Subcriticality, Criticality and Supercriticality
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.
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A dump truck drives over the scarred earth of the Rossing Uranium Mine in Namibia.
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.
In designing a nuclear bomb, engineers need the mass of uranium to be very supercritical so that all of the U-235 atoms in the mass split in a single microsecond. Think of it as all the kernels in a bag of popcorn popping at once, as opposed to sequentially.
In a nuclear reactor, however, the last thing you (and the rest of the world) want is all your atoms splitting at once. But the reactor core needs to be slightly supercritical so that plant operators can raise and lower the temperature of the reactor. The control rods give the operators a way to absorb free neutrons so operators can maintain the reactor at a critical level. We'll talk more about this next.
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How do engineers control the criticality of the uranium? The amount of U-235 in the mass (the level of enrichment) plays a role, as does the shape of the mass itself. If the shape of the mass is a very thin sheet, most of the free neutrons will fly off into space rather than hitting other U-235 atoms. As such, a sphere is the optimal shape, and you'd need 2 pounds (0.9 kg) of uranium-235 in it in to achieve a critical reaction. This amount is therefore referred to as the critical mass. For P-239, the critical mass is about 10 ounces (283 grams).
How do technicians control these levels? Find out on the next page.
