Nuclear power plants rely on a surprisingly wide range of chemical elements, from the uranium fuel at the core to the lead and boron in shielding walls. Some elements sustain the chain reaction, others regulate it, and still others protect workers and the environment from radiation. Here’s a breakdown of the key elements and the roles they play.
Uranium: The Primary Fuel
Uranium is the workhorse of nuclear energy. Natural uranium mined from the earth is more than 99% uranium-238, a slowly decaying isotope that cannot sustain a chain reaction on its own. The remaining 0.7% is uranium-235, the isotope that actually splits (fissions) when struck by a neutron, releasing enormous amounts of energy. Nearly all commercial reactors use fuel enriched to between 3% and 5% uranium-235, a grade called low-enriched uranium. A newer category, enriched between 5% and 10%, is being developed to let reactors run longer between refueling cycles.
Thorium and Plutonium: Alternative Fuels
Thorium-232 is not fissile by itself, but when it absorbs a neutron it gradually transforms into uranium-233, which can sustain a chain reaction. This breeding process requires a starter fuel like uranium-235 or plutonium-239 to get going, making thorium reactor designs more complex. Plutonium-239, produced when uranium-238 captures a neutron in conventional reactors, can also serve as fuel. Both elements represent potential alternatives that could extend the world’s nuclear fuel supply far beyond what uranium alone offers.
Fluorine: Essential for Fuel Processing
Before uranium reaches a reactor, it must be enriched, and fluorine makes that possible. Mined uranium ore is converted into uranium hexafluoride, a gas made of uranium and fluorine atoms. In this gaseous form, the slightly lighter uranium-235 molecules can be separated from the heavier uranium-238 molecules using high-speed centrifuges. Without fluorine’s ability to form a stable gas with uranium at relatively low temperatures, the enrichment process used worldwide wouldn’t work.
Zirconium: The Fuel’s Protective Shell
Inside a reactor, uranium fuel pellets sit inside long, thin tubes called cladding. These tubes are made from zirconium alloys, chosen in the 1950s by the U.S. Nuclear Navy for a critical property: zirconium absorbs very few neutrons. That means more neutrons remain available to split uranium atoms, keeping the chain reaction efficient. Pure zirconium alone isn’t strong or corrosion-resistant enough, so it’s alloyed with small amounts of tin (up to 1.5%), iron, chromium, and nickel. The most widely used versions are Zircaloy-2 and Zircaloy-4.
More recent designs incorporate niobium into the zirconium base. Adding niobium dramatically reduces hydrogen absorption during corrosion, which otherwise causes the cladding to become brittle over time. This improvement allows fuel to stay in the reactor longer before replacement, improving fuel economy.
Boron, Hafnium, Silver, Indium, and Cadmium: Controlling the Reaction
Control rods are the reactor’s brake pedal. They slide into the core to absorb neutrons and slow or stop the chain reaction. The elements inside these rods are chosen specifically because they’re hungry for neutrons.
- Boron is one of the most effective neutron absorbers. It appears in control rods as boron carbide and is sometimes dissolved directly into reactor cooling water for fine-tuned control.
- Silver, indium, and cadmium are combined into a single alloy used in pressurized water reactor control rods. Each element absorbs neutrons across a different energy range, giving the alloy broad effectiveness.
- Hafnium is a newer alternative. It absorbs neutrons well, resists corrosion, and holds up under intense radiation, making it attractive for pressurized water reactors.
Hydrogen and Deuterium: Slowing Neutrons Down
A fission reaction releases fast-moving neutrons, but slow neutrons are far more likely to split another uranium atom. A moderator slows those neutrons to the right speed. In most commercial reactors, ordinary water does double duty as both coolant and moderator. The hydrogen atoms in water are close in mass to neutrons, so collisions transfer energy efficiently, like a billiard ball hitting another billiard ball.
Canada’s CANDU reactors use heavy water instead. Heavy water contains deuterium, a hydrogen isotope with one extra neutron. Deuterium absorbs fewer neutrons than regular hydrogen, so CANDU reactors waste less of their neutron supply. This efficiency is significant enough that CANDU reactors can run on natural, unenriched uranium, skipping the costly enrichment step entirely.
Sodium, Lead, and Bismuth: Liquid Metal Coolants
Next-generation reactor designs move beyond water cooling to liquid metals, which can operate at much higher temperatures without pressurization. Sodium is the most established option, with a boiling point of 883°C, allowing the reactor to run hotter and generate electricity more efficiently. The tradeoff is that sodium reacts violently with water and air, demanding careful engineering. Sodium-potassium alloys lower the melting point to just 12°C, making them easier to handle.
Lead and lead-bismuth alloys are the other leading candidates. Lead boils at roughly 1,743°C, essentially eliminating the risk of coolant boiling during operation. Lead-bismuth eutectic has a lower melting point, making it easier to start up and shut down. Gallium, with a melting point near room temperature (29.8°C) and exceptionally high thermal conductivity, is also being explored as an emergency coolant for severe accident scenarios.
Lead, Barium, Iron, and Boron: Radiation Shielding
Different types of radiation require different shielding strategies, and each calls for specific elements. Gamma rays, the most penetrating form of radiation from a reactor, require dense, heavy elements to block them. Shielding concrete in nuclear facilities uses heavyweight aggregates containing iron (from magnetite ore) or barium (from barite ore) to absorb gamma radiation effectively. Steel liners inside containment structures serve the same purpose.
Neutron shielding works differently. Light elements are better at absorbing slow and intermediate neutrons because their atomic nuclei are closer in mass. Boron and hydrogen are the primary choices here, often combined in borated polyethylene shields. Heavy atoms handle fast neutrons by slowing them down through collisions before the lighter elements absorb them. Many shielding systems layer these materials to handle the full spectrum of radiation coming from the core.
Fission Products: Elements Created Inside the Reactor
Splitting uranium doesn’t produce equal halves. The most common fragment masses cluster around 95 and 137 atomic mass units, creating a predictable set of radioactive byproducts. The most significant ones from a safety perspective are cesium-137, strontium-90, and iodine-131.
Cesium-137 and strontium-90 both have half-lives around 30 years (30.07 and 28.8 years, respectively), meaning they remain dangerously radioactive for centuries. Cesium-137 emits both beta particles and gamma rays, while strontium-90 mimics calcium in the body and can accumulate in bones. These two isotopes are considered the most hazardous long-term environmental contaminants from any nuclear accident.
Iodine-131 delivers a higher initial radiation dose but has a half-life of only 8 days, so it decays relatively quickly. Its danger lies in how readily the body absorbs it into the thyroid gland. Xenon and krypton, both noble gases, are also produced in large quantities but disperse into the atmosphere rather than accumulating in soil or water. Other fission products include cerium, zirconium (stable end products of decay chains), and dozens of other elements scattered across the middle of the periodic table.