[en] Fuels irradiated by a high energy deposition under an RIA condition fragment into fine particles due to melting of fuel pellets following mechanical energy generation by the interaction between the fragmented fuels and coolant. In order to clarify the mechanism of fuel fragmentation and mechanical energy generation, detailed observation of fragmented fuels and particle size distribution measurement were carried out for the fuels used in the NSRR test. It was verified that the particles of fragmented fuels become fine as increasing deposited energy, subcooling temperature and fuel/coolant volume ratio. Appearance of the fragmented fuels is characterized as follows; (1) spherical particles, (2) rubble type particles, (3) porous particles, and (4) shell type particles. The spherical particle has a large hollow inside the particle with 60 to 80 % in void ratio. Most of the rubble type particle has longitudinal transverse cracks on the surface. The porous particle has many porosities on the surface. The shell type particle is a curved thin-plate type particle with rusterless surface. (author)