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.