
Elastic scattering
neutron reaction when a nucleus deflects a neutron without absorbing the neutron and conserves kinetic energy

Inelastic scattering
neutron reaction when a nucleus deflects a neutron without absorbing the neutron and kinetic energy is not conserved (the target nucleus emits a gamma ray).

Radiative capture
a process a neutron is absorbed by a nucleus resulting in an excited compound nucleus. The compound nucleus returns to ground state by emitting gamma rays.

Fission
"neutron reaction when a neutron is absorbed by a nucleus

Explain how changing neutron energy will affect the magnitude of a cross section for a given isotope.
If the cross section (σa – absorption) is large, then there is a high probability of an interaction will occur between a nucleus and a neutron. But as the energy increases the cross section decreases, thus lowering the probability of an interaction

Using the Liquid Drop Fission Model, describe the fission process.
When a neutron strikes a nucleus, the neutron is absorbed and the nucleus is excited by the neutron binding energy plus kinetic energy (causing the nucleus to vibrate). If the added energy is sufficient to overcome the nuclear forces, fission will likely occur.

Fission
neutron reaction when a nucleus is absorbed by a nucleus, resulting in the splitting of the nucleus into two new atoms, some neutrons and gamma rays.

Critical energy
the minimum amount of energy for fission to occur

Instantaneous
energy released in the form of kinetic energy fission fragments, neutrons and instantaneous gamma rays. 83%

Delayed (decayed or residual heat):
energy release in the form of kinetic energy of beta particles, decay gamma rays and neutrinos.

Fissile material
a fuel type that will fission simply because of binding energy of a neutron (BE greater than EC).

Fissionable material
a fuel type that requires kinetic energy in addition to binding energy of a neutron for fission to occur (BE less than EC).

Given a graph of fission product yields, explain the shape of the distribution curve.
Upon fissioning, the target will split into two fission fragments, one lighter (A≈95) and one heavier (A≈139).

List six of the products produced by fission.
Kinetic energy of fission fragments; Kinetic energy of fission neutrons; Instantaneous gamma rays; Kinetic energy of beta particles; Decay gamma rays

Neutron flux (fast, thermal):
the number of neutrons passing through a unit area per unit time.

Flux distribution (radial, axial):
the spatial representation of the neutron flux level.

Reaction rate
the measure of how many reactions are occurring in a unit volume per unit time.

Reactor power
the rate at which energy is emitted as a result of nuclear fissions.

Prompt neutron
neutrons emitted within 1014 seconds of the fission event and are a direct result of the fission process.

Delayed neutron
neutrons born more than 1014 seconds after the fission event (average 12.7 sec).

Explain why thermal neutrons are required for Light Water Reactor operation.
They use fissile materials that have large absorption crosssections for thermal neutrons (slow) by U235.

Explain why a moderator is required in a Light Water Reactor core.
Since neutrons born from fission are fast, a moderator (hydrogen in water) must be used to slow fast neutron via scattering reactions.

Describe the mechanics of the neutron slowing down (moderation) and diffusion processes.
Collision (elastic scattering) of the fast neutron with the hydrogen atoms in the water moderator.

Describe the properties of an ideal moderator.
A high MR (moderator ratio) with a high microscopic cross section for absorption (?a).

Fast neutron
a neutron that has a kinetic energy greater than 0.1 MeV.

Intermediate neutron
neutron that has a kinetic energy between 0.1 MeV and 1 eV.

Slow neutron
neutron that has a kinetic energy less than 1 eV.

Thermal neutron
a neutron that is in thermal equilibrium with its surroundings (these can be fast, intermediate, or slow depending on the temperature of its surroundings).

Epithermal
same as intermediate neutron.

Fast fission factor (E):
accounts for the neutrons produced by fast fission.

Fast nonleakage probability factor (Lf)
the fraction of fast neutrons that do not leak out of the core while slowing down.

