Archive for June, 2011

Wrong about Radon

June 1, 2011

A lot has been said by those opposed to the Lynas rare earth plant about the danger it poses to the public. One horrific scenario is described by public health expert Dr T Jayabalan here:

He said the best defence from direct exposure to increased radioactivity was to “stay as far away as possible”.
However, this would not work with a second, more surreptitious killer that would be released when the ore containing the rare earth is crushed to remove the thorium.
Jayabalan explained that when the ore is crushed, it releases another radioactive element which exists as a gas – radon.
“Radon can travel thousands of miles… which means you are not protected anywhere in Malaysia because the wind will blow (it across the nation).”

Scary, isn’t it? A Bhopal or Chernobyl like disaster right in our own backyards. Worse still, any one of us could be the victims.

The only problem with the scenario that Dr Jayabalan describes here is that it is simply not possible. There are several isotopes of radon with varying half lives. Naturally occuring thorium found in rare earth ores decays into radon-220 (also known as thoron). Radon-220  has a half life of only 55 seconds and none of its subsequent decay products are gases. What this means is that any radon released would have decayed within a few minutes and settled close to its source as radioactive dust. This dust then further decays into non-radioactive lead in a matter of hours. There is virtually no chance of any appreciable amount of radon-220 reaching even the perimeter of the plant, much less across the nation as Dr Jayabalan claims.

Dr Jayabalan gives the impression here that there are somehow pockets of radon trapped in the ore that have accumulated and released when the ore is crushed. This is misleading. Because the half life of radon-220 is so short, whatever radon that may be trapped could only be radon that had been created just a few minutes before.

It may also help if we had a ballpark figure of  how much radon we are talking about here. Thorium-232 has a half life of 14 billion years. This means that in a year, if you take 1 tonne of thorium, less than 0.05 milligrams of that would have decayed and turned into radon. The planned capacity of the Gebeng plant is 22,000 tonnes per year, with thorium content claimed to be 0.16%. This comes to about 35 tonnes of thorium per year. There is some dispute about these numbers, so let’s be generous and take it as 100 tonnes. Assuming the unlikely scenario that a whole year’s supply of ore is left lying around exposed in the plant at any one time, we’re looking at less than 5 mg of radon being produced in a whole year. When dispersed into the air, given radon’s very short half life, this is negligible.