Introduction

A nuclear reactor produces and controls the release of energy from splitting the atoms of certain elements. In a nuclear power reactor, the energy released is used as heat to make steam to generate electricity. In a research reactor the main purpose is to utilize the actual neutrons produced in the core. In most naval reactors, steam drives a turbine directly for propulsion for a ship or submarine.

A nuclear power facility.

The principles for using nuclear power to produce electricity are the same for most types of reactors. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive the turbines which produce electricity (as in most fossil fuel plants).

There are several components common to most types of reactors:

• Fuel: Usually pellets of uranium oxide (UO₂) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.
• Moderator: This is material which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.
• Control rods: These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems involve adding other neutron absorbers, usually as a fluid, to the system.)
• Coolant: A liquid or gas circulating through the core so as to transfer the heat from it. In light water reactors the moderator functions also as coolant.
• Pressure vessel or pressure tubes: Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the moderator.
• Steam generator: Part of the cooling system where the heat from the reactor is used to make steam for the turbine.
• Containment: The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction inside. It is typically a meter-thick concrete and steel structure.

There are several different types of reactors as indicated in the following table.

Most reactors need to be shut down for refuelling, so that the pressure vessel can be opened up. In this case, refuelling is at intervals of 1-2 years, when a quarter to a third of the fuel assemblies are replaced with fresh ones. The CANDU and RBMK types have pressure tubes (rather than a pressure vessel enclosing the reactor core) and can be refuelled under load by disconnecting individual pressure tubes.

If graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium. Natural uranium has the same elemental composition as when it was mined (0.7% uranium-235 (²³⁵U), over 99.2% uranium-238 (²³⁸238U)), whereas enriched uranium has the proportion of the fissile isotope ²³⁵U increased by a process called enrichment, commonly to 3.5 - 5.0%. In this case, the moderator can be ordinary water, and such reactors are collectively called light water reactors. Because the light water absorbs neutrons as well as slowing them, it is less efficient as a moderator than heavy water or graphite.

Practically all fuel is ceramic uranium oxide (UO₂ with a melting point of 2800°C) and most is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod, the zirconium being hard, corrosion-resistant and permeable to neutrons . Numerous rods form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor core. In the most common reactors these are about 3.5 meters long.

Burnable poisons are often used, especially in boiling water reactors (BWRs), in fuel or coolant to even out the performance of the reactor over the time when fresh fuel is loaded to the time of refuelling. These are neutron absorbers which decay under neutron exposure, compensating for the progressive build-up of neutron absorbers in the fuel as it is burned. The best known is gadolinium, a vital ingredient of fuel in naval reactors that are very inconvenient to refuel, compelling the design of reactors capable of running more than a decade between refuellings.

Pressurized Water Reactor (PWR)

Figure 1: Pressurized Water Reactor

Pressurized water reactors (PWRs) are the most common type of nuclear reactor, with over 230 in use for power generation and several hundred more employed for naval propulsion. The design of PWRs originated for use as a submarine power plant. PWRs use ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit, which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive a turbine.

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies containing 80-100 tonnes of uranium.

Water in the reactor core reaches about 325°C; hence, it must be kept under about 150 times atmospheric pressure to prevent it from boiling. Pressure is maintained by steam in a pressurizer (see diagram). In the primary cooling circuit, the water is also the moderator—if any of it were converted to steam, the fission reaction would slow down. This negative feedback effect is one of the safety features of PWRs. The secondary shutdown system involves adding boron to the primary circuit.

The secondary circuit is under less pressure and the water boils in the heat exchangers, which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.

Boiling Water Reactor (BWR)

Figure 2: Boiling Water Reactor

The design of boiling water reactors (BWRs) is similar to that of the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core in steam form, equipping the reactor with less moderating effect and thus efficiency.

The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived, so the turbine hall can be entered soon after the reactor is shut down.

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary control system involves restricting water flow through the core so that steam in the top part reduces moderation.

Pressurized Heavy Water Reactor (PHWR or CANDU)

Figure 3: Pressurised Heavy Water Reactor

The CANDU (CANada Deuterium Uranium) reactor design has been developed since the 1950s in Canada. It uses natural uranium oxide (0.7% ²³⁵U) as fuel, thus requiring a more efficient moderator, in this case heavy water (deuterium oxide, D₂O).

