4 An overview of nuclear power

This chapter provides background on nuclear power: how it is used to generate energy; what kinds of reactor technologies exist; and how it has developed historically.

As Figure 4 shows, a nuclear power plant is very similar to a fossil power plant where heat produces steam that drives a turbine-generator. 
The main difference is how the initial heat is produced. In a nuclear plant, it comes from nuclear fission.

4.2 Nuclear fission

At the heart of each atom of any element is a nucleus, made up of protons and neutrons. In one naturally occurring form of uranium, known as U-235, the nucleus is likely to undergo fission when bombarded by neutrons with low kinetic energy. “Fission” means the nucleus breaks into two fragments, as shown in Figure 5. In turn, these fragments release energy (in the form of radiation), and also at least two more neutrons.

When the mass of all the products left after fission has taken place is added up, the result is very slightly less than the mass of the original nucleus. Part of the mass has become energy. Einstein’s famous equation, E=mc2, determines just how much energy can be released by a very small mass.

Under the right conditions, the neutrons released by the break-up of the nucleus go on to bombard other nuclei, causing more fission events. By arranging material appropriately a self-sustaining, controlled chain reaction can be produced.

Almost all commercial nuclear reactors are thermal reactors. This means the neutrons released by fission are ‘slowed down’ by passing them through a relatively light material such as hydrogen, deuterium or carbon.  In turn, this makes the neutron more likely to contact another uranium nucleus and cause it to fission.

These lighter materials are called moderators.  They can be light water (ordinary water composed of hydrogen and oxygen), heavy water (a rarer form of water found in nature which is composed of deuterium and oxygen), or graphite (carbon).

Energy released from fission causes the uranium fuel elements to heat up. A flow of liquid or gas fluid – the coolant – flows over the fuel elements, picking up heat from the fuel and using it to boil water into steam to power the generator.

It is a common misconception that a nuclear reactor has the potential to explode like an atomic weapon.  However the technologies for power and for weapons are fundamentally different. A nuclear weapon is designed to release energy extremely quickly and in enormous quantities. It would be physically impossible to generate such large and rapid energy releases using the arrangement of fuel required to sustain a controlled fission chain reaction over the long periods of time (hours, days and years) needed to produce electric power in a nuclear reactor.

4.2.1 Types of nuclear reactor

Reactor types vary according to the moderator used to control the speed of neutrons, the coolant employed to transfer heat to the generating cycle, and by the degree of U-235 enrichment in the nuclear fuel. These characteristics are inter-related: natural uranium fuel without enrichment needs a more effective moderator that can slow neutrons to a speed where more fission events can take place.

There are 443 reactors operating around the world today, and they can be classified into the following broad categories:

• The Pressurized Water Reactor (PWR) – approximately 60% of reactors world-wide.  This reactor type uses ordinary ‘light’ water as a moderator and also as the coolant. It has two separate coolant loops, one to remove heat from the reactor and the other to provide steam to a turbine that drives an electrical generator. The primary loop (which is in closest contact with the reactor core) is maintained under high pressure to keep it from boiling.
• The Boiling Water Reactor (BWR) – approximately 20% of reactors world-wide. This type also uses light water as a moderator and coolant, but has a single coolant loop in which the water is allowed to reach boiling temperature and produce steam.
• The Pressurized Heavy Water Reactor (PHWR) – approximately 11% of reactors world-wide. This type is predominantly based upon the CANDU reactor developed in Canada. It uses heavy water as a moderator and coolant, and natural uranium fuel. Like the PWR it uses two separate coolant circuits, one to remove heat from the reactor and the other to provide steam to a turbine that drives an electrical generator. The primary loop cooling the reactor is maintained at high pressure to limit the amount of boiling.
• Gas cooled reactors (GCR) – A few reactors of this type have operated commercially, mainly in the UK.  These reactors use solid graphite as a moderator and gas (either carbon dioxide or helium) as the coolant removing heat from the nuclear fuel. The gas reactors in the UK are being phased out. However, as will be discussed later, new gas reactors are either being developed or considered because they could provide high-temperature heat along with a wide range of potential process applications.

Table 2 summarizes the differences between these reactor types.

4.2.2 The development of nuclear power

Electricity generation using nuclear power is a well established technology, dating back more than 50 years to the early prototype commercial power plants in the UK and USA in the mid-1950s.

Commercial nuclear power development started after World War II when it was recognized that the large energy release associated with fissioning of atoms could be applied to peaceful uses, in particular the generation of electricity.

