Fission neutron yields

This reaction offers the advantage of a superior neutron yield of in a thermal reactor system. The abiHty to breed fissile from naturally occurring Th allows the world s thorium reserves to be added to its uranium reserves as a potential source of fission power. However, the Th/ U cycle is unlikely to be developed in the 1990s owing both to the more advanced state of the / Pu cycle and to the avadabiHty of uranium. Thorium is also used in the production of the cx-emitting radiotherapeutic agent, Bi, via the production of Th and subsequent decay through Ac (20). 

A FIGURE 22.8 A representation of nuclear fission. A uranium-235 nucleus fragments when struck by a neutron, yielding two smaller nuclei and releasing a large amount of energy. 

Crouch, E.A.C. (1977) Fission product yields from neutron-induced fission. Atomic Data Nuclear Data Tables, 19,417-532. 

Figure 8.17. Neutron yields as a function of the mass of the primary fission fragments (according to J. Terrell, Proc. IAEA Symp. Phys. Chem. Fission, Salzburg 1965, IAEA, Vienna, Vols. 2, 3).

Fast breeder reactors are not operated, as e.g. light-water reactors, with slow neutrons, but with unmoderated fast neutrons as they occur immediately upon nuclear fission. These fast neutrons are necessary to sustain the chain reaction. The neutron yield per fission is here larger, since more neutrons are left over for the breeding process, once the neutrons lost by absorption and leakage have been subtracted. They are absorbed by or which are... 

FIGURE 23.6 Nuclear fission of U-235. When o U-235 nucleus captures a neutron (red dot), it undergoes fission to yield two smaller nuclei. On the average, 2.4 neutrons are emitted for every U-235 nucleus that divides. 

More than 300 different nuclides have been observed as the primary products of fission. The term fission products usually refers to the primary fission products, i.e., the fission fragments and their daughters resulting from radioactive decay and neutron absorption. Only a few of the primary fission products are stable, the rest being beta-emitting radionuclides. As a fission-product radionuclide undergoes beta decay, its atomic number increases whereas its mass number remains constant. The direct yield of a fission-product nuclide is the fraction of the total fissions that yield this nuclide, essentially as a direct-fission fragment. The cumulative... 

In thermal-neutron reactors has an important advantage over or Pu in that the number of neutrons produced per thermal neutron absorbed, tj, is higher for than for the other fissile nuclides. Table 6.1 compares the 2200 m/s cross sections and neutron yields in fission of these three nuclides. Thorium has not heretofore been extensively used in nuclear reactors because of the ready avaUabihty of the U in natural or slightly enriched uranium. As natural uranium becomes scarcer and the conservation of neutrons and fissile material becomes more important, it is anticipated that production of U from thorium will become of greater significance. 

Persotmel working with plutonium must be protected by light shielding. The external radiation to be shielded includes ganunas from alpha and beta decay, internal conversion x-rays, ganunas, and neutrons from spontaneous fission, and neutrons from (a, n) reactions in materials of low atomic number. Neutron yields for various types and forms of plutonium are listed in Table 9.15. 

If multineutron evaporation is responsible for the independent yields of shielded nuclides, then, of course, these yields would not be predicted by any simple hypothesis for the division of charge in fission which does not take into account the distribution of fission neutron numbers. 

Early Work. The irradiated fuel, upon discharge from the reactor, comprises the residual unbumt fuel, its protective cladding of magnesium alloy, zirconium or stainless steels, and fission products. The fission process yields over 70 fission product elements, while some of the excess neutrons produced from the fission reaction are captured by the uranium isotopes to yield a range of hew elements—neptunium, plutonium, americium, and curium. Neutrons are captured also by the cladding materials and yield a further variety of radioactive isotopes. To utilize the residual uranium and plutonium in further reactor cycles, it is necessary to remove the fission products and transuranic elements and it is usual to separate the uranium and plutonium this is the reprocessing operation. 

One common radioactive source of neutrons is a transuranium element that undergoes spontaneous fission to yield neutrons. The most common example of this type of source consists of Cf (californium-... 

While the invention has been described with reference to uranium it should be noted that numerous compositions capable of fission to yield neutrons in greater... 

The chemical separation of U from thorium is readily accomplished with high purity. The fissionable iso-60 tope U will support a chain reaction, and has many desirable qualities. In particular, U gives a relatively high average neutron yield per fission, the value as presently known being about 2.37-2.4 neutrons per fission (average). 

When neutron losses can be reduced to the minimum, and when a fissionable isotope giving an average neutron yield per fission of substantially over two, such as U333 is used to support the reaction, conversion factors above unity may be obtained in a converter wherein U333, for 70 example, is fissioned to produce U333. This type of improved converter is known as a breeder reactor and is specifically no part of the present invention except insofar... 

In addition, the physical measurements (after some corrections for prompt neutron emission) allow one to obtain fragment mass distributions (prior to prompt neutron emission). A comparison of these fission fragment yield curves with fission product yield curves makes it possible to extract information on prompt neutron emission. This will be discussed in the next subsection. 

Fission fragments (pre-neutron emission). The yield curves discussed above refer to fission product yields after emission of prompt neutrons. As discussed above, the physical methods based on momentum conservation at scission (double energy, double velocity, or energy and velocity measurements) allow the measurement of the yield distribution of fission fragments (prior to prompt neutron emission). In these cases, simultaneous information is obtained on the kinetic energy of the fragments detected. 

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