Nuclear Physics 2

Question 1

How do you get the mass of a nucleus in this topic

Answer 1

Deal with protons and neutrons separately

Use the values to the full 4sf

Question 2

What is mass defect

Answer 2

The mass of the nucleus is less than the mass of its constituents for all nuclei regardless of being ions or isotopes
Given in kg and usually very very small

Question 3

Binding energy

Answer 3

Energy released when nucleons come together to form the nucleus (loss of mass means loss of energy)

Energy needed to split an atom into its constituent parts (nucleons) (increase in mass requires an input in energy)

Question 4

Where has the mass gone when the nucleus is formed in nuclear fusion

Answer 4

Released as energy

Question 5

How do you calculate the binding energy of an atom

Answer 5

Need to know the mass defect, found using the mass of the nucleons minus the mass of the nucleus

E=mc^2

E=Binding energy in Joules
m=Mass defect/difference in kg
c=Speed of light in a vacuum in m/s

Question 6

Why is it sometimes more useful to use alternative units to SI units such as MeV instead of J

Answer 6

Dealing with very small number and standard form

Question 7

1 atomic mass unit

Answer 7

The mass of 1/12 of a carbon 12 atom

1u=1.661x10^-27kg (formula sheet front page)

Question 8

1u in MeV, mass of a proton and mass of a neutron in u

Answer 8

ALL ON THE FORMULA SHEET FRONT PAGE

Question 9

How do you find binding energy when given in SI units

Answer 9

Find the mass defect in kg

Use E=mc^2 to find the binding energy in Joules

Question 10

How do you find binding energy when given in atomic mass units

Answer 10

Find the mass defect in u and then multiply by 931.3Mev to get binding energy in MeV

Question 11

Smaller nucleus binding energy per nucleon vs total binding energy compared to a larger nucleus

Answer 11

Smaller means a greater binding energy per nucleon but a smaller total binding energy

Question 12

Greater binding energy per nucleon means …. stable because…

Answer 12

More stable
Releases more energy when it is being formed
So requires more energy to separate the particles within the nucleus

Question 13

What is the peak of the average binding energy per nucleon against the number of nucleons in the nucleus

Answer 13

Iron 56

Question 14

How does the average binding energy per nucleon against number of nucleons in the nucleus explain why fission and fusion work

Answer 14

Fusion; joining lighter nuclei to make heavier nuclei releases energy
Fission; splitting heavier nuclei into lighter nuclei releases energy

Fusion releases more energy than fission due to a steeper gradient

Question 15

Explain nuclear energy states

Answer 15

Nucleus has an excited state which is unstable
Emitting one or more gamma photons after alpha or beta decay can lose the excess energy and return the nucleus to its ground state

Question 16

Nuclear fission

Answer 16

A heavy nucleus splitting into two lighter and more stable nuclei with the release of energy and neutrons

Question 17

Outline nuclear fission

Answer 17

Bombard heavy nucleus with neutrons
Absorbs neutrons
Splits into two light nuclei
Releasing two or three neutrons

Question 18

In terms of the average binding energy per nucleon curve against time, what is the effect of nuclear fission

Answer 18

End products have a larger binding energy per nucleon than the starting nucleus
Which occurs due to them releasing energy
Because mass is lost
And this mass lost is converted into energy

Question 19

Do you use mass difference or mass defect for nuclear fission calculations

Answer 19

Mass difference

Since not looking at a single nucleus

Question 20

mass on the left>mass on the right

Answer 20

Energy released

Condition for interaction to release energy

Question 21

What forms can energy be released in in nuclear fission

Answer 21

Kinetic energy of the two lighter daughter nuclei
Kinetic energy of the neutrons
Gamma rays given off at the time of the reaction
Energy carries away by neutrinos
Energy subsequently released by alpha or beta decay of the daughter nuclei since there is no guarantee they will be stable

Question 22

Explain the containment structure for harnessing the energy released in a fission reactor

