Hakan Ozaltun

EVALUATION OF U10MO FUEL PLATE IRRADIATION BEHAVIOR VIA NUMERICAL AND EXPERIMENTAL BENCHMARKING

This article analyzes dimensional changes due to irradiation of monolithic plate-type nuclear fuel and compares results with finite element analysis of the plates during fabrication and irradiation. Monolithic fuel plates tested in the Advanced Test Reactor (ATR) at Idaho National Lab (INL) are being used to benchmark the performance of proposed fuel for several high power research reactors. Post-irradiation metallographic images of plates sectioned at the mid-plane were analyzed to determine dimensional changes of the fuel and the cladding response. A constitutive model of the fabrication process and irradiation behavior of the tested plates was developed using the general purpose commercial finite element analysis package, ABAQUS. Using calculated burn-up profiles of irradiated plates to model the power distribution and including irradiation behaviors such as swelling and irradiation enhanced creep, model simulations allow analysis of plate parameters that are either impossible or infeasible in an experimental setting. The development and progression of fabrication induced stress concentrations at the plate edges was of primary interest, as these locations have a unique stress profile during irradiation. Additionally, comparison between 2D and 3D models was performed to optimize analysis methodology. In particular, the ability of 2D and 3D models to account for out of plane stresses which result in 3-dimensional creep behavior that is a product of these components. Results show that assumptions made in 2D models for the out-of-plane stresses and strains cannot capture the 3-dimensional physics accurately and thus 2D approximations are not representative. Stress-strain fields are dependent on plate geometry and irradiation conditions, thus, if stress based criteria is used to predict plate behavior (as opposed to material impurities, fine micro-structural defects, or sharp power gradients), unique 3D finite element formulation for each plate is required.Copyright

The Effects of Fabrication Induced Residual Stress-Strain States on the Irradiation Performance of Monolithic Mini-Plates

Effects of initial stress-strain states on irradiation performance of monolithic fuel plates were studied. The monolithic fuel plates consist of a high density low enrichment U-Mo fuel that is encapsulated in an Aluminum cladding. Because the fabrication involves multiple stages, there are concerns, if the irradiation performance of the plates is affected by the pre-irradiation stress-strain states. To investigate these concerns, a representative plate was evaluated for distinct initial stress-strain states. First, the foil preparation stage by co-rolling process was simulated. For this, a scaled version of the process was simulated to calculate the stress-strain profiles. These profiles were then used to incorporate initial states for the HIP process. Additional HIP simulations were also considered to evaluate the cases with stress-free foils prior HIP bonding. For the simulation of HIP process with initially stress-free co-rolled foils, several bonding temperatures were considered. Finally, the irradiation processes were simulated for all cases with distinct pre-irradiation stress-strain states. The stress-strain fields from the fabrication process were used to incorporate the initial states for the irradiation simulations. The resulted distortions, stress-strain fields and temperature profiles were extracted at the selected locations. Finally, a comparative evaluation was made to determine the sensitivity of the plate’s performance to the pre-irradiation stress-strain states. The irradiation simulations have revealed that the fabrication stresses in the fuel would be relieved relatively fast in reactor. The fuel foil would be essentially stress-free during irradiation. The stresses however, would develop at the shutdown stage. For the cladding material, the stresses continue to increase and additional plastic strains are generated as a result of fuel swelling. The study indicated that the stress-strain fields of the plates during irradiation are not affected by the initial stress state of the plates.Copyright

Evaluation of Blister Behavior for U10Mo Mini Fuel Plates With Cold Rolled Foils

This article aims to provide possible mechanical causes for the lowered blister temperatures of RERTR-12 and AFIP-4 fuel plates. Recent experimental investigations to determine the blister threshold temperatures have indicated lower thresholds for similar plates with comparable burn-up histories. Measured blister temperatures of roughly 100 °C lower compared to the previously tested plates may not be satisfactory for some plates. The primary differences between recent experiments and previous tests are: (1) An aggressive cold work process involving large thickness reduction ratios without normalization or full annealing (2) Subjecting the plates to a thermal cycling process prior to irradiation, and finally (3) A primarily frontal neutron flux as opposed to a transverse flux profile. It is believed that the stress field has implications to blister behavior. To investigate this claim, the stress-strain states for the fabrication procedure were evaluated. First, the residual stress profile caused by the cold rolling process was calculated. Modeling of the cold rolling process has shown confirmation of residual stresses of considerable magnitude and the existence of stress gradients with respect to foil thickness prior to the HIP process. Once calculated, these stress profiles were used as an initial condition for the fabrication process. Due to the variation in stress fields depending on location at which a foil is cut from the cold rolled plate, three representative regions were selected and implemented in the HIP simulation. Variation in stresses, depending on location of the cold rolled plate as well and variation in the through-thickness, results in a wide range of mechanical stress states. This suggests that inhomogeneous irradiation and thermal cycling behavior will result from the use of cold rolled foils. Additionally, these results suggest that there will be fundamental differences in fuel plate behavior observed between plates fabricated with cold rolled foils versus hot rolled and fully annealed foils.Copyright

Effects of the Foil Centering 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 plate-type fuels consist of a high uranium density LEU foil contained within diffusion barriers and encapsulated within a cladding material. To benchmark this new design, effects of various geometrical and operational variables on irradiation performance have been evaluated. For this work, the effects of fuel foil centering on the thermo-mechanical performance of the mini-plates were studied. To evaluate these effects, a selected plate from RERTR-12 experiments, the Plate L1P756, was considered. The fuel foil was moved within the fuel plate to study the effects of the fuel centering on stress, strain and overall shape of the fuel elements. The thickness of the fuel foil, thickness of the Zr-liners and total thickness of the plate were held constant, except the centerline alignment of the fuel foil. For this, the position of the fuel foil was varied from the center position to a maximum offset corresponding to the minimum allowable aluminum cladding thickness of 0.1524 mm. Results for various offset cases were then compared to each other and to the ideal case of a centered fuel foil. Fabrication simulations indicated that the thermal expansion mismatch results in warping of the fuel plate during fabrication as the fuel plate is cooled from the HIP temperature when the fuel is not centered. Even if the model is constrained during cooling to simulate the rigid HIP can surrounding the fuel plate during cooling, warping is observed when the constraint is removed. Similarly, irradiation simulations revealed that the fuel offset causes virtually all irradiation-induced swelling to occur on the thin-cladding side of the plate. This is observed even for the smallest offset that was considered. The total magnitude of the swelling is approximately same for all offsets values.Copyright

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

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.

