China
Africa
Explore chapters and articles related to this topic
Nuclear fuels and fuel cycles
Published in Kenneth Jay, Nuclear Power, 2019
The table on page 23 shows that the highest neutron yield is given by fast fission of plutonium-239, so it would seem best to try to make a reactor work with plutonium as fuel and uranium-238 as fertile material. However, there are no large supplies of plutonium in nature; we have to start with natural uranium. It would be possible to separate uranium-235 in a diffusion plant and use it to fuel a fast reactor in which uranium-238 was converted to plutonium; the table shows that the neutron yield would be high. But separation to the concentration necessary is expensive and the capital costs of fast reactors are high at present. To start with fast reactors would be like starting aviation with supersonic aircraft. An alternative to separating concentrated uranium-235 is to burn natural uranium in thermal reactors and to extract the plutonium formed, for use in other (including fast) reactors later. Another possibility is to follow an intermediate course and use, in thermal reactors, slightly enriched uranium in which the concentration of the 235 isotope has been increased to a few per cent.
Shielding Systems and Radiation Shields
Published in Robert E. Masterson, Nuclear Engineering Fundamentals, 2017
In the vicinity of the core and the reactor pressure vessel, the shielding system must be designed to convert the fission neutrons into thermal neutrons. Water is normally employed in this case because it is not only an effective moderator, but it can also carry away the heat. The iron in the reactor pressure vessel can be used to eliminate some of the fission neutrons that are not stopped by the water blanket around the core. Occasionally, a fast fission neutron will collide with an iron atom in what is known as an inelastic collision. Inelastic scattering is a surprisingly effective mechanism for slowing down the fission neutrons, but it does not occur as frequently as other types of collisions do. For example, if the nucleus of a heavy element (say, iron) absorbs a high-energy neutron and then reemits it, the kinetic energy of the incident neutron can be reduced by 40%, 50%, 60%, or even 80%. It can be shown that the average kinetic energy of a neutron <E′> coming out of an inelastic collision with an atomic nucleus of mass A is approximately
Nuclear Fuel Materials
Published in C. K. Gupta, Materials in Nuclear Energy Applications, 1989
The quality of the material is very important for the construction of an explosive device. Uranium used for this purpose is typically more than 90% enriched in 235U. Since 238U undergoes fast fission, it can contribute to the fission process. The lowest fissile content for the construction of nuclear weapons is in the range of 10 to 20%. Materials having an enrichment of about 20% are called special nuclear materials (SNM) and are treated in a different category in contrast to the reactor grade material. Weapon-grade plutonium is almost all 239Pu with less than 5% 240Pu. The undesirable isotopes are 238Pu, 240Pu, and 241Pu. With 238Pu, the undesirable feature is the high heating effect to to alpha decay which causes the temperature increase of the metal and brings about a metallurgical phase transformation that hampers the explosion of the device. With 240Pu, two problems arise. First, it dilutes the fissile 239Pu, and second, it is a strong source of spontaneous fission neutrons which makes for a much less efficient and less predictable device. With 241Pu, although appearing to be a superior fissile isotope, there is yet another problem. It decays, by beta emission, to americium with a half-life of 14.5 year, which in turn decays by alpha emission with a half-life of 433 year. Therefore, both 241Pu and 238Pu affect the shelf life of a weapon because of the heat and the gas released in the decay. Since plutonium is a relatively weak and brittle material, the alpha decay of containments leads to rapid degradation of the weapon which needs periodic replacement. In a commercial nuclear power-producing reactor, the fuel resides inside the reactor for a long duration which results in increased formation of 240Pu and other higher isotopes of plutonium. Such material is unsuitable for weapon purposes. The military production reactors discharge fuel much earlier (exposure is kept low) than the commercial ones, to yield a low proportion of even isotopes to 239Pu.
An Assessment of Heterogeneous Effects on System Reactivity for Criticality Safety Analyses with LEU+ and HALEU Materials
Published in Nuclear Technology, 2023
Philip H. Sewell, Robert B. Hayes
With a change from a homogeneous to heterogeneous system, the bulk of the fuel material is separated from the moderator. This causes a slight increase in fast flux within the fuel, resulting in a small increase in the fast fission factor (↑ ϵ), as fast neutrons born within the fuel are more likely to collide with uranium rather than water. Additionally, separating the moderator from the fuel allows neutron moderation to occur outside of the fuel region, resulting in fewer neutrons being absorbed in the 238U resonances. This results in an increase in resonance escape probability (↑ p). Similarly, as neutrons are slowed down in the separate moderator region, there is a greater chance of thermal neutron capture outside of the fuel and the thermal flux in the fuel will be slightly lower in comparison to a homogeneous system. As a result, the expected behavior of a switch from a homogeneous to a heterogeneous system is a decrease in the thermal utilization (↓ f). The magnitude of these effects is dependent on the degree of heterogeneity in the system. A discussion of these effects is provided in Chapter 10 and depicted in Figs. 3 through 10 of the textbook Nuclear Reactor Analysis.5 This reference states that heterogeneous effects for fuel pins with enrichment up to 5 wt% 235U can result in significant increases in k∞, primarily due to the thermal utilization factor and resonance escape probability.
Simulations of Pressure-Tube–Heavy-Water Reactor Cores Fueled with Thorium-Based Mixed-Oxide Fuels
Published in Nuclear Technology, 2018
Ashlea V. Colton, Blair P. Bromley
For comparison, energy release data for NUO2 are also shown in Table XVIII. For NUO2 fuel, approximately 53% of the energy released comes from the fission of the original 235U and the heavier isotopes created by neutron capture on 235U, while 47% of the energy released comes from the fission of plutonium isotopes (created by neutron capture on 238U), fast fission of 238U, and the fission of other heavy isotopes created by neutron capture on 238U. Only very trace amounts of energy are produced from the original 234U in the NUO2 (0.1% or less).
Nuclear Science for the Manhattan Project and Comparison to Today’s ENDF Data
Published in Nuclear Technology, 2021
The first measurements of plutonium fission were made for thermalized neutrons at Berkeley in 1941 (Ref. 13, pp. 355–356.), and reported in a Berkeley report A-33 by Seaborg, Segrè, Kennedy, and Lawrence. At thermal energies, Chamberlain et al.mO. CHAMBERLAIN et al., LA-21, quoted in Weigland, LA-21 as CN-469 but perhaps this is a typo and should be CF-469; the author has not yet been able to track this down, University of California, Berkeley (1942—a guess). found that the fission cross section of Pu was 1.87 times greater than U, an amount that was found later by DeWire at Los Alamos to be an overestimate owing to the neutrons not being completely thermalized (see the following, and Table VIII; the correct value is 1.28). A January 1943 letter from Chamberlain, Kennedy, Segrè, and Wahl to Manley (NSRC A84-019-49-9) reported slow neutron values for this ratio of 1.238 and 1.294, much improved. Later, Lawrence would comment that the fast fission plutonium cross section is ten times that of (238) uranium (Ref. 13, p. 368.), while a measurement by Seaborg and Segrè in 1941 found a factor of 3.4 (Ref. 27, p. 23). While these might seem to be contradictory claims, both could be correct depending upon the exact neutron energy, owing to the fast-changing U fission cross section as it rises from its threshold; today we assess this ratio to be 10 at 1.4 MeV, but 3.7 at 2 MeV.