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Nuclear and Hydropower
Published in Roy L. Nersesian, Energy Economics, 2016
The second-generation European pressurized reactor has been upgraded to the third-generation evolutionary power reactor (EPR) manufactured by a joint venture between Areva and Siemens. EPR has an output of 1,650 mW (1.65 gW) using a 5 percent enriched uranium oxide fuel or a 50 percent uranium plutonium mixed oxide fuel. A heavy neutron reflector surrounding the reactor core improves fuel utilization and protects the reactor pressure vessel against aging (metal fatigue). An axial economizer in each steam generator enhances steam pressure and plant efficiency. In addition to its enhanced and more efficient output, a major improvement incorporated into the EPR is safety built around a quadruple redundant safeguard of four independent emergency cooling systems. Its foundation is built to withstand the largest potential earthquake. A leak-tight containment system surrounding the reactor is augmented by an extra or secondary containment and cooling area built under the reactor base. This acts as a core catcher to handle the potential accident of a molten core penetrating the bottom of the primary reactor containment. The outer containment has a two-layer concrete wall, each 2.6 meters thick, designed to withstand an impact by aircraft and internal pressures from a reactor accident.31
Nuclear reactors and their fuel cycles
Published in R.J. Pentreath, Nuclear Power, Man and the Environment, 2019
In view of the above discussion; what are the basic requirements of a nuclear reactor for the generation of nuclear power? The reactor vessel consists of an active core in which the fission chain reaction is sustained; the core contains the reactor fuel – the fissile material – and a moderator if this is required to reduce the energy of the neutrons. The moderator must be constructed from materials of fairly low atomic mass because the nuclei of low-mass elements absorb a large fraction of a neutron’s energy and slow it down to the thermal neutron energy region very quickly. The ideal nucleus to remove the maximum fraction of a neutron’s energy is one of equal mass – that of hydrogen. A further consideration is that the material should not have a high neutron capture and thus mop up neutrons and prevent a chain reaction from occurring at all. This reduces the choice, from a practical point of view, to water, heavy water (deuterium oxide), carbon in the form of graphite, and beryllium. In some reactor designs the core is also blanketed in a moderating material which acts as a neutron reflector, to minimize the loss of neutrons from the system. The energy of fission is released as heat and reactor operation is dependent upon the ability to remove the heat produced as fast as it is generated. The coolant must therefore circulate through the reactor core so as to maintain, as far as possible, a uniform internal temperature. Because the coolant passes through the core of the reactor it, too, must be a material which will absorb a relatively low number of neutrons. Again both light and heavy water are used and a gas, carbon dioxide, is used in some types of reactor. At high temperatures a liquid metal, sodium, is used. In the majority of reactor designs the coolant transfers heat indirectly, by one mechanism or another, to water to create steam which is used to generate power in a conventional manner by means of a turbine.
Moderator and Reflector Materials
Published in C. K. Gupta, Materials in Nuclear Energy Applications, 1989
In a self-sustaining nuclear chain reaction that proceeds in a nuclear reactor, most of the neutrons which are produced during the fission process are used in the reactor (1) to sustain the chain reaction, (2) for control purposes, or (3) for breeding purposes. A certain number of neutrons found near the surface of a nuclear core, however, tend to escape from the region of the chain reaction and hence can be put to no useful purpose. This leakage of neutrons outside the assembly impairs the neutron economy, and consequently an additional amount of fissionable material must be added as fuel. This is costly and often requires that the core of the reactor be somewhat larger. To cut down on leakage losses, radial and axial reflector materials are used surrounding the reactive region. The purpose is to scatter back escaping neutrons into the core in both thermal and fast reactors without absorbing them. The ratio of neutrons scattered back from the surface of the reflector to the amount reaching the surface is called the albedo. A reflector basically is nothing more than a low-cross-sectional material, which permits neutrons to collide with its own nuclei without neutron absorption. When a neutron collides with any low-cross-section nucleus, its direction of motion is changed. This, in a sense, is a neutron getting reflected. A material having a low cross section surrounding the core of a reactor acts as a neutron reflector. The choice of material, of course, depends upon several factors. In a thermal reactor the reflector can be made of a moderating material. In this case, when a neutron is reflected back into the reactor core, it has given up much of its kinetic energy to the reflector-moderator and hence is more useful to the chain reaction than if it had been reflected with all its original kinetic energy. In a fast reactor, slowing down of neutrons is not desirable, and many materials that scatter neutrons strongly can be used as reflectors. The reflector should be an element of high mass (a nonmoderator) so that the neutrons will be reflected back into the core with most of their original kinetic energy. They can thus be of maximum benefit in helping to maintain the fast chain reaction. In fast reactors, therefore, the role of reflector may only be played by heavy materials such as iron, lead, bismuth, thorium, uranium, etc., that possess a high-scattering cross section for fast neutrons.
