BEHAVIOR OF MIXED-OXIDE FUEL-ELEMENTS DURING AN OVERPOWER TRANSIENT

A slow-ramp (0.1%/s), extended overpower (almost-equal-to 90%) transie nt test was conducted in EBR-II on 19 mixed-oxide fuel elements with c onservative, moderate, and aggressive designs. Claddings for the eleme nts were Type 316, D9, or PNC-316 stainless steel. Before the transien t, the elements were preirradiated under steady-state or steady-state plus duty-cycle (periodic 15% overpower transient) conditions to burnu ps of 2.5-9.7 at%. Cladding integrity during the transient test was ma intained by all fuel elements except one, which had experienced substa ntial overtemperature in the earlier steady-state irradiation. Extensi ve centerline fuel melting occurred in all test elements. Significantl y, this melting did not cause any elements to breach, although it did have a strong effect on the other aspects of fuel element behavior.