Cooling systems needed to prevent overheating have failed there, as they did at Fukushima number one, and meltdown is possible, Edano said.
But he said he hoped it would withstand a blast if it happened, as number one did.
Some radiation did escape from Fukushima number one, Edano admitted, but he claimed that they had not reached high enough levels to affect human health.
The effect of the joint disasters on the economy is expected to be considerable, with key ports closed, electricity supply threatened and companies, including carmakers, suspending operations.
The government met Sunday to assess the economic damage.
At least 1,000 people have died, according to officials. But police in Miyagi prefecture say that 10,000 people are unaccounted for in the small port town of Minamisanriku alone.
A man was saved 15 kilometres out to sea Sunday after the tsunami swept him and his house away. Hiromitsu Shinkawa, 60, was found clinging to a piece of roof.
China has sent a team of rescuers, despite its dispute with Japan over the Kuril islands and a US flotilla including aircraft carrier Ronald Reagan reached waters off the north-east coast on Sunday.
The earthquake appears to have shifted the main Japanese island, Hokkaido, about 2.5 metres, according to the US Geological Survey.
Strong aftershocks continued Sunday.