Neutron reactions fission

Radioactivity occurs naturally in earth minerals containing uranium and thorium. It also results from two principal processes arising from bombardment of atomic nuclei by particles such as neutrons, ie, activation and fission. Activation involves the absorption of a neutron by a stable nucleus to form an unstable nucleus. An example is the neutron reaction of a neutron and cobalt-59 to yield cobalt-60 [10198 0-0] Co, a 5.26-yr half-life gamma-ray emitter. Another is the absorption of a neutron by uranium-238 [24678-82-8] to produce plutonium-239 [15117 8-5], Pu, as occurs in the fuel of a nuclear... 

In early 1941, 0.5 )-lg of Pu was produced (eqs. 3 and 4) and subjected to neutron bombardment (9) demonstrating that plutonium undergoes thermal neutron-induced fission with a cross section greater than that of U. In 1942, a self-sustaining chain reaction was induced by fissioning 235u... 

The heart of the nuclear reactor boiler plant system is the reactor core, in which the nuclear fission process takes place. Nuclear fission is the splitting of a nucleus into two or more separate nuclei. Fission is usually by neutron particle bombardment and is accompanied by the release of a very large amount of energy, plus additional neutrons, other particles, and radioactive material. The generation of new neutrons during fission makes possible a chain reaction process and the subsequent... 

FIGURE 17.24 A self-sustaining chain reaction, in which neutrons are the chain carriers, takes place when induced fission produces more than one neutron per fission event. These newly produced neutrons can stimulate fission in increasingly greater numbers of other nuclei. 

Plutonium-239 also is produced from natural uranium by the so-called pile reactions in which irradiation of uranium-235 isotope with neutrons produces fission, generating more neutrons and high energy ( 200 MeV). These neutrons are captured by the uranium-238 to yield plutonium-239. 

The data of Table I are derived from early time radiochemical data reported by Stevenson (5). The linearity of the radionuclide ratios was first pointed out in that report. The aerial filter samples were taken at successively later times, 1 and 2, below the reported cloud base, and 3, 4, and 5 in the cloud. The tabulated values of rA correspond to atom ratio of isotope A to an arbitrary refractory isotope normalized by dividing by the atom ratio in which the two species were formed. Refractory species include the isotopes of the rare earths Eu and Tb as well as 45Ca, 89Zr, Sc, and others produced by neutron reactions on stable isotopes. The tabulation has been limited to fission product species. However, the... 

Additional interactions of neutrons with nuclei include die release of charged particles by neutron-induced nuclear disintegration, Commonly known reactions are n-p. n — d. and n—ct. In these cases, the incident neutrons may contribute part of their kinetic energy to the target nucleus to effect the disintegration. Hence, more than mere neutron capture is involved, Then, there is usually a lower threshold for the neutron energy below which the reaction fails to occur, Another important reaction involving neutrons is fission, which may occur under different conditions for eidier slow or fast neutrons with appropriate fissionable material. 

Example Problem In a certain nuclear reaction, a beam of lsO was combined with 233U nuclei to form a compound nucleus of 256Fm. The nuclei were produced with an excitation energy of 95 MeV. Calculate the nuclear temperature assuming that y = 1, and then the relative probability of neutron to fission decay of the excited system. 

Sketch the fission excitation function for the reaction of 232Th with neutrons. The fission barrier is 6.5 MeV, and the binding energies of the last neutron in 232Th and 233Th are 6.90 and 4.93 MeV, respectively. 

Of the fast neutrons produced in fission, some of them will be moderated to thermal energies and will induce other fission reactions while others will be lost. The ratio of the number of neutrons in the next generation to that in the previous generation is called the multiplication factor k. If the value of k is less than 1, then the reactor is subcritical and the fission process is not self-sustaining. If the value of k is greater than 1, then the number of fissions will accelerate with time and the reactor is supercritical. The goal of reactor operation is to maintain the system in a critical state with k exactly equal to 1. The extreme upper limit for the multiplication factor would correspond to the mean number of neutrons per fission ( 2.5 for 235U(n,f)) if each neutron produces a secondary fission. 

The second method makes use of the fact that when a subcritical quantity of an appropriate isotope, that is, 239Pu (or 235U), is strongly compressed, it can become critical or supercritical. The reason for this is that compressing the fissionable material, that is, increasing its density increases the rate of production of neutrons by fission relative to the rate of loss by escape. The surface area (or neutron escape area) is decreased, while the mass (upon which the rate of propagation of fission depends) remains constant. A self-sustaining chain reaction may then become possible with the same mass that was subcritical in the uncompressed state. 

The reaction products from the thermal neutron induced fission of Pu... 

Another application of track detectors is dosimetry of a particles and neutrons. For neutron dosimetry the track detectors may be covered with uranium foils in which the neutrons induce fission. Alternatively, the detectors may be covered with a foil containing B or Li, and the a particles produced by (n, a) reactions are recorded. 

For operation of nuclear reactors, the delayed neutrons (section 8.9) play an important role, because they cause an increase in the time available for control. The multiplication factor due to the prompt neutrons alone is kesi - P), P being the contribution of the delayed neutrons, and as long as / eff(l P) < 1, the delayed neutrons are necessary to keep the chain reaction going. In the fission of 0.65% of the fission neutrons are emitted as delayed neutrons from some neutron-rich fission fragments such as or Xe. 

Second, the release of neutrons during fission makes it possible for a rapid and continuous repetition of the reaction. Suppose that a single neutron strikes a one gram block of uranium-235. The fission of one uranium nucleus in that block releases, on an average, about two to three more neutrons. Each of those neutrons, then, is available for the fission of three more uranium nuclei, hi the next stage, about nine neutrons (three from each of three fissioned uranium nuclei) are released. As long as more neutrons are being released, the fission of uranium nuclei can continue. 

---