Chapter 04: Conventional Nuclear Power Reactors

  • This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” n January 18, 2011; and
  • The chapter was updated on November 29, 2015.

Here is a graph which illustrates the types of commercial reactors:

reactors

1.         NUCLEAR POWER REACTORS:

The main function of a nuclear reactor is to produce heat. The production of heat in a nuclear reactor is no different from producing heat by using a boiler in a conventional coal, gas or oil-fired power station.

Whether from a conventional boiler or a nuclear reactor, heat is required to turn water into steam. This steam is, needed to spin large turbines, which in turn drive generators that produce electricity. A major difference between a nuclear power station and a conventional fossil-fueled station is that there is no release of combustion products to the environment from a nuclear station.

All nuclear reactors operate on the same basic principle, although there are different kinds of nuclear reactors in use throughout the world. A nuclear reactor creates heat by splitting uranium atoms. This “fission” of uranium atoms is, called a “nuclear reaction.”

2.         CONVENTIONAL NUCLEAR POWER REACTORS:

Generation I nuclear power reactors were, developed in 1950-1960s, and none of those nuclear reactors are in operation today. These reactors refer to the early prototype and power reactors, such as Shipping Port, Magnox, Fermi1, and Dresden.

A generation II nuclear power reactor is a design classification for nuclear reactors, and refers to the class of commercial reactors built up to the end of the 1990s and this section covers those nuclear power reactors.

The nuclear power reactors covered under this category are, classified as “Thermal Reactors”.

These reactors are composed of fuel (Fission Material) with the following characteristics:

  • Moderating materials to slow neutrons to low velocities (to prevent capture by U238);
  • Heavy-walled pressure vessels to house reactor components;
  • Shielding to protect personnel;
  • Systems to conduct heat away from the reactor; and
  • Instrumentation for monitoring and controlling the reactors’ systems.

This type of reactor is mostly used for generating electricity. The first plutonium production reactors were thermal reactors using graphite as the moderator. Here is a brief description of each Generation II type based on the World Nuclear Association:

2.1     Pressurized Water Reactors (PWR):

These reactors were originally designed by Westinghouse Bettis Atomic Power Laboratory for military ship applications, then by the Westinghouse Nuclear Power Division for commercial applications. The first commercial PWR plant in the United States was Shipping Port, which operated for Duquesne Light until 1982.

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This is the most common type, with over 230 in use for power generation and several hundreds more employed for naval propulsion. The design of the PWR originated 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 the turbine. A PWR has fuel assemblies of 200-300 rods each arranged vertically in the core. A large reactor would have about 150-250 fuel assemblies with 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 boiling. Pressure is maintained by steam in a “pressurizer” as shown in the diagram (Figure: 4-2). In the primary cooling circuit, the water is also the moderator and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit.

The secondary circuit is under less pressure and the water here 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.

2.2     Boiling Water Reactors (BWR):

These reactors were originally designed by Allis-Chalmers and General Electric (GE).

Slide3

The General Electric design has survived, whereas all Allis-Chalmers units are now shutdown. The first GE US commercial plant was at Humboldt Bay (Near Eureka) in California. Other suppliers of the BWR design worldwide have included – ASEA-Atom, Kraftwerk Union, and Hitachi. Commercial BWR reactors may be found in Finland, Germany, India, Japan, Mexico, Netherlands, Spain, Sweden, Switzerland and Taiwan. Japan and Taiwan have the newest BWR units.

The BWRs typically allow bulk boiling of the water in the reactor. The operating temperature of the reactor is approximately 570F producing steam at a pressure of about 1,000 pounds per square inch. Current BWRs have electrical outputs of 570 to 1,300 MWe. As this time the PWR designs are about 33 percent efficient.

The BWR design has many similarities to 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 percent of the water in the top part of the core as steam.

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, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this protection tends to balance out in the savings due to the simpler design. Most of the radioactivity in the water is very short-lived (mostly N-16, with a 7 second half-life), 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 more steam in the top part reduces moderation.

2.3     Pressurized Heavy Water Reactor (PHWR):

The PHWRs have been promoted primarily in Canada and India, with additional commercial reactors operating in South Korea, China, Romania, Pakistan, and Argentina. Canadian-designed PHWRs are, often called, “CANDU” reactors. Siemens, ABB (now part of Westinghouse), and Indian firms have also built commercial PHWR reactors.

Slide4

PHWRs have been popular in several countries because they use less expensive, natural (not enriched) uranium fuels and can be built and operated at competitive costs. The continuous refueling process used in PHWRs has raised some proliferation concerns because it is difficult for international inspectors to monitor. Additionally, the relatively high Pu-239 content of the PHWRs spent fuel has also raised proliferation concerns. The importance of these claims is challenged by their manufacturers. PHWRs, like most reactors, can use fuels other than uranium and the ACR series of reactors is intended to use slightly enriched fuels. Particular interest has been shown in India in thorium-based fuel cycles.