These early developments investigated different nuclear reactor concepts, including designs with light water, heavy water, gas and liquid metal coolants, and various types of nuclear fuel design. A number of countries undertook development of reactors in the early stages, including the United States, Canada, the UK, France and Russia.

THE USA
In the United States, commercial nuclear reactor designs very rapidly focused on compact light-water cooled designs based upon the successful development of naval propulsion reactors. These compact designs required fuel to be enriched so that it has a higher content of the U-235 isotope. These naval propulsion designs developed into the successful light-water reactor designs – Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) – that have become the predominant commercial power reactors currently in use around the world.

CANADA
Development of the Canadian CANDU design was influenced by two factors:

• The country’s resources of uranium led to an early decision not to rely on uranium enrichment since costly enrichment technology would have to be acquired from abroad. Instead, Canada’s nuclear program was based on natural uranium fuel.
• Because natural uranium has less of the U-235 isotope, a more efficient design for slowing down the neutrons was needed. Canada had developed expertise with heavy water during World War II, and this was incorporated into reactor design.

CANDU reactors operate in Canada and a number of countries around the world. The majority of CANDU reactors in Canada are located in Ontario as a result of the collaboration between the provincial utility Ontario Hydro and the Federal Crown Corporation, Atomic Energy of Canada Limited (AECL), in developing and constructing the reactors in the period between 1960 and 1972.

EUROPE
In the UK and France early developments focused on two concepts:

• Magnox reactors used gas as a coolant and graphite to moderate neutron speed.
• The Steam Generating Heavy Water Reactor used a combination of light water for cooling and heavy water as a moderator. 

Neither of these two concepts was successful and the designs were abandoned. The UK continued development of gas-cooled designs. The Advanced Gas Reactor has been operated commercially but is to be phased out.  Following the oil crisis of the early 1970s, France committed to licensing the PWR technology offered by Westinghouse in the U.S. and rapidly built the second-largest nuclear power program in the world.

SOVIET UNION
In the early period, the Soviet Union developed a graphite-moderated/water cooled design, referred to as the RBMK reactor. This design did not require tight tolerances and could be constructed relatively quickly and at low cost.  These reactors were being deployed in a very ambitious program which was rapidly halted following the accident at the Chernobyl Unit 4 reactor in 1986.  Subsequently, Russia has focused reactor development and deployment on a PWR-type of reactor design known as VVER. Reactors of this type are found in former Soviet-bloc eastern European countries.

ASIA
Asia has seen a steady increase in the number of reactors brought into service over the past three decades.  Japan has licensed U.S. light-water technology and operates a significant number of PWR and BWR reactors.  In the past  decade the large Japanese conglomerates Toshiba, Hitachi and Mitsubishi have either bought U.S. vendors or formed alliances with them to develop new advanced Generation III reactor designs.(Generation III reactor designs are discussed in section 4.3).

South Korea has also developed a significant nuclear power program focused on PWR and CANDU reactors. Additionally, South Korea has developed an advanced Generation III design. More recently China has embarked upon a very ambitious nuclear power program based primarily on PWR technology, but also including two CANDU units. India with its large population and burgeoning economy has also embarked upon a major expansion of its nuclear power program.  India’s program primarily uses domestically developed reactors based upon CANDU technology.

4.2.3 Recent deployment of nuclear power generation
Figure 6 shows the number of reactors built in Canada and around the world from 1965 to 2007. Since the early 1990s no new reactors have been brought into service in North America. In the United States, this reflected the financial impact of the Three Mile Island (Details of the Chernobyl and Three Mile Island events are covered in Chapter 6 on nuclear safety).accident, which terminated orders for new nuclear units, led to the cancellation of a large number of units and resulted in significant regulatory delays in bringing into service any reactors that were not cancelled.

28 Source Figure 6: American Nuclear Society, Nuclear News, Reference Issue, July 2008
Figure 6 indicates that deployment of new nuclear power plants leveled off in North America in the mid-1980s after events at Three-Mile Island and Chernobyl (discussed more fully in Chapter 6), but has continued to climb in the rest of the world.

It is interesting to note that the accident at Three Mile Island Unit 2 in 1979 significantly dampened the growth of nuclear power in the U.S. but had very little impact outside of the U.S. In fact non-U.S. growth in nuclear power actually accelerated after the Three Mile Island accident.

Canada installed 12 nuclear units between 1979 and 1992, when the Darlington reactors were brought into service. However, there have been no units constructed in Canada since then. This was largely because of cost issues. Darlington incurred large cost overruns due to interest charges when construction schedules were set back.  Subsequently, the Ontario government felt that low demand growth did not justify the addition of more nuclear units.

