UPDATE 1-Japan may require back-up emergency power at nuke plants

* Emergency power at Fukushima plant was knocked out by tsunami, quake

* Many nuclear plant operators reconsidering plant safety (Adds details)

TOKYO, March 30 (Reuters) - Japan's government may require nuclear plant operators to have additional emergency power in place to cool reactors and spent nuclear fuel pools in case existing systems are knocked out, a ministry official said on Wednesday.

"We need to consider this especially after the latest incident at Fukushima Daiichi where external and emergency power stopped," the official said.

The magnitude 9.0 quake and subsequent tsunami on March 11 that crippled a nuclear plant at Fukushima has led many nuclear plant operators to reconsider plant safety and install power source cars at their plants.

Workers at Fukushima may have to struggle for weeks or months under extremely dangerous conditions to re-start cooling systems vital to control the reactors and avert a meltdown.

On Monday, highly contaminated water was found in concrete tunnels extending beyond one reactor, while at the weekend radiation was 100,000 times greater than normal in water inside another.

That poses a major dilemma for the operator Tokyo Electric Power , which wants to douse the reactors to cool them, but not worsen the spread of radiation . (Reporting by Osamu Tsukimori; Editing by Edwina Gibbs)