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Nuclear Reactor Technology
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NUCLEAR REACTOR TECHNOLOGY

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. The most common use of nuclear reactors is for the generation of electric energy and for the propulsion of ships. Heat from nuclear fission is used to raise steam, which runs through turbines, which, in turn, powers either ships' propellors or electrical generators. A few reactors manufacture isotopes for medical and industrial use, and some reactors are operated only for research.

How it works

Just as conventional power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the thermal energy released from nuclear fission.


Fission

When a large, fissile, atomic nucleus, such as uranium-235 or plutonium-239, absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, and free neutrons, collectively known as "fission products." A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.

The reaction can be controlled by using neutron poisons, which absorb excess neutrons, and neutron moderators, which reduce the velocity of fast neutrons, thereby turning them into thermal neutrons, which are more likely to be absorbed by other nuclei. Increasing or decreasing the rate of fission has a corresponding effect on the energy output of the reactor.

Commonly-used moderators include regular (light) water (75% of the world's reactors), solid graphite (20% of reactors), and heavy water (5% of reactors). Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.


Heat generation

The reactor core generates heat in a number of ways:

The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms.

Some of the gamma rays produced during fission are absorbed by the reactor, their energy being converted to heat.

Heat produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shut down.

A kilogram of uranium-235 (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram of coal).


Cooling

A nuclear reactor coolant, usually water but sometimes a gas or a liquid metal or molten salt, is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. But in some reactors, the water for the steam turbines is boiled directly by the reactor core; for example, the boiling water reactor.


Reactivity control

Main articles: Nuclear reactor control, passive nuclear safety, delayed neutron, iodine pit, SCRAM, and decay heat

The power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions.

Control rods that are made of a neutron poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission. So pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it.

At the first level of control in all nuclear reactors, a process of delayed neutron emission by a number of neutron-rich fission isotopes is an important physical process. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "prompt neutrons") released immediately upon fission. The fission products which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes.

Keeping the reactor in the zone of chain-reactivity, where delayed neutrons are necessary to achieve a critical mass state, allows time for mechanical devices or human operators to have time to control a chain reaction in "real time." Otherwise, the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction would be too short to allow for intervention.

In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission, so more neutron moderation means more power output from the reactors. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore, a less effective moderator.

In other reactors, the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors, power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to SCRAM the reactor in an emergency shut-down. These systems insert large amounts of poison (often boron, in the form of boric acid) into the reactor, to shut the fission reaction down if unsafe conditions are detected or anticipated.

Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit. Xenon-135 is normally produced in the fission process, and acts as a neutron-absorbing "neutron poison," which acts to shut the reactor down but can be controlled, in turn, within the reactor by keeping neutron and power levels high enough to destroy it as fast as it is produced.

The normal fission process also produces iodine-135, which, in turn, decays with a half life of under seven hours, to new xenon-135. Thus, if the reactor is shut down, iodine-135 in the reactor continues to decay to xenon-135, to the point that the new xenon-135 from this source ("xenon poisoning") makes re-starting the reactor more difficult, for a day or two, than when first shut down. (This temporary state is the "iodine pit.")

If the reactor has sufficient extra capacity, it can still be restarted before the iodine-135 and xenon-135 decay, but as the extra xenon-135 is "burned off" by transmuting it to xenon-136 (not a neutron poison), within a few hours, the reactor may become unstable as a result of such a "xenon burnoff (power) transient" and then rapidly become overheated, unless control rods are reinserted in order to replace the neutron absorption of the lost xenon-135. Failure to properly follow such a procedure was a key step in the Chernobyl disaster.

Reactors used in nuclear marine propulsion (especially nuclear submarines) often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run and, in addition, often need to have a very long core life without refueling. For this reason, many designs use highly-enriched uranium but incorporate burnable neutron poison directly into the fuel rods. This allows the reactor to be constructed with a high excess of fissionable material, which is, nevertheless, made relatively more safe early in the reactor's fuel burn cycle by the presence of the neutron-absorbing material, which is later replaced by naturally-produced, long-lived neutron poisons (far longer-lived than xenon-135), which gradually accumulate over the fuel load's operating life.


