Here's What It Really Means That Iran Enriched Uranium to 4.5%.

Pedro_P

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▼ Iran claims it has enriched uranium to 4.5%, breaking the limit of 3.67% set during the 2015 nuclear deal. The move was a response to the U.S. violating the terms of the deal under President Donald Trump's administration. But what does the enrichment news mean?
To a certain extent, this is a question with a simple, chemical answer. As the U.S. Nuclear Regulatory Commission explains on its  website, uranium comes in a few different forms (or "isotopes"). All of them have the same number of protons (92) but a different number of neutrons. By far, the most common such isotope in nature is uranium-238, which has 146 neutrons. On Earth, this isotope makes up 99.3% of any sample of naturally occurring uranium.
But for nuclear reactors(or bombs), that flavor isn't very useful. Dense clusters of uranium-238 don't tend to start nuclear chain reactions. The second most common isotope, however, uranium-235 (making up just about 0.7% of any sample of natural uranium and containing 143 neutrons), does tend to start nuclear chain reactions. In these reactions, the nuclei of the uranium atoms split into smaller nuclei and release neutrons. Those neutrons then cause other nuclei to split, releasing more neutrons for a self-sustaining "chain" reaction that emits enormous amounts of energy.
Enriching uranium is the process of sorting uranium-238 atoms out of a uranium sample such that the sample includes a higher proportion of uranium-235. (▪ ▪ ▪)

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