Re: Nuclear Energy

I had a couple of small problems with your question. Firstly it is **not** 

Biochemistry. Secondly, I think the question you are meaning to ask is about

uranium, not plutonium.



Natural uranium comes in two main varieties - Uranium-238 and Uranium-235. 

There are minute trace amounts of three other types as well, but they do not

matter. At present, 99.3% of uranium atoms on earth are U-238 and only 0.7% 

are U-235. These different varieties or 'isotopes' of uranium have exactly

the same chemical properties, but differ very slightly in a few properties

that depend on the weight of atoms, because U-235 atoms are a little bit

lighter than U-238. (Yes, they weigh about 235 dalton and 238 dalton 

respectively!).



But the fission nuclear reaction that drives a nuclear power station - or a

nuclear bomb for that matter - is something that only U-235 atoms will do;

not U-238.



In ordinary uranium, only one atom in 150 is U-235. These atoms are too few

and far between to make a nuclear chain reaction go. So uranium in nuclear

fuel has to be "enriched" so that the U-235 content is about 5% - one atom

in 20 uranium atoms is U-235. That material will allow a nuclear chain

reaction to go.



For the first atomic bombs, virtually pure U-235 was obtained using a special

type of mass spectrometer. Uranium was put in a furnace in a vacuum. An

electron beam was used to remove an electron from a uranium atom to make a

charged particle. This charged particle would then travel along a curved

path through a magnetic field in a vacuum to a collector plate. But the

lighter U-235 atoms would follow a different path from the heavier U-238

particles, and so collect on a different part of the collector plate.



The more usual method of separation is to use another property that depends

on the weight of an atom - gas diffusion. The rate at which a gas leaks

through a hole depends on the mass of the gas molecules - lighter molecules

escape faster. Uranium is prepared in the form of uranium hexafluoride, a 

very heavy gas. U-235 hexafluoride escapes out of a small leak about half

of one percent faster than U-238 hexafluoride. So the early part of the

escaping gas stream is very slightly richer in U-235 than the initial sample.



A single stage like this produces only a very slight enrichment, but by 

making the gas flow through a large number of leaks it can be arranged for

the first part of the gas stream that comes out to be very much richer in

U-235 hexafluoride than the original sample.



This is the basic way in which U-235 is enriched. There are some tricky parts

in the detail of the technology which are kept secret. Setting up the process

is quite expensive.



There may be other more modern processes that I am not aware of - this is 

not really an area that I work in.



On looking back at your original question, I have decided that it might be an

idea to say something about plutonium as well. Plutonium is a separate

element that has different chemical properties from uranium, and is therfore

quite easy to separate from uranium. When the nuclear reaction goes, U-235

atoms break up into smaller pieces and free neutrons. But for every U-235

atom in a piece of nuclear fuel, there are about 20 U-238 atoms. If one of

these absorbs a stray neutron, it will turn first into U-239, which will

decay within a few days via Np-239 (neptunium) to Pu-239 (plutonium).

So plutonium-239 can be obtained readily from used nuclear fuel.



Like U-235, plutonium-239 will undergo fission reactions. So it could be used 

as nuclear fuel or for nuclear bombs. Unfortunately, while its properties

make it easier than U-235 to use in bombs, they also make it more difficult

to use as a nuclear fuel in power stations, and so far the method has not 

been worked out in a completely safe and satisfactory way. So, as far as I

am aware, plutonium is not yet used as nuclear fuel in power stations.



John.