Abstract

U-10wt.%Mo (U10Mo) alloy as a part of monolithic fuel system is being developed under Material Management and Minimization Reactor Conversion (MMMRC) program, tasked with replacing high-enriched uranium (HEU) fuel with low-enriched uranium (LEU) fuel in civilian research and test reactors. Use of U10Mo fuel alloy entails a Zr diffusion barrier to avoid the undesirable interdiffusion and reactions between the U10Mo and Al-alloy cladding. To better understand the interaction between these fuel system constituents, microstructural development and diffusion kinetics in U-Mo-Zr, U-Zr and fuel plate assembly processed by co-rolling and hot isostatic pressing (HIP) were investigated using a variety of analytical techniques accompanying scanning electron microscopy and transmission electron microscopy. Phase constituents, microstructure and diffusion kinetics between U10Mo and Zr were examined using solid-to-solid diffusion couples annealed at 650 °C for 240, 480 and 720 hours. Concentration profiles were mapped as diffusion paths on the isothermal ternary phase diagram. Within the diffusion zone, single-phase layers of (γU,βZr) were observed along with a discontinuous layer of Mo2Zr between the βZr and βU layers. In the vicinity of Mo2Zr phase, islands of αZr phase were also found. In addition, acicular αZr and U6Zr3Mo phases were observed within the γU(Mo). Growth rate of the interdiffusion-reaction zone was determined to be 1.81 × 10-15 m2/sec at 650 °C, however with an assumption of a certain incubation period. Investigation for interdiffusion and reaction between U and Zr were carried out using solidto-solid diffusion couples annealed at 580, 650, 680 and 710 °C. The interdiffusion and reaction layer consisted of αU containing Zr acicular precipitate, α’ (oC4-variant) and (γU,βZr) solid solution at 650, 680 and 710 °C. The δ-UZr2 phase, instead of (γU,βZr) solid solution phase, was observed in the couple annealed at 580 °C. The interdiffusion fluxes and coefficients were determined for the αU, (γU,βZr) and δ-UZr2 (580 C only) phases using both Sauer-Freise and Boltzmann-Matano analyses. For the α’-phase with negligible concentration gradient, integrated interdiffusion coefficients were determined via Wagner method. Marker plane was found in (γU,βZr) (cI2) solid solution from the couples annealed at 650, 680 and 710 °C and δ-UZr2 from the couple at 580 °C. Intrinsic diffusion coefficients at the compositions corresponding to the marker plane were determined based on Heumann analysis: U intrinsically diffused an order magnitude faster than Zr. Arrhenius temperature-dependence, Darken relation, and comparison to existing literature data demonstrated consistency in results. Monolithic fuel plate assembly was fabricated by sequential process of (1) co-rolling to laminate the Zr barrier onto the U10Mo fuel alloy and (2) HIP to encase the fuel laminated with Zr, within the Al-alloy 6061 (AA6061). In this study, HIP process was carried out as functions of temperature (520, 540, 560 and 580 °C for 90 minutes), time (45, 60, 90, 180 and 345 minutes at 560 °C) with ramp-cool rate (35, 70 and 280 °C/hour). At the interface between the U10Mo and Zr, following the co-rolling, the UZr2 phase was observed to develop adjacent to Zr, and the αU phase was found between the UZr2 and U10Mo. Mo2Zr was found as precipitates mostly within the αU phase. Observable growth due to HIP was only observed for the (Al,Si)3Zr phase found at the Zr/AA6061 interface, however, with a large activation energy of 457 kJ/mol. Decomposition of γU into αU and γ’ was observed in the U10Mo alloy. The volume fraction of α and γ’ increased as the HIP temperature and ramp-cool rate decreased. The UC-UO2 inclusions within the U10Mo fuel alloy were observed, but the volume percent of the UC-UO2 inclusions within the U10Mo alloy, ranging from approximately 0.5 to 1.8, did not change as functions of HIP temperature and holding time. However, the inclusions located near the surface of the U10Mo alloy, were frequently observed to interfere the uniformity of interdiffusion and reaction between the U10Mo alloy and Zr diffusion barrier. The regions of limited interaction between the U10Mo and Zr barrier associated with UC-UO2 inclusions decreased with an increase in HIP temperature, however no significant trend was observed with an increase in HIP duration at 560 °C.

Notes

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Graduation Date

2016

Semester

Summer

Advisor

Sohn, Yongho

Degree

Doctor of Philosophy (Ph.D.)

College

College of Engineering and Computer Science

Department

Materials Science Engineering

Degree Program

Materials Science & Engineering

Format

application/pdf

Identifier

CFE0006371

URL

http://purl.fcla.edu/fcla/etd/CFE0006371

Language

English

Release Date

August 2016

Length of Campus-only Access

None

Access Status

Doctoral Dissertation (Open Access)

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