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Concentration/Density: Neutron reactions leading to scattering, capture or fission reactions are more likely to occur in dense materials; conversely, neutrons are more likely to escape (leak) from low density materials. Moderation: Neutrons resulting from fission are typically fast (high energy). These fast neutrons do not cause fission as ...
For example, the capture cross section of deuterium 2 H is much smaller than that of common hydrogen 1 H. [2] This is the reason why some reactors use heavy water (in which most of the hydrogen is deuterium) instead of ordinary light water as moderator : fewer neutrons are lost by capture inside the medium, hence enabling the use of natural ...
An exception is the uranium-233 of the thorium cycle, which has a good fission/capture ratio at all neutron energies. Fast-neutron reactors use unmoderated fast neutrons to sustain the reaction, and require the fuel to contain a higher concentration of fissile material relative to fertile material (uranium-238).
Nuclear cross sections are used in determining the nuclear reaction rate, and are governed by the reaction rate equation for a particular set of particles (usually viewed as a "beam and target" thought experiment where one particle or nucleus is the "target", which is typically at rest, and the other is treated as a "beam", which is a projectile with a given energy).
A less moderated neutron energy spectrum does worsen the capture/fission ratio for 235 U and especially 239 Pu, meaning that more fissile nuclei fail to fission on neutron absorption and instead capture the neutron to become a heavier nonfissile isotope, wasting one or more neutrons and increasing accumulation of heavy transuranic actinides ...
Manifest functions are the consequences that people see, observe or even expect. It is explicitly stated and understood by the participants in the relevant action. The manifest function of a rain dance, according to Merton in his 1957 Social Theory and Social Structure, is to produce rain, and this outcome is intended and desired by people participating in the ritual.
To be a useful fuel for nuclear fission chain reactions, the material must: Be in the region of the binding energy curve where a fission chain reaction is possible (i.e., above radium) Have a high probability of fission on neutron capture; Release more than one neutron on average per neutron capture.
Let us calculate the probability of avoiding resonance capture in homogeneous and heterogeneous environments natural uranium-graphite. In both media the ratio of carbon and 238 U nuclei N C /N S =215. The diameter of the uranium rod is d=3 cm. Taking into account that ξC=0.159, σ C a =4.7 barn, we calculate the following probability;