Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

Applied Nuclear Physics > Week 9: Nuclear fusion > Flashcards

Week 9: Nuclear fusion Flashcards

1
Q

What are the advantages of fusion over fission?

A

Less radioactive waste.

Shorter half life of waste so no need to store for geological periods.

Safer due to lack of risk of runaway criticality, core meltdown, release of large amounts of radioactivity.

Plentiful fuel.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What amount of energy is released in a fusion reaction?

A

~ 1 MeV per nucleon

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Where does the energy come from in fusion?

A

The binding energy of He-4 (and He-3) compared to the hydrogen isotopes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the temperature requirement for fusion?

A

Need sufficiently high temperatures that the ions can reliably breach the Coulomb barrier (which is at a minimum when hydrogen isotopes are used) to allow the nuclear reactions to take place.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are individual deuterium nuclei referred to as?

A

Deuterons, d.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What are individual tritium nuclei referred to as?

A

Tritons, t.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the most achievable fusion option?

A

D-T fusion.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is D-T fusion?

A

A tritium breeding programme using the neutrons from the fusion to cause:

n + 6Li —> t + alpha

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the key challenge with D-T fusion?

A

Each fusion reaction consumes one triton and produces one neutron, and as it is impossible to guarantee every neutron will produce a new triton some neutron multiplication will be needed.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How can the ions in a plasma be described?

A

Maxwell Boltzmann distribution.

(37)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the most probable speed of an ion in a plasma?

A

The most probable speed occurs at a kinetic energy of kT, which at room temperature is 0.025 eV.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What temperature is needed for thermonuclear fusion?

A

Thermonuclear fusion needs a temperature of approximately 10^8 K to occur at a rate that is practical for power production.

This corresponds to kT ~ 10 keV.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

For a mixture of two gases with n1 and n2 atoms per unit volume:

What is the probability of interaction between an atom of gas 1 and gas 2?

A

(38)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

For a mixture of two gases with n1 and n2 atoms per unit volume:

What is the reaction rate?

A

(39)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the reaction rate density for all the atoms of gas 1?

What assumption is made here?

A

(40)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

How can our model of reaction rate density for fusion be improved?

A

We can make our model more realistic by using an average value of the product of the speed and the cross section.

(41)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What condition must be met for a plasma to be sustained?

A

Must be enough heat power provided to make up for the losses.

Pheat = Ploss

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is the power generated from fusion labelled as?

A

Pfus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is the fusion energy gin factor?

A

The ratio of the fusion power produced to the power needed to sustain the plasma.

Qfus = Pfus / Pheat

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is the breakeven point?

A

The point at which the fusion power generated is equal to the heating power needed to maintain the plasma.

Pfus = Pheat —> Qfus = 1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is ignition in fusion?

A

A step beyond breakeven.

At this point the energy deposited in the plasma by the fusion reactions is equal to the energy losses.

No external energy needs to be supplied to maintain the plasma at this point.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Why must a D-T plasma produce five times the energy needed to sustain itself before reaching ignition?

A

The majority of the energy is carried by the lighter neutron rather than the heavier alpha particle.

Pfus = 5Pheat —> Qfus = 5

23
Q

What is the Lawson criterion?

A

To make useful fusion energy requires the energy output to be greater than the energy needed to create the plasma.

Efus > Eplasma

24
Q

What is the equation for total energy output by a sustained series of fusion reactions over a confinement time?

