19.7
Nuclear Power
Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium…
Nuclear fission releases a great deal of thermal energy, allowing electricity generation from a steam turbine.
Nuclear fuel is typically a fissile nuclide such as uranium-235 that produces more than one neutron per fission on average. Fast neutrons released by fission must be slowed down by neutron moderators because thermal neutrons start chain reactions in fissile fuel most efficiently.
Water is a good moderator because hydrogen nuclei and neutrons have comparable sizes, ensuring that neutrons lose substantial kinetic energy in the collision. Heavy water is even better, as deuterium already has a neutron and is unlikely to absorb another.
Moderators also function as a neutron reflector to keep neutrons in the core in an even distribution.
Because the spontaneous fission of uranium-235 or 238 is unpredictable, a neutron source is installed in a reactor to ensure controlled initiation of the chain reaction.
The status of the chain reaction is described by the neutron multiplication factor, k: the ratio of the number of neutrons produced by fission in a generation to the number of neutrons produced by fission in the previous generation.
When k is less than 1, the reactor is subcritical and the energy output is decreasing. When k is 1, the reactor is critical and the energy output is steady. When k is greater than 1, the reactor is supercritical and the energy output is increasing.
The chain reaction is controlled with control rods made of neutron-absorbing materials like boron or cadmium. Fully inserted control rods absorb a large number of neutrons, keeping the reactor subcritical. Withdrawing the control rods allows more and more fissions to occur.
Coolant, such as water, transfers heat away from the reactor core to make steam for the turbine. As the reactor heats up, the neutrons move faster and are less likely to cause fissions, which helps avoid overheating.
The core is shielded by materials like water and thick concrete layers. The overall core design and containment structure both depend on the specific type of reactor.
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Q1: Why do nuclear reactors need neutron moderators?
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.
Q2: What is the neutron multiplication factor and how does it control reactor power?
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.
Q3: How do control rods regulate the fission rate in a nuclear reactor?
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.
Q4: Why is uranium-235 preferred as nuclear fuel despite its low natural abundance?
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.
Q5: What role does reactor coolant play in nuclear power generation?
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.
Q6: How does a nuclear reactor's containment system protect against radiation?
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.
Q7: Why do nuclear reactors require an external neutron source?
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.