Core of nuclear reactor

The half-value dose is the dose at which a property has been reduced to one half of its original value. Electron accelerators of various designs or y-radiation sources based on radioactive cobalt 60 ( Co) are the radiation sources typically used for targeted electron irradiation. The changes in plastics caused by radiation are the same for all types of radiation. Irradiation does not turn plastics radioactive. The only exceptions are neutrons (that occur almost exclusively in the core of nuclear reactors). They can activate plastics. 

The core of nuclear reactors is an example of a component in an NCL in which flow in parallel channels occurs. The stability of the parallel-channel flow through the core must be investigated, and these investigations include effects of conjugate heat conduction and energy production by fission in the material adjacent to the fluid. Single- and two-phase flows under steady-state and transient conditions are important considerations relative to safety analyses of Gen IV machines. 

Due to the very high neutron cross section of boron is of particular interest in materials used in the core of nuclear reactors. 

Energy Use and Conservation. A variety of materials are needed for high performance thermal insulation, particularly as components of nuclear reactors. Replacements for asbestos fibers are needed for components such as reactor core flooring, plumbing, and packaging. The fibers must be very resistant to high temperatures with outstanding dimensional stabiHty and resistance to compression. 

The past safety record of nuclear reactors, other than the Soviet Chernobyl-type RBMK reactors, is excellent Excluding RBMK reactors, there had been about 9000 reactor-years of operation in the world by the end of 1999, including about 2450 in the United States.1 In this time there was only one accident involving damage to the reactor core, the 1979 Three Mile Island accident, and even at TMI there was very little release of radionuclides to the outside environment. 

Nuclear power plants are designed to prevent accidents such as meltdown by careful control of fuel-rod placement and positioning of control rods made of boron or other materials that have high affinity for neutrons. If the core of the reactor should become overheated, the fuel rods are... 

For example, one of the earliest types of nuclear reactors is the boiling water reactor (BWR) in which the reactor core is surrounded by ordinary water. As the reactor operates, the water is heated, begins to boil, and changes to steam. The steam produced is piped out of the reactor vessel and delivered (usually) to a turbine and generator, where electrical power is produced. 

Reactors with other functions are also in use. For example, a breeder reactor is one in which new reactor fuel is manufactured. By far the most common material in any kind of nuclear reactor is uranium-238. This isotope of uranium does not undergo fission and does not, therefore, make any direct contribution to the production of energy. But the vast numbers of neutrons produced in the reactor core do react with uranium-238 in a different way, producing plutonium-239 as a product. This pluto-nium-239 can then be removed from the reactor core and used as a fuel in other reactors. Reactors whose primary function it is to generate plutonium-239 are known as breeder reactors. 

The rate of the reaction is modulated by controlling the number and energy of the neutrons allowed to stay in the uranium filled core of the reactor. Control rods are used to modulate the nuclear reaction rate. Control rods are made from an element (cadmium metal is often used) that strongly adsorbs neutrons. The rods are installed in channels in the reactor. When the rods are fully inserted in the reactor, so many neutrons are adsorbed that little reaction can occur. As the rods are withdrawn, more and more neutrons can react and the reactions begin. The reaction rate is controlled by the depth and number of rods inserted in the reactor. 

Neutron-capture prompt-gamma ray activation analysis (PGAA) is a recent addition to the nuclear analytical arsenal. In this technique the instantaneous gamma ray emission from a sample is measured as it is irradiated in a flux of reactor neutrons (33,3, 35). Because the sample must be several meters from either the core of the reactor or (less commonly) from the detector, the sensitivity of this technique is generally poorer than in conventional NAA. However, it is possible to measure small quantities of many elements which do not give radioactive neutron-capture products, notably 0.01 mg of H, 50 ng B, and 1 mg P in an electronics context. 

Table 3.13 gives effective cross sections for thermal neutrons and other nuclear properties of the materials in the core of this reactor. These effective cross sections have been calculated by the procedure recommended by Westcott, which has been outlined in Chap. 2, from data provided by Westcott [W3] and Critoph [Cl]. To obtain appropriate nuclear reaction rates, these effective cross sections are to be multiplied by the thermal-neutron flux, wmb. where Hub is density of neutrons in the Maxwell-Boltzmann part of the spectrum and C is the average speed of the Maxwell-Boltzmann neutrons. 

Background. Natural convection driven by internal heat sources is of interest in geophysics, and the heat transfer associated with such motion is important in the design of tanks in which fermentation or other chemical reactions occur and in the safety analysis of nuclear reactors where a core meltdown is postulated. The last of these applications has led to the intensive study of internally generating horizontal fluid layers. 

In the period of 1998-99, two sets of experiments focused on problems of rapid decrease of concentration of boric acid in reactor coolant at nuclear reactor core inlet were performed at the University of Maryland, US, under the auspices of OECD. The situation, when there is an inadvertent supply of boron-deficient water into the reactor vessel, could lead to a rapid (very probably local) increase of reactor core power in reactor, operated at nominal power, or to a start of fission reaction in shut-down reactor (secondary criticality). In the above mentioned experiments the transport of boron-deficient coolant through reactor downcomer and lower plenum was simulated by flow of cold water into a model of reactor vessel. These experiments were selected as the International Standard Problem ISP-43 and organisations, involved in thermal — hydraulic calculations of nuclear reactors, were invited to participate in their computer simulation. Altogether 10 groups took part in this problem with various CFD codes. The participants obtained only data on geometry of the experimental facility, and initial and boundary conditions. 

The difference between both risk measures is that damage of the core of the reactor mostly results in keeping the radioactivity inside the containment of the nuclear power plant, so no fatalities and injuries follow this sort of accident. 

Several hundred uranium dioxide fuel assemblies make up the core of a reactor. For a reactor with an output of 1,000 MWe, a typical core contains about 75 t of low-enrichment uranium ( 3.5% U). During the operating cycle of a nuclear reactor, several competing processes determine the final radionuclide inventory in the spent fuel. These processes are... 

The is one of the radionuchdes produced by neutron-induced reactions in aU tjrpes of nuclear reactors. In a nuclear power facility the production of can occm in the fuel, the moderator, the coolant, and the core construction materials mainly by the reactions 0(n, a) C, and N(n, p) C. Part of the created in reactors is continuously released as air-borne effluents in various chemical forms (such as CO2, CO, and hydrocarbons) through the ventilation system of the power plant during normal reactor operation. Another part of the C produced is released into the atmosphere from fuel reprocessing plants. The enviromnental release of this reactor-derived C leads to an increase in atmospheric specific activity and hence, to an increased radiation dose... 

The deterministic approach to the design of nuclear reactors was rapidly supplemented by the development of probabilistic studies, referred to as PSAs and also as PRA. The first study of this kind carried out in the United States was published in 1975 (Rasmussen report—USNRC 1975) and provided the first assessment of the potential risk of core damage for two power reactors. The accident in 1979 at the Three Mile Island plant generated renewed interest in this type of study. One of the recommendations made after the accident was that probabilistic analysis techniques should be used to supplement conventional safety assessment procedures for NPPs, and that probabilistic objectives should be developed in order to facilitate the determination of acceptable safety levels for nuclear facilities. 

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