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Core of a nuclear reactor

A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs in a split second). Nuclear reactors are used for many purposes, but the most significant current uses are for the generation of electrical power and, in rare cases, for the production of plutonium for use in nuclear weapons. Currently all commercial nuclear reactors are based on nuclear fission, and are considered problematic by some for their safety and health risks. Conversely, some consider nuclear power to be a safe and pollution-free method of generating electricity. Fusion power is an experimental technology based on nuclear fusion instead of fission.

There are other devices in which nuclear reactions occur in a controlled fashion, including radioisotope thermoelectric generators, which generate heat and power by passive radioactive decay, and Farnsworth-Hirsch fusors, in which controlled nuclear fusion is used to produce neutron radiation.

File:Nuclear powerplant-01.jpg
A nuclear power station. The nuclear reactors are inside the two cylindrical containment buildings in the foreground—behind are the cooling towers (venting water vapor).

Applications

History

Enrico Fermi and Leó Szilárd, while both were at the University of Chicago, were the first to build a nuclear pile and demonstrate a controlled chain reaction on December 2, 1942. In 1955 they shared US patent number 2,708,656 for the nuclear reactor.

The first nuclear reactors were used to generate plutonium for nuclear weapons. Additional reactors were used in the navy (see United States Naval reactor) to propel submarines and aircraft carriers. In the mid-1950s, both the Soviet Union and western countries were expanding their nuclear research to include non-military uses of the atom. However, as with the military program, much of the non-military work was done in secret. On December 20, 1951, electric power from a nuclear powered generator was produced for the first time at Experimental Breeder Reactor-I (EBR-1) located near Arco, Idaho. On June 27, 1954, the world's first nuclear power plant generated electricity but no headlines--at least, not in the West. According to the Uranium Institute (London, England), the first reactor to generate electricity for commercial use was at Obninsk, Kaluga Oblast, Russia. The Shippingport Reactor (in Pennsylvania) was the first commercial nuclear generator to become operational in the United States. The Shippingport reactor was ordered in 1953 and began commercial operation in 1957.

Even before the 1979 Three Mile Island accident, new orders for nuclear plants in the U.S. had ceased for economic reasons primarily related to greatly extended construction times. As of 2004, no new nuclear plants have been ordered in the USA since 1978 , although that may change by 2010 (see Future Of The Industry below).

Surprisingly, and unlike the Three Mile Island accident, the 1986 Chernobyl accident did not increase regulations affecting Western reactors. This was because the Chernobyl reactors were known to be an unsafe design (RBMKs) without containment buildings and operated unsafely, and the West had nothing to learn from them . There was however political fallout: Italy held a referendum the next year (1987), and it was decided to shut down the country's four nuclear power plants .

In 1992 the Turkey Point Nuclear Generating Station was hit directly by Hurricane Andrew. Over $90 million of damage was done, largely to a water tank and to a smokestack of one of the fossil-fueled units on-site, but the containment buildings were undamaged .

The first organization to develop utilitarian nuclear power, the U.S. Navy, is the only organization worldwide with a totally clean record. This is perhaps because of the stringent demands of Admiral Hyman G. Rickover, who was the driving force behind nuclear marine propulsion. The U.S. Navy has operated more nuclear reactors than any other entity, other than the Soviet Navy, with no publicly known major incidents. Two U.S. nuclear submarines, USS Scorpion and Thresher, have been lost at sea, though for reasons not related to their reactors, and their wrecks are situated such that the risk of nuclear pollution is considered low.

The future of the industry

Some experts predict that electricity shortages, fossil fuel price increases and concern over Greenhouse gas emissions will renew the demand for nuclear power plants. Watts Bar 1, which came on-line in 1997, was the last U.S. commercial nuclear reactor to go on-line.

