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Energy, atoms, and neutrons
Published in Kenneth Jay, Nuclear Power, 2019
The notion of adjusting, in effect, the critical mass of a reactor by removing neutrons forms the basis of reactor control. The most usual method of removing neutrons from the core of a thermal reactor is to put into it rods of neutron-absorbing material; suitable materials include boron, cadmium, and hafnium. A large amount of rod in the core will mop up all the neutrons and stop the reaction. By withdrawing the rods sufficiently, conditions for a divergent reaction can be established and the neutron population will increase. This increase is allowed to continue until the reaction rate (i.e. the number of fissions a second) reaches the desired level; the control rods are then pushed in again just far enough to remove all surplus neutrons over the number needed to keep the rate steady. The total amount of control rod required in a reactor changes as the fuel becomes burnt, so a large reactor may have fifty control rods, most of which will be used to take care of these slow changes in reactivity. There will be a few rods to control the small changes that occur continually.
How Reactors Work
Published in Geoffrey F. Hewitt, John G. Collier, Introduction to Nuclear Power, 2018
Geoffrey F. Hewitt, John G. Collier
Given enough fissile material, such as 235U, fission leads to the production of a self-sustaining chain reaction in which the neutrons arising from a given fission cause other fission reactions, which in turn cause others, and so on. Each fission reaction produces either two or three neutrons (with an average of about 2.5 neutrons per fission). Since only one neutron is required to cause a fission, about 1.5 neutrons are available in excess. In a supercritical system, these neutrons progressively increase the rate of fission, which is the basis for an atomic bomb. In a nuclear reactor the excess neutrons are either absorbed or used to produce more fissile material. Thus, a nuclear reactor has a critical mass of fissile material in which a state is achieved where, on average, one of the neutrons arising from a fission causes just one further fission. We thus have a delicate balance from which a slight deviation would cause the chain reaction either to die away or to accelerate. Fortunately, there are inherent features of the nuclear reaction within nuclear reactors that prevent the uncontrolled acceleration of the fission process and allow control of the reactor. We shall return to this matter when we discuss the component parts of nuclear reactors in Section 2.3.
Nuclear Fission Energy
Published in Heinz Knoepfel, Energy 2000, 2017
The application of a reflecting blanket can reduce the critical mass by a factor of nearly 3, as indicated in Table 3.5 (the blanket reflects part of the otherwise escaping neutrons back into the reacting mass). Strong compression as obtained in sophisticated implosion devices can further reduce the critical mass; this is particularly true for plutonium. In conclusion, the critical mass can be as low as 2 to 3 kg for plutonium 239 and 8 to 15 kg for uranium 235; this corresponds, for example, to a plutonium sphere of 6-cm diameter at normal density.
TRU oxide sample reactivity worths measured in the FCA-IX assemblies with systematically changed neutron energy spectra
Published in Journal of Nuclear Science and Technology, 2023
Masahiro Fukushima, Shigeaki Okajima, Takehiko Mukaiyama
The composition and geometric characteristics of the ‘clean cores’ of the FCA-IX assemblies are shown in Table 3. In this paper, a ‘clean core’ refers to an ideal core without a radial channel for insertion of a sample or a fission chamber. The core layouts for these clean cores were provided in the reference [6]. The core volume ranges from a minimum of 33.6 litter in the assembly IX-7 to a maximum of 177.4 litter in the assembly IX-1. The critical mass of 235U varies from 96.13 kg to 215.73 kg. The thickness of the radial blanket ranges from 31.15 cm to 39.50 cm, and that of the axial blanket is fixed to 35.56 cm. All basic information on the FCA facility has been compiled in the International Handbook of Evaluated Reactor Physics Benchmark Experiments (IRPhEP) [17].
Improved Disposition of Surplus Weapons-Grade Plutonium Using a Metallic Pu-Zr Fuel Design
Published in Nuclear Technology, 2023
Braden Goddard, Aaron Totemeier
Although nuclear weapon designs are classified, there is likely a strong correlation between the mass of nuclear material in a weapon and how fissile it is (critical mass). One clear indication of this is the fact that the highly fissile nuclides 239Pu and 233U have a SQ value of 8 kg, while the less fissile 235U has a SQ value of 25 kg when it is in its highly enriched uranium form. The difference in critical mass as a function of plutonium burnup can be quite large. Plutonium with a burnup of 50 000 MWd/tonne HM has a critical mass approximately 25% to 40% larger than that of weapons-grade plutonium.39–42