Nuclear Physics

Question 1

Define binding energy

Answer 1

The ​binding energy​ of a nucleus is the ​energy required to separate the nucleus into its serperate protons and neutrons (or the energy released when a nucleus is formed from its constituents)

Question 2

Define mass difference

Answer 2

The mass difference of a nucleus is the difference between the total mass of the separate nucleons added together compared to the mass of the nucleus when all the nucleons are together in the nucleus

Question 3

What is nuclear fission?

Answer 3

The splitting of a large atomic nucleus into smaller nuclei
Spontaneous fission is very rare

Question 4

Explain the process of induced fission

Answer 4

Fuel= uranium-235
Thermal neutron is absorbed by uranium 235 nuclei, forming uranium 236
Causing the nucleus to split into two daughter nuclei
Releasing several fast-moving neutrons
These collide with other uranium-235 nuclei in the fuel -> chain reaction

Question 5

Define nuclear fusion

Answer 5

Nuclear fusion= The fusing together of two small nuclei to produce a larger nucleus

Question 6

Why must the nuclei involved in nuclear fusion have high kinetic energy?

Answer 6

In order to get close enough to fuse, nuclei must overcome the electrostatic repulsion of the protons in the nuclei
Also strong force, which is needed to fuse together the nuclei, only takes effect at very small distances in the nuclei

Question 7

Explain what is meant by a thermal neutron

Answer 7

Slow moving neutrons, that are in thermal equilibrium with the moderator

Question 8

Define critical mass

Answer 8

The minimum mass of fuel required to establish a self-sustaining chain reaction

Question 9

Explain why a chain reaction can only occur if the mass of uranium is at or above the critical mass

Answer 9

Number of neutrons escaping is proportional to surface area
As mass increases the number of neutrons escaping decreases
As surface area to volume ratio decreases
Hence fraction inducing fission increases as mass increases

Question 10

What is the function of the moderator in a nuclear reactor

Answer 10

The moderator is a material that surrounds the fuel rods and control rods inside the reactor core
The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum
The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’
This ensures neutrons can react efficiently with the uranium fuel

Question 11

What is the function of the fuel control rods in a nuclear reactor

Answer 11

The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods
Lowering the rods further decreases the rate of fission, as more neutrons are absorbed
Raising the rods increases the rate of fission, as fewer neutrons are absorbed
This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission
In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way so no reaction can take place

Question 12

What is the purpose of the coolant in a nuclear reactor

Answer 12

The purpose of coolant: To remove the heat released by the fission reactions

The coolant carries the heat to an external boiler to produce steam
This steam then goes on to power electricity-generating turbines

Question 13

What materials are used for the moderator and why?

Answer 13

Moderators must be made from light nuclei which are not fissionable and will not absorb neutrons but will absorb a large amount of energy from them
Graphite and water are commonly used for moderators

Question 14

What materials are usually used to make the control rods? and why

Answer 14

Control rods must be made with non-fissionable materials
This is so that they can absorb excess neutrons without decaying themselves
Boron and cadmium are commonly used for control rods

Question 15

What materials are usually used for the coolant? and why

Answer 15

Water is commonly used as a coolant. This is because it has a high specific heat capacity meaning it can transfer large amounts of thermal energy
Other materials used are molten salt or inert gases, such as helium

Question 16

Explain the safety features of a nuclear reactor?

Answer 16

Fuel is mainly enriched uranium, containing mining uranium-238, which absorbs the neutrons, reducing the rate of the reaction

Control rods are handled remotely to limit worker’s radiation exposure

Around the nuclear reactor is a thick concrete shielding, which stops radiation escaping

In an emergency, all control rods are dropped, absorbing all free neutrons are preventing further fission from occurring

Question 17

How is nuclear waste handled safely?

Answer 17

1. The waste is removed and handled remotely to limit exposure
2. Any material removed from the reactor is extremely hot, so it is put in cooling ponds for up to a year
3. any plutonium or usable uranium is removed from spent fuel rods in order to be recycled
4. The waste is then ​vitrified​ (encased in glass) and placed in ​thick steel casks​ and stored in ​deep caverns​ in ​geologically stable locations​, so that there is no chance of the waste coming free of its casing. Locations to store radioactive waste are chosen so that they make a ​minimal impact on the environment and people living in the area are consulted about it beforehand​.