Le 2011/04/15 12:00, Darren New a écrit :
> On 4/15/2011 8:49, Invisible wrote:

>  > Or somebody flies a passenger jet into it. Or...
>
> They're designed to resist that one.
Or a jet fighter going at full after burner speed with maximum ammo.

>
>> Uranium-235 has a halflife is 700 million years.
>
> U-235 isn't that dangerous, tho, unless you pile up enough to interact.
>
> What it also means is that if you spill 100 pounds of U-235 somewhere,
> it's going to take 700 million years for even 50 pounds of it to have
> emitted radiation. That's a very low level of radiation.
>
> The actual chemical properties are probably more dangerous than the
> radioactive properties if you spread it out widely enough.

True!
Uranium is a heavy metal, and is toxic the same way as lead or arsenide, 
but somewhat more reactive.
Plutonium is lso much more dangerous based on it's extreeme chemical 
toxicity than it's radiation, and it's much more radioactive than uranium.

>
>  > That's /halflife/, not the
>> time it takes to degrade completely, just the time for *half* of it to go
>> away. 700 million years is longer than that oil has been in the
>> ground. ;-)
>
> Which tells you that it isn't *that* dangerous or there wouldn't be any
> life in the ground.
>

And during all that time, the rate of emission steadily goes down.

The most dangerous radioactive materials are those with short half-life.
Then, if that material emmits almost all of it's radiation as alpha 
particles, you may dismiss it (twice ionised helium). It's stopped by 
any sheet of paper. They can only cause damage if they are inside your body.

Then, there is the beta particles (electrons). Not to penetrating. A 
plank of wood, almost any thickness of any metal, or about 1m of air 
will stop it.

The real problem are gamma rays and neutron emissions. Gamma rays need 
thick lead shielding to stop.
Neutrons can make other materials become radioactive. They can also 
travel prety far. But water, and several other materials, does stop them 
effeciently.


Alain

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