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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 11: Critical Mass and Nuclear Reactors

Subcritical Mass and Supercritical Mass

Figure 12-20. Subcritical Mass and Supercritical Mass

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Subcritical Mass and Supercritical Mass

Figure 12-20. Subcritical Mass and Supercritical Mass

In a supercritical mass, neutrons produced by fission have a high probability of causing subsequent fission reactions. In a subcritical mass, the neutrons escape the material without causing more fission. Every time there is a red branch, more neutrons are formed. Note how the size of the subcritical mass in blue only allows three fission reactions creating neutrons as compared to many more within the blue area for the supercritical mass.

A chain reaction will only be self-sustaining if the piece of fissile material is large enough. If it is too small, the neutrons end up flying out into empty space instead of colliding with nuclei inside. The amount needed to sustain the reaction is called the "critical mass." A smaller amount is termed a "subcritical mass"; a larger amount is called a "supercritical mass." (Figure 12-20)

Diagram of an Atomic Bomb

Figure 12-21. Diagram of an Atomic Bomb

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Diagram of an Atomic Bomb

Figure 12-21. Diagram of an Atomic Bomb

The two subcritical masses of uranium-235 are kept separate until the moment of detonation.

A nuclear bomb starts with two or more subcritical masses of uranium or plutonium. As soon as they are brought together, they form a supercritical mass, which explodes.

For the bomb to work, the subcritical masses must be propelled together extremely rapidly. This is done with a conventional (non-nuclear) explosive. (Figure 12-21)

A fission reaction can be slowed down to a consistent, stable rate; it doesn't have to happen all at once as in an explosion. This is what happens in a nuclear reactor; a controlled fission reaction generates electricity. The nuclear fuel is composed of uranium pellets inside metal tubes called "fuel rods." Each fuel rod is undergoing some fission, sending neutrons to neighboring rods, and causing their atoms to undergo fission.

Fuel Rods and Control Rods

Figure 12-22. Fuel Rods and Control Rods

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Fuel Rods and Control Rods

Figure 12-22. Fuel Rods and Control Rods

Lowering the control rods in a reactor core slows the reaction. Raising them allows the reaction to speed up and generate heat.

If nothing intervened, the chain reaction could rapidly speed up and get out of control; the fuel rods would overheat and cause a meltdown. To slow down the rate of reaction, control rods are lowered between the fuel rods. The control rods are made out of a material that can absorb neutrons such as graphite. When the control rods are between the fuel rods, the neutrons of one fuel rod cannot cause as much fission in a neighboring fuel rod, and the whole reaction slows down. (Figure 12-22)

The fuel rods and the control rods make up the core of a nuclear power plant. When the reactor is active, the fission chain reaction heats a liquid coolant surrounding the rods. The hot coolant is pumped through pipes to boil water in the steam generator; the pressurized steam flows through the steam line to the turbine, which powers the generator. The steam is then cooled and condensed by water from the cooling tower, and then pumped back to the steam generator. (Figure 12-23)

Schematic of a Nuclear Power Plant

Figure 12-23. Schematic of a Nuclear Power Plant

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Schematic of a Nuclear Power Plant

Figure 12-23. Schematic of a Nuclear Power Plant

This schematic shows how power is generated and transferred in a pressurized water reactor.

Glossary

Critical mass

The minimum amount of a substance needed to sustain a fission chain reaction.

Subcritical mass

An amount of a fissile material below the critical mass.

Supercritical mass

An amount of a fissile material above the critical mass.

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