Helium precipitation study in UO2 by Transmission Electron Microscopy

International audienc

Simulation-based investigation of unsteady flow in near-hub region of a Kaplan turbine with experimental comparison

ABSTRACT: This paper presents a detailed comparison of steady and unsteady turbulent flow simulation results in the U9 Kaplan turbine draft tube with experimental velocity and pressure measurements. The computational flow domain includes the guide vanes, the runner and the draft tube. A number of turbulence models were studied, including the standard k - epsilon, RNG k - epsilon, SST and SST-SAS models. Prediction of the flow behavior in the conical section of the draft tube directly below the runner cone is very sensitive to the prediction of the separation point on the runner cone. The results demonstrate a significant increase in precision of the flow modeling in the runner cone region by using unsteady flow simulations compare to stage simulation. The prediction of the flow in the runner cone region, however, remains delicate, and no turbulence model could accurately predict the complex phenomena observed experimentally

Why a steady void size distribution in irradiated UO2_2? A modeling approach.

International audienceIn UO$_2$ pellets irradiated in reactor, Xe nano-bubbles nucleate, grow, coarsen and finally reach a quasi steady state size distribution (transmission electron microscope observations typically report a concentration around 10$^{-4}$ nm$^{-3}$ and a radius around 0.5nm). This phenomenon is often considered as a consequence of radiation enhanced diffusion, precipitation of gas atoms and ballistic mixing. However, 4MeV Au ion irradiation of UO$_2$ thin foils at room temperature yields a nano-void population whose size distribution reaches a similar steady state, although quasi no foreign atoms are implanted nor significant cation vacancy diffusion expected at such temperature and ion energy. Atomistic simulations performed at low temperature support the assumption of heterogeneous nucleation 25keV sub-cascades produce defect aggregates and in particular voids that grow through sub-cascade overlapping. In this work a semi-empirical model is proposed to extend these results to the simulation of the size distribution evolution of a representative defect aggregates population in a fraction of a material grain under a cascade overlap regime. To account for the damage accumulation when cascades overlap, this model is based on simple rules inferred from the atomistic simulation results. It satisfactorily reproduces the TEM observations of nano-voids size and concentration, which paves the way for the introduction of a more realistic damage term in rate theory models

Multi-Objective optimization of runner blades using a multi-fidelity algorithm

A robust multi-fidelity design optimization methodology has been developed to integrate advantages of high- and low-fidelity analyses and alleviate their weaknesses. The aim of this methodology is to reach more efficient turbine runners with respect to different constraints, in reasonable computational time and cost. In such a framework, an inexpensive low-fidelity (inviscid) solver handles most of the computational burden by providing data for the optimizer to evaluate objective functions and constraint values in the low-fidelity phase. An open-source derivative-free optimizer, NOMAD, explores the search space. Promising candidates are selected among all feasible solutions using a filtering process. The proposed filtering process accounts for Pareto optimal solutions and considers solutions which are different in the design variable space and are dominant in their local territories. A high-fidelity (viscous) solver is used outside the optimization loop to accurately evaluate filtered solutions. Accurate information achieved by high-fidelity analyses is also employed to recalibrate the low-fidelity optimization. The developed methodology demonstrated its ability to redesign a Francis turbine blade for a given best efficiency operating condition. The original and optimized cases were evaluated and compared for a complete range of operating conditions by calculating the efficiency curves and losses of different components. The optimal blade has provided an efficient runner for the given operating conditions considering the design constraints.</jats:p

Atomic scale insights on the microstructure evolution of urania under irradiation

International audienceUrania is commonly used as a fuel in nuclear industry. Urania is heavily irradiated during its in-reactor stay, and faces drastic microstructural modifications, including few percent swelling and increase of dislocation density. Dislocations loops nucleate first [1] and transform with increasing fluence into lines. However, the early stages of their nucleation are hardly attainable experimentally. One commonly infers that their nucleation is related to the aggregation of point defects or defects clusters into dislocations. In the present paper [2], we clarify the first steps of the effect of irradiation on urania by means of molecular dynamics simulations using empirical potentials. The irradiation dose is simulated by continuous accumulation of Frenkel pairs at 600DC, skipping the cpu-expensive displacement cascades.Starting from a defectless urania, we observe the nucleation and growth of dislocations under Frenkel pairs accumulation. Detailed analysis shows a four stages evolution (i) an increase of point defects (ii) then the nucleation of Frank loops 13 from the aggregation of point defects, (ii) the transformation of Frank loops into perfect loops 12 (iv) and finally their stabilization as lines. Our simulations also show a swelling up to 3.2% during the first stage in which point defects are present. This swelling suddenly decreases to 1.5 percent in the second stage, as soon as dislocations nucleate. Both stage (iv) and swelling agree with experimental data [1,3] and therefore strengthen the four stages scenario of the microstructure evolution of urania under irradiation