Nuclear Reactors - NukeStation Terminus.

Nuclear Reactors are based on controlled chain reactions. The energy released in nuclear disintegration is used to heat water which in turn, rotates turbines connected to generators which then produce electricity. Uranium-235 is the most common nuclear fuel for reactors. Reactors also produce radioisotopes which have applications in the medical field for treating diseases like cancer. They are also used in agricultural field for irradiating foodstuffs like Potatoes in order to increase their shelf-life. They have many other application in scientific research. Some reactors are specially used for research purposes. They are called research reactors.

Click here for a virtual tour through a reactor.

There are mainly three types of reactors :

1) Light and Heavy Water Reactors
2) Propulsion Reactors
3) Breeder Reactors

Light and Heavy Water Reactors :

A variety of reactors are used for the production of electric power. They use different types of fuels, moderators, and coolants. In the US, generally power reactors use nuclear fuel in the form of uranium oxide isotopically enriched to about 3 percent uranium-235. The moderator and coolant are highly purified ordinary water. A reactor of this type is called a light water reactor (LWR).

In the pressurised water reactor (PWR), which is a version of the LWR system, the water coolant operates at a pressure of about 150 atm. It is pumped through the reactor core, where it is heated to about 325° C (about 620° F). The superheated water is pumped through a steam generator, where, through heat exchangers, a secondary loop of water is heated and converted to steam. This steam drives turbine generators, and then it is condensed and pumped back to the steam generator. The secondary loop is isolated from the reactor core water and, therefore, is not radioactive. A third stream of water from a lake, river, or Cooling Tower is used to condense the steam. The reactor pressure vessel is about 15 m (about 49 ft) high and 5 m (about 16.4 ft) in diameter, with walls 25 cm (about 10 in) thick. The core houses some 82 metric tons of uranium oxide contained in thin corrosion-resistant tubes, clustered into fuel bundles.

In the boiling water reactor (BWR), a second type of LWR, the water coolant is permitted to boil within the core, by operating at somewhat lower pressure. The steam produced in the reactor pressure vessel is piped directly to the turbine generator, condensed, and then pumped back to the reactor. Although the steam is radioactive, there is no intermediate heat exchanger between the reactor and turbine to decrease efficiency. As in the PWR, the condensor cooling water has a separate source, such as a lake or river.

The power level of an operating reactor is monitored by a variety of thermal, flow, and nuclear instruments. Power output is controlled by inserting or removing from the core a group of neutron-absorbing control rods. The position of these rods determines the power level at which the chain reaction is just self-sustaining.

A thick coat of radioactive absorbant concrete sheets is used as a safety measure against the radiation emitted from the reactor during operation and shutdown. Emergency core cooling systems to prevent core overheating in the event of malfunction of the main coolant systems, and a large steel and concrete containment building are additional safety measures.

Propulsion Reactors :

Nuclear power plants similar to the PWR are used for the propulsion plants of large surface naval vessels such as the 95,000-ton aircraft carrier, known as USS Nimitz, which joined the Atlantic Fleet in 1975 and was programmed to operate for at least 15 years without refueling. The basic technology of the PWR system was first developed in the U.S. naval reactor program directed by Adm. Hyman G. Rickover. Reactors for submarine propulsion are generally physically smaller and use more highly enriched uranium to permit a compact core. The U.S., Great Britain, Russia, and France have nuclear-powered submarines with such power plants.

Three experimental seagoing nuclear cargo ships were operated for limited periods by the U.S., Germany, and Japan. Although they were technically successful, economic conditions and restrictive port regulations brought an end to these projects. The Soviets built the first successful nuclear-powered icebreaker, namely Lenin, for use in clearing the Arctic sea-lanes.

Breeder Reactors:

Uranium, the natural resource on which nuclear power is based, occurs in scattered deposits throughout the world. Its total supply is not fully known and may be limited unless very low concentration sources such as granites and shale were to be used. Conservatively estimated U.S. resources of uranium lie in the range of 2 to 5 million metric tons. The lower amount could support an LWR nuclear power system providing about 30 percent of U.S. electric power for only about 50 years. The principal reason for this relatively brief life span of the LWR nuclear power system is its very low efficiency in the use of uranium: Only approximately 1 percent of the energy content of the uranium is made available in this system.

The key feature of a breeder reactor is that it produces more fuel than it consumes. It does this by promoting the absorption of excess neutrons in a fertile material. Several breeder reactor systems are technically feasible. The breeder system that has received the greatest worldwide attention uses uranium-238 as the fertile material. When uranium-238 absorbs neutrons in the reactor, it is transmuted to a new fissionable material, plutonium, through a nuclear process called beta decay. In beta decay a nuclear neutron decays into a proton and a Beta Particle .

Fission can occur when plutonium-239 itself absorbs a neutron, and on the average about 3 neutrons are released. In an operating reactor, one of these neutrons is needed continue chain reaction. On the average about 1 neutron is lost by absorption in the reactor structure or coolant. The remaining 2 neutrons can be absorbed in uranium-238 to produce more plutonium through the reactions in the third equation above.

The breeder system is also called the liquid metal fast breeder reactor (LMFBR). In order to maximize the production of plutonium-239, the velocity of the neutrons causing fission is kept fast during initial energy release. Any moderating materials, such as water, are excluded from the reactor. A molten metal (eg.liquid sodium) is the preferred coolant liquid as it has very good heat transfer properties, melts at about 100° C and does not boil until about 900° C. However, reactivity with air and water and the level of radioactivity induced in it in the reactor are high in case of Sodium.

Development of the LMFBR system began in the U.S. before 1950 known as EBR-1. In Great Britain, the Soviet Union, and France, working breeder reactors were installed, and experimental work continued in Germany and Japan.

In one design of a large LMFBR power plant, the core consists of thousands of thin stainless steel tubes containing mixed uranium and plutonium oxide fuel: about 15 to 20 percent plutonium-239, the remainder uranium. Surrounding the core is a region called the breeder blanket, which contains similar rods filled only with uranium oxide. The entire core and blanket assembly measures about 3 m (about 10 ft) high by about 5 m (about 16.4 ft) in diameter and is supported in a large vessel containing molten sodium that leaves the reactor at about 500° C (about 930° F). This vessel also contains the pumps and heat exchangers that aid in removing heat from the core. Steam is produced in a second sodium loop, separated from the radioactive reactor coolant loop by the intermediate heat exchangers in the reactor vessel. The entire nuclear reactor system is housed in a large steel and concrete containment building.

The first large-scale plant of this type for generation of electricity, was Super-Phenix, which started its operation in France in 1984. An intermediate-scale plant, the BN-600, was built in the USSR on the shore of the Caspian Sea for the production of power and the desalination of water. The British have a large 250-MW prototype in Scotland.

The LMFBR produces about 20 percent more fuel than it consumes. In a large power reactor enough excess new fuel is produced over 20 years to permit the loading of another similar reactor. In the LMFBR system about 75 percent of the energy content of natural uranium is made available, in contrast to the 1 percent in the LWR

Research Reactors :

The main applications of these reactors are in the fields education and training, research, and radioactive isotope production. Easy to start and shut down, these reactors generally operate at power levels near 1 MW.

A widely used type is called the swimming pool reactor. The core is partially or fully enriched uranium-235 contained in aluminum alloy plates, immersed in a large pool of water that serves as both coolant and moderator. Materials may be placed directly in or near the reactor core to be irradiated with neutrons. Various radioactive isotopes can be produced for use in medicine, research, and industry . Neutrons may also be extracted from the reactor core by means of beam tubes to be used for experimentation.

Top