Asked by | 21st Mar, 2009, 11:05: AM
When a U-235 atom splits, two or three neutrons fly off. If there are no other U-235 atoms around, then those free neutrons fly into space as neutron rays. However, if the U-235 atom is part of a mass of uranium, then there are plenty of other U-235 atoms nearby for the freewheeling neutrons to collide with. Will one or more of the free neutrons hit another U-235 atom? The answer to that question determines a nuclear reactor's status.
Critical mass: If, on average, exactly one of the free neutrons from each fission hits another U-235 nucleus and causes it to split, then the mass of uranium is said to be critical. The mass will exist at a stable temperature.
Subcritical mass: If, on average, less than one of the free neutrons hits another U-235 atom, then the mass is subcritical. Eventually, induced fission will end under these conditions and your source of power along with it.
Supercritical mass: If, on average, more than one of the free neutrons hits another U-235 atom, then the mass is supercritical. This will cause the reactor to heat up.
Answered by | 28th Mar, 2009, 06:31: PM
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