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 percents swelling and increase of dislocation density. Dislocations are identified as perfect dislocations loops and transform with increasing fluence into lines at sufficiently high temperature – i.e. at 600 °C. 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, we clarify the first steps of the effect of irradiation on urania by combining molecular dynamics simulations and experimental investigations

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

Atomic scale simulations provide insights on swelling induced by irradiations

International audienceRationale of the responses of materials to irradiation is commonly based on separate post-mortem analysis of experiments and primary damage states obtained by atomistic simulations of displacement cascades.We developed in recent years an atomic scale methodology that gives access to irradiation doses for materials in which displacements cascades boil down to point defects only. Irradiation dose is obtained by accumulation of these point defects mimicking time-consuming cascades overlap. Such methodology proved to be very efficient in providing atomic scale explanations of irradiation effects in term of swelling for different materials such as graphite or urania.We show for example in irradiated graphite [1] that the well-known anisotropic volume change characterized by a shrinking in basal plane and a swelling in the c-axis is not related only to the widening of graphene interlayer caused by interstitials. It relies also to wrinkling of graphene layers with same physical laws as for rippling of carpets or curtains. In irradiated urania [2], we also bring an atomic scale explanation to the well-known dilatation-contraction peak observed in the early stage of irradiations. It is related to the transformation of Frank loops which significantly contribute to the swelling into perfect dislocations which release strain