Nuclear Reactors
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 bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled.

The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the power in some ships. This is usually accomplished by methods that involve using heat from the nuclear reaction to power steam turbines.

Basics of reactor operation

Conventional thermal power plants all have a fuel source to provide heat. Examples are gas, coal, or oil. For a nuclear power plant, this heat is provided by nuclear fission inside the nuclear reactor. When a relatively large fissile atomic nucleus (usually uranium-235 or plutonium-239) is struck by a neutron it forms two or smaller nuclei as fission products, releasing energy and neutrons in a process called nuclear fission. The neutrons then trigger further fission. And so on. When this nuclear chain reaction is controlled, the energy released can be used to heat water, produce steam and drive a turbine that generates electricity.

It should be noted that a nuclear explosive involves an uncontrolled chain reaction, and the rate of fission in a reactor is not capable of reaching sufficient levels to trigger a nuclear explosion (even if the fission reactions increased to a point of being out of control, it would melt the reactor assembly rather than form a nuclear explosion).

Commercial nuclear reactions use uranium as fuel for producing energy. One pound of uranium produces as much energy as six tons of coal or 1200 gallons of oil. Nuclear fuel is also very cheap, costing just 1/2 cent per kilowatt-hour. Natural uranium is made up of two isotopes: U-235, which is the fissionable isotope but accounts for only 0.7% of natural uranium, and U-238, which makes up over 99% of natural uranium but does not fission. In order for natural uranium to be used as reactor fuel, it must be enriched to 3-5% U-235. The first step in the enrichment process converts uranium to a gas. Solid uranium reacts chemically with fluorine to produce UF6, uranium hexafluoride, which is a gas at room temperature. A gas chromatography process then increases the U-235 content, and the enriched UF6 is then converted to uranium dioxide, a solid, and pressed into ceramic pellets. Old uranium-containing nuclear weapons are also being used for fuel. The U-235 content of these weapons ranges from 20-90% but can be diluted to 3-5% and used as fuel.

The actual nuclear reaction takes place in what is called the reactor core. The uranium fuel pellets are put into tubes and then placed in the reactor. Neutrons are released and strike uranium atoms, which release their own neutrons and cause a chain reaction. Heat from fission turns water to steam, turning turbines to produce electricity. Control rods fit between the fuel rods and absorb neutrons. Inserting control rods into the reactor core slows down the reaction, whereas withdrawing them allows the reaction to speed up.

How Nuclear Reactors Work

When an atom undergoes fission it splits into smaller atoms, other particles and releases energy. It turns out that it is possible to harness the energy of this process on a large enough scale for it to be a viable way of producing energy.

The fundamental point about nuclear energy is that the energy content of 1 gram of Uranium is equivalent to approximately 3 tonnes of coal. This means that we need to consume about 3 million times less material with Nuclear Power compared to using Coal or any other Fossil Fuel. This reduces the volumes of fuel and waste of nuclear power compared to Fossil Fuels.

The first nuclear reactor

Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor. CP-1 was built on a racquets court, under the abandoned west stands of the original Alonzo Stagg Field stadium, at the University of Chicago. The first artificial, self-sustaining, nuclear chain reaction was initiated within CP-1, on December 2, 1942.

On December 2, 1942, CP-1 was ready for a demonstration. Before a group of dignitaries, a young scientist named George Weil worked the final control rod while Fermi carefully monitored the neutron activity. The pile went critical at 3:20 p.m. Fermi shut it down 33 minutes later.

Operation of CP-1 was terminated in February 1943. The reactor was then dismantled and moved to Red Gate Woods, the former site of Argonne National Laboratory, where it was reconstructed using the original materials, plus an enlarged radiation shield, and renamed Chicago Pile-2 (CP-2). CP-2 began operation in March 1943 and was later buried at the same site, now known as the Site A/Plot M Disposal Site.

Reactor types

There are two types of nuclear power in current use, they are:

  • Nuclear Fission

  • Radioisotope Thermoelectric

The nuclear fission reactor produces heat through a controlled nuclear chain reaction in a critical mass of fissile material.

All current nuclear power plants are critical fission reactors. The output of fission reactors is controllable. There are several subtypes of critical fission reactors, which can be classified as Generation I, Generation II and Generation III. All reactors will be compared to the Pressurized Water Reactor (PWR), as that is the standard modern reactor design.

They generally use uranium as fuel, but research on using thorium is ongoing. 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 reactors (thermal-spectrum) and Fast Neutron Reactors fast-spectrum).

The difference between fast-spectrum and thermal-spectrum reactors is that fast-spectrum reactors will produce less waste, and the waste they do produce will have a vastly lower half-life, but they are more difficult to build, and more expensive to operate. Fast reactors can also be breeders, whereas thermal reactors generally cannot.

Thermal reactors use slow or thermal neutrons. Most power reactors are of this type. Neutron moderator materials that slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, until they are thermalized, characterize these. Thermal neutrons have a far higher probability of fissioning uranium-235, and a lower probability of capture by uranium-238 than the faster neutrons that result from fission. 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.

Fast neutron reactors use fast neutrons to sustain the fission chain reaction. They are characterized by an absence of moderating material. Initiating the chain reaction requires enriched uranium (and/or enrichment with plutonium 239), due to the lower probability of fissioning U-235, and a higher probability of capture by U-238 (as compared to a moderated, thermal neutron). Fast reactors have the potential to produce less transuranic waste because all actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications. Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing.

