Bulent Sencer

U-Zr metallic fuels cladded in Fe-alloys are being considered for application in an advanced sodium-cooled fast reactor that can recycle the U-Zr fuels and minimize the long-lived actinide waste. To understand the complex fuel-cladding chemical interaction between the U-Zr metallic fuels with Fe-alloys, a systematic multicomponent diffusion study was carried out using solid-to-solid diffusion couples. The U-10 wt pct Zr vs pure Fe diffusion couples were assembled and annealed at temperatures, 903 K, 923 K, and 953 K (630 °C, 650 °C, and 680 °C) for 96 hours. Development of microstructure, phase constituents, and compositions developed during the thermal anneals were examined by scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectroscopy. Complex microstructure consisting of several layers that include phases such as U6Fe, UFe2, ZrFe2, a-U, ß-U, Zr-precipitates, ?, ?, and ? were observed. Multi-phase layers were grouped based on phase constituents and microstructure, and the layer thicknesses were measured to calculate the growth constant and activation energy. The local average compositions through the interaction layer were systematically determined, and employed to construct semi-quantitative diffusion paths on isothermal U-Zr-Fe ternary phase diagrams at respective temperatures. The diffusion paths were examined to qualitatively estimate the diffusional behavior of individual components and their interactions. Furthermore, selected area electron diffraction analyses were carried out to determine, for the first time, the exact crystal structure and composition of ?, ?, and ?-phases. The ?, ?, and ?-phases were identified as Pnma(62) Fe(Zr,U), I4/mcm(140) Fe(Zr,U)2, and P42/mnm(136) U3(Zr,Fe), respectively.

Metallic U-alloy fuel cladded in steel has been examined for high temperature fast reactor technology wherein the fuel cladding chemical interaction is a challenge that requires a fundamental and quantitative understanding. In order to study the fundamental diffusional interactions between U with Fe and the alloying effect of Cr and Ni, solid-to-solid diffusion couples were assembled between pure U and Fe, Fe–15 wt.%Cr or Fe–15 wt.%Cr–15 wt.%Ni alloy, and annealed at high temperature ranging from 580 to 700 °C. The microstructures and concentration profiles that developed from the diffusion anneal were examined by scanning electron microscopy, and X-ray energy dispersive spectroscopy (XEDS), respectively. Thick U6Fe and thin UFe2 phases were observed to develop with solubilities: up to 2.5 at.% Ni in U6(Fe,Ni), up to 20 at.%Cr in U(Fe, Cr)2, and up to 7 at.%Cr and 14 at.% Ni in U(Fe, Cr, Ni)2. The interdiffusion and reactions in the U vs. Fe and U vs. Fe–Cr–Ni exhibited a similar temperature dependence, while the U vs. Fe–Cr diffusion couples, without the presence of Ni, yielded greater activation energy for the growth of intermetallic phases – lower growth rate at lower temperature but higher growth rate at higher temperature.

Metallic uranium alloy fuels cladded in stainless steel are being examined for fast reactors that operate at high temperature. In this work, solid-to-solid diffusion couples were assembled between pure U and Fe, and annealed at 853 K, 888 K and 923 K where U exists as orthorhombic α, and at 953 K and 973 K where U exists as tetragonal β. The microstructures and concentration profiles developed during annealing were examined by scanning electron microscopy and electron probe microanalysis, respectively. U6Fe and UFe2 intermetallics developed in all diffusion couples, and U6Fe was observed to grow faster than UFe2. The interdiffusion fluxes of U and Fe were calculated to determine the integrated interdiffusion coefficients in U6Fe and UFe2. The extrinsic (KI) and intrinsic growth constants (KII) of U6Fe and UFe2 were also calculated according to Wagner’s formalism. The difference between KI and KII of UFe2 indicate that its growth was impeded by the fast-growing U6Fe phase. However, the thin UFe2 played only a small role on the growth of U6Fe as its KI and KII values were determined to be similar. The allotropic transformation of uranium (orthorhombic α to tetragonal β phase) was observed to influence the growth of U6Fe directly, because the growth rate of U6Fe changed based on variation of activation energy. The change in chemical potential and crystal structure of U due to the allotropic transformation affected the interdiffusion between U and U6Fe. Faster growth of U6Fe is also examined with respect to various factors including crystal structure, phase diagram, and diffusion.