Feynman-α Analysis Using BGO Gamma-Ray Detector in a University Training and Research Reactor
Published in Nuclear Science and Engineering, 2023
Masaki Goto, Tadafumi Sano, Kunihiro Nakajima, Takashi Kanda, Atsushi Sakon, Kengo Hashimoto
The UTR-KINKI reactor is a light water–moderated and graphite-reflected two-core reactor that has highly enriched uranium fuels (around 90% enrichment). The rated thermal power is only 1 W, and the UTR has no cooling system. The reactor configuration and detector location are shown in Fig. 1. The six fuel assemblies are loaded in the respective core tanks separated by 18 in. (45.7 cm) of graphite. The maximum thermal neutron flux in the graphite region is around 107 n/cm2/s, and neutron irradiation for various samples is available there. Additional graphite surrounding the cores acts as a neutron reflector. The reactor has four control rods in symmetrical positions, two of which are assigned as safety rods for reactor scram and manual shutdown and are referred to as safety rod 1 and 2, respectively. The others are a shim safety rod for scram and coarse adjustment of reactivity, and a regulating rod for auto operation and fine adjustment of reactivity. A neutron source is manually inserted in a source hole located between the two cores to start up the UTR.
Enhancement of Fast Neutron Detection in Liquid Scintillator Detectors
Published in Nuclear Technology, 2022
Gang Li, Ghaouti Bentoumi, Liqian Li
Neutron reflector has been used to increase neutron flux in areas such as nuclear reactors, accelerator-based neutron sources, nuclear weapons, etc. The reflector has also been incorporated into thermal neutron detector designs; however, it has not been applied to fast neutron detections before. Via experiments and simulations, this paper demonstrates the effect of fast neutron detection enhancement using reflectors. The enhancement in neutron detection by adding a graphite reflector surrounding the detector cell is predicted to be 50%. Care should be taken because the spatial and timing resolution would be degraded due to the extended coverage and extra scattering. This method is suitable for applications in which cost-effective and highly efficient fast neutron detection is desired, for example, in detecting the presence of fissile materials.
Novel Methodologies for Modeling the Net Fission-Poison Reactivity Transients to Accurately Predict Criticals and Hot-Startup ECPs for the MURR Core
Published in Nuclear Technology, 2018
Outside the outer pressure vessel, four axially banked control blades and one regulating blade for fine adjustments control the core’s excess reactivity. The control blade absorber material, a matrix of aluminum boron carbide (Al-B4C), is clad with aluminum (see Table I for control rod specifications). The regulating blade is made of stainless steel so that its total worth is less than 2% of a typical control blade. The curved blades (rods) are situated within an annular coolant gap outside the outer pressure vessel. During operations, the control rods are withdrawn in-gang axially to increase core reactivity. The extent of the control rod motion is 26 in. (66.04 cm) starting from slightly below the bottom of the fuel assemblies (i.e., the zero position). Immediately outside of the control blade region is the primary neutron reflector consisting of a beryllium sleeve (see Table I for beryllium specifications). The reflector is surrounded by an annulus of 11 graphite wedges. These wedges act as a secondary reflector, host additional sample irradiation positions, and provide access to the core for six neutron beam ports. The beam ports are located on three axial levels surrounding the core. Figure 1 shows an x-y cross section of the MURR core configuration at the core center plane. In this illustration, the control rods are not shown since they were modeled withdrawn higher than the plane of view.