Pavel Medvedev

SCANNING ELECTRON MICROSCOPY ANALYSIS OF FUEL/MATRIX INTERACTION LAYERS IN HIGHLY-IRRADIATED U-Mo DISPERSION FUEL PLATES WITH Al AND Al–Si ALLOY MATRICES

In order to investigate how the microstructure of fuel/matrix-interaction (FMI) layers change during irradiation, different U?7Mo dispersion fuel plates have been irradiated to high fission density and then characterized using scanning electron microscopy (SEM). Specifially, samples from irradiated U?7Mo dispersion fuel elements with pure Al, Al?2Si and AA4043 (~4.5 wt.%Si) matrices were SEM characterized using polished samples and samples that were prepared with a focused ion beam (FIB). Features not observable for the polished samples could be captured in SEM images taken of the FIB samples. For the Al matrix sample, a relatively large FMI layer develops, with enrichment of Xe at the FMI layer/Al matrix interface and evidence of debonding. Overall, a significant penetration of Si from the FMI layer into the U?7Mo fuel was observed for samples with Si in the Al matrix, which resulted in a change of the size (larger) and shape (round) of the fission gas bubbles. Additionally, solid fission product phases were observed to nucleate and grow within these bubbles. These changes in the localized regions of the microstructure of the U?7Mo may contribute to changes observed in the macroscopic swelling of fuel plates with Al?Si matrices.

Effects of the Shape of the Foil Corners on the Irradiation Performance of U10Mo Alloy Based Monolithic Mini-Plates

Monolithic plate-type fuel is a fuel form being developed for high performance research and test reactors to minimize the use of enriched material. These fuel elements are comprised of a high density, low enrichment, U-Mo alloy based fuel foil, sandwiched between Zirconium liners and encapsulated in Aluminum cladding. The use of a high density fuel in a foil form presents a number of fabrication and operational concerns, such as: foil centering, flatness of the foil, fuel thickness variation, geometrical tilting, foil corner shape etc. To benchmark this new design, effects of various geometrical and operational variables on irradiation performance have been evaluated. As a part of these series of sensitivity studies, the shape of the foil corners were studied. To understand the effects of the corner shapes of the foil on thermo-mechanical performance of the plates, a behavioral model was developed for a selected plate from RERTR-12 experiments (Plate L1P785). Both fabrication and irradiation processes were simulated. Once the thermo-mechanical behavior the plate is understood for the nominal case, the simulations were repeated for two additional corner shapes to observe the changes in temperature, displacement and stress-strain fields. The results from the fabrication simulations indicated that the foil corners do not alter the post-fabrication stress-strain magnitudes. Furthermore, the irradiation simulations revealed that post-fabrication stresses of the foil would be relieved very quickly in operation. While, foils with chamfered and filleted corners yielded stresses with comparable magnitudes, they are slightly lower in magnitudes, and provided a more favorable mechanical response compared with the foil with sharp corners.Copyright

Impact of Mechanical Constraints on the Irradiation Performance of U10Mo Monolithic Mini-Plates

For the conversion of high performance research reactors to low enrichment Uranium fuel, U-Mo alloy based fuels in monolithic form were proposed. These plate-type fuels consist of a high uranium density, low enrichment uranium (LEU) foil contained within a diffusion barrier, and encapsulated within a cladding. To benchmark this new design, effects of various geometrical and operational variables on irradiation performance have been evaluated. In this work, the effects of mechanical constraints on the thermo-mechanical behavior of a plate were studied. To evaluate these effects, a selected plate from RERTR-12 experiments (Plate L1P756) was simulated. Four distinct cases which represent four distinct welding conditions were considered. Evaluation of the stress-strain fields in the fuel elements revealed that mechanical constraints may impact the plate’s performance. These constraints include (a) inlet side, (b) outlet side, (c) both inlet and outlet sides; and finally, (d) entire long edges. Results of these cases were then compared with the ideal case. The peak stress-strain magnitudes, displacement, stress and strain profiles on the fuel elements are evaluated to make a comparative assessment. The results indicated that the cases with constraints on “inlet side only” and “outlet side only” yielded lower cladding strains compared with other cases. The difference on the displacement profiles on the fuel foil was not significant. Peak stresses on the foil did not change considerably. These results imply that the mechanical constraints effects peak cladding strains, while it does not cause significant effects on the fuel behavior.Copyright

Effects of the foil flatness on irradiation performance of U10Mo monolithic mini-plates

Monolithic plate-type fuels comprise of a high density, low enrichment, U10Mo fuel foil encapsulated in a cladding material. This concept generates several fabrication challenges such as flatness, centering or thickness variation. There are concerns, if these parameters have implications on overall performance. To investigate these inquiries, the effects of the foil flatness were studied. For this, a representative plate was simulated for an ideal case. The simulations were repeated for additional cases with various foil curvatures to evaluate the effects on the irradiation performance. The results revealed that the stresses and strains induced by fabrication process are not affected by the flatness of the foil. Furthermore, fabrication stresses in the foil are relieved relatively fast in the reactor. The effects of the foil flatness on peak irradiation stressstrains are minimal. There is a slight increase in temperature for the case with maximum curvature. The major impact is on the displacement characteristics. Furthermore, while the case with a flat foil produces a symmetrical swelling, if the foil is curved, more swelling occurs on the thin-cladding side and the plate bows during irradiation.

Microstructural Characterization of the Irradiated Nuclear Fuels

The microstructural characterization using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for the irradiated fuels played an important role to the understanding of fuel performance. Significant progresses have been made in recent years on SEM and TEM work for fuel development in reduced-enrichment for research and test reactors (RERTR) program [1, 2]. It is extremely challenging to prepare the samples from the highly radioactive irradiated fuel for high resolution microscopy analysis. For the complex microstructure of irradiated fuels, the traditional mechanical polishing tends to produce a smeared and disturbed surface making it difficult to reveal the original microstructure in SEM while the traditional TEM sample preparation often limits the ability to access the areas of interest for detailed analysis. The new development using the focused-ion-beam (FIB) lift-out and polishing technical at the Idaho National Laboratory (INL) demonstrated the great advantage in microstructural characterization for the irradiated nuclear fuels.

AECL/U.S. INERI - Development of Inert Matrix Fuels for Plutonium and Minor Actinide Management in Power Reactors Fuel Requirements and Down-Select Report

This report documents the first milestone of the International Nuclear Energy Research Initiative (INERI) U.S./Euratom Joint Proposal 1.8 entitled “Development of Inert Matrix Fuels for Plutonium and Minor Actinide Management in Light-Water Reactors.” The milestone represents the assessment and preliminary study of a variety of fuels that hold promise as transmutation and minor actinide burning fuel compositions for light-water reactors. The most promising fuels of interest to the participants on this INERI program have been selected for further study. These fuel compositions are discussed in this report.

U.S./EURATOM INERI - Development of Inert Matrix Fuels for Plutonium and Minor Actinide Management in LWRs -- Fuel Requirements and Down-Select Report

This report documents the first milestone of the International Nuclear Energy Research Initiative (INERI) U.S./Canada Joint Proposal entitled “Development of Inert Matrix Fuels for Plutonium and Minor Actinide Management in Power Reactors.” The milestone represents the assessment and preliminary study of a variety of fuels that hold promise as transmutation and minor actinide burning fuel compositions for light water reactors. The most promising fuels of interest to the participants on this INERI program have been selected for further study. These fuel compositions are discussed in this report.