We studied the interfacial phase formation and diffusion kinetics in uranium–iron diffusion couples. A comparison was made between polycrystalline uranium (U) bonded with polycrystalline iron (FeP) and polycrystalline uranium bonded with single crystalline Fe (FeSC). After thermal annealing at 575 °C, 600 °C, 625 °C and 650 °C, respectively, diffusion and microstructures at the interface were characterized by scanning electron microscopy and transmission electron miscopy. The presence of grain boundaries in iron has a significant influence on interface reactions. In comparison with U–FeP system, interdiffusion coefficients of the U–FeSC system are significantly lower and were governed by much higher activation energies. Integrated interdiffusion coefficients and intrinsic diffusion coefficients were obtained. The intrinsic diffusion coefficients show faster diffusion of iron atoms in both U6Fe and UFe2 intermetallic phases than uranium.

The mechanical properties of AISI 304 stainless steel irradiated for over a decade in the Experimental Breeder Reactor (EBR-II) were measured using miniature mechanical testing methods. The shear punch method was used to evaluate the shear strengths of the neutron-irradiated steel and a correlation factor was empirically determined to predict its tensile strength. The strength of the stainless steel slightly decreased with increasing irradiation temperature, and significantly increased with increasing dose until it saturated above approximately 5 dpa. An effective tensile strain hardening exponent was also obtained from the data which shows a relative decrease in ductility of steel with increased irradiation damage. Ferromagnetic measurements were used to observe and deduce the effects of the stress-induced austenite to martensite transformation as a result of shear punch testing.

AISI 304 stainless steel was irradiated at 416 °C and 450 °C at a 4.4 × 10-9 and 3.05 × 10-7 dpa/s to ~0.4 and ~28 dpa, respectively, in the reflector of the EBR-II fast reactor. Both unirradiated and irradiated conditions were examined using standard and scanning transmission electron microscopy, energy dispersive spectroscopy, and atom probe tomography on very small specimens produced by focused ion beam milling. These results are compared with previous electron microscopy examination of 3 mm disks from essentially the same material. By comparing a very low dose specimen with a much higher dose specimen, both derived from a single reactor assembly, it has been demonstrated that the coupled microstructural and microchemical evolution of dislocation loops and other sinks begins very early, with elemental segregation producing at these sinks what appears to be measurable precursors to fully formed precipitates found at higher doses. The nature of these sinks and their possible precursors are examined in detail.

We study the effects of radiation damage on interdiffusion and intermetallic phase formation at the interfaces of U/Fe, U/(Fe + Cr), and U/(Fe + Cr + Ni) diffusion couples. Magnetron sputtering is used to deposit thin films of Fe, Fe + Cr, or Fe + Cr + Ni on U substrates to form the diffusion couples. One set of samples are thermally annealed under high vacuum at 450 °C or 550 °C for one hour. A second set of samples are annealed identically but with concurrent 3.5 MeV Fe++ ion irradiation. The Fe++ ion penetration depth is sufficient to reach the original interfaces. Rutherford backscattering spectrometry analysis with high fidelity spectral simulations is used to obtain interdiffusion profiles, which are used to examine differences in U diffusion and intermetallic phase formation at the buried interfaces. For all three diffusion systems, Fe++ ion irradiations enhance U diffusion. Furthermore, the irradiations accelerate the formation of intermetallic phases. In U/Fe couples, for example, the unirradiated samples show typical interdiffusion governed by Fick’s laws, while the irradiated ones show step-like profiles influenced by Gibbs phase rules.

We have studied the radiation effects in Fe–Zr diffusion couples, formed by thermal annealing of a mechanically bonded binary system at 850 °C for 15 days. After irradiation with 3.5 MeV Fe ions at 600 °C, a cross sectional specimen was prepared by using a focused-ion-beam-based lift out technique and was characterized using scanning/transmission electron microscopy, selected-area diffraction and X-ray energy dispersive spectroscopy analyses. Comparison studies were performed in localized regions within and beyond the ion projected range and the following observations were obtained: (1) the interaction layer consists of FeZr3, FeZr2, Fe2Zr, and Fe23Zr6; (2) large Fe23Zr6 particles with smaller core particles of Zr-rich Fe2Zr are found within the a-Fe matrix; (3) Zr diffusion is significantly enhanced in the ion bombarded region, leading to the formation of an Fe–Zr compound; (4) grains located within the interaction layer are much smaller in the ion bombarded region and are associated with new crystal growth and nanocrystal formation; and (5) large a-Fe particles form on the surface of the Fe side, but the particles are limited to the region close to the interaction layer. These studies reveal the complexity of the interaction phase formation in an Fe–Zr binary system and the accelerated microstructural changes under irradiation.