Plutonium-239, fission cross-section

The discovery of Pu has been described in detail by Seaborg in his Plutonium Story (chapter 1 of the book The Transuranium Elements 1958). First, the separation of Pu from Th caused some difficulties, because both elements were in the oxidation state 4-4. After oxidation of Pu(IV) by persulfate to Pu(VI), separation became possible. Pu is produced in appreciable amounts in nuclear reactors (section 14.1), but it has not immediately been detected, due to its low specific activity caused by its long half-life. After the discovery of Pu, plutonium gained great practical importance, because of the high fission cross section of Pu by thermal neutrons. Very small amounts of Pu are present in uranium ores, due to (n, y) reaction of neutrons from cosmic radiation with The ratio Pu/ U is of the order of 10 In 1971, the longest-lived isotope of plutonium, Pu (ri/2 = 8.00 lO y) was found by Hoffman in the Ce-rich rare-earth mineral bastnaesite, in concentrations of the order of 10 gAg-... 

Consider, for example, a PWR containing 80 te of uranium oxide enriched to 3%, with an average moderator temperature of 310 C. The effective specific volume of uranium oxide pellets will be 1.11 x 10 m /kg. The fission cross-section at a moderator temperature of 20°C/293 K, t/(293), is 582 x 10 m for U-235, while the corresponding figure for plutonium-239 is 743 x 10 m. Calculate NfOf at the start of life and at the end of life, and hence deduce the variation in the constant of proportionality, a. 

In a PWR operating with plutonium recycle the thermal-neutron flux is lower than for uranium fueling because of the hi er fission cross section for plutonium. As a result, less C is produced by thermal-neutron activation within the fuel, as drown in Table 8.11. 

Am. The uotope Am is formed by the decay of Pu. It undergoes alpha decay, with a half-Ufe of 458 years, to form Np. Isotopically pure Am can be extracted from aged reactor-grade plutonium. Irradiation of separated Am is the basis of technology to produce gram quantities of Cm. This is also one route to the production of the transcurium elements. However, the first neutronfission cross sections and which result in considerable heat evolution as contrasted to the production of transcurium isotopes from the irradiation of plutonium rich in the isotope Pu. 

The calculation results are satisfactory even when using simplified calculation schemes with lumped fission products. The reasons for the improvement in the bumup component are linked to differences in the energy release per fission and the fission cross section of Plutonium-239, which for the same power normalisation induce a different fluence. Also of importance are the fission product cross sections and the fact that complete decay chains are explicitly treated. However the decay component remains mispredicted. 

The net reactivity effeot of ir -humout and plutonium formation is positive in spite of a less-ttaan-one-for-one replacement of fissionable atoms by the conversion px ocess. This increase reeuUa.from tbe hi r fission cross section and neutron yield of compared to u The net eacposure effect, including... 

A successful effort was, however, mounted on a plutonium compound, PuSb (Lander et al. 1986). Because the most common isotope Pu has an enormous capture (and fission) cross section (total 1000 b) for thermal neutrons, it is useful only for small crystals used in diffraction experiments. A crystal was therefore made at Karlsruhe of the rare Pu (thermal absorption 18.5 b) isotope, which was kindly loaned by the US Department of Energy. 

Typical curves for the decay of and the buildup of plutonium isotopes in a natural uranium reactor are shown in Fig. 4.3. It will be seen that, beyond an exposure of approximately 7500 MW d/tonne of natural uranium, the concentration of Pu exceeds that of Because the fission cross section of is higher than that of the majority of fissions in the reactor will be taking place in plutonium rather than uranium even before the 7500-MW d/tonne exposure is reached. 

B. R. Leonard, S. M. Hauser, E. J. Seppi, "The Ratio of Plutonium-239 to Uranium-235 Fission Cross Sections from 0.020 to l.,0 Electron Volts, HW-30128, December 1, 1953. 

The density reactivity coefficients tend to increase with bumup. This is due to plutonium buildup in the fuel Pu has a larger thermal neutron absorption cross section, fission cross section, and the resonance absorption cross section than U. Although the density reactivity coefficient decreases with increasing water density, it is kept positive for all density region (i.e., the void reactivity coefficient is negative). Hence, the core can secure the inherent safety characteristics. 

