Neutron capture by uranium

The core decay heat is essentially composed of the decay heat of the fission products, the decay heat of the decay chain of uranium-239 and neptunium-239 produced by neutron capture by uranium-238, the decay heat of other actinides, the control rods and the structural materials and the heat generated by the residual fissions and by neutron capture by the fission products. The heat of the residual fissions is generally... 

The intensity of neutron fluxes during the thermonuclear explosion proved to be much higher than it had been expected. This made possible the processes of neutron capture by uranium discussed above. Uranium-253 and uranium-255 emitted 7 and 8 beta particles, respectively, and converted into isotopes 99 and H00 of elements 99 and 100. Their half-lives proved to be short but sufficient for analysis (20 days and 22 hours). 

Plutoqium present in the earth at its time of formation has long since decayed because of its relatively short half-life (/j/2 = 24,360 y for Pu). Most Pu in the environment is derived from nuclear-weapons testing or from nuclear wastes (cf. Hanson 1980 Kathren 1984). However, small amounts of natural Pu are produced through neutron capture by (see Eq [13.13]). Analyses of Pu in a number of uranium ore deposits have shown it to be near secular equilibrium with (see Section 13.1.6), with a weighted average Pu/U atomic ratio of (3.1 0.4) x 10, which nearly equals (3.0 0.5) x 10 , the ratio at secular equilibrium (Curtis et al. 1992,1994). 

Figure 23.8 shows two types of fission reactions. For a chain reaction to occur, enough uranium-235 must be present in the sample to capture the neutrons. Otherwise, many of the neutrons will escape from the sample and the chain reaction will not occur, as depicted in Figure 23.8(a). hi this situation the mass of the sample is said to be subcritical. Figure 23.8(b) shows what happens when the amount of the fissionable material is equal to or greater than the critical mass, the minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction, hi this case most of the neutrons will be captured by uranium-235 nuclei, and a chain reaction will occur. 

In a typical breeder reactor, nuclear fuel containing uranium-235 or plutonium-239 is mixed with uranium-238 so that breeding takes place within the core. For every uranium-235 (or plutonium-239) nucleus undergoing fission, more than one neutron is captured by uranium-238 to generate plutonium-239. Thus, the stockpile of fissionable material can be steadily increased as the starting nuclear fuels are consumed. It takes... 

Np. The isotope Np is formed in considerable quantities in reactors, by the nuclide chains initiated by (n, y) reactions in and by ( , 2n) reactions in Neutron capture by Np leads through Np to Pu, which is the principal alpha-emitting constituent of plutonium in power reactors. To produce Pu for use as a heat source for thermoelectric devices, neptunium has been recovered from irradiated uranium to form target elements for further irradiation in reactors. Commercial processes designed for this recovery are discussed in Chap. 10. 

In the reactor, the process of decay of a heavy nucleus supersaturated with neutrons is initiated by the nuclear-neutron collision because of electrical neutrality even a low-energy neutron can reach the nucleus vicinity, where it can be captured by the short-acting nuclear potential. The result is the creation of a compound nucleus that shortly decays with mission of more neutrons. Nuclear reactors operate by the forced fission of uranium nuclei. There is a number of possible ways in which fission can occur, e.g., decay with the formation of Rb and Cs nuclei and emission of four neutrons instead of a single neutron captured by the U nucleus ... 

Another major turning point in the history of nuclear science came with the discovery of fission by Otto Hahn and Fritz Strassmann in December 1938 (Hahn and Strassmann 1939a, b). In several laboratories in Rome, Berlin, and Paris, a complex series of P-decay chains resulting from neutron irradiation of uranium had been investigated since 1934, and these chains had been assigned to putative transuranium elements formed by neutron capture in uranium with subsequent P" transitions increasing the atomic numbers (see Sect. 1.2.3). But then evidence appeared that known elements in the vicinity of uranium, such as radium, were produced as well. When Hahn and Strassmaim attempted to prove this by a classical fractional crystallization separation of radium from barium serving as its carrier, the radioactivity turned out to be barium, not radium hence, new and totally unexpected type of nuclear reaction had to be invoked. 

