Fissile and fertile atoms

Enrichment = mass of fissile atoms/mass of fissile and fertile atoms (i. e. in U-based fuels + all Pu isotopes in U/Pu-based fuels)... 

The fuel utilization is referred to as bumup. The bumup may be expressed as the percentage of fuel used before it must be replaced. For example, 1 % bumup means that for each ton of fuel 10 kg of the fissile plus fertile atoms have been consumed (in fission and capture). However, usually the fuel bumup is given in amount of energy obtained per ton of initially present fuel atoms (in case of mixed U - Pu fuels per ton of initial heavy metal. 

Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split. 

When Pu absorbs a neutron, the more probable reaction is fission, but some atoms capture a neutron to produce fertile Pu. Upon further irradiation this captures another neutron to produce fissile Pu. When Pu absorbs a neutron, either fission may take place or Pu may be formed. Pu also decays with a half-life of 13.2 years to nonfissile Am. Pu is neither fissile nor fertile and, like U, is a poison. When it absorbs a neutron, Pu is formed, which decays with a half-life of 5 h to nonfissile Am. 

We shall assume that a representative unit volume of this design contains atoms of a single fissile species (e.g., U) with absorption cross section ajjf and Ng atoms of a single fertile material (e.g., with absorption cross section Og. The unit volume is assumed also to contain steady-state amounts of Xe, Sm, and other fission products with cross sections above 10,000 b, which build up to equilibrium concentration in a few days at the neutron fluxes typical of power reactors. It is assumed that no other fission products are present to an extent sufficient to affect the neutron balance. The items that affect the thermal-neutron... 

The used fiiel elemmts may later be reprocessed to recover the remaining amount of fissile material as well as any fertile material or regarded as waste fertile atoms are those which can be transformed into fissile ones, i.e. " Th and U, which through neutron capture and jS-decays form fissile and Pu, respectively. The chemical reprocessing removes the fission products and actinides other than U and Pu. Some of the removed elements might be valuable enough to be isolated although this is seldom done. The mixed fission products and waste actinides are stored as radioactive waste. The recovered fissile materials may be refabricated (the U may require re-enrichment) into new elements for reuse. This "back-end" of the nuclear fuel cycle is discussed in Chapter 21. 

The conversion ratio of a reactor is the production rate of fissile atoms (by conversion of fertile atoms) divided by the consumption rate of fissile atoms (by fission or other neutron-induced destruction). A reactor with a conversion ratio greater than unity is sometimes called a breeder reactor. Unlocking all of the potential energy in and 32xh will require high-conversion reactors. 

Table I indicates that the average number of neutrons that are released during fission is greater than 2, although it is dependent on the energy of the neutron causing fission. Of these two or more neutrons one will have to be used to maintain the chain reaction, the remaining one or more can be used for (1) leakage from the reactor core, (2) capture in structure and moderator materials, and (3) production of fissile atoms from fertile atoms.

URANIUM. A naturally occurring element, with atomic number 92 and symbol U. Isotopes of uranium find use in nuclear weapons as well as in nuclear power generation. The most prominent isotopes are uranium-233, uranium-234, uranium-235, and uranium-238. Uranium-233 is a man-made radioisotope, produced most often by neutron bombardment of thorium-232. It is significant largely because it is a fissile isotope. Uranium-234 is radioactive but is neither fissile nor fertile. Natural uranium contains about 0.0054 percent uranium-234. 

Plutonium (symbol Pu atomic number 93) is not a naturally occurring element. Plutonium is formed in a nuclear reaction from a fertile U-238 atom. Since U-238 is not fissile, it has a tendency to absorb a neutron in a reactor, rather than split apart into smaller fragments. By absorbing the extra neutron, U-238 becomes U-239. Uranium-239 is not very stable, and undergoes spontaneous radioactive decay to produce Pu-239. 

The FUJI-233U does not produce TRU including alternative nuclear materials such as Np, Am and Cm moreover, the FUJI MSR can incinerate such materials if required. Specifically, the FUJI-23 3U does not produce any significant amounts of Pu by virtue of the lower atomic number of the fertile fuel isotope ( Th versus U). Annual amounts of the fissile material loaded to the primary circuit in the U-Th MSR cycle are small compared with the Pu- U cycle ... 

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