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300s School/NUCE 2100 - Fundamentals of Nuclear Engineering/2024-09-17 Module 5 Nuclear Fission Basics.md
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@ -36,6 +36,8 @@ Rate of increase = Production - absorbed - leakage
$$k = \frac{\text{Number of neutrons in one generation}}{\text{Number of neutrons in preceding generation}}$$
Measures how many neutrons produced by each neutron born.
There are more formulas in the slides. K can be based on material properties
## Energy Dependence
## Energy Dependence and Moderating
Fission is most efficiently caused by thermal neutrons (energy < 1eV), but neutrons from fission are born at something like \>2MeV. 7 orders of magnitude reduction -- usually kinetic losses by neutrons colliding with things.
Different neutron moderators require a different number of collisions to bring the energy down to thermal. The closer the mass is to that of a neutron, the better. Hydrogen is best, then beryllium, then carbon (graphite).
Different neutron moderators require a different number of collisions to bring the energy down to thermal. The closer the mass is to that of a neutron, the better. Hydrogen is best, then deuterium, then beryllium, then carbon (graphite).