Разделение изотопов и применение их в ядерном реакторе

 

A second method of laser separation is known as MLIS, Molecular Laser Isotope Separation. In this method, an infrared laser is directed at uranium hexafluoride gas, exciting molecules that contain a U-235 atom. A second laser frees a fluorine atom, leaving uranium pentafluoride which then precipitates out of the gas. Cascading the MLIS stages is more difficult than with other methods because the UF5 must be refluorinated (back to UF6) before being introduced into the next MLIS stage. Alternative MLIS schemes are currently being developed (using a first laser in the near-infrared or visible region) where an enrichment of over 95% can be obtained in a single stage, but the methods have not (yet) reached industrial feasibility. This method is called OP-IRMPD (Overtone Pre-excitation - IR Multiple Photon Dissociation).

 

Finally, the SILEX process, developed by Silex Systems in Australia, has recently been licensed to General Electric for the development of a pilot enrichment plant. The method uses uranium hexafluoride as a feedstock, and uses magnets to separate the isotopes after one isotope is preferentially ionized.

 

Chemical methods

 

Although isotopes of a single element are normally described as having the same chemical properties, this is not strictly true. In particular, reaction rates are very slightly affected by atomic mass.

 

Techniques using this are most effective for light atoms such as hydrogen. Lighter isotopes tend to react or evaporate more quickly than heavy isotopes, allowing them to be separated. This is how heavy water is produced commercially (see Girdler sulfide process for details). Lighter isotopes also disassociate more rapidly under an electric field. This process in a large cascade was used at the heavy water production plant at Rjukan.

 

One candidate for the largest kinetic isotopic effect ever measured at room temperature, 305, may eventually be used for the separation of tritium (T). The effects for the oxidation of triated formate anions to HTO were measured as:k(HCO2-) = 9.54 M−1s−1 k(H)/k(D) = 38

k(DCO2-) = 9.54 M−1s−1 k(D)/k(T) = 8.1

k(TCO2-) = 9.54 M−1s−1 k(H)/k(T) = 305

 

 

 

Gravity

 

Isotopes of Carbon, Oxygen, and Nitrogen can be purified by chilling these gases or compounds nearly to their liquification temperature in very tall columns (200 to 700 feet tall - 70 to 200 meters). The heavier isotopes sink and the lighter isotopes rise, where they are easily collected. The process was developed in the late 1960s by scientists at Los Alamos National Laboratory. This process is also called "cryogenic distillation".

 

The SWU (separative work unit)

 

Separative Work Unit (SWU) is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, i.e. the extent of increase in the concentration of the U-235 isotope relative to the remainder.

 

The unit is strictly: Kilogram Separative Work Unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. The effort expended in separating a mass F of feed of assay xf into a mass P of product assay xp and waste of mass W and assay xw is expressed in terms of the number of separative work units needed, given by the expression SWU = WV(xw) + PV(xp) - FV(xf), where V(x) is the "value function," defined as V(x) = (1 - 2x) ln ((1 - x) /x).

 

Separative work is expressed in SWUs, kg SW, or kg UTA (from the German Urantrennarbeit )

1 SWU = 1 kg SW = 1 kg UTA

1 kSWU = 1.0 t SW = 1 t UTA

1 MSWU = 1 kt SW = 1 kt UTA

 

If, for example, you begin with 100 kilograms (220 pounds) of natural uranium, it takes about 60 SWU to produce 10 kilograms (22 pounds) of uranium enriched in U-235 content to 4.5%

 

Isotope Separators for Research

 