25
What is the energy required to create a plasma?
If the plasma has an equal number of ions and electrons in thermal equilibrium, each with an average energy of 1/2 kT, then the energy required to create the plasma is, Eplasma = 3nkT, where n is the number of ions (or electrons).
26
What is the triple product?
n * confinement time * T
27
What are the two main methods for containing plasmas?
Magnetic confinement Inertial confinement
28
How does magnetic confinement of a plasma work?
A low pressure plasma is confined in a toroidal chamber using magnetic fields, the combination of which (on average) keeps the plasma in the centre of the toroid.
29
Give an example of where magnetic confinement is used?
Tokomaks, which is the Russian acronym for "toroidal chamber with magnetic coils".
30
Why is a magnetic field used to confine a plasma?
Without confinement, the ions and electrons of the plasma will move in all direction, hitting the wall of the chamber and plasma will be lost as the energy is discharged. A magnetic field is used to apply a force to the charged particles.
31
What is the problem with toroidal geometry for magnetic confinement?
Using a toroidal geometry with a toroidal magnetic field means that the particles can travel around the loop forever without hitting the wall. The problem is that the toroidal field is not uniform and gets weaker towards the edges of the chamber. This means that the plasma will not be properly confined.
32
How is a poloidal magnetic used to solve the problem with the toroidal magnetic field in magnetic confinement of a plasma?
Adding a poloidal magnetic field twists the toroidal field making the particles move in helical paths around the axis of the torus.
33
How is a poloidal field created in a tokomak?
The poloidal field is created by the current induced in the plasma, which acts as the secondary coil of a transformer where the primary coil is the inner poloidal field coils.
33
How is a plasma heated?
The current induced in the plasma provides several megawatts of resistive heating. Additional radio-frequency heating is provided at the electron or ion cyclotron frequencies to give resonant energy absorption. Neutral beam injection (NBI) is also used to add energy (and fuel).
34
How does plasma lose energy?
The plasma will lose energy by particles escaping and the emission of photons.
35
What is plasma energy loss proportional to?
sqrt(T)
36
What is a disruption in a plasma?
A sudden loss of plasma, which frequently goes out with a bang.
37
Why do disruptions occur in a plasma?
The plasma collapses due to loss of energy into surrounding materials such as the wall or diverter.
38
Why are disruptions in a plasma a problem?
The shock of a disruption can easily cause damage to water and gas feed-throughs, vacuum seals, etc, so they are to be avoided.
39
Why was the idea of inertial confinement of a plasma developed?
ICF was developed in response to the difficulties in achieving stable plasma confinement in a magnetic field.
40
What is the basic approach to inertial confinement?
Using lasers to heat an outer layer of material that confines and compresses fuel to the point at which fusion occurs.
41
How does inertial confinement work (in detail)?
Laser beams rapidly heat the inside surface of the hohiraum. X-rays from the hohiraum create a rocket-like blow off of capsule surface, compressing the inter-fuel portion of the capsule. During the final part of the implosion, the fuel core reaches 100 times the density of lead and ignites at 1 million degrees C. Thermonuclear burn spreads rapidly though the compressed fuel, yielding many times the input energy.
42
What are the fuel pellets used in ICF?
Hollow spheres of frozen deuterium and tritium, surrounded by an ablator surface.
43
Why is power production not continuous in ICF?
Each "shot" requires a new pellet, so power production is not continuous.
44
State one advantage and one disadvantage of fusion compared to fission for power production.
Advantage: Less waste Shorter waste half-life Safer Plentiful fuel Disadvantage: Much harder to achieve
45
List three possible outcomes of D-D fusion.
4He + gamma 3He + n t + p
46
State the reaction used to produce tritium for D-T fusion fuel.
n + 6Li ---> t + alpha
47
State the equation for the Lawson criterion in a D-T plasma.
(43)
48
What does break even mean?
The point at which the fusion power generated is equal to the heating power needed to maintain the plasma.
49
What does ignition mean?
The energy deposited in the plasma from fusion processes is equal to heating power needed to maintain the plasma.
50
Sketch a torus and label the toroidal and poloidal directions.
(44)
51
List three methods to heat the plasma in a tokomak.
Resistive heating Radio-frequency heating Natural beam injection
52
Briefly explain how X-rays are generated to heat the fuel in indirect drive inertial confinement fusion.
Lasers are used to heat a heavy metal cylinder called the hohlraum, causing the emission of X-rays that heat the outer surface of the fuel.

Applied Nuclear Physics (12 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions