Why does a nuclear fission reaction generally not result in an uncontrolled chain reaction?

Answer 1

A nuclear fission reaction generally does not result in an uncontrolled chain reaction because of the release of binding energy. This contributes heat and energy into the reaction, which tends to reduce the density of the fissile material, i.e. making it larger, and thus making it subcritical.

The hard part in creating an uncontrolled fission reaction is in holding the fissile material in a supercritical geometry long enough to convert all of it. This requires enormous pressure and high technology.

Also, the result of a fission reaction includes neutrons that are often too energetic to properly go on and create subsequent reactions. In a controlled reaction, a moderator is required to "slow down" the neutrons. In an uncontrolled reaction, the dynamics are such that only prompt neutrons are needed to support the chain reaction, a state we call super prompt criticality, but in order to sustain this, the enrichment of the fuel must be above a certain level, typically greater than 20 percent U-235. Modern weapons are in the high 80's and 90's percent.

Answer 2

Nuclear fission reactions are controlled in reactors by using materials that absorb excess neutrons, like control rods. This prevents a chain reaction from running out of control. Additionally, reactors are carefully designed and operated to maintain the conditions necessary to sustain controlled fission reactions without escalating into an uncontrolled chain reaction.