Introduction

Today many nations are considering an expanded role for nuclear power in their energy portfolios.  This expansion is driven by concerns about global warming, growth in energy demand, and relative costs of alternative energy sources.  In 2008, 435 nuclear reactors in 30 countries provided 16% of the world’s electricity.  In January 2009, 43 reactors were under construction in 11 countries, with several hundred more projected to come on line globally by 2030.

A Nuclear Power Plant

Courtesy of R2 Controls

Nuclear reactors produce energy through a controlled fission chain reaction (see The First Chain Reaction).  While most reactors generate electric power, some can also produce plutonium for weapons and reactor fuel.  Power reactors use the heat from fission to produce steam, which turns turbines to generate electricity.  In this respect they are similar to plants fueled by coal and natural gas.  The components common to all nuclear reactors include a fuel assembly, control rods, a coolant, a pressure vessel, a containment structure, and an external cooling facility.

The speed of the neutrons in the chain reaction determines the reactor type (Figure 1).  Thermal reactors use slow neutrons to maintain the reaction.  These reactors require a moderator to reduce the speed of neutrons produced by fission.  Fast neutron reactors, also known as fast breeder reactors (FBR), use high speed, unmoderated neutrons to sustain the chain reaction.

Figure 1 – Types of Nuclear Reactors

 

Thermal reactors operate on the principle that uranium-235 undergoes fission more readily with slow neutrons than with fast ones.  Light water (H2O), heavy water (D2O), and carbon in the form of graphite are the most common moderators.  Since slow neutron reactors are highly efficient in producing fission in uranium-235, they use fuel assemblies containing either natural uranium (0.7% U-235) or slightly enriched uranium (0.9 to 2.0% U-235) fuel.  Rods composed of neutron-absorbing material such as cadmium or boron are inserted into the fuel assembly.  The position of these control rods in the reactor core determines the rate of the fission chain reaction.  The coolant is a liquid or gas that removes the heat from the core and produces steam to drive the turbines.  In reactors using either light water or heavy water, the coolant also serves as the moderator.  Reactors employing gaseous coolants (CO2 or He) use graphite as the moderator.  The pressure vessel, made of heavy-duty steel, holds the reactor core containing the fuel assembly, control rods, moderator, and coolant.  The containment structure, composed of thick concrete and steel, inhibits the release of radiation in case of an accident and also secures components of the reactor from potential intruders.  Finally, the most obvious components of many nuclear power plants are the cooling towers, the external components, which provide cool water for condensing the steam to water for recycling into the containment structure. Cooling towers are also employed with coal and natural gas plants.

Reactor Fundamentals

It is important to realize that while the U-235 in the fuel assembly of a thermal reactor is undergoing fission, some of the fertile U-238 present in the assembly is also absorbing neutrons to produce fissile Pu-239.  Approximately one third of the energy produced by a thermal power reactor comes from fission of this plutonium.  Power reactors and those used to produce plutonium for weapons operate in different ways to achieve their goals.  Production reactors produce less energy and thus consume less fuel than power reactors.  The removal of fuel assemblies from a production reactor is timed to maximize the amount of plutonium in the spent fuel (Figure 2).  Fuel rods are removed from production reactors after only several months in order to recover the maximum amount of plutonium-239.  The fuel assemblies remain in the core of a power reactors for up to three years to maximize the energy produced.  However it is possible to recover some plutonium from the spent fuel assemblies of a power reactor.

Figure 2 – The Fate of Plutonium in a Thermal Reactor

The power output or capacity of a reactor used to generate electricity is measured in megawatts of electricity, MW(e).  However, due to the inefficiency of converting heat into electricity, this represents only about one third of the total thermal energy, MW(t), produced by the reactor.  Plutonium production is related to MW(t).  A production reactor operating at 100 MW(t) can produce 100 grams of plutonium per day or enough for one weapon every two months.

Another important property of a reactor is its capacity factor.  This is the ratio of its actual output of electricity for a period of time to its output if it had been operated at its full capacity.  The capacity factor is affected by the time required for maintenance and repair and for the removal and replacement of fuel assemblies.  The average capacity factor for U.S. reactors has increased from 50% in the early 1970s to over 90% today.  This increase in production from existing reactors has kept electricity affordable.

Thermal Reactors

Currently the majority of nuclear power plants in the world are water-moderated, thermal reactors.  They are categorized as either light water or heavy water reactors.  Light water reactors use purified natural water (H2O) as the coolant/moderator, while heavy water reactors employ heavy water, deuterium oxide (D2O).  In light water reactors, the water is either pressured to keep it in superheated form (in a pressurized water reactor, PWR) or allowed to vaporize, forming a mixture of water and steam (in a boiling water reactor, BWR).  In a PWR (Figure 3), superheated water flowing through tubes in the reactor core transfers the heat generated by fission to a heat exchanger, which produces steam in a secondary loop to generate electricity.  None of the water flowing through the reactor core leaves the containment structure.  In a BWR (Figure 4), the water flowing through the core is converted to directly to steam and leaves the containment structure to drive the turbines.  Light water reactors use low enriched uranium as fuel.  Enriched fuel is required because natural water absorbs some of the neutrons, reducing the number of nuclear fissions.  All of the 103 nuclear power plants in the United States are light water reactors; 69 are PWRs and 34 are BWRs.

