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Our colleagues from the UK Science Media Centre gathered these comments from scientists.  As of March 17 09:00 JST.


Science Media Centre (UK) ongoing rapid reaction: Japan


On predictions for a radioactive plume:
Dr. Jim Smith, Reader in Environmental Physics at the University of Portsmouth, said:
“The shape of any plume of material obviously depends on how high it goes – related to temperature of the damaged fuel – and weather.  After Chernobyl, more volatile elements such as radiocaesium and radioiodine spread across Europe.  Less volatile elements such as strontium-90 and plutonium were discharged as microscopic hot particles – tiny bits of reactor material – and were mainly deposited within 10 km of the plant.
“Rainfall washout is an important factor in depositing radionuclides to the ground.  Japan should have an emergency response model which can predict real-time plume dispersion for, say, a loss of all the Iodine-131 in the fuel.  This should be made public if it exists.  I would be amazed if the plume resulted in big radiation doses in Tokyo, but modelling of the plume dispersion is needed to confirm this.”
Prof Steve Jones, independent nuclear and environmental consultant, said:
“The plume moves, of course, with the wind.  It spreads vertically within the boundary layer of the atmosphere – this is the region up to an altitude a few hundred to a couple of thousand metres in which turbulence is formed by friction with the ground and temperature differences between the ground and the air mass.
“If the release occurs at a height above ground (or if there is some thermal buoyancy) the plume will travel some distance before the vertical dispersion brings it to ground level – this distance will depend on effective release height and atmospheric stability but may be a kilometre or more.
“As the plume travels it is also dispersed laterally – as a very rough guide, most of the activity will be found within a sector centred on the source and expanding at an angle of about 30 degrees.
“Over a few tens of kilometres (in flat terrain) the trajectory will be approximately a straight line in the wind direction.  Over long distances – 100 km or more – the trajectory will tend to follow the wind direction along the isobar contours you see on a pressure chart. Terrain may of course modify the ground level wind direction, and may also increase vertical dispersion, and the plume will follow this.
“Over longer distances and longer times the behaviour gets very complex, as the activity effectively follows the 'package' of air into which it was released but it also disperses – to track it you need the sort of computer models used to forecast the weather.  The Met Office can do this and I wouldn't be surprised if they're doing that at the moment.
“If there is a great deal of thermal energy associated with the release, the initial plume rise will take it straight through the boundary layer and it may be transported great distances in the laminar airflows higher in the atmosphere. I don't think this is (yet) the situation at Fukushima.”
Tony Ennis, Fellow of the Institution of Chemical Engineers and independent environmental safety consultant, said:
“The movement of the plume depends on several factors.
“The radioactive material being dispersed into the atmosphere is in the form of heavier than air particles and thus it will eventually fall to ground.  The distance that the particles will travel is obviously dependent on wind and weather conditions.  Since wind speed increases with the height above ground, the higher the particles are ejected, the further they will travel before landing.
“The particles are ejected from the core in the plume of hot air rising from the fire. Hot air is less dense than cold air and hence rises.  The hotter the plume, the faster and further it will rise. Thus, the height which the particles will reach is a function of the size and weight of the particle, the temperature of the hot gases from the fire and the velocity of the fire plume. 
“At the present time, there appears to be comparatively little heat being generated by the fires (in comparison to say, Chernobyl or even Buncefield).  Both of these events were large fires where particles were carried up to thousands of metres above ground level and were carried hundreds of miles by the strong winds that exist at high altitudes. The particles are not, therefore, being ejected into the high altitude winds but are tending to stay in the lower velocity winds that exist closer to the ground.
“Given the conditions existing in the area at the present time, it is reasonable to expect that radioactive particles may travel a few tens of kilometres in the direction of the prevailing wind.  As the plume gets further away from the source, so the amount of radiation will fall off down to a ‘safe’ level.
“The situation is not, however, totally straightforward, since a low wind speed will increase the amount of radioactive particles deposited close to the site since they will not be carried away.  A high wind speed will carry the particles further but may also have the effect of diluting the plume faster thus reducing the hazardous range.  The worst condition would probably be a low wind speed with stable atmospheric conditions as the plume will be diluted slowly in this case and would result in a higher deposition of radioactive particles downwind of the source.
“Another complicating factor is the possibility of rain which would tend to wash the particles out of the cloud.  Thus, rain around the reactors would potentially be beneficial.
“If, however, the size of the fire increases or the amount of radiation being emitted increases (e.g. as a result of a meltdown of the core or a fire within the core), then the hazardous zone associated with the plume will increase.”
On the question of radiation spreading from Fukushima:
Tony Roulstone, Course Director, MPhil in Nuclear Energy, Department of Engineering, University of Cambridge said:
“The concern that people have about radiation is recognised – it is unseen and has uncertain but potentially frightening effects. The other problem is that this accident does not appear to have a quick resolution leading the anxiety that is ramped up by all sorts of comments and unfiltered information. The good news about radiation is it is simple to measure and to measure at levels vastly below that significant to human health.
“The facts for Tokyo as reported are that radiation levels have risen for about ½ to 1 micro Sieverts per hour. This is the sort of variability in the natural background radiation between different regions that have hard and soft rock in the earth. One can say that the level is noteworthy but it is not significant.
“Finally, international experts in the field are saying that the cores in the reactor do not have sufficient energy to spread the contamination very widely and for that reason the exclusion zones that have been set up by the Japanese authorities are appropriate even for the worst outcome.”
