After the tsunami: Second explosion at Fukishima
There are reports that 11 people were injured, one seriously, in the explosion. But the key thing is that the reactor core remains intact, and radition levels remain below permissable limits, Japanese officials have said.
Nevertheless, the fear of a nuclear catastrophie - however low the chances are that that may actually become a reality - has gripped the world.
Thousands of people have already taken part in an anti-nuclear protest in Germany. There is also set to are set to be protests today in Paris from groups saying nuclear power is dangerous.
In the US, Senator Joe Lieberman said the country should "put the brakes on" building nuclear power stations "until we can absorb what has happened in Japan."
"Nuclear meltdown looms", warns the Huffington Post.
The single word most symbolic of these worries - Chernobyl - is trending on Twitter.
The Japanese ambassador to the UK, Keiichi Hayashi, put it starkly:
"There were explosions in Fukushima plant, but the important thing is the containment vessel, the inner site, has not been damaged. If it's damaged it could be a major major disaster. But that's not happened yet."
It's perhaps that last part of the quote - particularly the use of the word "yet" - that sounds especially ominous.
However, the chances of that actually happening are, scientists explain, actually very slim.
Dr Josef Oehmen, a research scientist at MIT in Boston, has written a very long - but worth bearing with - blog entitled Why I Am Not Worried About Japan's Nuclear Reactors.
The key point he makes is that though the outer buildings have exploded, they are designed to do so and their core remains solidly contained:
"The plant is safe now and will stay safe. Japan is looking at an INES Level 4 Accident: Nuclear accident with local consequences. That is bad for the company that owns the plant, but not for anyone else."
The BBC's Richard Black has a very good basic explainer on what has happened so far, and what is likely in the future.
He makes the point that "environment groups are beginning to feature Fukushima in their energy communications - and whatever actually happens at the site, it is likely to become a major card in campaigns to promote renewable energy above nuclear."
Does the risk of nuclear meltdown at Fukushima - however slight - concern you? Do you back the idea of "put the brakes on" when it comes to nuclear power in your country? Or do you think nuclear should remain a key way of supplying energy?