Japan was fighting to stop fuel rods in two earthquake-damaged reactors at the Fukushima Daiichi plant from overheating after some controlled radiation leaks into the air to relieve pressure.
The government said a building housing one reactor was at risk of exploding after a blast blew the roof off the first the day before at the complex, 240 km (150 miles) north of Tokyo.
The fear is that if the fuel rods do not cool, they could melt the container that houses the core, or even explode, releasing radioactive material into the wind.
Here are comments from experts:
PADDY REGAN, NUCLEAR PHYSICIST, SURREY UNIVERSITY, UK
(Speaking on Sky News)
"If there is a meltdown, which basically would mean the fuel inside the reactor becomes so hot that basically the cladding that holds it in place ... melts, that can release the radioactivity that comes from the fission cells."
"That could only be released into the environment if that material gets out of the main pressure vessel of the reactor."
He said there were only two ways that could happen; the first would be if a small amount of pressure in the vessel was vented to stop it exploding.
"The second one is if you had a really severe enormous over pressure that it blew the top of the reactor off the main vessel.
"That's what happened at Chernobyl that is not what has happened in the first reactor here," he said, adding the explosion there was caused by a build-up of super hot heated steam.
"That explosion is what blew away the outside of the first reactor but left the core of the main reactor effectively ... untouched. If that is the same for the second reactor, then (it) shouldn't be a huge problem."
He said it was "not a great thing" to have a core melt but as long as the main core of the reactor vessel maintained structural integrity then radioactive material would not get out into the environment.
He said pumping seawater round the reactor as a coolant would take away the heat and stop any further core meltdown.
"It means they shouldn't then have too release any steam pressure from the reactor which should mean that there is no excess radiation released into the environment."
MARCO RICOTTI, NUCLEAR PLANT SPECIALIST AND PROFESSOR AT THE
POLYTECHNIC INSTITUTE OF MILAN
(From an interview in Italian newspaper Corriere Della Sera)
"At Chernobyl the reactor had reached a huge increase in power, there was no safety container and there was not enough time to evacuate people. At Fukushima the reactor was turned off, there was a safety container and there was enough time to move away the surrounding population."
Pressure in Fukushima had doubled the normal levels.
"To lower it, technicians have let out vapor in the air twice. In simple terms you can think of the vent of a pressure cooker. The aim is to maintain the integrity of the safety container."
(Reporting by Michael Holden in LONDON, Catherine Hornby in ROME, Fredrik Dahl in VIENNA)