Recent studies on the formation of the rim structure and on polygonization in LWR fuel

Abstract. At cross sectional burnups in excess of- 40 MWd/kgUO2, a grain subdivision process occurs at the outer rim of LWR U02 fuel. A similar phenomenon can be observed in the Pu-rich inclusions in MOX fuel. Three phenomena are usually observed in the resulting "Rim zone": formation of small grains of sub^m size, formation of pores of about um size, and reduction of the signal of fission Xe in EPMA measurements. The mechanisms for rim-structure formation are still not fully understood, and it is not completely obvious, whether the above three processes occur simultaneously or one after the other in sequential steps. Also, two different types of small grains have recently been found at CEA in France. The present state of knowledge on rim structure formation in LWR fuel is reviewed, as well as recent progress in observing and understanding the formation of nanocrystals and of polygonization in other ceramics. Controlled ion irradiation techniques, including fission product ions at fission energy (e.g. 72 MeV iodine ions) have been applied to U02 and to simulated high burnup U02, so-called SIMFUEL. The process of polygonization, i.e. the rearrangement of dislocations into dislocation walls forming low energy "sub-boundaries " and rather perfect, but slightly misoriented subgrains, was studied in these experiments. The same experiments confirmed the extreme radiation stability Of U02 and demonstrated effects of fission spikes: fission-induced bubble formation, re-solution of fission