Tokyo Electric Power Co plans to fill a leaking nuclear reactor at the Fukushima Daiichi power plant with sea water to cool it down and reduce pressure in the unit, Japan's top government spokesman said on Saturday.

"The nuclear reactor is surrounded by a steel reactor container, which is then surrounded by a concrete building," Chief Cabinet Secretary Yukio Edano said. "The concrete building collapsed. We found out that the reactor container inside didn't explode."

Japan earlier in the day warned of a meltdown at the reactor at the plant, damaged when a massive earthquake and tsunami struck the northeast coast, but said the risk of radiation contamination was small.

"We've confirmed that the reactor container was not damaged. The explosion didn't occur inside the reactor container. As such there was no large amount of radiation leakage outside," he said.

"At this point, there has been no major change to the level of radiation leakage outside (from before and after the explosion), so we'd like everyone to respond calmly," Edano said.

"We've decided to fill the reactor container with sea water. Trade minister Kaieda has instructed us to do so. By doing this, we will use boric acid to prevent criticality."

Edano said it would take about five to 10 hours to fill the reactor core with sea water and around 10 days to complete the process.

Edano said due to the falling level of cooling water, hydrogen was generated and that leaked to the space between the building and the container and the explosion happened when the hydrogen mixed with oxygen there. – Reuters