Fission

Question 1

Define critical energy

Answer 1

The minimum excitation energy for a specific nuclide to fission.

Question 2

Define microscopic cross-section

Answer 2

The effective target area that single nucleus presents to a bombarding particle

Question 3

Describe the energy released from fission by the decay energy method

Answer 3

Use the conservation of mass-energy method for each decay chain present. Sum the energy released for each decay chain.

Question 4

Define fissionable material

Answer 4

Any material capable of fission

Question 5

What is the primary source of intrinsic source neutrons weeks after a reactor shutdown

Answer 5

1. Transuranic
2. Spontaneous
3. Photo-neutron

Question 6

Define macroscopic cross-section

Answer 6

The target area that is presented by all of the nuclei of a given nuclide contained in 1 cm³ of the material

Question 7

Define intermediate neutrons

Answer 7

A neutron that has energy between a fast neutron and a slow (thermal) neutron

Question 8

On average how much energy is released immediately following fission of U-235

Answer 8

On average 187 MeV out of 200 MeV

Question 9

Define mean free path

Answer 9

• The average distance of travel by a neutron before absorption or scatter.
• The inverse of macroscopic cross-section.

Question 10

Describe which fission product nuclides are most likely to result from fission of U-235

Answer 10

• Cesium-140
• Rubidium-93

Question 11

Define thermal neutron flux

Answer 11

The number of thermal neutrons crossing a unit area in a given amount of time

Question 12

Define atomic density

Answer 12

The number of atoms of a given type per unit volume of material

Question 13

Describe spontaneous fission source neutrons (intrinsic)

Answer 13

• Atoms that spontaneously fission
  
  • U-235
  • U-238
  • Curium-242
  • Curium-244

Question 14

How do changes in neutron flux and macroscopic cross-section affect reaction rates

Answer 14

• As neutron flux or macroscopic cross-section increase, reaction rates increase
• As neutron flux or macroscopic cross-section decrease, reaction rates decrease

Question 15

Define Barn

Answer 15

The unit of measure for cross-section equal to 10^-24 cm²

Question 16

How is heat produced as a result of fission

Answer 16

The kinetic energy of the fission fragments, beta particles, and fission neutrons interacting with their surroundings

Question 17

Describe elastic scattering including conservation principles

Answer 17

Neutron collides with a nucleus and bounces off at a different trajectory. Conservation of momentum determines the resultant velocities.

Question 18

What is the primary source of intrinsic source neutrons at the beginning of core life

Answer 18

1. Photo-neutron
2. Spontaneous fission
3. Transuranic

Question 19

What is the purpose of installed neutron sources

Answer 19

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.

Question 20

Explain the fission process using the liquid drop model of a nucleus

Answer 20

Question 21

Describe the relationship between neutron flux and reactor power

Answer 21

• As neutron flux increases reactor power rises
• As neutron flux decreases reactor power decreases

Question 22

Describe Aplha-neutron source neutrons (intrinsic)

Answer 22

Alpha particles from decay interact with oxygen-18 and boron-11

Question 23

Define neutron flux

Answer 23

The number of neutrons passing through the unit area per unit time

Question 24

Describe the energy released from fission by the conservation of mass-energy method

Answer 24

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.

Question 25

How does the absorption and scattering cross section of typical nuclides vary with neutron energies in the 1/v region and resonance absorption region

Answer 25

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)

Question 26

Define fissile material

Answer 26

A fissionable material that DOES NOT need additional kinetic energy to reach the critical energy for fission to occur

Question 27

Explain the shape of the binding energy per nucleon versus mass number curve

Answer 27

Question 28

Define fast neutrons

Answer 28

Newly ejected neutrons with high energy

Question 29

Describe the energy released from fission by the change in binding energy method

Answer 29

Compare the binding energy of the products to the binding energy of the reactant. The difference in energy is the energy released following fission

Question 30

What is the primary source of intrinsic source neutrons immediately after reactor shutdown

Answer 30

1. Photo-neutron 2. Transuranic 3. Spontaneous fission

Question 31

What is the significance of the binding energy per nucleon vs mass number curve to fission energy

Answer 31

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.

Question 32

Describe Photo-neutron source neutrons (intrinsic)

Answer 32

The interaction of gamma rays and deuterium nucleus

Question 33

Define fast neutron flux

Answer 33

The number of fast neutrons crossing a unit area in a given amount of time

Question 34

Describe radiative capture

Answer 34

Incoming neutron interacts with the target nucleus forming a compound nucleus which then decays to its ground state and decays away

Question 35

Define fertile material

Answer 35

Materials that undergo transmutation to become fissile material

Question 36

Define excitation energy

Answer 36

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.

Question 37

Describe particle ejection

Answer 37

Incoming neutron interacts with the target nucleus forming a compound nucleus which then ejects a particle

Question 38

Describe fission

Answer 38

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

Question 39

Describe inelastic scattering including conservation principles

Answer 39

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.

Question 40

What is the purpose of source neutrons

Answer 40

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.

Question 41

Describe transuranic source neutrons (intrinsic)

Answer 41

Elements with an atomic number greater than uranium (92) that produce neutrons from either spontaneous fission or alpha-neutron reactions.

Question 42

Explain binding energy per nucleon

Answer 42

The binding energy of a nucleus divided by the total number of nucleons (i.e., protons + neutrons) in the nucleus

Question 43

Define thermal neutrons

Answer 43

Very low kinetic energy neutrons which add essentially no kinetic energy to a reaction.

Question 44

On average how much energy is released by delayed fission energy following fission of U-235

Answer 44

On average 13 MeV out of 200 MeV

Question 45

Describe radial and axial neutron flux distribution

Answer 45

* 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.