8.4 - Nuclear Fusion & Fission

Question 1

What does E=mc² mean?

Answer 1

Mass can be converted into energy and energy into mass.

Question 2

What is the mass defect defined as?

Answer 2

The difference between an atom’s mass and the sum of the masses of its protons and neutrons.

Question 3

How can the mass defect be calculated?

Answer 3

Δm = Zmₚ + (A - Z)mₙ - mₜₒₜₐₗ
Z = proton number
(A - Z) = neutron number
mₜₒₜₐₗ =measured mass of the nucleus

Question 4

Why does the mass defect exist?

Answer 4

Mass is lost through energy (E=mc²). The forces of repulsion between the protons in the nucleus takes energy. Known as BINDING ENERGY. As it holds the nucleons together as a nucleus.

Question 5

What is binding energy defined as?

Answer 5

The amount of energy required to separate a nucleus into its constituent protons and neutrons.

Question 6

What is the atomic mass unit (u) defined as?

Answer 6

The mass of exactly 1/12 of an atom of carbon-12.
1u = 931.5MeV

Question 7

How do you calculate the average binding energy per nucleon?

Answer 7

avg binder energy per nucleon = binding energy / nucleon number

Question 8

When is the average binding energy per nucleon at a maximum?

Answer 8

When the nucleon number = 50

Question 9

Where do the most stable nuclei occur?

Answer 9

Around the maximum point on the graph when A=50 (so iron, as it has a nucleon number of 56).

Question 10

Where does fusion occur on an average binding energy per nucleon by nucleon number graph?

Answer 10

Before iron as fusion is when you combine small nuclei. This increases the average binding energy per nucleon dramatically, which means a lot of energy is released during nuclear fusion.

Question 11

Where does fission occur on an average binding energy per nucleon by nucleon number graph?

Answer 11

After iron as its the splitting of large nuclei. The 2 new nuclei numbers are smaller so there is an increase in the average binding energy per nucleon. Energy is released during fission. (not as much as fusion tho)

Question 12

What is nuclear fusion defined as?

Answer 12

The fusing together of 2 small nuclei to produce a larger nucleus.

Question 13

Why do nuclei need lots of energy to fuse?

Answer 13

• All nuclei are positively charged - so there will be an electrostatic repulsion between them.
• Nuclei can only fuse if they overcome this force and get close enough for the attractive force of the strong interaction to hold the, together.

Question 14

How much energy is needed to fuse nuclei?

Answer 14

1MeV of Ek needed. (a LOT of energy)

Question 15

What is nuclear fission defined as?

Answer 15

The splitting of a large atomic nucleus into smaller nuclei.

Question 16

How is fission induced?

Answer 16

By firing neutrons at a nucleus.

Question 17

What is the binding energy equal to in fission?

Answer 17

The amount of energy released in forming the nucleus (calculated using E=mc²)

Question 18

What is the process of fission?

Answer 18

• neutrons are fired at a nucleus and the nucleus splits into 2 or more daughter nuclei.
• during fission, neutrons are ejected from the nucleus, which in turn, can collide with other nuclei which triggers a cascade effect.
• This leads to a chain reaction which lasts until all of the material has undergone fission, or the reaction is halted by a moderator

Question 19

Why does fusion occur at low values of A?

Answer 19

Attractive nuclear force between nucleons&raquo_space; repulsive electrostatic forces between protons

Question 20

Why does fission occur at high values of A?

Answer 20

Repulsive electrostatic forces between protons begin to dominate which can often break apart the nucleus rather than hold it together.

Question 21

Where is helium on the average binder energy per nucleon by nucleon number curve?

Answer 21

It is much higher up on than expected and is just above the line of best fit.

Question 22

When does induced nuclear fission occur?

Answer 22

When a stable nucleus splits into small nuclei due to the absorption of a slow-moving neutron

Question 23

How does induced fission occur with uranium-235?

Answer 23

A single neutron is fired at a uranium-235 atom which creates a uranium-236 atom which immediately decays into 2 daughter nuclei and 3 neutrons.

Question 24

What are thermal neutrons?

Answer 24

The neutrons involved in induced fission.
They have low kinetic energy and are slow-moving.

Question 25

Why are neutrons fired slowly at uranium-235 atoms?

Answer 25

They would not be absorbed by the atom if it was fast-moving. The neutron would rebound off the atom.

Question 26

What are thermal neutrons defined as?

