Formation and growth of intragranular fission gas bubbles in UO2 fuels with burnup of 6-83 GWd/t

The detailed characteristics of intragranular fission gas bubbles in UO fuel pellets (burnup: 6-83 GWd/t) before and after postirradiation annealing at 1600 and 1800°C have been examined by TEM and SEM fractography. In the base-irradiated fuels, a high density of small bubbles of about 2 nm in diameter precipitated uniformly in the matrix. When increasing burnup above 44 GWd/t, larger bubbles of 10-20 nm newly appeared in addition to the small bubbles. On heating at high temperatures, bubble growth was saturated within a few minutes. Enhanced coarsening of bubbles was found preferentially near the grain boundaries in the middle burnup fuels of 16-28 GWd/t and throughout the grains in the high burnup fuels of 44 and 83 GWd/t. The bubble growth during annealing was associated with a remarkable decrease of the bubble number density, and the relationship between bubble density N and mean diameter d was expressed as N α d. The coarsening was attributed to coalescence via bubble migration for moderately large bubbles of up to 50-60 nm, and to Ostwald ripening accompanied by a sufficient vacancy supply from external vacancy sources such as free surfaces, grain boundaries or irradiation-induced sub-grain boundaries for huge bubbles above 100 nm

Corrosion behavior of unirradiated oxide fuel pellets in high temperature water

In order to elucidate fuel wash-out behavior after the degraded secondary failure of LWR fuel rods, corrosion experiments in high temperature and high pressure water of 340°C and 15 MPa were conducted for unirradiated oxide fuels, two each of undoped UO with two different grain sizes, 5 wt% GdO- and 10 wt% GdO-doped UO. In degassed water of dissolved oxygen concentration (DO) equal to 10 ppb, no corrosion or oxidation was found, but the corrosion became noticeable above DO = 3 ppm. Pellet oxidation proceeded with UO formation along the grain boundaries and subsequent UO growth into enveloped UO grains. Consequently, the grain boundary penetration rate in the oxygenated water followed a diffusion-controlled parabolic law. Mismatch in the O/U ratio between the grain boundaries and interiors led to weakened boundaries, which is the cause of fuel wash-out in heavily degraded fuel rods. Both increases of grain size and GdO content were found to provide improved corrosion resistance