Sir John Beddington, Chief Scientific Officer for the UK government, said in a transcribed conversation with the British Embassy in Tokyo

“Let me now talk about what would be a reasonable worst case scenario. 

If the Japanese fail to keep the reactors cool and fail to keep the pressure in the containment vessels at an appropriate level, you can get this, you know, the dramatic word “meltdown”.

But what does that actually mean?  What a meltdown involves is the basic reactor core melts, and as it melts, nuclear material will fall through to the floor of the container.

There it will react with concrete and other materials … that is likely… remember this is the reasonable worst case, we don’t think anything worse is going to happen. 

In this reasonable worst case you get an explosion. You get some radioactive material going up to about 500 metres up into the air.  Now, that’s really serious, but it’s serious again for the local area. 

It’s not serious for elsewhere even if you get a combination of that explosion it would only have nuclear material going in to the air up to about 500 metres. 

If you then couple that with the worst possible weather situation i.e. prevailing weather taking radioactive material in the direction of  Greater Tokyo and you had maybe rainfall which would bring the radioactive material down - do we have a problem? 

The answer is unequivocally no.  
Absolutely no issue. 
The problems are within 30 km of the reactor. 

And to give you a flavour for that, when Chernobyl had a massive fire at the graphite core, material was going up not just 500 metres but to 30,000 feet. 
It was lasting not for the odd hour or so but lasted months, and that was putting nuclear radioactive material up into the upper atmosphere for a very long period of time. 

But even in the case of Chernobyl, the exclusion zone that they had was about 30 kilometres. 
 And in that exclusion zone, outside that, there is no evidence whatsoever to indicate people had problems from the radiation. 

The problems with Chernobyl were people were continuing to drink the water, continuing to eat vegetables and so on and that was where the problems came from. 

That’s not going to be the case here.  So what I would really re-emphasise is that this is very problematic for the area and the immediate vicinity and one has to have concerns for the people working there. Beyond that 20 or 30 kilometres, it’s really not an issue for health.”
 

Prof Steve Jones, independent nuclear and environmental consultant, said:


“The radiation dose rates reported so far from the vicinity of the plant are consistent with a significant release of radioactvity, but so far on a scale very much lower than Chernobyl.

However, without an estimate of the quantity of individual radionuclides released, or measurements of radionuclide concentrations in air or in deposits on the ground, it is not possible to make any very meaningful estimates of the possible radiation dose to affected members of the public.”
 

Dr Richard Wakeford, Dalton Nuclear Institute and Visiting Professor of Epidemiology, University of Manchester, said:

“Words like ‘apocalypse’ and ‘catastrophe’ used about the situation at the Fukushima nuclear power plant are utterly inappropriate for the position there, as far as the radiation exposure of members of the public are concerned.

Such irresponsible statements could produce unnecessary panic and flight, leading to further loss of life. No expert would use terms like these to describe the situation at Fukushima.
 
“The Japanese authorities have taken the appropriate actions - evacuation of the surrounding area, a food ban, and the distribution of iodine tablets - should a serious release of radioactive material occur. 
The levels of radiation experienced by the public at present should be no cause for concern.

To put radiation doses into context, many Japanese undergo CT scans for cancer screening purposes, and these scans produce radiation doses of about 10 millisieverts (10,000 microsieverts) - much more than they are receiving from the Fukushima reactors.

“Radiation sickness and other early effects of radiation exposure are caused by high doses of radiation (about 1,000 millisieverts - 1,000,000 microsieverts), much higher than the doses received around the Fukushima reactors.
The actions taken by the Japanese authorities are designed to reduce the long-term risks. The main objective if there were to be a release of radioactive iodine is to limit the exposure of the thyroid glands of young children - stopping the consumption of contaminated foods and ‘blocking’ the thyroid with stable iodine are very effective ways of doing this.”
 

Prof Paddy Regan, Professor of Nuclear Physics at the University of Surrey, said:

“Is Tokyo safe?

Tokyo is approx 200 km from the edge of the Fukushima site. This means that, assuming that any radiation is spread out evenly if was to get airborne, the dose of radiation would be 1 part in approx. 40,000 (i.e., 1 forty thousandth) of that seen at the edge of the plant (assumes that the edge of the plant is 1 km from the source).

 If the activity at the edge of the plant is ~1mSv / hour (and reducing), this translates to ~(1/40,000) mSv/hour at 200 Km (=0.000025 mSv/hour). If this radiation kept up at this level for a full year (also extremely unlikely), this would translate to an ADDITIONAL dose of ~0.2 mSv/year for people in Tokyo (or about the same as a chest X-ray and about 1/10th of the annual dose UK people get from the environment).
 
“Note that these are all UPPER estimates as they assume the radiation levels will stay at these levels (they should drop as the time elapses since the reactors were switched off.
Perhaps more importantly, they also assume that the plume can reach Tokyo at all which is very, very unlikely (more likely just to get to the edge of the ~20 km exclusion zone).

These are also an over estimate most likely as any plume seems to be blown over the sea anyway.
 
“Executive summary:  Tokyo is safe from radiation at the current levels.
Even the max values quoted so far (spikes at ~200 msV/hour briefly at one on the reactors) translate to a max of ~40 mSv per year which is ~20msV, but still below the dose likely to cause significant increases in cancer.”

Source:  Australian Science Media Centre (AusSMC)   www.aussmc.org