Nuclear Plant Safety

How Does a Nuclear Power Plant Work?

Safety is at the heart of nuclear technology. Understanding how a plant works helps to demonstrate its several inherent protective features.

Fission: the Nuclear Reaction

A nuclear power plant houses one or more nuclear reactors, which are used to split heavy atoms, like uranium, to generate heat in a process called fission. Fission occurs when a fuel atom is hit by a neutron, causing the fuel atom to split. In today’s reactors, low-energy neutrons, called thermal neutrons, are used to split uranium-235 fuel. When the uranium-235 atom splits, two or three new neutrons are created with very high energy. These high-energy neutrons, called fast neutrons, have to be decreased in energy before they can cause more uranium-235 fissions.

The Chain Reaction “Sweet Spot”

Nuclear reactors operate using a chain reaction of neutrons and fissions. Each fission produces multiple neutrons, so if every neutron caused more fissions, then the chain reaction would increase in size rapidly: one fission would produce three neutrons, which cause three fissions, releasing nine neutrons, and so forth. If one fission creates, on average, 0.5 neutrons, then within a fission or two, all the neutrons would be consumed and the reaction would stop. Nuclear reactors operate safely at the “sweet spot” where one neutron from each fission leads to another fission, on average. Critical reactors are in the state where for every fission, one neutron is created that leads to an additional fission, creating a stable chain reaction. When we talk about a nuclear reactor being “critical,” this is what we mean. When a reactor operator increases or decreases the power level of the reactor, they have to take the reactor into a configuration above or below the critical level. When increasing power, the reactor is taken supercritical with just barely more than one fission occurring from each prior fission (typically only one or two extra fissions for every hundred atoms). When decreasing in power, the opposite occurs and the reactor is taken subcritical to produce one or two less fissions for every hundred atoms. In all cases, the reactors are very tightly controlled.

Components and Heat Transfer

The heat produced in the nuclear fission reaction is transferred from the reactor through the plant’s cooling systems to generate steam, which spins a turbine and generates electricity. Specifically, what are the parts of a nuclear plant, and how do they work?

Reactor Core

The core of a nuclear reactor is where the nuclear reaction takes place, fissioning fuel atoms to release heat. The core is a dense collection of nuclear fuel that has been fabricated into fuel rods and assemblies.

Fuel Rods

Fuel Rods are thin and long, with a diameter of about 1 cm and a length of about 4 m. They contain nuclear fuel — usually, low-enriched uranium, which contains 3-5% uranium-235 — in a form that remains chemically stable at high temperatures, most commonly uranium dioxide (UO₂). The fuel is contained inside the fuel cladding, which is made of alloys with high melting temperatures and low neutron interaction properties including aluminum, stainless steel, and zirconium. The fuel and cladding are designed and sealed to prevent the radioactive fission products that are created within the fuel material from escaping, serving as the first physical safety barrier of a nuclear reactor. A collection of over 150 fuel rods forms a fuel assembly. Assemblies group together the rods, making them easier to keep track of and move. A commercial-size reactor may contain between 150 and 250 fuel assemblies.

Reactor Vessel and Coolant

The reactor core is located inside a reactor vessel, which is the second physical safety barrier separating it from the outside world. Reactor vessels in the operating fleet are designed to withstand high temperatures and pressures. These reactor vessels are usually called reactor pressure vessels and a typical nuclear reactor operates at a pressure of about 150 atmospheres. Inside the reactor pressure vessel, the reactor core is submerged in a circulating coolant. For most reactors today, liquid water is used as the coolant. Due to the high pressures inside the reactor pressure vessel, the water can be heated to a temperature of about 300 degrees Celsius without boiling. In some of the advanced reactors being demonstrated and commercialized, novel coolants are able to maintain liquid form at very high temperature but at ambient or near-ambient pressures. For these reactor vessels, the same requirements to withstand high pressures may be alleviated.

Moderator and Control Rods

In most nuclear reactors today, the coolant also serves as a moderator. A moderator mediates a nuclear reaction by “slowing down” neutrons — in other words, decreasing their energy to the appropriate level to cause more fissions — by colliding with them. To carefully control the rate of the reaction in the core, retractable control rods can be inserted and removed between fuel assemblies. These are designed to be fully inserted into the core in the event of loss of power or other failures, so that they will stop the chain reaction in accident scenarios no matter what occurs.

Heat Transfer and Power Generation

In the reactor vessel, the coolant absorbs the heat produced by the reactor core. It is then circulated through the primary coolant loop. In pressurized water reactors, the primary loop leads to a heat exchanger, which is used to transfer heat from the primary coolant loop to a secondary coolant loop. The coolant in the secondary loop does not receive radiation and has a low risk of contamination. After passing through the heat exchanger, the primary loop is cooled and cycles back into the reactor core. The secondary coolant loop leads to a steam generator, converting the hot water into steam, which then spins a turbine to generate electricity. The hot steam is then collected in a condenser, which converts it back to liquid water which can be cycled repeatedly through the loop. The primary coolant is subject to significant neutron radiation in the core. In pressurized water reactors, it doesn’t contact other pieces of machinery because it could contaminate them with small amounts of radioactivity. However, boiling water reactors accept this additional contamination by boiling water in the primary loop to directly drive a turbine generator. The turbine receives small levels of contamination, but it allows the removal of an intermediary coolant loop.

Containment

In the context of nuclear energy, containment can have two meanings. As a verb, containment means preventing the spread of radioactivity. As a noun, containment refers to the mechanisms surrounding the reactor core and reactor vessel. Most notably, operating reactors are contained in a large, strong building of thick reinforced concrete, which is designed and certified to completely prevent the spread of radiation in case of a reactor accident. The containment building also protects and isolates the reactor from external events such as hurricanes, earthquakes, tornadoes, tsunamis, and even airplane impacts. This reinforced structure is the third and ultimate physical safety barrier around the nuclear reactor. Nuclear plants are contained on dedicated pieces of land. Environmental radioactivity-measuring devices are placed in and around the plant to monitor and ensure that no radioactivity is escaping from the reactor vessel or reactor building. The land is guarded to prevent clandestine intruders to the nuclear facility, and operates with a local network disconnected from the internet, eliminating the risk of cyberattacks.