As of 2004, the immediate future of the industry in many countries still appeared uncertain, the most notable exceptions being Japan, China and India, all actively developing both fast and thermal technology, South Korea and the United States, developing thermal technology only, and South Africa and China, developing versions of the Pebble Bed Modular Reactor (PBMR). Finland and France actively pursue nuclear programs; Finland has a new AREVA plant under construction. Japan has an active nuclear construction program with new units brought on-line in 2005. In the U.S., three consortia responded in 2004 to the U.S. Department of Energy's solicitation under the Nuclear Power 2010 Program and were awarded matching funds - the Energy Policy Act of 2005 authorized subsidies for up to six new reactors, and authorized the Department of Energy to build a reactor to produce both electricity and hydrogen. As of the early 21st century, nuclear power is of particular interest to both China and India to serve their rapidly growing economies - both are developing fast breeder reactors. See also future energy development.

On September 22, 2005 it was announced that two sites in the U.S. had been selected to receive new power reactors (exclusive of the new power reactor scheduled for INL) - see Nuclear Power 2010 Program.

It is possible that the first new nuclear power plant to be built in the United States since the 1970s may be installed in the remote town of Galena, Alaska. The town's City Council approved the idea, and Toshiba proposed to install its model 4S "nuclear battery" in Galena free of charge as a test.

See also nuclear power phase-out, nuclear energy policy.

Method of operation

All commercial nuclear reactors produce heat through nuclear fission. In this process, the nucleus of an element such as uranium splits into two smaller atoms. This occurs naturally in radioactive elements, but it can be induced artificially by making some atoms absorb a neutron. This causes the nucleus to become unstable and makes it split apart very quickly.

The fission process for a uranium atom yields two smaller atoms, one to three fast-moving free neutrons, and energy. Uranium fission therefore releases more neutrons than it requires, and the reaction can become self sustaining if conditions are appropriate. This is called a chain reaction.

When a neutron is captured by a fissionable nucleus, it may cause fission immediately, or it may lead to an unstable species which undergoes fission a short time later. A mass of fissionable material is said to be a critical mass if each fission event leads to one or more fission events on average. A mass is said to be prompt critical if the immediate fission events are sufficient to carry on a chain reaction. A prompt critical mass will rapidly release an exponentially increasing amount of heat and cannot be controlled. Nuclear reactors are (with the exception of certain speculative subcritical reactors) designed to contain critical masses that are not prompt critical, so that control systems can react quickly enough to maintain a steady rate of heat production.

The neutrons released by fission are moving quickly. Such "fast neutrons" are not easily absorbed by fissionable nuclei. Some reactors are designed to work with these neutrons, but most reactors use a neutron moderator to slow these neutrons down so that they are more easily absorbed. Such neutrons are often slowed until they are in thermal equilibrium with the reactor core; as a result, they are called thermal neutrons (or slow neutrons).

The amount of heat produced by a reactor is a crucial parameter. It may be controlled by adjusting the amount of neutron moderator in the reactor core, control rods consisting of neutron absorbers may be used to control the output, or the physical arrangement of the fuel may be changed. The Doppler broadening effect also serves to reduce the rate of fission as the temperature increases. Many reactors use several methods, both for control and for emergency shutdown.

Reactor design

See also Nuclear power plant and nuclear reactor physics

A nuclear reactor is designed to carry out nuclear fission reactions on a large scale. This produces heat, fission products, and intense neutron radiation. In a nuclear power plant, that heat is used to do useful work. Some reactors, whether experimental or military, are designed with no concern for making use of the generated heat, as their goal is to make use of the neutron radiation to transmute elements. In either case, for all current nuclear reactors, it is essential that a nuclear chain reaction be continually sustained.

In a sustained nuclear chain reaction, the fission of a single fuel nucleus releases a few neutrons. These neutrons initially carry a great deal of energy (and are therefore called fast neutrons). These neutrons may be captured immediately by another fuel nucleus, or they may interact with a neutron moderator or a neutron absorber. The likelihood that a fast-moving neutron is captured by a fuel nucleus is relatively low, so it is often necessary to slow down the neutrons. This is done by allowing the neutrons to scatter off nuclei of a neutron moderator. After a few such scattering events, the neutron radiation has a thermal energy spectrum (that is, they are moving with the same average energy as a gas at the same temperature as the reactor core) and is much more easily captured by a fuel nucleus.

