Title: Some Facts About Neutron and Reactor Physics
Abstract: Chapter 2 describes some facts about neutron and reactor physics needed for the understanding of Chaps. 3 – 10 . It starts with the radioactive decay and the definitions of the decay constant and the half-life. It continues with the explanation of the fission process for fissile nuclear isotopes, e.g. U-233, U-235, or Pu-239 and the fission energy release by creation of fission fragments (products), prompt fission neutrons and delayed neutrons and radiation (β-particles, γ-rays and antineutrinos). This is followed by the definition of reaction rates of neutrons with other atomic nuclei, the presentation of measured microscopic cross sections for absorption, capture and fission as well as the definition of the macroscopic cross section and the neutron flux. In LWR cores the fuel is arranged heterogeneously in lattice cells together with a moderator (water) in order to slow down the fission neutrons with high kinetic energy to kinetic energies in the range of 0.025 eV (thermal energy). This is most effective if the enriched uranium fuel is put in cylindrical rods which are arranged in e.g. a square grid. The optimization of the geometrical distance between the fuel rods leads to important safety characteristics of LWR cores: the negative fuel Doppler coefficient and the negative coolant (moderator) coefficient. The definition of the criticality factor or effective multiplication factor, keff, allows a characterization whether the reactor core is operated in steady state condition or whether it is subcritical or even supercritical. The criticality or effective multiplication factor, keff, can be changed by moving or by insertion or withdrawing of absorber material (boron, cadmium, gadolinium, indium, silver, hafnium, erbium) in the core. This allows control of the reactor. The reactor core is controlled always in a keff range where the delayed neutrons are dominating. The delayed neutrons are therefore of highest importance for the control of the reactor. During reactor operation over months and years the initially loaded U-235 in the low enriched uranium fuel will be consumed, neutron absorbing fission products will build up or other heavy nuclei with masses above U-235 and Pu-239 will be created. This decreases the criticality of the effective multiplication factor keff. This burnup effect on the criticality factor keff is accounted for by the design of the reactor core. The enrichment of the initially loaded fuel is increased such that keff becomes slightly >1. This is balanced by absorber materials (moveable absorber rods, burnable neutron poisons, e.g. gadolinium or boric acid) which keep the reactor core always at keff ≥ 1. After shutdown of the reactor the gradually decaying fission products and the radioactive decay of higher actinides creates afterheat in the reactor core. This afterheat (decay heat) must be transferred by the coolant water to outside coolant towers or to river or sea water.
Publication Year: 2014
Publication Date: 2014-01-01
Language: en
Type: book-chapter
Indexed In: ['crossref']
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Cited By Count: 1
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