How Do Nuclear Reactors Work?

Nuclear fission reactors are machines that facilitate a sustained fission reaction in suitable materials, usually for the purpose of electrical power production. Most nuclear fission reactors currently in use are uranium-235 burning thermal spectrum light water reactors, so I shall describe this type first, then look at the alternatives.


Light Water Reactors:

Light water reactors get their name from their moderating fluid, which is ordinary water. This is called “light water” because the water molecules have ordinary hydrogen atoms with one proton and no neutrons as the H2 component of H2O. Some nuclear reactors use heavy water as a moderator, which has deuterium (heavy hydrogen), with one proton and one neutron, as the H2 component. Heavy water reactors will be described later. There are two basic types of light water reactor in popular use, the Pressurised Water Reactor (PWR) and the Boiling Water Reactor (BWR). The PWR is the most common.

Pressurised Water Reactors:

Modern PWRs are built around certain basic components:

Reactor Core:

The reactor core is that part of the reactor where the nuclear fuel assemblies are located and the fission reaction takes place. The shell of the core is known as the pressure vessel in a light water reactor, as part of its function is to contain water and steam at high pressure.

Nuclear Fuel:

The fuel for a nuclear reactor generally consists of pellets of uranium oxide embedded within zirconium alloy (zircaloy) tubes, which are arranged into bundles called fuel assemblies. The uranium in the fuel has been enriched to consist of 3-5% fissile U-235 rather than the natural level of 0.7% U-235.

Moderator:

The moderator is a substance which slows neutrons emitted naturally by the uranium fuel to thermal velocities, enabling them to be captured by U-235 nuclei and initiate fission reactions. Moderator material needs a high proportion of very light elements in order to absorb the kinetic energy of the neutrons rather than just reflecting them with little loss of velocity. Hydrogen in water (H2O) is well suited to this purpose, and light water is used as the moderator in most current reactors.

Control Rods:

Control rods are long rods inserted between fuel assemblies for the purpose of absorbing neutrons before they can participate in a fission reaction, thus controlling the reaction rate. They are made of elements with high neutron absorption cross sections, such as silver, indium, cadmium, boron, and others.

Coolant:

The coolant is a fluid which keeps the temperature of the reactor core under control by removing heat. It also serves the critically important function of delivering this heat to a secondary cooling loop which drives a turbine to produce electric power. Because water is both a highly effective moderator and coolant, it serves both roles in a light water reactor.

-Primary coolant loop. The primary coolant loop pumps water in a continuous cycle through the reactor core, then through the steam generator of the secondary loop, then back into the core for the next cycle. The water is pressurised to remain liquid at operating temperature. There is no exchange of water between the primary loop and the secondary loop.

-Secondary coolant loop. The secondary coolant loop contains water which is boiled in the steam generator using heat from the primary loop. The steam is then used to drive a turbine for electricity production before being cooled by the tertiary loop and recondensed to flow through the steam generator again. There is no exchange of water with the primary or tertiary loop.

-Tertiary coolant loop. The tertiary loop is an open cycle which takes water from a source outside the plant and uses it to cool the secondary loop by condensing the steam after it has driven the turbine. This water has no direct contact with water in the secondary or primary loop. It is used once, then discharged into the environment, usually at a temperature a few degrees higher than it had when taken into the plant.

The basic features listed above show how a nuclear reactor works in broad outline; Fuel bundles are placed in the reactor core. Neutrons emitted naturally by the uranium fuel are slowed by the moderator to thermal velocity and then set off subsequent fission reactions which liberate more neutrons to continue the process. The energy released by these reactions heat the coolant, which transfers heat energy through the primary and secondary loops to drive a turbine for power production. The fission reaction rate is managed with the assistance of neutron-absorbing control rods. Excess heat is taken away by the tertiary cooling loop.



There are other important components to a nuclear plant not directly related to power generation.

Containment Structure:

Containment refers to the barriers built into the reactor to shield the environment from radioactive material. Modern light water reactors have multiple layers of containment. The first level is the ceramic structure of the uranium oxide fuel pellets which trap fission products within the lattice. The next is the zircaloy cladding for the fuel. The third is the reactor vessel and cooling system. The final containment is the steel-reinforced concrete structure known as the containment building in which the reactor is situated. This is the ultimate backup if all else fails, and is designed to withstand high internal pressure to contain the steam released by a breach of the pressure vessel. They are also now designed to withstand attacks from outside such as missile strikes and impacts of large aircraft.

Radwaste Facility:

Nuclear plants need a facility to store used fuel after it has been removed from the core. The used fuel contains highly radioactive fission products which must be sequestered from the environment for some time before it can be moved to permanent storage. The first unit in the radwaste system is the spent fuel pool. This is a pool about forty feet deep where the spent fuel is kept while it cools down for several years. Spent fuel initially has a high temperature, maintained by the decay of high-level fission products. Storage in the spent fuel pool helps to prevent melting, and also provides shielding from the high level radioactivity of the spent fuel.

After about a decade the shortest lived and most highly radioactive fission products have decayed away, and the spent fuel can be removed from the pool to be sealed in steel-reinforced concrete containers for above ground storage.


Boiling Water Reactors:

BWRs are similar to PWRs, but are operated at a lower pressure. Instead of maintaining the water in the pressure vessel as a pressurised liquid, the water in a BWR is allowed to partially boil and drive a steam turbine directly, without using a secondary coolant loop. Because the turbine is in direct contact with steam from the reactor core, extra containment precautions are taken in the turbine hall of a BWR.

In the United States, the civilian nuclear power plant fleet consists of 69 PWRs and 35 BWRs.

Alternative reactor types:

There are many possibilities for reactor design beyond the ordinary light water variety. We will look at these in turn in subsequent essays.

-Heavy water reactors.

-Fast breeder reactors.

-Gas-cooled reactors.

-Molten salt reactors.

-Aqueous reactors.

-Small modular reactors.