The Problem: A Mountain of Lynas Waste ?

A reader once asked ” Would it be safe to say 132,000,000,000 bq will be resulting from 22,000 tonnes of Lynas waste even though it has only 6 bq per gm?”

Well, Thorium-232 produces only alpha rays (gamma rays from daughter isotopes are insignificant, please see below) which cannot pass through even a thin piece of paper or the surface of the skin.

If you have a lump of pure Thorium-232, all you need to do is to wrap it up in newspaper and that will stop all the alpha radiation from getting out!

Since alpha particles of Thorium-232 which decay with energies of 3.8 to about 4.8 MeV can travel only a couple of centimetres in air, only less than 50 micron in fluids and much less in solid material, even if you have a huge pile of Lynas waste, the external radiation would not increase by much because only a tiny percentage of thorium-232 atoms directly on the surface of the pile will be able to radiate out their alpha particles.

Those just below the surface will remain inside the pile. Each alpha particle will just pick up 2 electrons and be converted to the simple non-radioactive helium gas.

Even those apha particles from the surface will be converted to harmless helium gas after a few centimetres through the air.

So all that your mountain of Lynas waste will only “radiate” harmless non-radioactive Helium gas.

The radiation of 6 bq/gm from the weakly radioactive Thorium-232 in the Lynas waste is so low that even IAEA do not consider it significant and as such can be transported without any special permission.

One reader stated that although Thorium-232 may not be dangerous, all their “daughter” isotopes have very short half-lives like Radon-220 and these are highly radioactive, so how could we say that Thorium-232 is not dangerous.

Well, the reason is obvious. But I find great difficulty in explaining to the layman the reason why when an element with an extremely long half-life that decays into isotopes with very short half-life, there is minimal accumulation of the short half-life isotopes and as such do not build up into a large enough quantity to pose any significant hazard.

I will try to explain by this simple analogy. If there are 2 million people waiting outside a stadium and 1 million people managed to get into the stadium after 14,000,000,000 years (half-life of Thorium-232), it means that you will see only 1 person getting into the stadium every 14,000 years.

But once in the stadium the half-life of the new person (half-life of Radon-220 from the Thorium-232 decay chain is only 55 seconds) is so short when compared with those outside (Thorium-232) that he has left the stadium almost immediately. Getting in is extremely slow but getting out is extremely fast!

Since only 1 person gets into the stadium every 14,000 years, if you look into the stadium you will see it as empty most of the time!

It is just like trying to fill up a bucket by a slow, slow drop by drop drip and the bucket has a big hole at the bottom. If you look into the bucket you will see practically little or no water in it!

Since the Lynas waste product has only 6 Bq/gm of radioactivity, the chance of detecting any significant amount of radon gas is pretty slim indeed.