Nuclear fission can only occur in some types of heavy and unstable nuclei such as in the uranium -235 and plutonium -239.

核裂变只能在某种类型的重核及不稳定核素内发生，如铀- 235及钚- 239。

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In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed.

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From a public health perspective, the most important isotopes are short-lived isotopes of iodine (like Iodine-131), Cesium-137, Strontium-90, and possibly Plutonium-239.

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Each fission reaction splits the nucleus of an atom of uranium-235 or plutonium-239 into two smaller atoms and releases a great deal of energy.

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