Nuclear Fission Powered

Pressurized water reactors (PWR)

These are reactors cooled and moderated by high pressure, liquid (even at extreme temperatures) water. They are the majority of current reactors, and are generally considered the safest and most reliable technology, although Three Mile Island is a reactor of this type. This is a thermal neutron reactor design.

Boiling water reactors (BWR)

These are reactors cooled and moderated by water, under slightly lower pressure. The water is allowed to boil in the reactor. The thermal efficiency of these reactors can be higher, and they can be simpler And even potentially more stable and safe. Unfortunately, the boiling water puts more stress on many of the components, and increases the risk that radioactive water may escape in an accident. These reactors make up a substantial percentage of modern reactors. This is a thermal neutron reactor design.

Pressurized Heavy Water Reactor (PHWR)

A Canadian design, (known as CANDU) these reactors are heavy-water-cooled and -moderated Pressurized-Water reactors. Instead of using a single large containment vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These reactors are fuelled with natural uranium and are thermal neutron reactor designs. PHWRs can be refueled while at full power, which makes them very efficient in their use of uranium (it allows for precise flux control in the core). Most PHWR's exist within Canada, but units have been sold to Argentina, China, India (pre-NPT), Pakistan (pre-NPT), Romania, and South Korea. India also operates a number of PHWR's, built after the 1974 Smiling Buddha nuclear weapon test.

RBMKs

A Soviet Union design, built to produce plutonium as well as power, the dangerous and unstable RBMKs are water cooled with a graphite moderator. RBMKs are refuelable and employ a pressure tube design instead of a PWR-style pressure vessel. Notably, they are too large and powerful to have containment buildings. Chernobyl was an RBMK.

Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor

These are generally graphite moderated, and CO2 cooled. They have a high thermal efficiency compared with PWRs and an excellent safety record. There are a number of operating reactors of this design mostly in the United Kingdom, older designs are either shut down or will be with in the near future. However the AGRs have an anticipated life of a further 10 to 20 years. This is a thermal neutron reactor design.

Super Critical Water-cooled Reactor (SCWR)

This is a theoretical reactor design that is part of the Gen-IV reactor project. It combines higher efficiency than a GCR with the safety of a PWR, though it is perhaps more technically challenging than either. The water is pressurized and heated past its critical point, until there is no difference between the liquid and gas states. A CWR is similar to a BWR, except there is no boiling (as the water is critical), and the thermal efficiency is higher as the water behaves more like a classical gas. This is an epithermal neutron reactor design.

Liquid Metal Fast Breeder Reactor (LMFBR)

This is a reactor design that is cooled by liquid metal, and totally unmoderated. These reactors can function much like a PWR in terms of efficiency, and don't require much high-pressure containment, as the liquid metal doesn't need to be kept at high pressure, even at very high temperatures. All three use/used liquid sodium. These reactors are fast neutron, not thermal neutron designs. These reactors come in two types:

  • Lead Cooled. Using lead as the liquid metal provides excellent radiation shielding, and allows for operation at very high temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons are lost in the coolant And the coolant does not become radioactive. Unlike sodium, lead is mostly inert, so there is less risk of explosion or accident, but such large quantities of lead may be problematic from toxicology and disposal points of view. Often a reactor of this type would use a lead-bismuth eutectic mixture. In this case, the bismuth would present some minor radiation problems, as it is not quite as transparent to neutrons, and can be transmuted to a radioactive isotope more readily than lead.

  • Sodium Cooled. Most LMFBRs are of this type. The sodium is relatively easy to obtain and work with, and it also manages to actually remove corrosion on the various reactor parts immersed in it. However, sodium explodes violently when exposed to water, so care must be taken, but such explosions wouldn't be vastly more violent than (for example) a leak of superheated fluid from a CWR or PWR.

Aqueous homogeneous reactor

Aqueous homogeneous reactors (AHR) are a type of nuclear reactor in which soluble nuclear salts (usually uranium sulfate or uranium nitrate) have been dissolved in water. The fuel is mixed with the coolant and the moderator, thus the name "homogeneous" ('of the same physical state') The water can be either heavy water or light water, both which need to be very pure. A heavy water aqueous homogeneous reactor can achieve criticality (turn on) with natural uranium dissolved as uranium sulfate. Thus, no enriched uranium is needed for this reactor. The heavy water versions have the lowest specific fuel requirements (least amount of nuclear fuel is required to start them). Even in light water versions less than 1 pound (454 grams) of plutonium-239 or uranium-233 is needed for operation. Neutron economy in the heavy water versions is the highest of all reactor designs.

Their self-controlling features and ability to handle very large increases in reactivity make them unique among reactors, and possibly safest.

Aqueous homogeneous reactors were sometimes called water boilers, although they are not boiling water reactors. They seem to be boiling their water, but in fact this bubbling is from the production of hydrogen and oxygen as the radiation, and especially the fission particles, dissociate the water into its constituent gases. They were widely used as research reactors as they were self-controlling, have very high neutron fluxes and were easy to manage.

Radioisotope Thermoelectric Generator

The radioisotope thermoelectric generator produces heat through passive radioactive decay. Some radioisotope thermoelectric generators have been created to power space probes, some lighthouses, and some pacemakers. The heat output of these generators diminishes with time; the heat is converted to electricity by thermocouples.

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