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

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)

Neutron Production Fission 235U + 1η → 236U → 2 FF + ~2.5 1 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

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

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

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

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

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

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

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

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

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

Magnox

AGR

PWR

BWR

RBMK

CANDU