LETTER | Lynas has repeatedly stated that the specific (radio)activity of their water leach purification (WLP) residues is low (but still above Malaysia’s regulatory limit of 1 becquerel per gm of material [the becquerel (Bq) is a measure of radioactivity, equal to the number of nuclear decays per second]:
“The WLP residue, although classified as radioactive material, has the same radioactivity level as the feedstock material (rare earth ore concentrates) used in the Lamp process (about 6 Bq/g of Th)”.
(More accurately, this should read 6Bq/g of WLP - pure Th232 has a specific activity of 4070 Bq/gm of thorium, so 1655ppm of Th232 in WLP residues would contribute 6.7 Bq/gm of WLP).
But saying that each gram of WLP contributes 6Bq of radioactivity amounts to saying that Th232 decays in a single step to a stable element which is not radioactive. Clearly, this is not the case as is evident from the decay chain for Th232 below:
In a stable equilibrium, the number of nuclear decays for each of the subsequent radioactive progenies in the Th232 decay series is equal to the number of nuclear decays of Th232.
Hence the specific activity of WLP would be 10x the Bq counts contributed solely by Th232 nuclear decays (followed by nine other nuclear decays in the decay chain of progenies in the figure above).
In line with this, p.38 of the Radiological Impact Assessment (Nuklear Malaysia, June 2010) stated that Lynas' refinery would produce "32,000 tons per year of water leach purification residue (WLP) with radioactivity concentration of 61 Bq/g containing 1,655ppm (6.62 Bq/g) thorium-232 and 22.5ppm (0.28 Bq/g) of uranium-238".
It is noteworthy that the RIA arrived at this estimate despite this qualification:
"All but one of the daughter products of thorium-232 is a solid. The one exception is radon-220, an isotope of radon, but commonly referred to as thoron [half-life 55 seconds]. There is a possibility of thoron being able to emanate from the concentrate, the residue or thorium bearing contaminated materials so that the entire radioactive series may not be in secular equilibrium. When in secular equilibrium the thorium-232 radioactive series has an activity ten times the activity of thorium-232") (p.41)
Likewise, the Preliminary Environmental Impact Assessment includes a table on page 5-55 which states that the WLP contains 1655ppm of Thorium Oxide and 22.5ppm of Uranium Oxide, for a total specific (radio)activity of 62.0 Bq/g of WLP, i.e. 10 times the specific activity announced.
Lynas should explain why it is taking the boundary case (equivalent to a one-step decay of Th232 to a non-radioactive progeny), rather than in a decay chain including nine other radioactive progenies, occurring in a low-permeability clayey mass of WLP residues, which would retain much of the short-lived Thoron 220 and its decay progenies, and thus approximate a closed system tending towards secular equilibrium.
The writer represents the Citizens' Health Initiative.
The views expressed here are those of the author/contributor and do not necessarily represent the views of Malaysiakini.