Resonance escape probability factor (p)
the fraction of neutrons that are not absorbed while slowing to thermal energy.

Thermal nonleakage probability factor (Lth)
the probability that a thermal neutron will not leak out of the core.

Thermal utilization factor (f)
the ratio of the number of thermal neutrons absorbed in the fuel to the number of thermal neutrons absorbed in the core.

Reproduction factor (n)
the number of fast neutrons produced from fission compared to the number of thermal neutrons absorbed.


Subcritical
keff < 1.0, negative p (reactivity)

Supercritical
keff > 1.0, positive p (reactivity)

Define effective multiplication factor (keff) and discuss its relationship to the state of the reactor.
The factor by which the number of neutrons produced in one generation is multiplied to determine the number of neutrons produced from fission in the next generation.

kexcess
the amount by which the total installed keff exceeds 1.0.

Define shutdown margin
amount of reactivity by which a xenonfree, cold (68°F) reactor is or would be subcritical if all but the highest worth control rod were fully inserted.

reactivity
The fractional change in neutron population per generation.

For prompt delayed neutrons, contrast:
1. generation sources
2. generation times
3. average kinetic energy
4. effects on the fission process
5. leakage probabilities
 1. generation sources: fission
 2. generation times: within 1014 seconds
 3. average kinetic energy: 2 MeV
 4. effects on the fission process: little chance to produce fission
 5. leakage probabilities: greater probability to leak due to # of collisions to slow neutrons

For prompt delayed neutrons, contrast:
1. generation sources
2. generation times
3. average kinetic energy
4. effects on the fission process
5. leakage probabilities
 1. generation sources: daughter of fission product
 2. generation times: 12.7 seconds
 3. average kinetic energy: 0.5 MeV
 4. effects on the fission process: greater chance to produce fission
 5. leakage probabilities: less probability to leak due to # of collisions to slow neutrons

Define startup rate
measure of the rate of change of reactor power in decades per minute (DPM).

Define reactor period
the time (sec) required for power to increase by a factor of e (2.71828).

Define and explain the fuel temperature (Doppler) coefficient of reactivity
Defined as the “change in reactivity per unit change in fuel temperature”. FTC is always at a desirable negative value.

Describe the effects of boration/dilution on reactivity
Boration adds a negative reactivity / Dilution (decrease boron concentration) adds positive reactivity.

Explain Doppler broadening
Raising fuel temperature causes the nuclei to vibrate more rapidly, effectively broadening the energy range of neutrons that may be resonantly absorbed.

Define and explain the void coefficient of reactivity.
Steam bubbles, or voids, have an effect of reducing moderator density with a result similar to increasing moderator temperature (negative coefficient). Negligible in a PWR.

Explain the concept and characteristics of subcritical multiplication.
the process of utilizing source neutrons and fuel to maintain a constant neutron population with keff less than 1.0.

Define and explain the moderator temperature coefficient (MTC) of reactivity
The change in reactivity per unit change in the temperature (°F) of the moderator. Negative MTC desirable but may be positive at BOL due to high boron concentration.

Explain resonance absorption and identify the factors that affect it.
 · At specific neutron energy levels, the slowing down neutron is absorbed and removed from life cycle.
 · U238 and buildup of Pu240 (plus others) at EOL have high microscopic cross sections for absorption.
 · High mod temp keeps neutrons at resonance energies longer (requires more collisions in less dense water) increasing neutron absorption.
 · High fuel temp widens resonance absorption energy range increasing neutron absorption.

Explain prompt critical
Prompt critical is the point when the reactor goes critical without delay neutrons.

Explain the effect of delayed neutrons on reactor control.
The delayed neutrons length the average neutron generation times to slow the reactor response to reactivity changes. Without delayed neutron, the reactor will be highly supercritical so rapidly that power changes are lost thus creating a bomb.

State the relationship between reactivity and effective multiplication factor.
If keff=0 there is no reactivity

