Fissile nuclides

One of the many problems of nuclear power is the availability of fuel uranium-235 reserves are only about 0.7% those of the nonfissile uranium-238, and the separation of the isotopes is costly (Section 17.12). One solution is to synthesize fissile nuclides from other elements. In a breeder reactor, a reactor that is used to create nuclear fuel, the neutrons are not moderated. Their high speeds result in... 

Fig. 2. Schematic illustration of the ideal open nuclear fuel cycle (NRC 2003). In this case, there is no reprocessing. Interim storage may last for tens of years so that the heat and radioactivity are much less prior to handling and final disposal. The spent fuel still contains fissile nuclides, such as 235U and 239Pu (generated by neutron capture reactions with 238U).

Nuclear Fuels. There are tw o broad categories of nuclear fuels (1) the fissile nuclides previously mentioned and (2) die fissionable nuebdes,... 

Further improve proliferation resistance (PR) and safety. The essential elements to enhance nonproliferation of nuclear weapons or to suppress harmful usage of nuclear power are to (i) decrease the global inventory of separated fissile nuclides, including the existing warheads (ii) make the... 

Plutonium-239. Plutonium-239 represents a fortuitous phenomenon. Whereas U-235 is the only significant fissile nuclide in nature, its major isotope, U-238, captures a neutron to produce another fissile nuclide, plutonium-239. A substantial amount of the energy produced during the life of uranium fuel is produced by the conversion of U-238 to Pu-239 which subsequently fissions. This process provides the basis for the nuclear breeding cycle. 

The energy output of a nuclear reactor is characterized by the bum-up which is usually given in megawatt-days (MW d) per ton of fuel. By use of natural uranium a burn-up of about 10 MW d per ton is achieved. This corresponds to the fission of about 13 kg of fissile nuclides per ton of fuel, the greatest part being Pu produced by reaction (11.4) The fission of leads to a decrease of its concentration below the natural isotopic abundance of 0.72%. From the economic point of view, only the recovery of plutonium is of interest. 

Highly enriched uranium containing >90% is, in general, only used in research reactors. The production of fissile nuclides is negligible, and the maximum burn-up is of the order of 10 MW d per ton of fuel, corresponding to the consumption of about 13% of the fuel. Recovery of the remaining is of economic interest. 

Mixtures of uranium and plutonium may be used instead of weakly enriched uranium in thermal reactors and are applied in fast breeder reactors, which are operated with the aim of producing more fissile material than is consumed by fission. The main fissile nuclide is Pu, which is continuously reproduced according to reaction (11.4) from In fast breeder reactors operating with about 6 tons of Pu and about 100 tons of U a net gain of fissile Pu may be obtained. The bum-up is about 10 MW d per ton of fuel, and reprocessing with the aim to recover the plutonium is expedient. 

The main aim of reprocessing is the recovery of fissile and fertile material. If U or U-Pu mixtures are used as fuel, the fissile nuclides are and Pu, and the fertile nuclide is Reprocessing of these kinds of fuel closes the U-Pu fuel cycle. The U-Th fuel cycle is closed by reprocessing of spent fuel containing mixtures of U and Th. In the case of final storage of the spent fuel elements, the fuel cycle is not closed fissile and fertile nuclides are not retrieved for further use. 

Nuclear reactors used to synthesize fissile nuclides for fuel and weapon use. 

Obtained primarily from the ore pitchblende, UO2, by reducing the oxide to the metal and then enriching or increasing the fraction of the fissile nuclide, uranium-235... 

Nuclear properties of the three principal fissile nuclides are summarized in Table 3.1. The property 7 given in Table 3.1 is of interest in relation to the possibility of using these fissile nuclides in a breeder reactor. If a reactor is designed carefully for neutron economy, it is possible under certain conditions to generate fissile material at a rate equal to or greater than the consumption rate of fissile material. Such a reactor can be operated as a true breeder if the newly formed fissile material is returned to the reactor. The minimum requirements of a fuel to... 

Another important objective is to follow the changes in reactivity that take place as fissile nuclides are depleted or formed from fertile nuclides, and as neutron poisons are formed through buildup of fission products or burned out through reaction with neutrons. 

The cross section of low-cross-section fission products, Op, has a single constant value independent of the fissile nuclide from which the fission products were produced and independent of the flux time to which the fission products were exposed. This assumption is an oversimplification, because the yield of individual fission products is different from each fissile nuclide, and individual fission products with hi er cross sections tend to be converted to those of lower cross section as irradiation progresses. Walker [Wl] has given tables from which may be determined the effective cross sections of fission products from U, U, Pu, and Pu as a function of the flux and flux time to which the fission products have been exposed. 

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. 

Compared with Pu, the other synthetic fissile nuclide, has the advantage that it can be denatured, made less available for use as a nuclear explosive, by isotopic dilution with in a mixture containing less than 12 percent Production of a nuclear explosive from such a mixture would require costly and difficult isotope separation (Chap. 14). No similar means exists for denaturing Pu, which can be more readily separated from by chemical reprocessing (Chap. 10). 

Proportion of fertile nuclide ( U, Th, or Pu) diluting fissile nuclide... 

For given fuel composition (factors 1 and 2 speciHed), the simplest but most restrictive condition to ensure subcriticality is one of items 3, 4, S, or 6 (limitation of mass, dimensions, volume, or concentration of fissile material). These soolled single-parameter limits for fissile nuclides are spelled out in American National Standard ANSI N16.1-1975 [A4], They were abstracted in Table 4.11 of Chap. 4 and are amplified somewhat in Sec. 8.2, following. These single-parameter limits give the largest mass, size, volume, or concentration that will be safely subcritical no matter what other criticality-limiting conditions may be present. 

Table 10.25 Single-parameter limits for uniform aqueous solutions containing fissile nuclides...

This latter explanation of fine structure is a plausible one. However, complete fission yield curves for the neutron-induced fission of other fissile nuclides are needed to add further light on this problem. 

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