Paths of some neutrons in a thermal reactor

A nuclear reactor that uses a moderator is called a slow or thermal reactor, and it is normally categorized according to the type of moderator. Common moderators are heavy water and ordinary light water. Some reactors also use graphite, although it has a number of problems (see, for example, the Windscale fire and the Chernobyl accident). A reactor that is not moderated is called a fast reactor. The higher neutron flux allows some nuclear reactions to occur that are difficult to arrange in a slow reactor. In particular, it is possible to transmute thorium and other isotopes into usable fuel isotopes. Such a reactor can potentially produce more fuel than it consumes; for this reason fast reactors are sometimes called "breeder reactors".

A computer simulation of reactor core temperature. The white circles are fuel rods.

When a neutron is captured by a fuel nucleus, the nucleus may undergo fission immediately, it may remain in an unstable state for a short while before undergoing fission, or it may fail to undergo fission at all. Fission events that occur immediately are called "prompt" fission events, and if there are enough prompt events for the reaction to be self-sustaining without the delayed fission events, then the reactor is said to be prompt critical. In such a situation, the amount of fission in the reactor will grow exponentially and very quickly; the result would be a large explosion (although not one comparable to a nuclear weapon). Thus a stable nuclear reactor must be maintained in a critical but not prompt critical state. Controls are also essential to ensure that the temperature does not rise so high that the reactor is damaged or destroyed.

A nuclear reactor is controlled by adjusting the configuration of neutron absorbers in and around the core, the configuration of the neutron moderator (if any), and sometimes the configuration of the fuel itself. The most common arrangement is to include neutron-absorbing control rods which can be partially inserted into the reactor in order to damp its reaction. Such control rods normally require sophisticated monitoring equipment, so a number of advanced reactor designs (such as the pebble-bed reactor) have tried to build in passive safety systems which require no action by electronic, mechanical, or human agents to prevent plant overheating (see Passively safe).

In any nuclear reactor, some sort of cooling is necessary. In a nuclear power plant, the cooling system must be designed so that it can make use of the heat released. Most nuclear reactors use water as a coolant, either in a pressurized liquid form or by boiling into steam. Since water acts as a moderator, fast reactors cannot be cooled with water. Molten sodium or sodium salts are in current use. Reactors designed for transmutation only may simply release the heat to the environment.

Safety

By regulation, as part of the design of any nuclear reactor provisions must be made for operator errors or failure of critical equipment. For this reason the "Defense in Depth" concept is employed to ensure operability of all systems when required for safety. All systems in nuclear plants have three main safety objectives:

  • Control of Reactivity (ability to control the amount of neutron flux in the fuel either mechanically or chemically),
  • Maintenance of Core Cooling (maintaining an adequate supply and backup supply of coolant to the core region) and
  • Maintenance of Barriers to Release of Radiation (fuel cladding, primary barrier, containment and attenuation devices).

Where Systems, Structures and Components (SSC) are required to perform any duties supporting the three safety functions, they are provided with frequent inspection, operational or functional tests, and increased design, purchase and repair scrutiny as part of a Quality Assurance (QA) plan. Part of the design of these SSC includes redundancy (having multiple backup components), provision of independent systems (such as a requirement to have two or more separate systems performing the same function in parallel) "voting" on an interpretation of a signal, fail-safe design (knowing how a SSC will fail and what effect it will have on companion SSC) monitoring instrumentation and protection against "Common Mode Failure". Common Mode Failure prevents a single failure from affecting both "trains" or systems of independent, redundant equipment. Engineering performance is tested on a frequent basis (surveillance) to provide assurance (QA) of readiness to perform its designed function. It should be noted that many of these same design features are mandated on commercial airliners.

