Scattering Reactions

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 + 0 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 + 1 + 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 + ~2.51 η+ Q FF = fission fragment 2.5 = 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 10-14 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 • Xenon – 135 • Samarium – 149 • Control Rods 1n • Boric Acid

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