Hasbeen,

A power station of 2.6MW output capacity (the size of Eraring or Bayswater, NSW,) feeds about six tonne of Uranium through its boilers every year. How much goes into the atmosphere and how much to flue dust, and then partly to blended cement, I don't know.

Lake Macquarie during a drought would contain about 3 tonnes (the amount in all ocean water) plus whatever enters from Eraring. But it is not dangerous. The half life of U^235 is 700 million years and for U^238 it is about 5 times that.

Because of those long half lives neither in very dangerous unless concentrated in a pure form. Pure yellow cake is 98.28% U238 and only 0.72% U235 so is virtually harmless.

From a decay point of view the carbon 14 in your body is possibly a greater risk as the half life is only 5750 years but it depends on the decay reaction and carbon is a beta decay whereas U^235 is a neutron and I am not across the relative danger but suspect the neutron.

Basically the shorter the half life the greater the initial problem but the faster the problem fades away. Almost every cubic metre of the earth's surface contains a couple of ccs of Thorium but because of the long half life it is not a problem. Thorium converts to U^235 if bombarded with neutrons and therefore can be to used to replenish the U233 which decays and is lost as it produces the heat from fission in a nuclear power station.

The thorium reactor starts with a U^233 core which is replaced as it decays as it produces energy and by neutron bombardment converts Thorium 232 to Uranium 233. That is why a thorium reactor is not a significant hazard.

Even plutonium in small quantities is not all that dangerous. One physicist challenged Ralph Nader to eat as much caffeine as the physicist was prepared to eat plutonium. Nader was no mug, knew the relative risks