On detection of process (pressure, temperature, radiation, flow, etc) indications outside of a normal range an alarm will sound and be "acknowledged" in the control room, where an operator makes adjustments. If the alarming parameters exceed set points further, the reactor, turbine or generator may provide a fault signal which automatically places the system in a safer (lower energy) mode and may terminate operations without operator control. In the case of a generator or turbine fault, steam will be limited or shut off and the turbine will slow. If the problem is not corrected quickly, a SCRAM will occur automatically inserting the control rods into the reactor core and slowing the neutron flux by over 99% in seconds. The plant can be restarted, but only after an investigation is completed.

Each facility operates to a set of license conditions (Final Safety Analysis Report, or FSAR) specific to the units' design, location and environment. The license conditions, condensed in a set of Technical Specifications, describes the limits of power, certain process parameters, staff, training and qualifications, minimum available equipment and other physical and administrative requirements which must be in place in order to operate the reactor. Violation of the license conditions may result in fines and inability to operate the facility - also, since the license conditions constitute part of a federal license, plant personnel may face criminal charges.

Types of reactors

NC State's PULSTAR Reactor is a 1 MW pool-type research reactor with 4% enriched, pin-type fuel consisting of UO2 pellets in zircaloy cladding.
The control room of NC State's Pulstar Nuclear Reactor.

A number of reactor technologies have been developed. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that are used to sustain the fission chain reaction.

  • Thermal (slow) reactors use slow or thermal neutrons. These are characterised by having moderating materials which are intended to slow the neutrons until they approach the average kinetic energy of the surrounding particles, that is, until they are thermalised. Thermal neutrons have a far higher probability of fissioning U-235, and a lower probability of capture by U-238 than the faster neutrons that result from fission do. As well as the moderator, thermal reactors have fuel (fissionable material), containments, pressure vessels, shielding, and instrumentation to monitor and control the reactor's systems. Most power reactors are of this type, and the first plutonium production reactors were thermal reactors using graphite as the moderator. Some thermal power reactors are more thermalised than others; Graphite (ex. Russian RBMK reactors) and heavy water moderated plants (ex. Canadian CANDU reactors) tend to be more thoroughly thermalised than PWRs and BWRs, which use light water (normal water) as the moderator (due to the extra thermalization, these types can use natural uranium/unenriched fuel).
  • Fast reactors use fast neutrons to sustain the fission chain reaction, and are characterised by the lack of moderating material. They require highly enriched fuel (sometimes weapons-grade), or plutonium in order to reduce the amount of U-238 that would otherwise capture fast neutrons. Some are capable of producing more fuel than they consume, usually by converting U-238 to Pu-239. Some early power stations were fast reactors, as are some Russian naval propulsion units, and construction of prototypes is continuing, see fast breeder, but overall the class has not achieved the success of thermal reactors in any application. An example of this type of reactor is the Fast Breeder Reactor (FBR).

Thermal power reactors can again be divided into three types, depending on whether they use pressurised fuel channels, a large pressure vessel, or gas cooling.

  • Pressure vessels holding steam heated by the reactor are used by most commercial and naval reactors. This serves as a layer of shielding and containment.
  • Pressurised channels are used by the RBMK and CANDU reactors. Channel-type reactors can be refuelled under load, which has advantages and disadvantages discussed under CANDU reactor.
  • Gas-cooled reactors are cooled by a circulating inert gas, usually helium, but nitrogen and carbon dioxide have also been used. Utilisation of the heat varies, depending on the reactor. Some reactors run hot enough that the gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger to make steam for a steam turbine. The pebble bed reactor uses a gas-cooled design.

Since water serves as a moderator, it cannot be used as a coolant in a fast reactor. Most designs for fast power reactors have been cooled by liquid metal, usually molten sodium. They have also been of two types, called pool and loop reactors.

Current families of reactors

Obsolescent types still in service

Obsolescent types no longer in service

Advanced reactors

More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor (ABWR), two of which are now operating with others are under construction. The best-known radical new design is the Pebble Bed Modular Reactor (PBMR), a High Temperature Gas Cooled Reactor (HTGCR). Other possible designs exist on the drawing board, notably the energy amplifier, awaiting political support and funding. Some, such as the Integral Fast Reactor (IFR), have been cancelled due to a political climate unfavorable to nuclear power.

