Fertile nuclides

The fission neutrons at birth have energies of approximately 1 to 2 MeV In a thermal reactor the neutron energy is rapidly reduced through collisions with light nuclei to thermal (—.02 to 1 eV), to promote for more efficient capture. Besides the nuclear fuel, there are many other materials in the reactor core also competing for the neutrons, including the moderator (the material used to slow down or thermalize the neutrons), fertile nuclides that produce additional fissile material (discussed in a later section), neutron poisons present in control rods, the coolant, fuel element cladding, and other structural materials. 

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. 

One of the principal objectives of fuel-cycle analysis is to follow quantitatively the changes in concentration of fissile and fertile nuclides and fission products during neutron irradiation. 

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. 

Thorium is important in nuclear technology as the naturally occurring fertile nuclide from which neutron capture produces fissile by the succession of reactions... 

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

TABLE 19.2. Nuclear data for some fissile and fertile nuclides see also Figs. 19.3, 19.5 and 20.5)... 

A fertile nuclide by definition means a non-fissile nuchde, which, however, can be converted into a fissile nuclide via neutron-induced nuclear reactions (p. 2807). 

Nuclear reactors can be designed on the basis of their fuel cycle such that they breed more fissile nuclides than what they use. Breeder reactors can utilize uranium, thorium, and plutonium resources more efficiently. There are two types of breeder reactors (1) fast neutron spectmm breeder and (2) thermal neutron spectmm breeder reactors, which are designed based on (99.2% natural abundance) and Th (100% natural abundance), respectively. Fertile nuclides and Th capture neutrons and trans-form, respectively, to fissile nuclides Pu and U. Through this process, which is known as breeding, the reactor produces more fissile nuclides than what it consumes. Fast-breeder reactors (FBRs) can also be used in order to transmute the long-lived... 

The blanket, which surrounds the reactor core, is the region containing the fertile nuclides. 

The NAS has requested that only reactor concepts that do not contain any uranium or thorium be examined. Eliminating from the reactor prevents the production of more Pu. Similarly, eliminating thorium from the reactor prevents the production of The main reason for not including fertile nuclides is to preclude the production of additional weapons materials. A plutonium-only based fuel provides the highest plutonium destruction rate. 

All reactors without fertile materials at the same power level and capacity factor destroy plutonium at the same rate. In addition, this rate is faster than the rate of any reactor containing fertile materials (uranium or thorium), because fertile materials produce more weapons materials. All plutonium isotopes release about the same amount of energy per fission (within 2%, and nearly independent of the incident neutron energy). Without fertile nuclides in the system, all of the fission energy comes from plutonium. Therefore, if the reactor is operated at a certain power level, the total amount of plutonium will decrease linearly in time. Thus, all reactors... 

A fissionable nuclide such as is calledyissi/c tPu is also fissile. A nuclide such as fU/ which can be converted into a fissile nuclide, is said to be fertile. In a breeder nuclear reactor, a small quantity of fissile nuclide provides the neutrons that convert a large quantity of a fertile nuclide into a fissile one. (The newly formed fissile nuclide then participates in a self-sustaining chain reaction.)... 

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