4.2.4 Current situation

As of mid-2008, construction is underway on projects that will increase the number of reactors world-wide to approximately 491 within the next six years. The distribution and types of nuclear reactors operating in different regions of the world are summarized in Table 3.

Canada has a total of 22 nuclear power reactors currently in service, of which 20 are in Ontario (with 18 operating and 2 in a laid-up state) and 1 in each of Quebec and New Brunswick. All of these reactors are CANDU Pressurized Heavy Water reactors.

Additionally there are research reactors located at AECL’s Chalk River Laboratory (the 135-MW NRU reactor and a low-power 100W reactor ZED-2). Other research reactors are located at universities, including the second largest in North America at McMaster University, and a number of smaller SLOWPOKE research reactors, including one at the University of Alberta.

4.3 New nuclear reactor designs
A new generation of nuclear reactor designs, often referred to as Generation III reactors, are about to be deployed over the next decade. These include:

• The Advanced CANDU Reactor (ACR-1000) designed by Atomic Energy of Canada Ltd.,
• The AP-1000, an advanced PWR designed by Westinghouse,
• The EPR, an advanced PWR designed by the French nuclear company, AREVA, and
• The ESBWR, an advanced boiling water reactor designed by General Electric.

These reactors feature enhanced safety, including ‘passive’ safety systems (Safety is discussed in more detail in Chapter 6), and improved economics.  Passive features do not rely on external sources of power to keep them functioning. Instead they rely on natural processes such as natural circulation (associated with temperature differences in fluids) steam generation and steam condensation to remove heat from systems.

Also under development is a new generation of reactors, often referred to as Generation IV reactors, which also incorporate improved safety and non-proliferation features.   (The latter make it even more difficult to divert materials for non-power generation purposes). The most advanced of these is the Pebble Bed Modular Reactor (PBMR) currently under development in South Africa. This high-temperature gas reactor uses graphite as a moderator and helium as a coolant and has ball-shaped graphite-coated fuel (the “pebbles”). The reactor is designed for flexible applications such as producing either 165 MW of electrical power or 400 MW of thermal heat for process applications (e.g. hydrogen generation, water desalination or oil sands recovery).

4.4 Environmental aspects of  nuclear power
This section discusses the air and water aspects of  nuclear power. Issues specifically related to nuclear fuel and waste are discussed in subsequent chapters.

4.4.1 Air
Nuclear power has attracted renewed interest recently because it does not emit carbon dioxide (CO2) or other air pollutants during operation, unlike fossil fuel-based forms of electricity generation. Considering the entire life cycle (including mining, processing, uranium enrichment, fuel fabrication and transport), the emission of CO2 from nuclear power generation is similar in magnitude to the life-cycle emissions from renewable energy sources such as wind power.

The majority of the life-cycle emissions for nuclear power result from mining and enrichment, assuming the energy these processes require comes from fossil fuelled power stations. However, if uranium enrichment used the more efficient centrifuge process and nuclear generated electricity in place of fossil fuelled generation, then life-cycle CO2 emissions from nuclear power would be substantially lower.

4.4.2 Water
Nuclear power plants, like fossil-fuelled power plants, require cooling to condense the steam exiting the large turbines. This cooling is provided by cold water flowing through the tubes of the turbine condenser. The heat transferred to the condenser cooling water is released to the environment by one of three possible means:   

• Once-through cooling extracts water from a river, lake or ocean. The amount of water extracted in a year for the reference 800-MW nuclear plant would be in the range of 600 to 1400 million cubic meters.  Of this, about 10 million cubic meters would be lost to evaporation while the rest was returned to the body of water. Once-through water cooling has various effects on the environment: damage to aquatic life at intakes; discharge of warmer water into the parent body of water; and the impact of chlorine, which is used to control corrosion and accumulation of microbes and minerals, on aquatic life.
• Heat release to the atmosphere by evaporative cooling towers. For the 800-MW reference plant, this type of cooling would require 20 to 30 million cubic meters of water, of which about 17 million cubic meters would be lost to evaporation. Environmental impacts arise from periodic blow-down discharge of water containing chlorine and other chemicals used to control corrosion and the accumulation of microbes and minerals.
• Heat release to the atmosphere by dry air fan cooling.  Forced air cooling does not require any water for cooling but does consume some of the electricity generated by a power plant to drive the fans.  Although less efficient than direct water cooling or evaporative cooling, this is a good option for areas where there is limited water supply.

The volumes of water required by various cooling systems and the environmental impacts are similar to those for fossil-fuelled plants. Cooling water is not in contact with nuclear fuel and so cannot release radioactivity into the environment.

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