Effects of the Foil Flatness on the Stress-Strain Characteristics of U10Mo Alloy Based Monolithic Mini-Plates

The effects of the foil flatness on stress-strain behavior of monolithic fuel mini-plates during fabrication and irradiation were studied. Monolithic plate-type fuels are a new fuel form being developed for research and test reactors to achieve higher uranium densities. This concept facilitates the use of low-enriched uranium fuel in the reactor. These fuel elements are comprised of a high density, low enrichment, U–Mo alloy based fuel foil encapsulated in a cladding material made of Aluminum. To evaluate the effects of the foil flatness on the stress-strain behavior of the plates during fabrication, irradiation and shutdown stages, a representative plate from RERTR-12 experiments (Plate L1P756) was considered. Both fabrication and irradiation processes of the plate were simulated by using actual irradiation parameters. The simulations were repeated for various foil curvatures to observe the effects of the foil flatness on the peak stress and strain magnitudes of the fuel elements. Results of fabrication simulations revealed that the flatness of the foil does not have a considerable impact on the post fabrication stress-strain fields. Furthermore, the irradiation simulations indicated that any post-fabrication stresses in the foil would be relieved relatively fast in the reactor. While, the perfectly flat foil provided the slightly bettermorexa0» mechanical performance, overall difference between the flat-foil case and curved-foil case was not significant. Even though the peak stresses are less affected, the foil curvature has several implications on the strain magnitudes in the cladding. It was observed that with an increasing foil curvature, there is a slight increase in the cladding strains.«xa0less

Effects of Thickness of Zirconium Liner on Stress-Strain Characteristics of U10Mo Monolithic Plates

The effects of the thickness of Zirconium liner on stress-strain behavior of monolithic fuel mini-plates during fabrication and irradiation processes were studied. Monolithic plate-type fuel elements is a new fuel form being developed for research and test reactors to achieve higher uranium densities which allows the use of low-enriched uranium fuel in reactor core. These fuel elements are comprised of a high density, low enrichment, U–Mo alloy based fuel foil encapsulated in a cladding material made of Aluminum. Early RERTR experiments indicated that the presence of an interaction layer between the fuel and cladding materials causes mechanical problems. To minimize the fuel/cladding interaction, employing a diffusion barrier between the cladding and the fuel materials was proposed. Current monolithic plate design employs a 0.025 mm thick, 99.8% pure annealed Zirconium diffusion barrier between the fuel foil (U10Mo) and the cladding materials (AL6061-O). To benchmark the irradiation performance, a number of plates were irradiated in the Advanced Test Reactor (ATR) with promising irradiation performance. To understand the effects of the thickness of the Zirconium diffusion barrier on the stress-strain behavior of the plates during fabrication, irradiation and shutdown stages, a representative plate from RERTR-12 experiments (Plate L1P7A0) was selected and simulated. Both fabrication and irradiation stages were considered. Simulations were repeated for various Zirconium thicknesses to understand the effects of the thickness of the diffusion barrier. Results of fabrication simulations indicated that Zirconium thickness has noticeable effects on foil’s stresses. Irradiation simulations revealed that the fabrication stresses of the foil would be relieved rapidly in the reactor. Results also showed that Zirconium thickness has little or no effects on irradiation and shutdown stresses.Copyright

Finite Element Simulations of Monolithic Plates for the Conversion of High Performance Research and Test Reactors: NBSR, MITR, MURR and AFIP

This article presents evaluation of the stress-strain characteristics of U10Mo alloy based monolithic fuel plates for the conversion of high power research and test reactors. Monolithic plate-type fuel is a new fuel form being developed to achieve higher uranium densities within the reactor core to allow the use of low-enriched uranium fuel in high performance reactors. For this work, irradiation behavior of four different reactor plates (NBSR, MURR, MITR and AFIP plates) with different foil and cladding geometries were benchmarked against each other. For each plate, three distinct cases were considered: (1) fabrication induced residual stresses (2) thermal cycling of fabricated plates and finally (3) mechanical behavior under proposed irradiation conditions. Given that the temperatures approach the melting point of the cladding during the fabrication and thermal cycling, high temperature material properties were incorporated to improve accuracy. Residual stress fields due to the fabrication process (Hot Isostatic Pressing) were computed first. Solutions of fabrication simulations were used as initial states for the irradiation and thermal cycling simulations. For the thermal cycling simulation, an elasto-plastic material model with thermal creep was used. The transient irradiation behavior was formulated by a fully coupled thermal-structural interaction. Temperature fields on the plates were used to compute the thermal stresses. Volumetric swelling and irradiation creep of the foil were considered. The irradiation analysis showed that the stresses evolve rapidly in the reactor. It was found that the stress field of the fuel elements is dependent on the plate geometry, especially the foil thickness. Furthermore, the foil-cladding thickness ratio is the determining factor for the mechanical behavior. The compressive stresses of the foil are reduced with an increasing foil-cladding thickness ratio. The cladding deformation becomes severe for the plates with thicker foils.Copyright