A news item:
"An International Atomic Energy Agency report confirmed Iran's refusal to heed Security Council demands to freeze uranium enrichment and stated that the Islamic republic has enriched about 630 kilograms of low-enriched uranium suitable for nuclear fuel. Some analysts contend that the amount is enough to upgrade into a nuclear weapon."
What is low-enriched uranium and how can you make a bomb from it?
Natural uranium is about 0.7% U-235, 99.4% U-238 and the balance U-234. It is the U-235 that is capable of sustaining a nuclear chain reaction, necessary for making a bomb. In fact, U-235 is the only naturally occurring isotope that can sustain a nuclear chain reaction. But the 0.7% content of natural uranium is loo low a concentration for making a bomb. The percentage has to be increased using a method such as a centrifuge, to a level much higher, at least 20%. The graph below shows the kilograms of uranium needed to make a chain reaction vs the amount of enrichiment. At 6 percent the amount is infinite. At 20%, 400 kg will do it. At 80 % a few kg is more than enough. Low-enriched uranium is that which is less than 20% enriched. Well on its way to bomb-grade, but not practical for making a bomb. If the Iranian LEU is 10% U-235, continued refining will give enough to make at least one bomb.
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