Radioactivity....

[mrjaffa]

Uranium is considered radioactive, correct? More radioactive than most elements that is.

I thought this would have meant it would have a very short half life, but it doesn't. Billions of yeas in fact.

So does the half life of an element have no reflection on how radioactive it is?

[JulianO]

Uranium is pretty un-radioactive in the scheme of things. It has a half life of around a billion years, so it is very slow to decay.

This is useful, because there is still some around - so we can use it in things. There are elements with MUCH shorter halflives, which are therefore much more radioactive. Uranium you can sort of handle almost without gloves - though I wouldn't advise it. You need a lot of it to make any decent power, not as much as coal or oil, but a reactor full. Other isotopes are much more raidoactive, releasing their energy more quickly, and so can be used for short lived power sources.

The problem is you can't mine it, you have to make it.

So Uranium is one of the few naturally occurring elements with a long enough half life to still be with us even after the 4 billion years of the earth. Most of the others appear and disappear fleetingly, either as decay products from U and similar (potassium 40 is another long lived one), or from events like cosmic rays or similar (which make ¹⁴C for instance used in carbon dating)

[Knight of Clear Skies]

So does the half life of an element have no reflection on how radioactive it is?

As a rule of thumb, the shorter the half-life, the more radioactive an isotope is. However, there are different types of decay, and the products of some isotopes may themselves be radioactive. For example, Radon gas is produced (indirectly) by the decay of Uranium and Thorium.

What makes Uranium-235 suitable for use in a nuclear reactor is that it is fissile - capable of sustaining a chain reaction, via neutron production and capture. Most types of radioactive decay do not emit neutrons.

As an aside, the first nuclear reactors on Earth were natural, caused by geological processes which concentrated uranium ores and moderated by periodic infiltration of groundwater.

[mrjaffa]

Thanks for this guys. Interesting stuff. So the shorter the half life, the more radioactive something is. Uranium is just very suitable for nuclear fission and found naturally even though it has a long half life.

I wonder if we'll see Nuclear Fusion reactors in our lifetime.

[acey]

Uranium is a naturally occuring metal, found in a mineral called pitchblende which has been mined for centuries - Uranium was first isolated from it in 1789. It is perfectly safe to handle. A particular isotope of Uranium, U235, which occurs in small quantities in natural uranium, is of use in fusion reactions (power generation or bombs). So raw mineral is refined to give uranium metal, which is then refined to produce the required isotope.

The amount of radioactivity emitted by a substance is not straightforward, because there are different types of emissions (alpha, beta, gamma, plus other stuff such as neutrons), and there are different processes going on inside any radioactive substance, so all types get emitted in varying amounts over time. Radioactive substances can be dangerous because of toxicity rather than the rate of radioactive decay: Plutonium-239 has a half-life of 24,200 years and is considered one of the most toxic substances known to man; Iodine-131 has a half-life of 8 days and is used to diagnose and treat thyroid disease.

The SI unit of radioactivity is the Becquerel, defined as a decay rate of one nucleus per second. It's related to half-life via an equation that can be found at the Wikipedia link below. Note that it also depends on the mass of the sample and its atomic mass. But for one mole of substance (eg 235 grams of Uranium-235 or 131 grams of Iodine-131) the radioactivity is inversely proportional to the half life.

http://en.wikipedia.org/wiki/Becquerel

[JulianO]

On a clear day you can see a perfectly good fusion reactor, one that we use for most of our power sources on Earth

There are a confusing number of radiation units. Besides the Becquerel, there are Sieverts and Grays (plus older Rad and Rem) which are used for working out biological exposure. There is also the Curie, and the Rutherford - both obsolete.

(acey - I think you mean fission not fusion above!)

[endure]

BBC2 have just broadcast an excellent programme about Marie Curie. It's repeated next Monday 13th. Well worth watching (or recording).

http://www.bbc.co.uk/programmes/b01s954d

[acey]

(acey - I think you mean fission not fusion above!)

Oops - thanks for that!

[Macavity]

From: http://en.wikipedia....iki/Alpha_decay

By 1928, George Gamow had solved the theory of the alpha decay via [quantum] tunneling. The alpha particle is trapped in a potential well by the nucleus. Classically, it is forbidden to escape, but according to the (then) newly-discovered principles of quantum mechanics, it has a tiny (but non-zero) probability of "tunneling" through the barrier and appearing on the other side to escape the nucleus.

Random / Cool (Coxoid?) fact to ponder:

As with most things "quantum mechanical", there is a small but calculable tendancy for "everything" (above a certain atomic number) to be theoretically unstable - To decay eventually. Just that the half life becomes immesurably large - Gold is "radioactive" but no gold nucleus has been observed to decay. At least (Hopefully) on human timescales...

Aside: I have (happily) held a slab of Depleted Uranium in my hand - OK, it was oil-coated, well wrapped in an airtight plastic bag etc. Not sure I'd do this: http://www.angelfire...d/chapter1.html - But (grimly) fascinating.

[mrjaffa]

Thanks Acey. And everyone else. A good read :-)