Environmental group Greenpeace warned Saturday that quake damage to two atomic plants means "Japan is in the middle of a nuclear crisis with potentially devastating consequences".
Japan scrambled Saturday to prevent nuclear accidents at two atomic plants where reactor cooling systems failed after a massive earthquake Friday, and ordered 45,000 people living near one and 3,000 near the other to evacuate.
Cooling systems have malfunctioned at two plants, the Fukushima No. 1 and No. 2 plants, both located in an area about 250km northeast of greater Tokyo, an urban area of 30 million people.
Operator Tokyo Electric Power said Saturday it had released radioactive steam to reduce pressure from No. 1.
"Releasing any amount of radiation into the atmosphere risks the health of people in the surrounding area," said Greenpeace International head of nuclear campaign Jan Beranek in a statement emailed to AFP.
"The fact that the Fukushima nuclear power plant is leaking, or has been forced to deliberately release, contaminated gases from the reactor into the atmosphere means that all of the physical protection that was supposed to isolate radioactivity from the environment has failed."
"How many more warnings do people need to get before they understand that nuclear reactors are inherently hazardous?" asked Beranek.
"We are told by the nuclear industry that things like this cannot happen with modern reactors, yet today Japan is in the middle of a nuclear crisis with potentially devastating consequences."
"While the immediate focus is on minimising radiation release and keeping local people safe, this is yet another reminder of the inherent risks of nuclear power, which will always be vulnerable to a potentially deadly combination of human error, design failure and natural disaster."