Chapter 26 - Nuclear Physics

Question 1

State the 2 interpretations of the E =mc^2 equation

Answer 1

1. Mass is a form of energy.

2. And that energy has a mass. A change in energy must lead to a direct change in mass.

Question 2

Define mass defect

Answer 2

The difference between the mass of the completely separated nucleons and the mass of the nucleus.

Question 3

Define binding energy

Answer 3

This is the minimum energy required to completely separate a nucleus into its constituent protons and neutrons.

Question 4

What is binding energy a measure of

Answer 4

The stability of a nucleus

Question 5

How to identify a more stable system

Answer 5

It has a higher binding energy per nucleon.

The more energy required to break up a bond; the stronger that bond is due to stability.

Question 6

One very important thing to note

Answer 6

Stability is based on the

Binding energy PER NUCLEON.

Question 7

What does a graph of BE per nucleon against A tell us

Answer 7

All the atoms before iron causes fusion.

All after Iron causes fission.

Question 8

What is the most stable isotope

Answer 8

Iron- it has the highest BE per nucleon.

This is why it is often the most inner core of a star.

Question 9

Define a thermal neutron

Answer 9

They have been slowed down and their mean kinetic energy is similar to the thermal energy of particles in the reactor core.

Question 10

Nuclear fission with uranium

Answer 10

They shoot a neutron into an atom of Uranium-235, turning into a U-236 which then breaks down into Barium, Krypton and 3 neutrons.

Question 11

Define the induced fission

Answer 11

The process of bombarding stable nuclei with neutrons and making them unstable and fissionable

Question 12

What do the fuel rods do

Answer 12

They are enriched with uranium.

Question 13

What does the moderator do

Answer 13

It is there to slow down the fast neutrons produced.
Fast neutrons have a low chance on success, but as they collide with protons in the water, they slow down and transfer a significant amount of kinetic energy.

Question 14

What do the control rods do

Answer 14

They absorb the neutrons to slow down or stop the fission. Commonly made up of boron or cadmium. If they are all the way in, then they can completely stop the fission from happening.

Question 15

What is the uranium ore made up of

Answer 15

99% is non-fissionable U-238. It can absorb a neutron to become radioactive U-239.
1% is fissionable U-235. It can absorb a neutron and become fissile U-236.

Question 16

Define fissile

Answer 16

It just means that the material is able to undergo nuclear fission

Question 17

How are radioactive isotopes stored

Answer 17

Buried deep underground, in locations that are geologically stable and secure from attack.

Question 18

Define a fission neutron

Answer 18

Neutrons released during a fission event, and are capable of inducing further fission reaction.

Question 19

Define an uncontrolled chain reaction

Answer 19

All fission neutrons produce further fission events, giving rise to energy released at an exponential rate.

Question 20

Define a controlled chain reaction

Answer 20

Only 1 fission neutron produces another further fission event, giving rise to a constant rate of energy production

Question 21

What happens in a nuclear power plant?

Answer 21

They produce a CONSTANT power output.

Question 22

What does fusion do

Answer 22

It binds 2 or more atoms together under extremely high pressures and temperatures together.

Question 23

Proton-proton cycle

Answer 23

1. 2 protons fuse together - producing a deuterium nuclear, an electron and a neutrino.
   (this produces 2.2MeV of energy)
2. The deuterium nucleus fuses with a proton and a 3 2 He is produced. This releases 5.5MeV of energy.
3. The 3 2 He combines with another 3 2 He, producing a 4 2 He + 2 protons.

This cycle repeats.

Question 24

Temperature of the Sun

Answer 24

1.4 x 10 ^7 K.

Question 25

Density of the Sun

Answer 25

1.5 X 10 ^5 kgm -3.

Question 26

Define Nuclear fusion

Answer 26

The process by which 2 light nuclei combine to produce a larger nucleus.

Question 27

What happens to the BE/ nucleon in nuclear fusion

Answer 27

The binding energy per nucleon increases, the nucleons become tightly bound and it is more stable now.

Question 28

Does fusion release more energy than fission

Answer 28

Yes, 4-8 times more.

Question 29

Why can only lighter nuclei undergo fusion

Answer 29

Because their nuclei’s positive charges are a lot smaller, so you have to do less work against the electrostatic forces of repulsion to fuse them together.
By fusing them together, the mass increases, meaning that the binding energy per nucleon also increases.