperature. This result is consistent with previous macro-scale fracture tests of W-1%Ta alloy at room temperature. The future goal is to extend this technique at elevated temperatures using our hot nanoindenter (up to 750 °C). This will provide quantitative analysis of the fracture process of W-Ta alloys at real reactor operating environment. By comparing micro- with macro-fracture toughness, this will also shed light on the feasibility of using micro-fracture tests to probe bulk fracture toughness

The effect of intracrystalline water on the mechanical properties of olivine at room temperature

The effect of small concentrations of intracrystalline water on the strength of olivine is significant at asthenospheric temperatures but is poorly constrained at lower temperatures applicable to the shallow lithosphere. We examined the effect of water on the yield stress of olivine during low-temperature plasticity using room-temperature Berkovich nanoindentation. The presence of water in olivine (1,600 ppm H/Si) does not affect hardness or yield stress relative to dry olivine (≤40 ppm H/Si) outside of uncertainty but may slightly reduce Young’s modulus. Differences between water-bearing and dry crystals in similar orientations were minor compared to differences between dry crystals in different orientations. These observations suggest water content does not affect the strength of olivine at low homologous temperatures. Thus, intracrystalline water does not play a role in olivine deformation at these temperatures, implying that water does not lead to weakening in the coldest portions of the mantle

Origin of age softening in the refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) are emerging materials with potential for use under extreme conditions. As a newly developed material system, a comprehensive understanding of their long-term stability under potential service temperatures remains to be established. This study examined a titanium-vanadium-niobium-tantalum alloy, a promising RHEA known for its superior high-temperature strength and room-temperature ductility. Using a combination of advanced analytical microscopies, Calculation of Phase Diagrams (CALPHAD) software, and nanoindentation, we investigated the evolution of its microstructure and mechanical properties upon aging at 700°C. Trace interstitials such as oxygen and nitrogen, initially contributing to solid solution strengthening, promote phase segregation during thermal aging. As a result of the depletion of solute interstitials within the metal matrix, a progressive softening is observed in the alloy as a function of aging time. This study, therefore, underscores the need for a better control of impurities in future development and application of RHEAs

Measuring fracture resistance behaviour of tungsten using chevron-notched micro-cantilevers

The fracture properties of tungsten-tantalum alloy (W-1%Ta), one of the candidate material for the plasma facing components in proposed fusion reactor design, were investigated using microscale chevron-notched cantilever bending tests at both ambient and high temperature (700 °C) conditions. Finite element analysis (FEA) were used to optimise the crack stability of the chevron notch design and obtain the geometry-dependent stress-intensity factors (SIFs) and stiffness vs. crack length relationships. Room temperature chevron-notched micro-cantilevers of single-crystal silicon were used to validate the FE-calculated SIFs. The average fracture toughness (KIc) measured from Si cantilevers was 0.85±0.04 MPa·m0.5 , which was in good agreement with previously reported macroscopic values (0.7~1 MPa·m0.5 ). This result showed no size-effect in fracture toughness of the intrinsically brittle silicon, and chevron-notched micro-cantilever was an effective micro-specimen geometry for measuring fracture toughness. Stable crack growth (SCG) accompanied with plastic deformation were observed in most room temperature tested chevron-notched W-1%Ta micro-cantilevers. Linear-elastic fracture mechanical analysis (LEFM) previously used for the brittle Si cantilevers will underestimate the true fracture toughness of the semi-brittle W-1%Ta, hence an elastic-plastic fracture mechanical analysis (EPFM) was employed to measure the fracture toughness using the fracture resistance curve (J-R curve) approach. The average EPFM-calculated fracture toughness at crack instability (KQc) was 25.8±2.3 MPa·m0.5 , whereas the average LEFM-calculated fracture toughness was 4.3±0.2 MPa·m0.5 . The EPFM-calculated microscopic KQc were significantly higher than previously reported macroscopic value, possibly due to the larger crack tip plastic zone to specimen size ratio. Using Irwin’s estimation, the crack tip plastic zone radius of W-1%Ta micro-cantilevers ranged from 200~1500 nm, which were significantly larger than the crack tip plastic zone radius of the intrinsically brittle silicon (~5 nm), suggesting the size effect seen in fracture toughness might arise from the larger plastic zone to specimen size ratio. Chevron-notched W-1%Ta micro-cantilevers up to 700 °C were tested using a high- temperature nanoindenter (Micro Materials® NanoTest Xtreme). Prior testing, the temperature of the indenter and sample were carefully matched to minimise the thermal drift effects. The EPFM-calculated KQc increased gradually with temperature, until a sharp increase (40.5±3.2 MPa·m0.5) was observed at 700 °C. This microscopic brittle-to-ductile transition temperature (BDTT) was significantly higher than the macroscopic BDTT (700 vs. 200 °C), possibly due to the higher strain rate used and the tantalum addition. The amount of SCG also increased with temperature, ranging from 500 nm at RT to 800 nm at 700 °C. The larger microscopic KQc (compared to macroscopic KIc in the same temperature regime) and SCG were probably caused by the combination of the extended crack tip plastic zone and extensive dislocation shielding effects, due to the thermally-activated dislocation activities. This thesis has demonstrated micro-fracture tests using chevron-notched cantilevers are capable of measuring the fracture resistance curves from the semi-brittle W-1%Ta. Due to larger crack tip plastic zone to specimen size ratio, the EPFM-calculated KQc were significantly higher than macroscopic values. A specimen size effect to fracture toughness is suggested, which should be carefully considered when performing micro-fracture testing of ductile materials.</p

