Lecture 3: Basic Reactor Physics

Question 1

How is neutron flux calculated?

Answer 1

Neutron number density x Neutron speed

Question 2

How does neutron current differ from neutron flux?

Answer 2

Neutron current takes the direction of neutrons into account (+ or - matters)

Question 3

Define source rate?

Answer 3

The rate of production of neutrons per unit volume

Question 4

Why is moderation so important?

Answer 4

Probability of successful fission of fuel depends on the neutron energy

Question 5

What is the average lifetime of a neutron?

Answer 5

10^-3 seconds

Question 6

What is average number of neutrons produced per reaction?

Answer 6

2.59

Question 7

What is the typical uranium contents of fuel?

Answer 7

96% U238
4% U235

Question 8

Define criticality:

Answer 8

neutrons in = # neutrons out

Steady state. Reactivity = 0.

Question 9

How is the rate of neutron number density calculated?

Answer 9

= Production rate - Absorption rate - Rate of flow of neutrons away from location

Question 10

Define the diffusion approximation

Answer 10

Neutrons tend to drift from regions of high flux to low flux at a rate proportional to the flux gradient.

Question 11

Where is the power distribution typically maximum?

Answer 11

At centre of vessel

Question 12

How is the spatial distribution of thermal power and neutron flux related?

Answer 12

Lots of neutrons
Lots of fission
Lots of fragments colliding with lattice
Location of greatest energy generation

Question 13

Define the multiplication factor?

Answer 13

Ratio of:
Rate of neutron production this generation
Rate of neutron loss this generation

Question 14

What symbol is used to define multiplication factor?

Answer 14

k

Question 15

What are the three multiplication factor states?

Answer 15

k < 1: Subcritical (ramp-down)
k = 1: Critical (steady-state)
k > 1: Supercritical (ramp-up)

Question 16

What is k_infinity?

Answer 16

A material property to describe the multiplication factor if a reactor were infinitely large, therefore no leakage.

Question 17

How to calculate k_infinity?

Answer 17

Ratio of:
Rate of neutron production this generation
Rate of neutron absorption this generation

Question 18

What is a prompt neutron?

Answer 18

Neutrons produced instantaneously at the time of fission

Question 19

What are delayed neutrons?

Answer 19

Neutrons emitted after a delay as part of the decay process for fission fragments

Question 20

What are the approximate timings for a prompt and delayed neutron?

Answer 20

Prompt: 10^-4 s
Delayed: 0.1 s

Question 21

Name two prompt feedback mechanisms:

Answer 21

Resonance absorption and doppler broadening
Fuel expansion

Question 22

Name a delayed feedback mechanism:

Answer 22

Moderator heating

Question 23

What is the importance of reactivity feedback mechanisms?

Answer 23

Required when neutron production is too high and must be reduced.

Question 24

How does resonance absorption and doppler broadening effectively slow neutron production?

Answer 24

Increase the temperature of U238, thus improving effectiveness of resonances in capturing neutrons.

Question 25

How does fuel expansion effectively slow neutron production?

Answer 25

Expanding the fuel reduces the number density reducing fission probability

Question 26

How does moderator heating effectively slow neutron production?

Answer 26

As moderator temp is increased, its number density increases, reducing its ability to moderate so doesn’t slow down neutrons as well and reduces fission.