The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes that form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube is designed so that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit.

A CANDU fuel assembly consists of a bundle of 37 half-meter-long fuel rods (ceramic fuel pellets in zircaloy tubes) along with a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat (though this circuit is not shown in the diagram in Figure 3.

Advanced Gas-cooled Reactor (AGR)

Figure 4: Advanced Gas-cooled Reactor

Advanced gas-cooled reactors (AGRs) are the second generation of British gas-cooled reactors, using graphite moderators and carbon dioxide (CO₂) as a coolant. AGRs are fuelled with uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C, and then past steam generator tubes outside the core, but still inside the concrete and steel pressure vessel. Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen into the coolant.

The AGR was developed from the Magnox reactor, also graphite-moderated and CO₂-cooled, and a number of these are still operating in the UK. They use natural uranium fuel in metal form.

Light Water Graphite-moderated Reactor (RBMK)

RBMK (high-power channel reactor, reaktor bolshoy moshchnosti kanalniy), is a Soviet design developed from plutonium production reactors. RBMKs employ long (7 meter) vertical pressure tubes running through a graphite moderator, and is cooled by water that is allowed to boil in the core at 290°C, much as in a BWR. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 meters long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorbtion without inhibiting the fission reaction, and problems due to positive feedback can arise, which is why they have never been built outside the Soviet Union.

Advanced reactors

Several generations of reactors are commonly distinguished. Generation I reactors were developed in 1950-60s and relatively few are still running today. They typically ran on natural uranium fuel and were graphite-moderated. Generation II reactors are typified by the present U.S. fleet and are the most in operation elsewhere. They typically use enriched uranium fuel and are mostly cooled and moderated by water. Generation III are the Advanced Reactors, the first three of which are in operation in Japan with others under construction or ready to be ordered. They are developments of the second generation designs equipped with enhanced safety features.

Generation IV nuclear reactors are still on the drawing board and will not be operational before 2020 at the earliest, probably later. They will tend to have closed fuel cycles and burn the long-lived actinides now forming part of spent fuel, so that fission products are the only high-level waste. Many will be fast neutron reactors.

More than a dozen Generation III advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs, some are more radical departures. The former include the Advanced Boiling Water Reactor, two of which are now operating with others under construction. The best-known innovative design is the Pebble Bed Modular Reactor, using helium as coolant, at very high temperature, to drive a turbine directly.

Considering the closed fuel cycle, Generation I-III reactors recycle plutonium (and possibly uranium), while Generation IV designs are expected to have full actinide recycling capability.

Fast neutron reactors

Fast neutron reactors (only one in commercial service) do not have a moderator and utilize fast neutrons, generating power from plutonium while making more from the ²³⁸U isotope in or around the fuel. While fast neutron reactors get more than 60 times as much energy from the original uranium compared with normal reactors, they are expensive to build and await resource scarcity to spur further development.

Primitive reactors

The world's oldest known nuclear reactors operated at what is now Oklo in Gabon, West Africa. About 2 billion years ago, at least 17 natural nuclear reactors achieved criticality in a rich deposit of uranium ore. Each operated at about 20 kW thermal. At that time, the concentration of ²³⁵U in all natural uranium was 3.7 percent instead of 0.7 percent as at present. ²³⁵U decays much faster than ²³⁸U, whose half-life is about the same as the age of the Earth. These natural chain reactions, started spontaneously by the presence of water acting as a moderator, continued for about 2 million years before finally dying away.

During this long reaction period, about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements were generated in the orebody. The initial radioactive products have long since decayed into stable elements but close study of the amount and location of these has shown that there was little movement of radioactive wastes during and after the nuclear reactions. Thus, plutonium and the other transuranics remained immobile.

Further Reading

• College of Engineering University of Wisconsin - Madison. Nuclear Reactors.
• Wilson, P.D. (Ed.), 1996. The Nuclear Fuel Cycle: From Ore to Waste. Oxford University Press. ISBN: 0198565402