Figure 3 – Pressurized Water Reactor

 

Courtesy of the Uranium Information Centre

 

Figure 4 – Boiling Water Reactor

(1) the core inside the reactor vessel creates heat, (2) a steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat, (3) the steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line, and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity.

Courtesy of the U.S. Nuclear Regulatory Commission

Heavy water reactors use D2O as the coolant/moderator, allowing natural, unenriched uranium to be used as the fuel.  This is possible because D2O absorbs fewer neutrons than H2O.  The heat transfer system is similar to that of the PWR, with the steam generator located within the containment structure.  The heavy water reactor shown in Figure 5, known as CANDU, was developed in Canada and sold globally.  The cost advantage in fuel is offset by the expense of producing D2O by a chemical exchange process or electrolysis.  The individual fuel assemblies of a heavy water reactor can be replaced without shutting down the reactor, thus eliminating the down time involved with refueling a light water reactor.  However, spent fuel produced by a heavy water reactor contains more plutonium and tritium than that from light water reactors.  This, coupled with the difficulty in monitoring a continuously fueled reactor, causes concerns about the proliferation of nuclear weapons.  Thus, heavy water is classified as a "sensitive material" because a nation possessing it can produce plutonium directly from natural uranium, eliminating the need for uranium enrichment.

Figure 5 – Heavy Water Reactor

Courtesy of Atomic Energy of Canada Limited

Another type of thermal reactor is graphite moderated and gas cooled.  Twenty-six Magnox reactors (Figure 6), employing pressurized carbon dioxide as the coolant, were built in the United Kingdom but are currently being phased out of service.  These gas cooled reactors have the same advantages as the heavy water reactors in that they can use natural uranium fuel and be fueled continuously.

Figure 6 – Gas Cooled Reactor

(Courtesy Osterreichisches Ökologie-Institut)

The pebble bed represents a new design for a gas cooled reactor (Figure 7).  It uses helium as the coolant and a fuel consisting of low enriched uranium dioxide coated with silicon carbide and pyrolitic carbon encased in small graphite spheres.  The graphite serves as the moderator.  Fresh fuel is added at the top of the reactor and the spent fuel removed from the bottom, allowing continuous operation.  The helium coolant is maintained at high temperatures and pressures, which increases the efficiency of heat transfer and power production.  South Africa is leading the effort to develop this reactor technology.

Figure 7 – Pebble Bed Reactor

Courtesy of the European Nuclear Society

 

Fast Neutron Reactors

In contrast to thermal reactors, the neutrons in a fast neutron reactor (or fast breeder reactor, FBR) are not slowed by the presence of a moderator (Figure 8).  The coolant, usually a liquid sodium or lead, is a substance that does not slow or absorb neutrons.  It also has excellent heat transfer properties, which allow the reactor to be operated at lower pressures and higher temperatures than thermal reactors.

Figure 8 – Fast Neutron Breeder Reactor

Courtesy of Nationmaster

An FBR is configured and operated to produce more fuel than it consumes.  Fast neutrons are readily absorbed by fertile uranium-238, which then can undergo successive beta emissions to become fissile Pu-239.  Thorium-232 is another fertile isotope that can absorb neutrons and produce fissile uranium-233 by beta emissions.  These fissile isotopes can be reprocessed for nuclear reactor fuel or weapons.  Because fast neutrons are not as efficient in producing fission as slow ones, FBRs require uranium oxide containing 20% U-235, plutonium oxide, or a mixture of these oxides, known as MOX, as fuel.

Originally FBRs were thought to be a means of extending global uranium resources by producing fissile Pu-239 or U-233 as reactor fuel.  However, problems with reactor operations and material components combined with the discovery of new uranium deposits mean that FRBs are not economically competitive with existing thermal reactors.  FBR research has produced technical advances but the limiting factor continues to be the price of FBR-produced reactor fuel versus the cost of uranium fuel.  FBRs are more complex than other types of reactors and also raise concerns about the proliferation of plutonium for use in nuclear weapons.

Summary of Reactor Types

Table 1 summarizes the characteristics of the reactors discussed above.

