Thorium exists almost exclusively as Thorium-232 (more than 99%), and so I will omit the calculation for Thorium-230 and Thorium-229.
The radioactivity for a gram of Thorium-232 can be calculated via the following formula:
Activity, A = (M/Mo)*NA*λ
λ = (ln 2/t1/2)
So the formula becomes
A = (M/Mo)*NA*(ln 2/t1/2)
t1/2 = half-life of Thorium-232
M = mass of thorium (in grams)
Mo = molecular mass of Thorium-232
NA = Avogadro's number
If we have a gram of Thorium-232,
A = (1/232)*6.022 x 10E23*(ln 2/14000000000)
= 4070 Becquerel
That's not a huge number to be honest. Bananas are radioactive too. The unstable radioisotope Potassium-40 in bananas has a half-life of 1.25 billion years, but it only makes up 0.0117% of the total Potassium content in a banana. So a typical 120gram banana has roughly 15 Becqueral, and 273 bananas produce the same amount of activity as a gram of pure Thorium-232.
The figure you quote for the specific activity of Th-232 of 4070 Bq/g is correct. However this relates to PURE Thorium-232. The Lynas rare earths concentrate contains only 0.16% Thorium-232: 4070 x 0.16% = ~6 Bq/g.
Which means every gram of soil mined by LYNAS produces 6 disintegrations per second. That's less than half the radioactivity of a banana.
Does it mean if we could afford to stand by the fruit stall selling several bunches of bananas for hours on end and expose ourselves to plenty of banana-radiation, then perhaps we're ready for thorium?
And according to LYNAS,
The LAMP is completely different to the Bukit Merah rare earths plant. The Bukit Merah plant processed "monazite" from the waste of tin mines, which is very different to the rare earths we are processing. There are now much higher standards in place which mean Bukit Merah could never be repeated.
"Monazite" typically contains 6~7% thorium i.e. 40 times more than the thorium contained in the soil mined by LYNAS. Hence it is much safer than the Bukit Merah rare earths plant.
|Monazite sand. Image: geology.com|