Uranium

Isotopes

Naturally occurring uranium is composed of three major isotopes, 238U, 235U, and 234U, with 238U being the most abundant (99.3% natural abundance). All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being 238U with a half-life of 4.5 × 109 years, 235U with a half-life of 7 × 108 years, and 234U with a half-life of 2.5 × 105 years. 238U is an α emitter, decaying into Lead-206.

Related Topics:
Isotope - Natural abundance - Radioactive - Radioisotope - Half-life - Decaying

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Uranium isotopes can be separated to increase the concentration of one isotope relative to another. This process is called "enrichment" (see enriched uranium). To be considered "enriched" the 235U fraction has to be increased to significantly greater than 0.711% (by weight) (typically to levels from 3% to 7%). 235U is typically the main fissile material for nuclear power reactors. Either 235U or 239Pu are used for making nuclear weapons. The process produces huge quantities of uranium that is depleted of 235U and with a correspondingly increased fraction of 238U, called depleted uranium or "DU". To be considered "depleted", the 235U isotope concentration has to have been decreased to significantly less than 0.711% (by weight). Typically the amount of 235U left in depleted uranium is 0.2% to 0.3%. This represents anywhere from 28% to 42% of the original fraction of 235U.

Related Topics:
Enriched uranium - Nuclear power - Nuclear weapon - Depleted uranium

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Given that the half life of 235U is considerably shorter than 238U, the "depleted" uranium is still significantly radioactive, as is the natural uranium after refining.

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Another way to look at this is as follows: CANDU reactors use natural uranium (0.71% fissile material). From Pressurized water reactors (PWRs) of typical design (most USA reactors are PWR) we note the fuel goes in with about 4% 235U and 96% 238U and comes out with about 1% 235U, 1% 239Pu and 95% 238U. If the 239Pu were removed (fuel reprocessing is not allowed in the USA) and this were added to the "depleted uranium" then we would have 1.2% fissile material in the reprocessed "depleted uranium" and at the same time have 1% fissile material in the left over "spent" fuel. Both of these would be considered "enriched" fuels for a CANDU style reactor.

Related Topics:
CANDU reactor - Pressurized water reactor

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