Fission by fast neutrons

When a uranium or plutonium nucleus undergoes fission, it splits into two fragments of unequal size. Figure 2.1 shows yield/mass curves for fission of 235U and 239Pu by slow neutrons. Fission by fast neutrons gives slightly different distributions, with increased prob-... 

Mass number Fission by sJow neutrons Fission by fast neutrons ... 

In the operating reactor, it is assumed that the total poisoning ratio for all hi -cross-section fission products from U and the U caused to undergo fission by fast neutrons from U, q u, has the same value as in the reference design ... 

In addition, fission products come from other fissile species such as Pu and Pu, each of which has its own total poisoning ratio q for high-cross-section fission products formed from that species and from caused to undergo fission by fast neutrons from it ... 

A primary object of the present invention is to provide a breeder system wherein a nuclear fission chain reaction is utilized to produce fissionable material at a rate 10 greater than the rate of consumption of fissionable material within the chain reacting composition. This is accomplished by neutron bombardment of fertile material adapted to undergo nuclear reaction productive of fissionable material as hereinafter described. Fertile iso-15 topes as herein defined are isotopes such as and U238 which are converted to thermally fissionable isotopes, and Pu 39, respectively, by nuclear reaction under neutron bombardment. These fertile isotopes are fissionable by fast neutrons and substantially nen-fission-20 able by slow neutrons (below about 1000 e.v.) and absorb neutrons fast or slow to undergo the above-mentioned nuclear reactions. 

After the bum-up indicated, the amount of that underwent fission is 20 kg. In addition, 1.6 kg of U was fissioned by fast neutrons. The amount of 22.2 kg of U captured neutrons and decayed to Pu. Of this 12.1 kg underwent fission again. Of the remaining 10.1 kg of Pu, 5.2 kg remained as such until the end of exposure, the rest of 4.9 kg captured more neutrons forming 240.24i,242,243py 24ip underwent fission. Finally 5 kg of... 

The important physical constants regarding the fission process for the uranium and thorium isotopes and by-products are given in Table I. As is indicated natural thorium ( Th) is only fissionable by fast neutrons with an energy greater than 2MeV (2 x 10 eV). Even then, however, the probability of capturing a neutron that will cause fission (uy) is very low. 

The most abundant isotope in natural uranium (99.3%) is only fissionable by fast neutrons with an energy above 1 MeV, but the less abundant isotope has a very high fission probability for slow, so-called thermal neutrons as well as being fissionable by fast neutrons. Both natural isotopes only fissionable by fast neutrons, Th and can be considered as fertile materials since by capturing a neutron they can be converted into the fissionable or Pu and Pu, respectively, all having more or less the same fission properties as... 

In fast (neutron) reactors, the fission chain reaction is sustained by fast neutrons, unlike in thermal reactors. Thus, fast reactors require fuel that is relatively rich in fissile material highly enriched uranium (> 20%) or plutonium. As fast neutrons are desired, there is also the need to eliminate neutron moderators hence, certain liquid metals, such as sodium, are used for cooling instead of water. Fast reactors more deliberately use the 238U as well as the fissile 235U isotope used in most reactors. If designed to produce more plutonium than they consume, they are called fast-breeder reactors if they are net consumers of plutonium, they are called burners . 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

In the 1941 paper with Yu. B. Khariton [40], the problem of the critical size of a sample of 235 U in the fission of nuclei by fast neutrons was considered. The calculations showed that, in order to sustain a chain fission reaction by fast neutrons in a sample of 235 U surrounded by a heavy neutron reflector, it is sufficient to have only ten kilograms of pure 235U isotope. Here also a theory is given which allows calculation of the critical mass of... 

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... 

The nuclide, " 92 U, is fissionable by fast-moving neutrons but is not fissile. 

The fission of one nucleus of produce about 200 MeV of usable energy, to be compared with 4 eV produced by the oxidation of one C atom. During the overall process, a huge amount of heat is produced through a controlled nuclear chain reaction in a critical mass of fissile material. Potential future developments (fission in fast neutron reactors (breeders), also known as fourth generation nuclear, which... 

Pu. The isotope Pu is produced by neutron capture in Pu. It is not fissionable by thermal neutrons, but, like all other plutonium isotopes, it fissions with fast neutrons. Pu is converted to a fissionable nuclide by neutron capture. Therefore, like Th and it is a fertile material. It undergoes alpha decay, with a half4ife of 6580 years, to form which then decays to Th, the parent of the 4n decay series discussed in Chaps. 6 and 8. Like the other even-mass plutonium isotopes, Pu produces neutrons by spontaneous fission. It is present in greater concentration in reactor plutonium than any of the other even-mass plutonium isotopes. 

U only by fast neutrons, and that each fission produces two or three new neutrons while large amounts of energy are released. The possibility of producing nuclear weapons and building nuclear power stations is considered in several countries. 

It is obvious that the neutron energy spectrum of a reactor plays an essential role. Figure 19.4 shows the prompt (unmoderated) fission neutron spectrum with 2 MeV. In a nuclear explosive device almost all fission is caused by fast neutrons. Nuclear reactors can be designed so that fission mainly occurs with fast neutrons or with slow neutrons (by moderating the neutrons to thermal energies before they encounter fuel). This leads to two different reactor concepts - the fast reactor and the thermal reactor. The approximate neutron spectra for both reactor types are shown in Figure 19.4. Because thermal reactors are more important at present, we discuss this type of reactors first. 

Uranium fission caused by neutrons was forced or artificial. Not each uranium nucleus could be split and not each neutron could produce fission. When scientists had studied the fission mechanism in more detail they understood that the intensity of fission was higher under the effect of slow neutrons and if the uranium isotope with a mass number of 235 was used. The other uranium isotope, uranium-238, experienced fission only when bombarded by fast neutrons. Can there be a natural process similar to artificial uranium fission N. Bohr thought about that and put forward a hypothesis about possible spontaneous uranium fission (without external energy being transferred to the nuclei). 

If the interference is not too important it can be corrected for. If uranium or other fissionable material is present in the sample, the fission products with high fission yields (Sr, Mo, Zr, Ce, Ba) can induce important positive errors. The interference can easily be estimated when the uranium content is known. In FNAA, secondary interference reactions may occur when fast neutrons interact with other elements and produce particles that induce a nuclear reaction that forms the same indicator nuclide. These particles are usually protons ejected by fast neutrons from a matrix with a high hydrogen content. Examples are ... 

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