The reactor core

The starting material for the fabrication of UO2 standard fuel is enriched UFe, which is supplied by the enrichment plant in pressure cylinders (the preceding steps of fabrication starting from uranium ore have been described, among others, by Peehs, 1996). From this material, the UO2 compound is made either by a wet or by a dry process. Among the wet processes, the ADU (ammonium diuranate) process is mainly used, in which UFe gas is hydrolyzed in aqueous ammonia solution to form ammonium diuranate. Besides ADU, the AUC (ammonium uranyl carbonate) process is applied in which UFe is hydrolyzed in an aqueous solution 

The dimensions of the fuel rods differ according to the plant and the design of the fuel assembly. In the 1300 MWe plants the total length of the fuel rod amounts 

Usually, uranium fuel assemblies consist of fuel rods with identical enrichment. In mixed-oxide fuel assemblies, however, fuel rods containing different plutonium contents are frequently combined in order to achieve a homogeneous power distribution over the whole assembly an example of such an arrangement is shown in Fig. 1.6. The Mox fuel assemblies are designed to be fully compatible with the dimensions of uranium fuel assemblies thus, both types can be freely interchanged in refuelling the reactor core. 

The fuel rods are joined to the fuel assembly by the fuel assembly structure (see Fig. 1.7.), which connects the fuel assembly top and bottom end pieces. In an 18 X 18 array, 300 fuel rods are fixed in position by the spacer grids, with nine of them being distributed over the length of the bundle. In earlier designs the material of the spacer grids was Ni-coated Inconel X 713 for reasons of neutron economy and in order to minimize the amount of material forming long-lived radionuclides in the neutron field, Zircaloy-4 is now commonly used. Only the very small spacer 

Inside the reactor pressure vessel, the reactor core is fixed in position by the core internal structure consisting of the core shroud and an upper and lower core grid plate (see Fig. 1.3.). In a 1300 MWe plant the reactor core itself consists of 193 fuel assemblies arranged in a geometrical pattern inside the reactor pressure vessel as an example, a core scheme also containing mixed-oxide fuel assemblies is shown in Fig. 1.8. 

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Whereas addition of hydrogen to feedwater helps solve the O2 or ECP problem, other complications develop. An increase in shutdown radiation levels and up to a fivefold increase in operating steam plant radiation levels result from the increased volatiUty of the short-Hved radioactive product nitrogen-16, N, (7.1 s half-life) formed from the coolant passing through the core. Without H2 addition, the in the fluid leaving the reactor core is in the form of nitric acid, HNO with H2 addition, the forms ammonia, NH, which is more volatile than HNO, and thus is carried over with the steam going to the turbine. 

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

Most nuclear reactors use a heat exchanger to transfer heat from a primary coolant loop through the reactor core to a secondary loop that suppHes steam (qv) to a turbine (see HeaT-EXCHANGETECHNOLOGy). The pressurized water reactor is the most common example. The boiling water reactor, however, generates steam in the core. 

Size requirements are limited by packaging considerations for neutron irradiation. Typically, polyethylene or quartz containers are used to contain the sample in the reactor core. For example. Si wafers are cleaved into smaller pieces and dame sealed... 

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

The reactor core was made up of stacks of hexagonal graphite blocks. Each fuel element block had 210 axial fuel holes and 108 axial coolant holes (Section 5, Fig. 14). The fuel particles were formed into a fuel compact (Section 5.3) and sealed into the fuel channels. 

The partial containment system had many rooms in one room, with a pressure capability of about 26 psi, was the reactor core. The steam drums were in two rooms four main recirculation pumps were in each of two rooms. 

Any release of radioactive material affecting the public requires temperature above the melting point of the materials to deform the reactor core and confining structures This section lists the barriers preventing release, presents scoping calculations that illustrate the conditions and time scale of concern. Conjectures are presented as to how core melt might happen. The section concludes with information about the partial core melt that occurred at TMI-2. 

The mean frequencies of events damaging more than 5% of the reactor core per year were found to be Internal Events 6.7E-5, Fire 1.7E-5, Seismic 1.7E-4, and total 2,5E-4. Thus, within the range of U. S. commercial light water reactors The core damage frequency itself, is only part of the story because many N-Reactor accident sequences damage only a small fraction of the core. The... 

Plasma in the reactor core is self heated by the a-particles which have the remaining 20% of the energy produced. 

The heart of the nuclear reactor boiler plant system is the reactor core, in which the nuclear fission process takes place. Nuclear fission is the splitting of a nucleus into two or more separate nuclei. Fission is usually by neutron particle bombardment and is accompanied by the release of a very large amount of energy, plus additional neutrons, other particles, and radioactive material. The generation of new neutrons during fission makes possible a chain reaction process and the subsequent... 

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. 

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