In the first days after the Fukushima Daiichi nuclear power station began spewing radiation in mid-March, the plant’s operator and Japanese safety regulators studiously avoided the word “meltdown”.

Yes, they said, uranium fuel rods in the tsunami-hit facility’s reactors might have been damaged after cooling systems failed. But the official view was that the rods were still mostly intact – and radioactive material was safely contained inside their zirconium sheaths.

Now, a little over two months later, new information on the state of Fukushima Daiichi’s three overheated reactors is making the m-word impossible to avoid. Fuel inside the cores, it is now understood, melted far more quickly and extensively than was initially believed – disintegrating just a few hours after the tsunami knocked out the plant’s electricity and cooling systems.