Japan warns on quake deaths rise
The government's swift imposition of a protection zone may have kept any health impact downAs it was almost bound to do at some point, Japan's nuclear safety agency has uprated its assessment of the Fukushima power station incident from a level four to a level five.
These are categories on the International Nuclear and Radiological Events Scale (INES), which runs from zero (nothing happened, essentially) to seven, a "major accident".
So far, Chernobyl is the only seven-rated incident in nuclear history.
Level five is defined as an "accident with wider consequences".
So what is the worst-case scenario for those "wider consequences" at Fukushima?
What clues are there either from that level five rating, or from the situation on the ground, as to how things might transpire - whether it will in the end prove to have been a disaster or a distraction from the serious and widespread impact of the tsunami?
"The worst-case scenario would be where you have the fission products in stored canisters or in the reactors being released," said Professor Malcolm Sperrin, director of medical physics and clinical engineering at Royal Berkshire Hospital, UK.
"Radiation levels would then be very high around the plant, which is not to say they'd reach the general public.
"And we're definitely not in the situation where we're going to see another Chernobyl - that possibility has long gone."
Distant adviceThe level five rating applies specifically to the nuclear reactors in buildings 2 and 3 at Fukushima, rather than to the spent fuel cooling ponds that have lost water and where the stored fuel is heating up.
That implies that the regulators believe the main source of radioactivity coming from the plant has been the reactors.
Certainly, one of the the spikes in readings earlier in the week appeared to co-incide with damage to reactor number 2, believed to be a crack in the containment system - the symptoms being a sharp release of steam and an abrupt drop in pressure.
On Thursday and Friday, radiation levels around the plant appeared much more stable.
And although elevated readings have been noted in some locations 30km from Fukushima, there has been nothing outside the 30km protection zone that has appeared to pose a danger to health.
Despite this, a number of governments have advised their citizens to stay much further away - or in the case of the UK, to consider doing so.
However, when the UK's chief scientific adviser explained the reasoning to BBC News on Thursday, he was still painting a worst-case scenario that appeared some way short of apocalyptic.
"The worst-case scenario would see the ponds starting to emit serious amounts of radiation, with some of the reactors going into a meltdown phase," he said.
"We put that together with [a possible scenario of] extremely unfavourable weather conditions - wind in the direction of Tokyo, for example.
"Even in that situation, the radiation that we believe could come into the Tokyo area is such that you could mitigate it with relatively straightforward measures, for example staying indoors and keeping the windows closed."
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