1. Scattering Reactions
Scattering reactions - the incident particle collides with the target nucleus
Elastic scattering – a collision between a particle and nucleus where the kinetic energy and momentum is conserved
0η1 + ZXA → ZXA + 0 η1
Inelastic scattering – a collision between a particle and a nucleus where a portion of the kinetic energy is used to raise the target nucleus to a higher energy level
0 η1 + ZXA → ZYA+1 * → ZXA η1 + γ

2. Absorption Reactions
Absorption reactions – the incident radiation transfers energy by becoming part of the nucleus
Radiative capture – incident particle is captured by nucleus and excess energy is carried off by gamma emission
0η1 + ZXA → ZYA + 1 * → zXA γ
Particle emission – incident particle is absorbed by the nucleus and the resultant excitation energy is high enough that a particle is ejected from the nucleus
0η1 + ZXA → ZYA + 1 * → Y + γ + p (p = particle; mass number of resultant nucleus is dependent on particle ejected, i.e. proton, neutron, or alpha)
Fission – particle is absorbed by a heavy nucleus splitting the nucleus into fission fragments with the emission of neutrons
0η1 + ZXA → ZYA + 1 → 2FF + 0η1(s) + γ + E (FF = fission fragments; average number of neutrons released per fission is ~2.5)

3. Neutron Production Fission 235U + 1η → 236U → 2 FF + ~2.5 1 η + Q
FF = fission fragment = average number
Alpha neutron reactions (α,η)
4 α + 9Be → 12C + 1 η
Photoneutron reactions (γ,η)
γ + 9Be → 8Be* + 1 η
*excited state

4. Neutron Energy Classification
Fast neutrons > 0.1 MeV
Intermediate neutrons 1.0 eV to 0.1 MeV
Slow neutrons < 1.0 eV
Thermal neutron kinetic energy equals the atoms of the material around them

5. Prompt vs. Delayed Neutrons
Prompt neutrons are released when the fission event occurs
Delayed Neutrons are released after the fission event
Within seconds
10-14 seconds up to 89 seconds
Average energy 2 MeV
Average energy 0.5 MeV
Occur in 99.36% of fission events
Occur in 0.64% of fission events

6. Moderator Properties Atomic mass close to that of a neutron
Small atomic cross section for neutron absorption
Large cross section for neutron scatter
Chemically non-reactive with reactor materials
Provides a large energy loss per collision
Stable isotope to avoid changing cross section characteristics

7. Reactor States
Subcritical – the number of neutrons in a generation is less than the number of neutrons in the previous generation
Critical – the number of neutrons in a generation is equal to the number of neutrons in the previous generation
Supercritical – the number of neutrons in a generation is greater than the number of neutrons in the previous generation

8. Keff Keff = 1, the reactor is critical
Keff < 1, the reactor is subcritical
Keff > 1, the reactor is supercritical

9. Reactivity Coefficients
Changes in reactivity coefficients add positive or negative reactivity to the reactor system
Moderator temperature coefficient
Fuel temperature coefficient
Pressure coefficient
Void coefficient

10. Neutron Absorbers and Poisons
Control Rods 1n
Boric Acid
Xenon – 135
Samarium – 149

11. Reactor Power Control
Reactor power is controlled and adjusted by careful management of:
Neutron flux
Reactivity coefficients
Control rods
Boron concentration (PWR)

12. Enrichment Natural Uranium = 0.7 % 235U
Slightly enriched = 0.8 % to 3 % 235U
Low enriched = 3 % to 5 % 235U

13. Magnox

14. AGR

15. PWR

16. BWR

17. RBMK

18. CANDU