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Issues Facing New Nuclear Build
Published in William J. Nuttall, Nuclear Renaissance, 2022
Approximately 38% of the fission neutrons go on to produce new fissions in the fuel. It is this proportion that must be controlled precisely. As each uranium-235 fission produces on average 2.59 neutrons per fission it is important for a reactor controlled to be in criticality that 1/(2.59) (i.e. 38.6%) go on to produce further fissions. Controlling the process so precisely is achieved by means of neutron-absorbing control rods. These can be inserted into the reactor core to soak up surplus neutrons if it is noticed that the reactor is starting to run above perfect criticality. The deeper the rods are inserted into the core, the lower the reactor criticality. If the control rods are fully inserted the nuclear reaction stops completely and the reactor is said to be shut down.
Chain-Growth Polymerization
Published in Timothy P. Lodge, Paul C. Hiemenz, Polymer Chemistry, 2020
Timothy P. Lodge, Paul C. Hiemenz
Chain reactions do not go on forever. The fog may clear and the improved visibility ends the succession of accidents. Neutron-scavenging control rods may be inserted to shut down a nuclear reactor. The chemical reactions that terminate polymer chain growth are also an important part of the polymerization mechanism. Killing the reactive intermediate that keeps the chain going is the essence of a termination reaction. Some interesting polymers can be formed when this termination process is suppressed; these are called living polymers, and will be discussed extensively in Chapter 4.
Thermal Energy Production in Nuclear Power Plants
Published in Robert E. Masterson, Nuclear Reactor Thermal Hydraulics, 2019
Control rods are made from materials such as boron and cadmium that absorb excess neutrons and make it easier to control the nuclear chain reaction. Control rods change the power profile in both the radial and axial directions, and they tend to depress the power generation rate in their immediate vicinity. This effect is illustrated for a single control rod in Figure 5.29. Hence, the core power profile returns to its original shape as soon as a control rod is withdrawn. Reactors contain literally hundreds of control rods, and the control rods are placed strategically throughout the core to assist with power control. In most commercial PWRs, one out of every four fuel assemblies contains a cluster of control rods, which is sometimes called a control rod bank. In BWRs, larger cruciform-shaped control rods are surrounded by four fuel assemblies. The placement of these control rods is shown in Figure 5.40.
Development of the NuScale Power Module in the INL Modelica Ecosystem
Published in Nuclear Technology, 2021
Konor Frick, Shannon Bragg-Sitton
Xenon is a side of effect of the fission process. Fuel reactivity insertion is a byproduct of the fuel temperature and is a negative reactivity source as fuel temperature rises. Control rods are neutron absorbers that can increase or decrease reactivity by being moved into and out of the core to compensate for the effects of the others. The moderator temperature coefficient is designed to be negative such that, as the water in the core heats up, there is a negative reactivity insertion. Finally, boron is inserted via a CVCS to uniformly decrease reactor power. Of these, only the boron concentration and control rod insertion length are directly controllable. Both operate on relatively large timescales when compared with the other mechanisms.
Versatile Test Reactor Conceptual Core Design
Published in Nuclear Science and Engineering, 2022
The core design contains six primary control rods and three secondary control rods. The primary rods are moved during normal operation to adjust for changes in reactivity and to control the power level of the core. The secondary rods are fully withdrawn from the fuel region during normal operation and are fully inserted when the reactor is shut down in order to provide an additional shutdown margin, for example, during refueling operations. Each control rod system is sufficient by itself to safely shut down the reactor from any anticipated state. Each control rod is operated independently from other control rods in order to allow for fine control of the reactor.
Radiation protection evaluation and spontaneous discharge performance of betavoltaic tritium batteries using well-aligned titanium dioxide nanotube arrays
Published in Radiation Effects and Defects in Solids, 2023
Yi-Sin Chou, Yu-Chang Liu, Yu-Zhen Zeng, Shi-Chern Yen
Control rods are used in nuclear reactors to control the fission rate of nuclear fuel (uranium or plutonium) that can absorb many neutrons without decaying on their own. Selected elements have different neutron capture cross sections for neutrons of different energies (14). The Chernobyl nuclear power plant is a graphite water-cooled reactor. The fuel rods are placed in a graphite buffer to slow down the neutrons and maintain the chain reaction, and the control rods control the power produced by the reactor. Using graphite as a neutron absorber is inefficient compared to chemical elements such as boron, cadmium, silver, hafnium or indium (15). C-14 comes from an abandoned nuclear reactor with activated graphite moderator around the fuel rods. It was originally used as a neutron retarder (16). Moreover, C-14 can also be produced by using other materials through electron beam irradiation (17). C-14 has become a common main radiation source used in nuclear batteries. Prototype cells for Nanodiamond Batteries (NDBs) were first presented at the University of Bristol’s annual lecture (18). The battery is equipped with a semiconductor package C-14, using waste graphite containing C-14, combined with general carbon materials (19). The beta particles of C-14 contain very low radiation intensity, and the periphery is covered with non-radioactive synthetic diamond material. Therefore, there is no risk of radiation leakage, and the radiation dose is even lower than that of a banana (20). Common nuclear power plants, such as boiling water reactor (BWR), pressurized water reactor (PWR), and heavy water reactor (HWR), operate using thermal neutrons. Graphite is less used as a control rod materials for nuclear reactors, so not much C-14 radioactive waste is produced (21).