Heavy water reactors now in commercial operation use heavy water as moderators and coolants. The Canadian firm, the Atomic Energy of Canada Limited (AECL), has also recently proposed a modified PHWR (the ACR series) which would only use heavy water as a moderator. Light water would cool these reactors. No successful effort has been made to license commercial PHWRs in the United States.

CANDU-specific features and advantages include:

  • Use of natural uranium as a fuel:
    • CANDU is the most efficient of all reactors in using uranium: it uses about 15% less uranium than a pressurized water reactor for each megawatt of electricity produced;
    • Use of natural uranium widens the source of supply and makes fuel fabrication easier. Most countries can manufacture the relatively inexpensive fuel;
    • There is no need for uranium enrichment facility;
    • Fuel reprocessing is not needed so costs, facilities and waste disposal associated with reprocessing are avoided; and
    • CANDU reactors can be fueled with a number of other low-fissile content fuels, including spent fuel from light water reactors. This reduces dependency on uranium in the event of future supply shortages and price increases.
  • Use of heavy water as a moderator:
    • Heavy water (deuterium oxide) is highly efficient because of its low neutron absorption and affords the highest neutron economy of all commercial reactor systems. As a result chain reaction in the reactor is less possible with natural uranium fuel; and
    • Heavy water used in CANDU reactors is readily available. It can be, produced locally, using proven technology. Heavy water lasts beyond the life of the plant and can be re-used.
  • CANDU reactor core design:
    • Reactor core comprising small diameter fuel channels rather that one large pressure vessel;
    • Allows on-power refueling – extremely high capability factors are possible;
    • The moveable fuel bundles in the pressure tubes allow maximum burn-up of all the fuel in the reactor core; and
    • Extends life expectancy of the reactor because major core components like fuel channels are accessible for repairs when needed.

2.4     Advanced Gas-Cooled Reactors (AGR):

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

Slide1

The AGR was developed from the Magnox reactor, is graphite moderated and CO2 cooled, and two of these are still operating in UK. They use natural uranium fuel in metal form.  The newer Advanced Gas Cooled (AGR) Reactors use a slightly enriched uranium dioxide clad with stainless steel. Carbon dioxide is the coolant gas used. Two key advantages of this design are:

  • Higher operating temperature with a higher thermal efficiency; and
  • Not susceptible to accidents of the type possible with water cooled/moderated reactors.

2.5     Light Water Graphite Moderated Reactor (RBMK):

In 2003, several of these reactors were still operating in the Soviet Union, but there were no plans to build any more, and there is international pressure to close those that remain. The RBMK was the culmination of the Soviet program to produce a water-cooled power reactor based on their graphite-moderated plutonium production reactors. The first of these, AM-1 (For Atom Mirny, Russian for “peaceful atom”) was designed to produce 5MWe (30MW thermal) and delivered power to Obninsk from 1954 until 1959.

Slide5

Ordinary (light) water absorbs neutrons readily and so removing water from the core (such as happens when it boils and is replaced by steam), tends to increase the rate at which the nuclear reaction proceeds. In a water-moderated reactor this effect is countered by the reduction in moderation, but in the RBMK, the moderating effect of the water is small compared to that of the graphite, so the overall effect is positive. This is called a “positive void coefficient”. The RBMK as designed also had a “positive power coefficient”, meaning that an increase in reactor power tends to further increase the rate of reaction. Large positive void and power coefficients can produce runaway conditions and have not been permitted in other reactor designs; however, it was not possible to eliminate them from the RBMK if natural uranium fuel was to be used.

The RBMK was also intended to use recycled uranium from reprocessed PWR fuel which has a low remaining enrichment. In this configuration it was also unstable. These characteristics brought the RBMK to the world’s notice in 1986, when one of the four RBMK reactors at Chernobyl exploded in the worst civilian nuclear accident to date.

Since that accident remaining RBMK have been operated with a reduced number of fuel elements containing more highly enriched fuel, enabling them to operate relatively safely but defeating the original concept. Control systems have also been improved, in particular to eliminate the graphite tips on the control rods, which produced an immediate increase in power when the rods were first inserted. This design feature is blamed for triggering the first actual explosion when the emergency shutdown button was pressed in an attempt to shut down the already out of control reactor during the Chernobyl disaster.

2.6  Fast Breed Nuclear Reactor:

Because slow neutrons are more likely to split uranium atoms, most reactor types are designed to make use of them. In contrast, fast breeder reactors (FBRs) use fast neutrons to convert materials such as uranium-238 and thorium-232 into fissile materials, which then fuel the reactor. This process, combined with recycling, has the potential to increase available nuclear fuel resources in the very long term. FBRs operate mainly in Russia.

fast

The modern small modular reactor (SMR) is designed to be built economically in factory-like conditions (rather than onsite), and with capacities between approximately 10 MWe and 300 MWe.

 

Resources:

  1. NU – TeachNuclear – Types of Reactors;
  2. Nuclear Technology Exploring Possibilities – Major Reactor Types;
  3. Canadian Nuclear Association – Major Reactor Types;
  4. World Nuclear Association – Nuclear Power Reactors.

mir

 

 

Dr. Mir F. Ali

Chapter 5 …

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