The 85-centimetre (2.9 foot) long object, believed to be an unexploded bomb dropped by the United States during the war, was discovered by workers constructing a parking lot close to the facility's reactors, a spokesman for Fukushima operator Tokyo Electric Power Co (TEPCO) said.
TEPCO called police immediately upon finding the object, suspending construction work and roping off the area around one kilometre (0.62 mile) from the reactors, he added.
There was no impact on ongoing decommissioning operations at the nuclear plant, which suffered meltdowns in March 2011 after a powerful earthquake spawned a huge tsunami.
Unexploded US bombs and shells are still occasionally found in Japan more than 70 years after the conflict ended in 1945, particularly on the southern island of Okinawa where an extremely bloody battle took place in the war's closing months.
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