Electrical power generation

The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam, which will then drive a steam turbine that generates electricity.

EARLY REACTORS

The neutron was discovered in 1932. The concept of a nuclear chain reaction brought about by nuclear reactions mediated by neutrons was first realized shortly thereafter, by Hungarian scientist Leó Szilárd, in 1933. He filed a patent for his idea of a simple nuclear reactor the following year, while working at the Admiralty in London. However, Szilárd's idea did not incorporate the idea of nuclear fission as a neutron source, since that process was not yet discovered. Szilárd's ideas for nuclear reactors using neutron-mediated nuclear chain reactions in light elements proved unworkable.

Inspiration for a new type of reactor, using uranium, came from the discovery by Lise Meitner, Fritz Strassman, and Otto Hahn, in 1938, that bombardment of uranium with neutrons (provided by an alpha-on-beryllium fusion reaction, a "neutron howitzer") produced a barium residue, which, they reasoned, was created by the fissioning of the uranium nuclei. Subsequent studies in early 1939 (one of them by Szilárd and Fermi) revealed that several neutrons were also released during the fissioning, making available the opportunity for the nuclear chain reaction that Szilárd had envisioned six years previously.

On August 2, 1939, Albert Einstein signed a letter to President Franklin D. Roosevelt (written by Szilard), suggesting that the discovery of uranium's fission could lead to the development of "extremely powerful bombs of a new type," giving impetus to the study of reactors and fission. Szilárd and Einstein knew each other well and had worked together years previously, but Einstein had never thought about this possibility for nuclear energy until Szilard reported it to him at the beginning of his quest to produce the Einstein-Szilard letter to alert the U.S. government.

Shortly after, Hitler's Germany invaded Poland in 1939, starting World War II in Europe. The U.S. was not yet officially at war, but in October, when the Einstein-Szilard letter was delivered to Roosevelt, he commented that the purpose of doing the research was to make sure "the Nazis don't blow us up." The U.S. nuclear project followed, although with some delay, as there remained skepticism (some of it from Fermi) and also little action from the small number of officials in the government who were initially charged with moving the project forward.

The following year, the U.S. Government received the Frisch–Peierls memorandum from the UK, which stated that the amount of uranium needed for a chain reaction was far lower than had previously been thought. The memorandum was a product of the MAUD Committee, which was working on the UK atomic bomb project, known as "Tube Alloys," later to be subsumed within the Manhattan Project.

Eventually, the first artificial nuclear reactor, Chicago Pile-1, was constructed at the University of Chicago, by a team led by Enrico Fermi, in late 1942. By this time, the program had been pressured for a year by U.S. entry into the war. The Chicago Pile achieved criticality on December 2, 1942, at 3:25 p.m. The reactor support structure was made of wood, which supported a pile (hence, the name) of graphite blocks, embedded in which was natural uranium-oxide "pseudospheres," or "briquettes."

Soon after the Chicago Pile, the U.S. military developed a number of nuclear reactors for the Manhattan Project, starting in 1943. The primary purpose for the largest reactors (located at the Hanford Site in Washington state) was the mass production of plutonium for nuclear weapons. Fermi and Szilard applied for a patent on reactors on December 19, 1944. Its issuance was delayed for 10 years because of wartime secrecy.

"World's first nuclear power plant" is the claim made by signs at the site of the EBR-I, which is now a museum near Arco, Idaho. This experimental LMFBR, operated by the U.S. Atomic Energy Commission, produced 0.8 kW in a test on December 20, 1951, and 100 kW (electrical) the following day, having a design output of 200 kW (electrical).

Besides the military uses of nuclear reactors, there were political reasons to pursue civilian use of atomic energy. U.S. Pre...