One of the reactors at Japan's Fukushima Daiichi nuclear plant likely suffered a substantial meltdown of its core, operator Tokyo Electric Power Co. said Thursday, offering a fresh assessment of the reactor that suggests it came closer than the operator had previously revealed to a catastrophic meltdown.

It is likely that the fuel rods that form the core of Reactor No. 1 had more than half melted in March, Tepco spokesman Junichi Matsumoto said Thursday. That assessment came after Tepco this week determined that both of the...