In early 1941, 0.5 )-lg of Pu was produced (eqs. 3 and 4) and subjected to neutron bombardment (9) demonstrating that plutonium undergoes thermal neutron-induced fission with a cross section greater than that of U. In 1942, a self-sustaining chain reaction was induced by fissioning 235u... 

The technologically most important isotope, Pu, has been produced in large quantities since 1944 from natural or partially enriched uranium in production reactors. This isotope is characterized by a high fission reaction cross section and is useful for fission weapons, as trigger for thermonuclear weapons, and as fuel for breeder reactors. A large future source of plutonium may be from fast-neutron breeder reactors. 

Xenon occurs in the atmosphere at trace concentrations. It also occurs in gases from certain mineral springs. Xenon also is a fission product of uranium, plutonium, and thorium isotopes induced by neutron bombardment. The radioactive fission product, xenon-135, has a very high thermal neutron cross-section. The element has been detected in Mars atmosphere. 

Pu and can be used as nuclear explosives, because they have sufficiently high cross sections for fission by fast neutons. By use of the equations in section 11.1, it can be assessed that, in the absence of a reflector, a sphere of about 50 kg uranium metal containing 94% or a sphere of about 16 kg plutonium metal ( Pu) is needed to reach criticality. If a reflector is provided, the critical masses are about 20 kg for and about 6 kg for Pu. The critical masses for are similar to those for Pu. 

The critical mass of fissile material required to maintain the fission process is roughly inversely proportional to the neutron-absorption cross section. Thus the critical mass is lowest for plutonium in thermal reactors, larger for the uranium isotopes in thermal reactors, and much greater in fast reactors. For this reason, as well as others, thermal reactors are the preferred type except when breeding with plutonium is an objective then a fast reactor must be used. 

As inadiation progresses, the effective cross section of fission products from each fuel nuclide decreases. This decrease is partially offset by greater production of fission products from plutonium, which have a higher cross section than those from U. The constant value of Op = 80 given in Table 3.13 takes these two effects approximately into account. 

Using the data for the Wuigassen reactor and the thermal neutron capture cross-section (Table 19.4) it can be calculated how many kg Pu should be formed per t U at a bum-up of27 500 MWd/t. (a) Make this calculation assuming that plutonium disappears only through fission in Vu. (b) According to Table 21.2 each t U from a PWR contains 8.69 kg Pu why is your result much lower ... 

The breeding ratio values given in the fifth column of 50 Table I are based on the nuclear properties of plutonium and uranium and take into account neutrons lost by leakage from the reactor as well as those neutrons lost by capture in the coolant and in the metallic protective jacket enclosing the fission metal. It is assumed that the 65 coolant has an absorption cross section per cubic centimeter which is about that of natural uranium, and that the absorption cross section per cubic centimeter of the jacket metal is about that of natural uranium. 

Although the CSEWG nuclear data benchmarks are adequate for testing Uranium cross sections, more accurate plutonium experiments are needed. The benchmark set should also be expanded to include temperature-dependent experiments, (e.g., Rensselaer Polytechnic Institute and Stanford U self-indication measurements), as well as experiments sensitive to moderator properties (e.g., fission neutron age in water) and to control material properties (e.g., criticality of gadolinium loaded lattices). A considerable number of existing experiments performed in Europe and Japan as well as in the United States diould be considered as candidates for extending the data and methods validation benchmark sets. 

The radial distribution of the fission products over the cross section of the fuel pellet is primarily governed by the profile of the thermal neutron flux. However, there are two effects leading to variations in the distribution of some of the fission products. The first one of these effects is the preferential formation of plutonium in the outermost pellet zones due to epithermal neutron capture in the nucleus. This effect is more pronounced in high-bumup fuel than in fuel with a lower burnup. Fig. 3.7. shows an alpha autoradiographic image of a fuel pellet cross section where the enhanced a activity in the outer ring can be seen it must nonetheless be pointed out that the greatest fraction of this a activity is not due to the... 

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