Radioactive beams of specific isotopes are widely used in the fields of experimental physics, biology and materials science. The production and formation of these radioactive atoms into an ionic beam for study is an entire field of research carried out at many laboratories throughout the world. The first isotope separator was developed at the Copenhagen Cyclotron by Bohr and co-workers using the principle of electromagnetic separation. Today, there are many laboratories around the world which supply beams of radioactive ions for use. Arguably the principal Isotope Separator On-Line (ISOL) is ISOLDE at CERN, which is a joint European facility spread across the Franco-Swiss border near the city of Geneva. This laboratory uses mainly proton spallation of uranium carbide targets to produce a wide range of radioactive fission fragments that are not found naturally on earth. During spallation (bombardment with high energy protons), a uranium carbide target is heated to several thousand degrees so that radioactive atoms produced in the nuclear reaction are released. Once out of the target, the vapour of radioactive atoms travels to an ionizer cavity. This ionizer cavity is a thin tube made of a low work function metal allowing for collisions with the walls to liberate a single electron from a free atom. Once ionized, the radioactive species are accelerated by an electrostatic field and injected into an electromagnetic separator. As ions entering the separator are of approxiamtely equal energy, those ions with a smaller mass will be deflected by the magnetic field by a greater amount than those with a heavier mass. This differing radius of curvature allows for isobaric purification to take place. Once purified isobarically, the ion beam is then sent to the individual experiments. In order to increase the purity of the isobaric beam, laser ionization can take place inside the ionizer cavity to selectively ionize a single element chain of interest. At CERN, this device is called the Resonance Ionization Laser Ion Source (RILIS). Currently over 60% of all experiments opt to use the RILIS to increase the purity of radioactive beams.

 

Beam Production Capability of ISOL Facilities

 

As the production of radioactive atoms by the ISOL technique depends on the free atom chemistry of the element to be studied, there are certain beams which cannot be produced by simple proton bombardment of thick actinide targets. Refractory metals such as tungsten and rhenium do not emerge from the target even at high temperatures due to their low vapour pressure. In order to produce these types of beams, a thin target is required. The Ion Guide Isotope Separator On Line (IGISOL) technique was developed in 1981 at the University of Jyvaskyla cyclotron laboratory in Finland. In this technique, a thin uranium target is bombarded with protons and nuclear reaction products recoil out of the target in a charged state. The recoils are stopped in a gas cell and then exit through a small hole in the side of the cell where they are accelerated electrostatically and injected into a mass separator. This method of production and extraction takes place on a shorter timescale compared to the standard ISOL technique and isotopes with short half-lives (sub millisecond) can be studied using an IGISOL. An IGISOL has also been combined with a laser ion source at the Leuven Isotope Separator On Line (LISOL) in Belgium. Thin target sources generally provide significantly lower quantities of radioactive ions than thick target sources and this is their main drawback.

 

As experimental nuclear physics progresses, it is becoming more and more important to study the most exotic of radioactive nuclei. In order to do so, more inventive techniques are required to create nuclei with extreme proton/neutron ratios. The most promising technique to date is by using multiple targets. By first producing a radioactive beam by an ISOL method and then reaccellerating it to make it hit a secondary thin target, very exotic nuclei can be produced. The National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University is a good example of such a facility. The higher the energy of interaction, generally the more exotic the nucleus produced. It then becomes necessary to be able to slow these nuclei down once they have been produced. Pioneers at the Japanese facility RIKEN were the first to use a giant gas catcher and novel electric fields to do this, which is becoming the standard technique.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Nuclear reactor

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 electrical power and for the power in some ships. Heat from nuclear fission is used to raise steam, which runs through turbines, which in turn powers either ship's propulsion or electrical generators. There are also other less common uses as discussed below.

Contents 1 How it works 1.1 Fission 1.2 Heat generation 1.3 Cooling 1.4 Reactivity control 1.5 Electrical power generation 2 History - Early reactors 3 Components 4 The people in a nuclear power plant 5 Reactor types 5.1 Classifications 5.1.1 Classification by type of nuclear reaction 5.1.2 Classification by moderator material 5.1.3 Classification by coolant 5.1.4 Classification by generation 5.1.5 Classification by phase of fuel 5.1.6 Classification by use 5.2 Current technologies 5.3 Future and developing technologies 5.3.1 Advanced reactors 5.3.2 Generation IV reactors 5.3.3 Generation V+ reactors 5.3.4 Fusion reactors 6 Nuclear fuel cycle 6.1 Fueling of nuclear reactors 7 Natural nuclear reactors

 

 

 

 

 

 

 

 

 

 

How it works

An induced nuclear fission event. A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. Though both reactors and nuclear weapons rely on nuclear chain reactions, the rate of reactions in a reactor is much slower than in a bomb.