Table 1 – Reactor Characteristics

Reactor Type	Function	Coolant	Moderator	Chemical Form of Fuel	Fuel Enrichment Level*
Thermal
Boiling Water	electricity	light water	light water	uranium dioxide	low enriched uranium
Pressurized Water	electricity, nautical power	light water	light water	uranium dioxide	low enriched uranium
Heavy Water	electricity, plutonium production	heavy water	heavy water	uranium dioxide or uranium metal	natural, unenriched uranium
Gas Cooled Graphite Moderated	electricity, plutonium production	carbon dioxide or helium	graphite	uranium dicarbide or uranium metal	slightly enriched or natural uranium
Water Cooled Graphite Moderated	electricity, plutonium production	light water	graphite	uranium dicarbide or uranium metal	slightly enriched uranium
Pebble Bed Gas Cooled Graphite Moderated**	electricity	pressurized helium	graphite and silicon carbide	uranium dioxide or thorium dioxide	low enriched uranium
Fast Neutron
Fast Neutron Breeder	electricity, plutonium production	molten sodium or lead	none required	various mixtures of plutonium dioxide and uranium dioxide	various mixtures of plutonium dioxide and uranium dioxide

*Percentage of U-235 isotope in the fuel compared to U-238 isotope. Natural uranium contains 0.7% U-235, slightly enriched uranium from 0.8 to 3.0% U-235, and low enriched uranium from 3.0 to 5.0% U-235.
** Currently under development.

Reactor Safety Concerns

Reactor safety has been called into question by two major accidents, one in 1979 at the Three Mile Island (TMI) nuclear power plant in Pennsylvania and the other in 1986 at Chernobyl RBMK (Reactor Bolshoy Moshchnosty Kanalnyin) in the former Soviet Union (Figure 9).  The latter was the worst reactor accident in history, with 31 people dying of direct radiation poisoning and thousands more exposed to high doses of radiation over a period of nearly 20 years.  Disputes still fester regarding total damage of the Chernobyl disaster, although an extensive study reported by a UN sponsored group of scientists indicates that the damage was somewhat less than earlier feared.  The Chernobyl reactor used a graphite moderator and water coolant without a containment structure.  Although some of the design flaws were corrected after this accident, RMBKs are considered to be the most dangerous reactors.

 

Figure 9 - Chernobyl Nuclear Power Plant Following the 1986 Accident

Courtesy of the U.S. Department of Energy

Although the TMI accident was much smaller in scope and resulted in no deaths, it had a profound psychological impact on the public’s view of commercial nuclear power.  The TMI plant was a pressurized light-water reactor, whereas Chernobyl had a reactor design similar to the one used for the reactors at Hanford, Washington, which produced plutonium for the United States nuclear weapons.  Both accidents involved human error overriding safety features built into the reactor systems.  Features of the new generations of reactors including passive safety systems and fewer pipes and valves are designed to minimize the potential for accidents and to limit the damaging effects should such an accident occur.

 A Brief History of Nuclear Power Reactors

The first nuclear reactors were used to produce plutonium for nuclear weapons.  These water-cooled, graphite reactors operated in the United States from 1944 to 1982 and were also used in the Soviet Union during the Cold War.  Nuclear reactors were first used to power a submarine, the USS Nautilus, in 1954.  That same year the Obninsk 5 megawatt nuclear power plant in the Soviet Union became the first reactor to be linked to an electrical grid.  The first commercial nuclear power plants went online at Calder Hall in the United Kingdom in 1956 and Shippingport, Pennsylvania in 1957.  The number of nuclear power reactors grew at a rapid rate before leveling off in the late 1980s.

In addition to the reactors for generating electricity mentioned above, there are also 220 reactors powering ships and submarines.  Another 284 reactors operating in 56 nations are used for research in a variety of areas.  Approximately 20% of the electricity used in the United States is generated by 103 nuclear reactors, although no new reactors have been placed in operation since the 1970s. France leads the world in the generation of electricity with more than 70% coming from nuclear sources. France, Japan, Russia, and Great Britain reprocess spent reactor fuel from commercial reactors.

Interest in nuclear power has increased driven in large part by concerns over energy supplies and climate charge.  Reactor designs have evolved from the first generation of reactors cited above to the generation II systems that included the PWR, BWR, and Magnox reactors of the 1970s and 1980s (Figure 10).  Table 2 includes some of the designs for generation III and III+ reactors.  These systems are touted to be more economical to build and operate, as well as containing improved passive safety features.  Figures 11 and 12 show some of the advances in reactor design for the European Pressured Water Reactor and the Westinghouse AP1000 PWR.

 

Figure 10 – Evolution of Nuclear Reactors

 

Table 2 –Designs for Generation II and III+ Reactors

Design	Manufacturer	Approximate Capacity (MWe)	Reactor type	Status (2008)
AP 600	Westinghouse	650	PWR	Certified
AP1000	Westinghouse	1117	PWR	Certified
ABWR	GE and Tosuiba	1371	BWR	Certified
System 80+	Westinghouse	1300	PWR	Certified
ESBWR	GE	1550	BWR	Undergoing Certification
EPR	AREVA NP	1600	PWR	Pre-certification
Pebble Bed Modular	Westinghouse, Eskom	180	High Temperature Graphite	Pre-certification

Courtesy of the U.S. Energy Information Administration

 

Figure 11 – European Pressured Water Reactor

Courtesy of Areva

 

Figure 12 – Design Features of the Westinghouse AP1000

Courtesy of Westinghouse

Finally, designs for Generation IV reactors are being developed for deployment by 2030.  These reactor designs will continue to address concerns associated with economics, safety, proliferation, waste, and reactor security.

Complete Bibliography on Nuclear Reactors from Alsos Digital Library for Nuclear Issues