Fission Flashcards
Define critical energy
The minimum excitation energy for a specific nuclide to fission.
Define microscopic cross-section
The effective target area that single nucleus presents to a bombarding particle
Describe the energy released from fission by the decay energy method
Use the conservation of mass-energy method for each decay chain present. Sum the energy released for each decay chain.
Define fissionable material
Any material capable of fission
What is the primary source of intrinsic source neutrons weeks after a reactor shutdown
- Transuranic
- Spontaneous
- Photo-neutron
Define macroscopic cross-section
The target area that is presented by all of the nuclei of a given nuclide contained in 1 cm3 of the material
Define intermediate neutrons
A neutron that has energy between a fast neutron and a slow (thermal) neutron
On average how much energy is released immediately following fission of U-235
On average 187 MeV out of 200 MeV
Define mean free path
- The average distance of travel by a neutron before absorption or scatter.
- The inverse of macroscopic cross-section.
Describe which fission product nuclides are most likely to result from fission of U-235
- Cesium-140
- Rubidium-93
Define thermal neutron flux
The number of thermal neutrons crossing a unit area in a given amount of time
Define atomic density
The number of atoms of a given type per unit volume of material
Describe spontaneous fission source neutrons (intrinsic)
- Atoms that spontaneously fission
- U-235
- U-238
- Curium-242
- Curium-244
How do changes in neutron flux and macroscopic cross-section affect reaction rates
- As neutron flux or macroscopic cross-section increase, reaction rates increase
- As neutron flux or macroscopic cross-section decrease, reaction rates decrease
Define Barn
The unit of measure for cross-section equal to 10-24 cm2
How is heat produced as a result of fission
The kinetic energy of the fission fragments, beta particles, and fission neutrons interacting with their surroundings
Describe elastic scattering including conservation principles
Neutron collides with a nucleus and bounces off at a different trajectory. Conservation of momentum determines the resultant velocities.
What is the primary source of intrinsic source neutrons at the beginning of core life
- Photo-neutron
- Spontaneous fission
- Transuranic
What is the purpose of installed neutron sources
Since intrinsic neutrons are not abundant at the beginning of core life, installed neutron sources ensure instruments are operable. Examples are Antimony-Beryllium (most common), californium-252, alpha-beryllium, photon-beryllium.
Explain the fission process using the liquid drop model of a nucleus
Describe the relationship between neutron flux and reactor power
- As neutron flux increases reactor power rises
- As neutron flux decreases reactor power decreases
Describe Aplha-neutron source neutrons (intrinsic)
Alpha particles from decay interact with oxygen-18 and boron-11
Define neutron flux
The number of neutrons passing through the unit area per unit time
Describe the energy released from fission by the conservation of mass-energy method
Compare the mass of the reactant to the mass of the products. The difference in mass multiplied by 931.5 MeV/amu gives the energy released following fission.
How does the absorption and scattering cross section of typical nuclides vary with neutron energies in the 1/v region and resonance absorption region
Scattering and absorption cross-sections are the largest in the 1/v region (thermal range) and at certain peaks in the intermediate range (resonance range)
Define fissile material
A fissionable material that DOES NOT need additional kinetic energy to reach the critical energy for fission to occur
Explain the shape of the binding energy per nucleon versus mass number curve
Define fast neutrons
Newly ejected neutrons with high energy
Describe the energy released from fission by the change in binding energy method
Compare the binding energy of the products to the binding energy of the reactant. The difference in energy is the energy released following fission
What is the primary source of intrinsic source neutrons immediately after reactor shutdown
- Photo-neutron
- Transuranic
- Spontaneous fission
What is the significance of the binding energy per nucleon vs mass number curve to fission energy
The higher the binding energy per nucleon the more stable the nuclide. Above a mass number of 60 fission creates energy. Below a mass number of 60 fusion creates energy.
Describe Photo-neutron source neutrons (intrinsic)
The interaction of gamma rays and deuterium nucleus
Define fast neutron flux
The number of fast neutrons crossing a unit area in a given amount of time
Describe radiative capture
Incoming neutron interacts with the target nucleus forming a compound nucleus which then decays to its ground state and decays away
Define fertile material
Materials that undergo transmutation to become fissile material
Define excitation energy
The measure of how far the energy level of a nucleus is above its ground state. Must be equal to the critical energy for fission to occur.
Describe particle ejection
Incoming neutron interacts with the target nucleus forming a compound nucleus which then ejects a particle
Describe fission
Incoming neutron interacts with the target nucleus forming a compound nucleus which then splits into two fission fragments and between typically 2 to 3 neutrons
Describe inelastic scattering including conservation principles
Some kinetic energy is transferred to the target nucleus as excitation energy. The kinetic energy of the outgoing neutron and nucleus is reduced by the amount of kinetic energy that is transferred to excitation energy.
What is the purpose of source neutrons
Source neutrons are not fission products. They help monitor the fission process during reactor startup and help ensure that instrumentation is operable. Source neutrons can either be intrinsic or installed.
Describe transuranic source neutrons (intrinsic)
Elements with an atomic number greater than uranium (92) that produce neutrons from either spontaneous fission or alpha-neutron reactions.
Explain binding energy per nucleon
The binding energy of a nucleus divided by the total number of nucleons (i.e., protons + neutrons) in the nucleus
Define thermal neutrons
Very low kinetic energy neutrons which add essentially no kinetic energy to a reaction.
On average how much energy is released by delayed fission energy following fission of U-235
On average 13 MeV out of 200 MeV
Describe radial and axial neutron flux distribution
- Axial flux is measured from the top to the bottom of the core
- Radial flux is measured across the core
- The goal is to maintain flux distribution near the center of the core.
