6 research outputs found
Why a steady void size distribution in irradiated UO? A modeling approach.
No full textInternational audienceIn UO 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 nm 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 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
Atomic scale insights on the microstructure evolution of urania under irradiation
No full textInternational 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