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 reduces 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 shutdown.

A kilogram of uranium-235 (U-235) converted via nuclear processes contains approximately three million times the energy of 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

 

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

Control rods that are made of a nuclear 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.

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 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.

 

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.

 

History - Early reactors

 

The concept of a nuclear chain reaction was first realized by Hungarian scientist Leó Szilárd in 1933. He filed a patent for his idea of a simple nuclear reactor the following year.

The first artificial nuclear reactor, Chicago Pile-1, was constructed at the University of Chicago by a team led by Enrico Fermi in 1942. It achieved criticality on December 2, 1942 at 3:25 PM. The reactor support structure was made of wood, which supported a pile of graphite blocks, embedded in which was natural Uranium-oxide 'pseudospheres' or 'briquettes'. Inspiration for such a reactor was provided by 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 revealed that several neutrons were also released during the fissioning, making available the opportunity for a chain reaction. Shortly after the discovery of fission, Hitler's Germany invaded Poland in 1939, starting World War II in Europe, and all such research became militarily classified. On August 2, 1939 Albert Einstein wrote a letter to President Franklin D. Roosevelt 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.

Soon after the Chicago Pile, the U.S. military developed nuclear reactors for the Manhattan Project starting in 1943. The primary purpose for these reactors was the mass production of plutonium (primarily at the Hanford Site) for nuclear weapons. Fermi and Leo Szilard applied for a patent on reactors on 19 December 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. President Dwight Eisenhower made his famous Atoms for Peace speech to the UN General Assembly on December 8, 1953. This diplomacy led to the dissemination of reactor technology to U.S. institutions and worldwide.

The first nuclear power plant built for civil purposes was the AM-1 Obninsk Nuclear Power Plant, launched on June 27, 1954 in the Soviet Union. It produced around 5 MW (electrical).

After World War II, the U.S. military sought other uses for nuclear reactor technology. Research by the Army and the Air Force never came to fruition; however, the U.S. Navy succeeded when they steamed the USS Nautilus (SSN-571) on nuclear power January 17, 1955.

The first commercial nuclear power station, Calder Hall in Sellafield, England was opened in 1956 with an initial capacity of 50 MW (later 200 MW).

The first portable nuclear reactor "Alco PM-2A" used to generate electrical power (2 MW) for Camp Century from 1960.

 

Components

 

The control room of NC State's Pulstar Nuclear Reactor.

The key components common to most types of nuclear power plants are:

Nuclear fuel

Nuclear reactor core

Neutron moderator

Neutron poison

Coolant (often the Neutron Moderator and the Coolant are the same, usually both purified water)

Control rods

Reactor vessel

Boiler feedwater pump

Steam generators (not in BWRs)

Steam turbine

Electrical generator

Condenser

Cooling tower (not always required)

Radwaste System (a section of the plant handling radioactive waste)

Refueling Floor

Spent fuel pool

Nuclear safety systems

Reactor Protective System (RPS)

Emergency Diesel Generators

Emergency Core Cooling Systems (ECCS)

Standby Liquid Control System (emergency boron injection, in BWRs only)

Containment building

Control room

Emergency Operations Facility

Nuclear training facility (usually contains a Control Room simulator)

 

The people in a nuclear power plant

 

Nuclear power plants typically employ just under a thousand people per reactor (including security guards and engineers associated with the plant but possibly working elsewhere).

Nuclear engineers

Reactor operators

Health physicists

Emergency response team personnel

Nuclear Regulatory Commission Resident Inspectors

In the United States and Canada, workers except for management, professional (such as engineers) and security personnel are likely to be members of either the International Brotherhood of Electrical Workers (IBEW) or the Utility Workers Union of America (UWUA).

 

 

 

 

 

 

 

 

 

 

 

 

 

Reactor types

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.

 

Classifications

 

Nuclear Reactors are classified by several methods; a brief outline of these classification schemes is provided.

