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FACTS ABOUT NUCLEAR WEAPONS |
Materials that readily undergo fission are called fissile materials. Those usually used in nuclear weapons are uranium and plutonium. Many types of uranium and plutonium exist--these are called isotopes. The U-235 isotope of uranium the Pu-239 isotope of plutonium are used in weapons. (Section 9 tells you what these numbers mean.)
Naturally occurring uranium contains only 0.7% of U-235, the rest is not fissile and consists mainly of U-238. Natural uranium can be refined in many ways to increase the proportion of U-235. This process of increasing the percentage of U-235 is called enrichment.
Some reactors (and all Indian ones) run on natural uranium. Others need uranium with about 2% to 5% U-235--this is called enriched uranium.
Weapons need uranium enriched to the level of 80% to 90% U-235. Such uranium is called highly enriched uranium (HEU).
Plutonium does not occur naturally and has to be made in nuclear reactors. It contains the isotopes Pu-239, Pu-240 and other higher isotopes. For weapons applications, Pu-240 and higher isotopes are contaminants. (They make the fission start too early, reducing the total yield of the weapon.)
The normal reactor grade plutonium, produced in power reactors, contains about 65% Pu-239. Weapons require above 94% of Pu-239. Making this in a normal reactor is not energy-efficient, so usually weapons grade plutonium is produced in ``research'' reactors.
It is generally felt that it should be possible to make low-yield weapons (upto a few kilotons) from reactor grade plutonium.
Some weapons experts are of the opinion that, with a sophisticated design, a reactor grade plutonium fission weapon can have as much yield as one made with weapons grade plutonium (upto about 20 kilotons). However, a reactor grade weapon would use more plutonium for the same yield. Reactor grade plutonium is also more difficult to handle and engineer.
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