Stereological evolution of the rim structure in PWR-fuels at prolonged irradiation: Dependencies with burn-up and temperature

Detailed characterisations of high burn-up structures in oxide fuels

“…This graph provides more or less the same information as that given in Table 2, as these porosity measurements were used to evaluate the molar volume of the fission gases in the bubbles. When comparing MOX and UO 2 peripheries, the porosity tends to be slightly larger in the MOX agglomerates, though its increase at very high burn-up remains rather consistent with what is seen in the UO 2 rim at lower burn-ups This increase in porosity is more than twice the standard overall fuel swelling ($0.6%/10 GWd/t), which surprisingly was also the porosity increase rate measured in [28] for very high burn-up UO 2 fuels. The porosity in the mid-radius agglomerates seems to increase at a slightly faster rate (an increase in the molar volume can be observed in Table 2).…”

“…It clearly shows the increase in the size of these bubbles and, with the appearance of very large bubbles by coalescence, the formation of bubbles more than twice the volume of the other large bubbles. Still, it is rather unclear whether the decrease in the amount of bubbles only results from this coalescence or if ripening should be taken into account as recommended in [28]. Such ripening would imply resolution, by the fission spikes, of fission gas atoms, from rather large (more than 1 lm) and highly pressurised bubbles with a high diffusion of these gases in the grains or at the grain boundaries.…”

Evolution of porosity in the high-burnup fuel structure

“…While the mechanisms of formation of such microstructures are still open to debate [4,5], theoretical consensus and experimental evidence exist on their evolution under irradiation. In particular, the porosity is observed to increase with burnup and the presence of 'extra-large' pores (with sizes of 4-10 lm or more) in very high burnup samples has been detected [6]. Understanding the evolution of porosity during irradiation is of paramount importance since the porosity directly affects the thermal conductivity and the mechanical strength of the fuel [7,8].…”

Lattice contraction in the rim zone as controlled by recrystallization: Additional evidence

“…1, already published in [2] and given here just for comparison, correspond to a standard LWR-fuel with well developed rim-structure. Typical power history and irradiation temperatures were provided in [10]. Fig.…”

“…Local burn-up calculated with Appolo 2-Code. Typical power ratings and central temperatures[10]: at average burn-up %35 GWd/tM, k % 275 W/cm, T c % 1150°C; at average burn-up ffi60-65 GWd/tM, k % 175 W/cm, T c % 920°C. Original lattice parameter, porosity and burn-up data from Ref [2]…”