 

Classification by type of nuclear reaction

 

Nuclear fission. All commercial power reactors are based on nuclear fission. They generally use uranium and its product plutonium as nuclear fuel, though a thorium fuel cycle is also possible. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that sustain the fission chain reaction:

Thermal reactors use slowed or thermal neutrons. Almost all current reactors are of this type. These contain neutron moderator materials that slow neutrons until their neutron temperature is thermalized, that is, until their kinetic energy approaches the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher cross section (probability) of fissioning the fissile nuclei uranium-235, plutonium-239, and plutonium-241, and a relatively lower probability of neutron capture by uranium-238 (U-238) compared to the faster neutrons that originally result from fission, allowing use of low-enriched uranium or even natural uranium fuel. The moderator is often also the coolant, usually water under high pressure to increase the boiling point. These are surrounded by reactor vessel, instrumentation to monitor and control the reactor, radiation shielding, and a containment building.

Fast neutron reactors use fast neutrons to cause fission in their fuel. They do not have a neutron moderator, and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly enriched in fissile material (about 20% or more) due to the relatively lower probability of fission versus capture by U-238. 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 (see fast breeder or generation IV reactors).

Nuclear fusion. Fusion power is an experimental technology, generally with hydrogen as fuel. While not suitable for power production, Farnsworth-Hirsc fusors are used to produce neutron radiation.

Radioactive decay. Examples include radioisotope thermoelectric generators as well as other types of atomic batteries, which generate heat and power by exploiting passive radioactive decay.

 

 

 

Classification by moderator material

 

Used by thermal reactors:

Graphite moderated reactors

Water moderated reactors

Heavy water reactors

Light water moderated reactors (LWRs). Light water reactors use ordinary water to moderate and cool the reactors. When at operating temperature, if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate. Graphite and heavy water reactors tend to be more thoroughly thermalised than light water reactors. Due to the extra thermalization, these types can use natural uranium/unenriched fuel.

Light element moderated reactors. These reactors are moderated by lithium or beryllium.

Molten salt reactors (MSRs) are moderated by a light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF₂.

Liquid metal cooled reactors, such as one whose coolant is a mixture of Lead and Bismuth, may use BeO as a moderator.

Organically moderated reactors (OMR) use biphenyl and terphenyl as moderator and coolant.

 

 

 

 

 

 

 

 

 

 

Classification by coolant

 

In thermal nuclear reactors (LWRs in specific), the coolant acts as a moderator that must slow down the neutrons before they can be efficiently absorbed by the fuel.

Water cooled reactor. There are 104 operating reactors in the United States. Of these, 69 are pressurized water reactors (PWR), and 35 are boiling water reactors (BWR).

Pressurized water reactor (PWR)

A primary characteristic of PWRs is a pressurizer, a specialized pressure vessel. Most commercial PWRs and naval reactors use pressurizers. During normal operation, a pressurizer is partially filled with water, and a steam bubble is maintained above it by heating the water with submerged heaters. During normal operation, the pressurizer is connected to the primary reactor pressure vessel (RPV) and the pressurizer "bubble" provides an expansion space for changes in water volume in the reactor. This arrangement also provides a means of pressure control for the reactor by increasing or decreasing the steam pressure in the pressurizer using the pressurizer heaters.

Pressurised heavy water reactors are a subset of pressurized water reactors, sharing the use of a pressurized, isolated heat transport loop, but using heavy water as coolant and moderator for the greater neutron economies it offers.

 

Boiling water reactor (BWR)

BWRs are characterized by boiling water around the fuel rods in the lower portion of a primary reactor pressure vessel. A boiling water reactor uses 235U, enriched as uranium dioxide, as its fuel. The fuel is assembled into rods that are submerged in water and housed in a steel vessel. The nuclear fission causes the water to boil, generating steam. This steam flows through pipes into turbines. The turbines are driven by the steam, and this process generates electricity. During normal operation, pressure is controlled by the amount of steam flowing from the reactor pressure vessel to the turbine.

 

Pool-type reactor

Liquid metal cooled reactor. Since water is a moderator, it cannot be used as a coolant in a fast reactor. Liquid metal coolants have included sodium, NaK, lead, lead-bismuth eutectic, and in early reactors, mercury.

 

Sodium-cooled fast reactor

Lead-cooled fast reactor