There is subterranean production of chlorine-36 and the world average chlorine contents of granite and basalt have been given as around 50 and 200 ppm, respectively. Sedimentary rocks have variable contents ranging from 10 ppm in sandstones to 20,000 ppm in deep-sea limestones. Rock outcrops are exposed to the cosmic neutron flux so that some chlorine-36 results from neutron capture by chlorine-35, but, below a few meters, it is ineffectual. Nonetheless, some chlorine-36 results from an in situ neutron flux in rock matrices caused by (a,n) reactions triggered by alpha particles from uranium and thorium radioactive decay systems. This flux may be of the order of 10 cm s (Kuhn et al. 1984). 

With short reactor irradiation times, relatively pure Pu is formed from natural or low-enriched uranium. The cross section for thermal neutron capture by Pu is about 1,020 barns, and a sizable fraction produces Pu rather than fission ... 

Uranium-233 (V, = -1.59 10 years) is formed through neutron capture by Th (mainly in thorium-fueled nuclear reactors) followed by emission of two beta particles. Alternatively, may be formed by alpha decay of Np that in turn is produced through a sequence of nuclear reactions in uranium-fueled reactors. The °Th decay product of is the relatively short-lived (t = -7880 years) so the ratio between these nuclides is linear for thousands of years after purifications. Alpha spectrometry and mass spectrometry can be used for measuring the Th/ U ratio. In special cases, this pair of nuclides cau serve as a chrouometer for nuclear proliferation, particularly if fissile is produced in reactors fueled with thorium. 

They showed that element 93 has chemical properties similar to those of uranium and not those of an eka-rhenium as suggested on the basis of the periodic table of that time. To distinguish it from uranium, element 93 was reduced by SO2 and precipitated as a fluoride. This new element was named neptunium after Neptune, the planet discovered after Uranus. In 1952, trace amounts of Np were found in uranium of natural origin, formed by neutron capture in uranium. 

Element 94 was named plutonium after the planet discovered last, Pluto. In 1941, the first 0.5 /rg of the fissionable isotope Pu were produced by irradiating 1.2 kg of uranyl nitrate with cyclotron-generated neutrons. In 1948, trace amounts of Pu were found in nature, formed by neutron capture in uranium. In chemical studies, plutonium was shown to have properties similar to uranium and not to osmium as suggested earlier. The actinide concept advanced by G. T. Seaborg, to consider the actinide elements as a second / transition series analogous to the lanthanides, systematized the chemistry of the transuranium elements and facilitated the search for heavier actinide elements. The actinide elements americium (95) through fermium (100) were produced first either via neutron or helium-ion bombardments of actinide targets in the years between 1944 and 1955. 

Doppler feedback is a key process in reactor stability that takes place at the level of the atomic nucleus. In nuclear reactors, Doppler feedback is due to thermal vibration of uranium-238 nuclei, which makes the nuclei appear bigger to passing neutrons. Hence neutrons are more readily captured by uranium nuclei when the fuel temperature increases. Nuclear reactor fuel is normally only 2.5-3% fissile (heat-producing) uranium-235. The other 97-97.5% is uranium-238, which is non-fissile -i.e., it does not fission easily to produce heat. Instead, uranium-238 captures neutrons and becomes uranium-239, which with a half-life of just over two days becomes plutonium-239 (which is again fissile). The key point is that the immediate effect of higher fuel temperature is that non-fissile uranium-238 mops up more neutrons within the reactor core without producing more heat. 

Moderator (water and graphite) slows down neutrons to enable their capture by uranium-235... 

Neptunium-237 is also formed by neutron capture in uranium-235 ... 

The important isotope Pu is produced by the ton in nuclear reactors. Excess neutrons from the fission of uranium-235 are captured by uranium-238 to yield plutonium-239 ... 

In research reactors, especially those using low enrichment fuel, neutron capture by continuously produces Np (half-life 2.3 d), which contributes a large fraction (up to 30%) of the total gamma activity of the core at shutdown. Np is transmuted by beta decay into Pu. Small quantities of other actinides are produced by neutron capture by other isotopes of uranium and their decay products. 

In a reactor fueled with natural uranium, slightly more than 99% will be (there may be small amounts of During reactor operation, neutron capture by the results in the... 

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