EEE会議（どうやって核爆弾を作りますか？）.........................................2003/7/26

 

 

 

Alok Jha
The Guardian, Thursday June 19, 2003

In principle it's very easy - get a critical mass of radioactive material, sit
back and watch the runaway nuclear reaction go. But luckily for us it's the
first part - getting the radioactive material - that is the biggest stumbling
block.

Earlier this week, Iran joined the growing list of countries suspected of
developing nuclear weapons. On Monday the European Union said Iran's nuclear
reactors could make the radioactive raw materials needed for a nuclear bomb and
demanded weapons inspectors be allowed in.

"You cannot make a nuclear bomb without fissile material," says Andrew Furlong,
of the Institute of Chemical Engineers. And for an average thermonuclear device,
the necessary material is plutonium or enriched uranium.

Uranium, a naturally-occurring heavy metal, comes as uranium 238 or 235. Both
are radioactive and will decay into other elements, given time, but only the
latter can be forcibly split when neutrons are fired at it. This is the basis of
a nuclear bomb.

When an atom breaks apart, it gives out energy and more neutrons, which can then
split other atoms. Get enough atoms splitting and you have the chain reaction
needed for a bomb blast.

But natural uranium overwhelmingly consists of the 238 isotope, which bounces
back any neutrons striking it - useless then for a bomb. To make a bomb, natural
uranium needs to be treated to concentrate the 235 isotope within it.

And this is where the problems really begin. For every 25,000 tonnes of uranium
ore, only 50 tonnes of metal are produced. Less than 1% of that is uranium 235.
No standard extraction method will separate the two isotopes because they are
chemically identical.

Instead, the uranium is reacted with fluorine, heated until it becomes a gas and
then decanted through several thousand fine porous barriers. This partially
separates the uranium into two types. One is heavily uranium 235, and called
"enriched" while the rest is the controversial "depleted" uranium used to make
conventional weapons.

To make a nuclear reactor, the uranium needs to be enriched so that 20% of it is
uranium 235. For nuclear bombs, that figure needs to be nearer 80 or 90%. Get
around 50kg of this enriched uranium - the critical mass - and you have a bomb.
Any less and the chain reaction would not cause an explosion.

You could use plutonium instead. According to Keith Barnham, a physicist at
Imperial College, this is the preferred material because it makes much lighter
weapons that can be mounted on to missiles.

Plutonium is produced as a by-product in nuclear reactors and only around 10kg
is needed for a bomb. An average power plant needs about a year to produce
enough and expensive reprocessing facilities are required to extract the
plutonium from the fuel.

With the basic material, life gets easier. The bomb will explode once the
critical mass of uranium or plutonium is brought together. So, to begin with,
and to make sure that it doesn't explode in the hands of its owners, the bomb
needs to keep the metal separated into two or more parts. When the weapon is in
place and ready to go off, these sub-critical masses need only be thrown
together - and this can be done with conventional explosives.

The chain reaction, explosion and familiar mushroom cloud then take care of
themselves.

Guardian Unlimited c Guardian Newspapers Limited 2003
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