Answer 26

Neutrons which are in thermal equilibrium with their surroundings. (ie in nuclear reactors neutrons are slowed down until they’re in thermal equilibrium with the moderator.)

Question 27

How are thermal neutrons kinetic energies connected with their temperature? (equation from thermal physics)

Answer 27

E = 3/2kT
k = boltzmann constant
T = thermodynamic temperature

Question 28

What is the critical mass defined as?

Answer 28

The minimum mass of fuel required to maintain a steady chain reaction.

Question 29

What are the 2 factors that need to be controlled when a nuclear reactor is producing energy at the required rate?

Answer 29

• The number of free neutrons in the reactor
• The energy of the free neutrons

Question 30

What are the 3 main components of a nuclear reactor?

Answer 30

• A moderator
• Control rods
• Coolant

Question 31

What is the purpose of the moderator?

Answer 31

To slow down the neutrons.
(fast moving = won’t be absorbed)

Question 32

What are moderators made out of and why?

Answer 32

Made of materials such as water and graphite as they are poor absorbers of neutrons.

Question 33

How does the moderator slow down the fast-moving neutrons?

Answer 33

The neutrons collide with the molecules of the moderator, causing them to lose some momentum.
They are slowed until they’re in thermal equilibrium with the moderator.

Question 34

What is the purpose of control rods?

Answer 34

To absorb neutrons.

Question 35

What are control rods made of?

Answer 35

A material which absorbs neutrons without becoming dangerously unstable themselves.
(commonly boron and cadmium.)

Question 36

How is the number of neutrons absorbed controlled?

Answer 36

By varying the depth of the control rods in the fuel rods.
- lowering = decreases rate of fission as more neutrons are absorbed
- raising - increasing rate of fission, fewer neutrons absorbed

Question 37

What do the control rods do if a nuclear reactor needs to be shut down?

Answer 37

The control rods are lowered all the way so no reaction can take place.

Question 38

What is the purpose of coolant?

Answer 38

To transfer thermal energy efficiently between the water systems of a nuclear power plant. - remove heat

Question 39

What are the water systems in a nuclear power plant regulated by?

Answer 39

The heat exchanger.

Question 39

What is the nuclear reactor surrounded by?

Answer 39

A thick concrete (or lead) case, which acts as shielding, preventing any radiation from escaping.

Question 39

Where does the heat produced by the reactor go?

Answer 39

Used to make steam for powering electricity-generating turbines.

Question 40

Why are the waste products of nuclear power stations so harmful?

Answer 40

Uranium has a very long half-life (billions of years) and therefore produces harmful radiation well above background radiation levels for a long time.

Question 40

How is nuclear waste stored?

Answer 40

In water tanks or sealed underground. (prevents damage to people and environment.

Question 41

What is the fuel used in nuclear reactors?

Answer 41

U-238 enriched with U-235 as U-235 is the isotope that undergoes fission.
U-238 isotope absorbs fission neutrons which helps to control the rate of fission reactions.

Question 42

What are 3 things that are done to reduce power plant worker’s exposure to radiation?

Answer 42

• fuel rods handled remotely (machines)
• the shielding around the nuclear reactor
• the emergency shut-down - when fuel rods and fully lowered.

Question 43

What is low level nuclear waste and where is it stored?

Answer 43

Clothing, gloves and tools which may be lightly contaminated.
Only radioactive for a few years. Encased in concrete and stored only a few metres underground.

Question 44

What is intermediate-level waste and where is it stored?

Answer 44

• daily used items and the fuel rods themselves.
• longer half-life than low-level waste. Encased in cement in steel drums and stored underground

Question 45

What is high-level waste?

Answer 45

• unusable fission products (from fission of uranium-235 or spent fuel rods)
• very dangerous, radioactive for 1000s of years

Question 46

How is high-level waste treated?

Answer 46

• sent to cooling ponds (still extremely hot).
• isotopes of plutonium and uranium harvested to be used again
• waste is mixed with molten glass and made solid - vitrification
• encased in containers made of steel, lead or concrete
• stored VERY deep underground

Question 47

What are 3 benefits of nuclear energy?

Answer 47

• Power stations produce no polluting gases
• highly reliable
• Require far less fuel as uranium provides far more energy per kg compared to fossil fuels.

Question 48

What are 2 risks of nuclear fuels?

Answer 48

• the production of radioactive waste is very dangerous and expensive
• nuclear meltdown is v bad.