Fukushima – Projections for a radioactive plume

Our colleagues at the UK SMC gathered the comments below. Journalists are free to use these quotes directly in their coverage.

Science Media Centre (UK) ongoing rapid reaction: Japan

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.”