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Fast neutrons released by fission must be slowed down to cause reliable fission in uranium-235. Neutron moderators like water and heavy water reduce neutron speed through collisions, allowing thermal neutrons to efficiently trigger nuclear chain reaction and critical mass conditions. Moderators also function as neutron reflectors, keeping neutrons evenly distributed in the reactor core.

The neutron multiplication factor, k, is the ratio of neutrons produced in one fission generation to the previous generation. When k is less than 1, the reactor is subcritical and energy decreases. When k equals 1, the reactor is critical with steady output. When k exceeds 1, the reactor is supercritical and energy increases.

Control rods contain neutron-absorbing materials like boron or cadmium. Inserting rods absorbs slow neutrons, reducing fission rates and decreasing power output. Withdrawing rods allows more fissions to occur, increasing energy production. In emergencies, fully inserting all control rods shuts down the chain reaction.

Uranium-235 produces more than one neutron per fission on average, sustaining chain reactions. Though naturally occurring at only 0.7 percent by weight, most power reactors enrich fuel to 3-5 percent uranium-235. This enrichment provides sufficient fissile material for self-sustaining reactions in modern reactors containing millions of fuel pellets.

Reactor coolant transfers heat from the fission reaction to external boilers and turbines, where thermal energy converts to electricity. Water is the most common coolant, though specialized reactors use molten sodium, lead, or molten salts. Two separate coolant loops often prevent contaminated coolant from reaching the steam turbine.

The containment system has three protective layers: a steel shell absorbs neutron radiation; high-density concrete shields absorb gamma rays and X-rays; additional shielding absorbs secondary radiation. Steel or concrete domes cover pressurized water reactors to contain radioactive materials during accidents.

Spontaneous fission of uranium-235 and uranium-238 is unpredictable and produces very few neutrons. An external neutron source, such as beryllium-9 paired with an alpha emitter like americium-249 or plutonium-239, initiates the fission chain reaction reliably and allows controlled reactor startup.