Nuclear weapons without 90%-enriched uranium

A correction. Some days ago I said that one needs to enrich uranium to at least 90% to build a bomb. However, the nuclear engeneer Sérgio Guerreiro Ribeiro remembered in "O Globo" (in Portuguese) that it is not necessarily so. It is also possible to make it with plutonium-239 (Pu-239), that is produced as “waste” in common nuclear power plants. And a bomb needs about 8 kg of Pu-239, against 25 kg of uranium-235. Plutonium produced normally in power plants have 60% of Pu-239 and a weapon needs 93%, but there are techniques to reach this proportion. Useful numbers to assess scenarios...

Numbers about bombs, Iran and so on

I have seen journalists saying that Iran will have a nuclear bomb soon because he is enriching uranium at 20%. Well, we have to take care with numbers. A nuclear weapon needs enrichment of 90%. Nuclear power plants use 5%. Reactors for research, 20%. In Nature, it appears at 0,71%. These numbers are the proportion of uranium-235 in the mixture of U-235 and U-238, the two main kinds ("isotopes") of U. According to Global Security, a set of 850 to 1000 ultracentrifuges spends 1 year to produce U enriched at 90% enough to produce a bomb (20-25 kg). Just numbers, but useful to assess scenarios.