High Burnup Structure in Nuclear Fuel : Impact on Fuel Behavior

When UO2 and (U,Pu)O2 fuels locally reach high burn-up, a major change in the microstructure takes place. The initial grains are replaced by thousands of much smaller grains, fission gases form micrometric bubbles and metallic fission products form precipitates. This occurs typically at the rim of the pellets and in heterogeneous MOX fuel Pu rich agglomerates. The high burn-up at the rim of the pellets is due to a high capture of epithermal neutrons by U leading locally to a higher concentration of fissile Pu than in the rest of the pellet. In the heterogeneous MOX fuels, this rim effect is also active, but most of the high burn-up structure (HBS) formation is linked to the high local concentration of fissile Pu in the Pu agglomerates. This Pu distribution leads to sharp borders between HBS and non-HBS areas (Fig.1).

When UO 2 and (U,Pu)O 2 fuels locally reach high burn-up, a major change in the microstructure takes place.The initial grains are replaced by thousands of much smaller grains, fission gases form micrometric bubbles and metallic fission products form precipitates.This occurs typically at the rim of the pellets and in heterogeneous MOX fuel Pu rich agglomerates.The high burn-up at the rim of the pellets is due to a high capture of epithermal neutrons by 238 U leading locally to a higher concentration of fissile Pu than in the rest of the pellet.In the heterogeneous MOX fuels, this rim effect is also active, but most of the high burn-up structure (HBS) formation is linked to the high local concentration of fissile Pu in the Pu agglomerates.This Pu distribution leads to sharp borders between HBS and non-HBS areas (Fig. 1).

Fig. 1: Sharp limit between HBS and non-HBS areas in a 55 GWd/t HM MOX MIMAS fuel (from [1]).
In these MOX fuels, the HBS forming at various radial positions, and not only at the rim, it was shown that the size of the new grains, of the bubbles and of the precipitates increase with the irradiation local temperatures.Other parameters have been shown to have an influence on the HBS initiation threshold, such as the irradiation density rate, the fuel composition with an effect of the Pu presence, but also of the Gd concentration in poisoned fuels, some of the studied additives, like Cr, and, maybe some of the impurities.However, not all the differences in the UO 2 HBS rim extent measured by different teams on various fuels have been explained [2][3].The effect of impurities may be the main reason for these differences, but it has not been documented enough yet.It has been shown recently, with examinations of a UO 2 fuel in which 235 U was heterogeneously distributed, that a high Pu concentration is not mandatory for HBS formation [4].
Several changes in the fuel behavior occur concomitantly with the HBS formation.An increase of the fission gas release and an increase in the fuel swelling rate are measured [3].It was shown by indirect and direct approaches that HBS formation was not the main contributor to the increase of fission gas release at high burn-up [1,[5][6].Indeed, studying the Kr/Xe ratio or some isotopic ratio in the gases collected during rod puncturing and using the differences in the gas productions as a function of the radial position, it was shown that the HBS areas were not the main source of the released gases.SIMS measurements of the local retention confirmed this trend.Nonetheless, the formation of a strong bonding between the fuel pellet periphery and the inner surface of the cladding, with the formation of the inner zirconia layer induces tensile stresses in the fuel during power decrease periods.This leads to radial cracking of the rim and, consequently, to some fission gas release.The amount of gases released by this mechanism never was evaluated.HBS formation does participate to fuel swelling increase, but it is not the only phenomenon involved, other areas showing also gas bubble formation.It must be noted that all porosity observed is not new, a local decrease of the large fabrication pores being measured.The pressure in the HBS bubbles has been evaluated.At 650 K, it is about 78 MPa, and decreases with the increasing burn-up.This corresponds to an average atomic volume of ~150 Å 3 , i.e. more than three times the UO 2 trivacancy volume, hence the increase in the swelling [1].
Impact of HBS on the fuel behavior during ramp on high burn-up fuels is unclear.The generalized fuel to cladding contact prior to the ramp is certainly the major parameter.HBS may play a role, but no direct evidence of that was found, though, prior to the ramps, high strain of the HBS can be seen in the large fabrication pores decrease as well as on free surface HBS major swelling.In loss of coolant accident (LOCA) type conditions, out of pile heating tests on fuel sections as well on specially designed experimental discs have shown, like during in-pile LOCA tests, the fragmentation of HBS [7][8].The areas prone to fragmentation are the HBS areas but also the high precipitation areas [9].The particular power history of the heterogeneous UO 2 , with an increase in the linear powers at high burn-up, led to a generalized fission gas release of the HBS spot bubbles.In addition, a generalized opening of grain boundaries is evidenced by Cs departures and deposits [4].
It could therefore be interesting to design a fuel that would be particularly resistant to HBS formation and to gas precipitation into large bubbles at high temperature.A large grain UO 2 fabricated by NFD (Japan), with a long oxidizing sintering, was shown to be locally highly resistant both to bubble formation at high temperature and to HBS [10].Expanding this to the whole fuel is an appealing idea.