Nuclear fuel cycle

Main article: nuclear fuel cycle

Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture of plutonium and uranium (see MOX). The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle.

Uranium is sampled and mined as other metals are, via open-pit mining or leach mining. Raw uranium ore found in the United States ranges from 0.05% to 0.3% uranium oxide. Uranium ore is not rare; the largest probable resources, extractable at a cost of US$80 per kilogram or cheaper, are located in Australia, Kazakhstan, Canada, South Africa, Brazil, Namibia, Russia, and the United States.

The raw ore is then milled, where it is ground and chemically leached. The resulting powder of natural uranium oxide is called "yellowcake". The yellowcake powder is then converted to uranium hexafluoride to prepare for enrichment.

Since under 1% of the uranium found in nature is the easily fissionable U-235 isotope, the uranium must be enriched to about 4% U-235, usually by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired onto pellet form. These pellets are stacked into tubes which are then sealed and called fuel rods. Many of these fuel rods are used in each nuclear reactor.

Fueling of nuclear reactors

The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full-power days in a reactor's operating cycle (between refueling outage times) is related to the amount of fissile uranium-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full-power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent," and is discharged and replaced with new (fresh) fuel assemblies. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor.

Not all reactors need to be shut down for refueling; for example, pebble bed reactors, molten salt reactors and CANDU reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows individual fuel elements to be moved about within the reactor core to places that are best suited to the amount of U-235 in the fuel element.

The amount of energy extracted from nuclear fuel is called its "burn up," which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.

Waste management

The final stage of the nuclear fuel cycle is the management of the still highly radioactive, "spent" fuel, which constitutes the most problematic component of the nuclear waste stream. After fifty years of nuclear power the question of how to deal with this material remains fraught with safety concerns and technical problems, and one of the most important lines of criticism of the industry is based on the long-term risks and costs associated with dealing with the waste.

Management of the spent fuel can include various combinations of storage, reprocessing, and disposal. In practice storage has been the primary modality so far. Typically the spent fuel rods are stored in a pool of water which is usually located on-site. The water provides both cooling for the still-decaying uranium, and shielding from the continuing radioactivity.

Another, more permanent method of disposal of high-level nuclear waste calls for the material to be buried deep underground in certain geological formations. The Canadian government, for example, is seriously considering this method of disposal, known as the Deep Geological Disposal concept. Under the current plan, a vault is to be dug 500 to 1000 meters below ground, under the Canadian Shield, one of the most stable landforms on the planet. The vaults are to be dug inside geological formations known as batholiths, formed about a billion years ago. The used fuel bundles will be encased in a corrosion-resistant container, and further surrounded by a layer of buffer material, possibly of a special kind of clay (bentonite clay). The case itself is designed to last for thousands of years, while the clay would further slow the corrosion rates of the container. The batholiths themselves are chosen for their low ground-water movement rates, geological stability, and low economic value. (See The Canadian Nuclear FAQ, Waste Management section, by Dr. Jeremy Whitlock)

Reprocessing is attractive in principle because (1) it can recycle nuclear fuel and (2) it can prepare the waste material for disposal. Considerable experience with reprocessing in France however, has indicated that a one way fuel cycle based on extracting and processing fresh supplies of uranium and storing the spent fuel is more economical than reprocessing.

Natural nuclear reactors

A natural nuclear fission reactor can occur under certain circumstances that mimic the conditions in a constructed reactor. The only known natural nuclear reactor formed 2 billion years ago in Oklo, Gabon, Africa. Such reactors can no longer form on Earth: radioactive decay over this immense time span has reduced the proportion of U-235 in naturally occurring uranium to below the amount required to sustain a chain reaction.

The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years.

These natural reactors are extensively studied by scientists interested in geologic radioactive waste disposal. They offer a case study of how radioactive isotopes migrate through the earth's crust. This is a significant area of controversy as opponents of geologic waste disposal fear that isotopes from stored waste could end up in water supplies or be carried into the environment.


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