Measuring fracture resistance behaviour of tungsten using chevron-notched micro-cantilevers

The fracture properties of tungsten-tantalum alloy (W-1%Ta), one of the candidate material for the plasma facing components in proposed fusion reactor design, were investigated using microscale chevron-notched cantilever bending tests at both ambient and high temperature (700 °C) conditions. Finite element analysis (FEA) were used to optimise the crack stability of the chevron notch design and obtain the geometry-dependent stress-intensity factors (SIFs) and stiffness vs. crack length relationships. Room temperature chevron-notched micro-cantilevers of single-crystal silicon were used to validate the FE-calculated SIFs. The average fracture toughness (KIc) measured from Si cantilevers was 0.85±0.04 MPa·m0.5 , which was in good agreement with previously reported macroscopic values (0.7~1 MPa·m0.5 ). This result showed no size-effect in fracture toughness of the intrinsically brittle silicon, and chevron-notched micro-cantilever was an effective micro-specimen geometry for measuring fracture toughness. Stable crack growth (SCG) accompanied with plastic deformation were observed in most room temperature tested chevron-notched W-1%Ta micro-cantilevers. Linear-elastic fracture mechanical analysis (LEFM) previously used for the brittle Si cantilevers will underestimate the true fracture toughness of the semi-brittle W-1%Ta, hence an elastic-plastic fracture mechanical analysis (EPFM) was employed to measure the fracture toughness using the fracture resistance curve (J-R curve) approach. The average EPFM-calculated fracture toughness at crack instability (KQc) was 25.8±2.3 MPa·m0.5 , whereas the average LEFM-calculated fracture toughness was 4.3±0.2 MPa·m0.5 . The EPFM-calculated microscopic KQc were significantly higher than previously reported macroscopic value, possibly due to the larger crack tip plastic zone to specimen size ratio. Using Irwinâs estimation, the crack tip plastic zone radius of W-1%Ta micro-cantilevers ranged from 200~1500 nm, which were significantly larger than the crack tip plastic zone radius of the intrinsically brittle silicon (~5 nm), suggesting the size effect seen in fracture toughness might arise from the larger plastic zone to specimen size ratio. Chevron-notched W-1%Ta micro-cantilevers up to 700 °C were tested using a high- temperature nanoindenter (Micro Materials® NanoTest Xtreme). Prior testing, the temperature of the indenter and sample were carefully matched to minimise the thermal drift effects. The EPFM-calculated KQc increased gradually with temperature, until a sharp increase (40.5±3.2 MPa·m0.5) was observed at 700 °C. This microscopic brittle-to-ductile transition temperature (BDTT) was significantly higher than the macroscopic BDTT (700 vs. 200 °C), possibly due to the higher strain rate used and the tantalum addition. The amount of SCG also increased with temperature, ranging from 500 nm at RT to 800 nm at 700 °C. The larger microscopic KQc (compared to macroscopic KIc in the same temperature regime) and SCG were probably caused by the combination of the extended crack tip plastic zone and extensive dislocation shielding effects, due to the thermally-activated dislocation activities. This thesis has demonstrated micro-fracture tests using chevron-notched cantilevers are capable of measuring the fracture resistance curves from the semi-brittle W-1%Ta. Due to larger crack tip plastic zone to specimen size ratio, the EPFM-calculated KQc were significantly higher than macroscopic values. A specimen size effect to fracture toughness is suggested, which should be carefully considered when performing micro-fracture testing of ductile materials.</p