The radioactive elements, uranium and thorium occur widely
in naturally occurring substances and at widely varying
concentrations. With half lives of the order of 1,000,000,000 years much of the
uranium and thorium present at the time of formation of the universe,
is still in existence. These elements are distributed throughout
the body of the planet but do become preferentially concentrated
in certain types of solid minerals and hence rocks.
The exploitation of minerals for commercial and domestic use will
therefore bring these radioactive elements into the sphere of life
occupied by workers and members of the public. It is management of
this interaction, which is the subject of the ZMC’s work.
Zircon is a very tough durable and resistant mineral and is not
affected by low temperature processes found on the earths surface.
Consequently the uranium and thorium in zircon is that given it
during crystallisation from the melt. The nature of the zircon crystal
inhibits the removal of uranium and thorium and is one of the
properties that allows this mineral to be used for dating of rocks.
The U and Th atoms are firmly locked in the zircon lattice and
the strength of this bond is reflected in the fact that the decay
products, e.g. polonium remains within the zircon crystal even
up to temperatures of 1200°C.
The uranium contents of commercial zircon are typically about
320 ppm with thorium about 150ppm. These produce
radioactivity levels of 58 Bq/gm total uranium chain activity and
6 Bq/gm total thorium chain activity. There are zircons which
contain up to several 1000 ppm of uranium/thorium however these
high levels render the zircon magnetic allowing it to be removed.
Most commercial zircon operations remove the high U/Th zircons
prior to final production.