After the uranium ore is mined and milled, it is refined into enriched uranium hexafloride, a highly toxic gas. The UF6 gas is then mixed with nitric acid to create a solid compound of uranium dioxide. Normally this compound contains 3-4% uranium 235. If the percentage of U-235 becomes to high, it is possible to achieve criticality, or the start of a nuclear reaction. The uranium dioxide is then formed into small pellets which are them pressed into fuel rods for use in nuclear reactors.