Fission energy, average

Neutrons. The number of neutrons per fission caused by thermal neutrons is between two and three. This number increases linearly with the kinetic energy of the neutron inducing the fission. The average energy of a neutron emitted in fission is about 2 MeV. More than 99 percent of the neutrons are emitted at the time of fission and are called prompt neutrons. A very small fraction is emitted as delayed neutrons. Delayed neutrons are very important for the control of nuclear reactors. 

The neutrons emitted have an average kinetic energy of 2 MeV. For the average of 2.5 neutrons emitted in fission of U by thermal neutrons, about 5 MeV of the fission energy is required. The emission of y-rays in the act of fission, prompt y-rays, accounts for another 6-8 MeV. The largest part of the fission energy is observed as the kinetic energy... 

Upper panel Decay chains from the irradiations of (a) (labeled k, I, m) (b) (n, o, p). The lifetimes and fission energies are averaged from all... 

In these very heavy atoms, spontaneous fission is often the predominant mode of decay, and it is also the process most liable to large errors in half-life prediction. Nix, in a letter of considerable practical interest, computes the total energy release in fission, the average number of neutrons per fission (v), and the neutron energy for a few superheavy nuclides. Some of these results are shown in Table 1 the inclusion of a plutonium isotope provides a comparison with a known actinide. 

Although Eq. (2) has been derived for a simplified model, its accuracy when used for the AGN-201 reactor power calibration will depend primarily on the average thermal-flux measurement and to a lesser extent on a correction made for nonthermal fission. The energy distribution of the neutrons in the core actually covers a wide range, from fission energies down to thermal. Before the neutrons are slowed down into the thermal region, some will be captured in and and hence cause additional... 

As long as the amount of fissionable material is less than the critical mass, the rate of fission events does not grow, and the rate of energy release remains low. In contrast, a sample behaves quite differently when the amount of fissionable material is larger than the critical mass. Above the critical mass, more than one neutron, on average, is recaptured for every fission that occurs. Now the number of fission reactions grows rapidly. As an illustration, consider what happens when two neutrons are recaptured from each fission reaction. As shown in figure 22-11 on... 

Cf spontaneous fissions have a fast neutron energy spectrum, shown in Figure 3, with an average energy of 2.2MeV. On average, 3.76 neutrons are emitted per spontaneous fission. The neutron emission rate is 2.34 X 10 n/(s-g)... 

Example Problem An important delayed neutron emitter in nuclear fission is 137I. This nuclide decays with a half-life of 25 s and emits neutrons with an average energy of 0.56 MeV and a total probability of approximately 6%. Estimate the energy of an excited state in 137Xe that would emit a 0.56-MeV neutron. 

Figure 11.19 Average total number of neutrons, vT, as a function of the mass number of the fissioning system for spontaneous and thermal neutron-induced fission. The values for thermal neutron-induced fission have been corrected to zero excitation energy (spontaneous fission) assuming dv jdt = 0.12 MeV-1.

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