Sir John Beddington, Chief Scientific Officer for the UK government, said in a transcribed conversation with the British Embassy in Tokyo (this was sent to us by our friends at BIS and is available here):
“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 Malcolm Joyce, Professor of Nuclear Engineering at Lancaster University said:
“The spread of airborne contamination is unlikely to be evenly distributed because this depends on the transport mechanism – i.e. whether via smoke or steam, the altitude the contamination reaches before significant dispersion takes place and the time period over which the contamination is evolved – as John Beddington commented earlier in the week, concerning the important distinction between this incident and Chernobyl.  I would expect the activity to be dispersed as a plume, probably teardrop-shaped but obviously this is very dependent on the prevailing winds.  These currently appear to be away from Tokyo. There are well-established simulation models to predict these plume dynamics.
“Concerning source of the radiation itself, there is firstly gamma shine, much of which I would expect to arise from soluble fission products such as caesium-137 and that which arises from the radioactive noble gases.  These are likely to be significantly dispersed by the action of the transport and its intensity thus reduced.  In the unlikely event that the plume were to drift the 200km in the direction of Tokyo, given the direction of prevailing winds and the scale of the plume which is much, much smaller than Chernobyl, there would be a second potential issue associated with the deposition and ingestion of the very short-lived iodine isotopes.”
What will happen if radiation levels increase to such levels that the workers have to withdraw from Fukushima completely?
Tony Roulstone, Course Director, MPhil in Nuclear Energy, Department of Engineering, University of Cambridge said:
“When the levels were 1000mSV it was understandable that people were withdrawn.  The radiation levels now seem to be fluctuating at a level well below this high level and TEPCO seems to be managing the dose and risk of its staff in light of the serious situation.  The current sea water cooling arrangement for units 1/2/3, while perhaps effective, could be called a jury rig and needs monitoring and management. If the station staff continue to do this the natural reduction of the fission product (decay) will allow the core to cool.  Currently the fission product heating is about 1/300th of the prior core power – and falling.  As we have seen with the interruption of cooling to unit 2 on Monday, continuous cooling is the top priority.”
Prof Barry Marsden, Professor of Nuclear Graphite Technology at the University of Manchester, said:
“The level of irradiation will not stop remedial action: it will just make things more difficult and increase the difficulties and time to manage the risk.
“The six reactors at Fukushima Dalichi first went critical between 1970 to 1979, the first is 460MW(t) the second five 784MW(t) and reactor six 1100MW(t)
“These reactors are designed to have ‘active’ backup systems.  In the case of failure power these is required to drive emergency cooling systems pumps, although in some Boiling Water Reactors (BWRs) there is a system where steam  escaping into the containment should be driven down into a pressure suppression pool condensing the steam and reducing the pressure. The tsunami obviously compromised power to the pant preventing adequate cooling. This loss of power many have also compromised the ability to monitor the situation in various parts of the plant. New reactor designs are now design to include “passive” safety systems that are designed to shut down and cool fuel without the need for power being available at the plant.
“At Fukushima Daiichi it was the vulnerability of the auxiliary cooling systems and difficulty in getting emergency power quickly to the plant that has caused the problems not the reactors as such themselves.
“As there was no a catastrophic explosion as was the case with Chernobyl the release of activity is likely to stay local to the plant, hence the implementation of the exclusion zones.”
Dr. Jim Smith, Reader in Environmental Physics at the University of Portsmouth, said:
“This would be the nightmare scenario.  It may be possible to use a very heavy lifting helicopter to drop water on the plant if it was fitted with lead shielding for the pilot.  Depending on the gamma ray energy, about 1 cm thickness of lead is needed to reduce the radiation exposure by half.  So a 5 cm thick shield would reduce dose by about 30 times, for example.  The problem, of course, is that lead is heavy – allowing less remaining payload for water.  Alternatively remote controlled devices could be used, though this was tried at Chernobyl and failed because of the intense radiation field.”
On the long-term view:
Prof. Rolf Aalto, a geographer at the University of Exeter, has studied tsunamis and works as a geochronologist with fallout radionuclides on a weekly basis.
"The geological evidence in Japan (and elsewhere around the Pacific) indicates a history of giant tsunamis over the past several thousand years.  Unfortunately, an engineering and political decision was made to design protection and plan cities around a hypothesized 5m tsunami – about the size of those experienced in Japan over the last century.  However, it was not a surprise to geologists that a tsunami two to three times larger appeared, following a massive local earthquake.  Although the scale of both the earthquake and tsunami were exceptional, they were both well within the realm of what can occur within that tectonic setting.
"Regarding the longer impacts from radioactive fallout, on Wednesday morning it was reported that reactor no. 3 had ruptured, increasing the possibility for substantial release of the plutonium-enriched fuel used in that unit.  Plutonium is the most dangerous known substance and there is now a real chance that it, plus a suite of other fallout radionuclides, could be released into the atmosphere.  The good news is that this material would probably be taken across the Pacific Ocean by prevailing winds and the vast majority would rain out into the ocean, resulting in a lower immediate risk to people.  However, there could potentially be some fallout on islands and other locations downwind depending on exact wind speed and direction.  It is crucial to track where these fallout radionuclides accumulate to assess direct threats to humans as well as indirect threats through the potential damage to the ecology of the ocean.  It would be especially important to document the radionuclide concentrations on soils and investigate the longer term implications for exchange cycles within these soils and throughout river basins.
“At the University of Exeter, academics have been researching fallout radionuclides for 30 years and have developed a superb academic laboratory for making the relevant measurements for both alpha ray (eg., Plutonium) and gamma ray (eg., Caesium) emitting radionuclides.  We are in discussions with stakeholders at probable downwind fallout locations and are ready to assist where needed.”


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