TRANSIENT ELONGATION OF A FRESH FUEL-ROD UNDER REACTIVITY INITIATED ACCIDENT CONDITIONS

In-pile experiments of fresh fuel rods under reactivity initiated acci dent (RIA) conditions have been performed in the Nuclear Safety Resear ch Reactor at the Japan Atomic Energy Research Institute in order to u nderstand the basic pellet cladding mechanical interaction (PCMI) beha vior. Rapid fuel pellet expansion due to a power excursion would cause radial and longitudinal deformation of the cladding. This PCMI could be one of the possible incipient failure modes of an embrittled claddi ng of a high burnup fuel under the RIA conditions. Basic PCMI behavior was studied by measuring cladding deformation of a fresh fuel rod wit hout complicated irradiation effects. The transient elongation measure ments of the fuel with two kinds of gap width indicated not only PCMI- induced cladding elongation, but also reduction of the pellet stack di splacement by the cladding constraint. In the tests under a high-press ure and high-temperature condition simulating an operation condition o f BWRs, additional ridge-type cladding deformation was generated due t o the axial collapse of the cladding. A preliminary analysis for inter pretation of the tests was made using a computer code for the transien t analysis of fuel rods, FRAP-T6.