Answer 22

1. Fuel rods made of uranium, which contains 0.7% U235 naturally, but enriched to 5%
2. Fission reaction occurs in the rods, releasing energy and increasing the temperature of rods
3. Neutrons released each fission go on to create further fissions, sustaining the reaction
4. Coolant pumped around core which transfers energy from fuel rods to pipes containing water
5. Water is heated by the coolant
6. Turns to steam
7. Steam drives a turbine which is connected to a generator
8. Generator creates an alternating current

Question 23

Purpose of coolant in nuclear fission reactors

Answer 23

In the centre of the reactor
Transfers heat from fuel rods to the pipes containing water that passes into reactor vessels
Passes through heat exchangers to maximise the efficiency of heat transfer to the water in order to make sure it boils and turns into steam before reaching the turbines

Question 24

Suitable coolant for nuclear fission reactors

Answer 24

Carbon dioxide gas

Question 25

Purpose of control rods in nuclear fission reactors

Answer 25

Affects the number of neutrons released
2.4 average neutrons released each fission
To maintain a steady rate 1.4 must be absorbed
Made of a material that absorbs neutrons
Putting control rods further into the reactor decreases the rate of reaction
Controlled by a computer to ensure the rate remains at a constant level

Question 26

Control rod functions in emergencies

Answer 26

Inserted all the way in to stop the rate of reaction

Question 27

Why are control rods necessary

Answer 27

If left alone a nuclear fission reaction will increase the number of fissions occurring per second since more neutrons released than used
Hence the amount of energy released increases
Dangerous because as temperature increases pressure will keep increasing and could lead to a meltdown

Question 28

Critical mass

Answer 28

When one fission creates one further fission

Question 29

Purpose of the moderator in a nuclear fission reactor

Answer 29

Slows the neutrons down
Neutrons released are travelling with a very high energy and travelling too fast to be absorbed by another uranium nucleus to start another fission reaction
Each fuel rod is inserted into a material designed to reduce the energy of the neutrons without stopping them completely
Must be a material neutrons can pass though whilst undergoing collisions in the nuclei in the moderator
Each collision reduces the energy of the neutrons, slowing them down

Question 30

Suitable material for a moderator

Answer 30

Water
Heavy water
Graphite

Question 31

What are thermal neutrons

Answer 31

Neutrons that have been slowed to sufficient speeds

To be absorbed by a uranium nucleus

Question 32

3 things safety features of nuclear fission reactions protect

Answer 32

Workforce
Wider community
Environment

Question 33

Safety features of a nuclear fission reactor

Answer 33

Reactor core is a thick steel vessel designed to withstand high pressure and temp in the core. Absorbs beta radiation and some of the gamma radiation and neutrons from the core

Core in a building with very thick concrete walls which absorb the neutrons and gamma radiation that escaped from the reactor vessel

Every reactor has emergency shut down system designed to insert control rods fully into the core to totally stop fission then flood the system to remove the heat

Sealed fuel rods are inserted and removed from reactor by means of remote handling devices

Question 34

Why are fuel rods more dangerous after than before

Answer 34

Before they only emit alpha radiation which can be absorbed
After they emit beta and gamma radiation due to the many neutron rich fission products that form
After contain plutonium isotope 239Pu which is a very active alpha emitter and can cause lung cancer if inhaled

Question 35

Why is nuclear waste so dangerous

Answer 35

If in an excited state then they may emit gamma rays
If they are still unstable they may emit further beta particles
Beta and gamma are more penetrating so need to be carefully contained

Question 36

How must the spent fuel rods be removed and stored

Answer 36

By remote control
Stored underwater in cooling ponds to remove excess thermal energy for up to a year since they continue to release heat and radiation

Question 37

What happens to the remaining plutonium and uranium

Answer 37

Separated to be recycled

Question 38

What happens to radioactive waste

Answer 38

Stored in sealed containers in deep trenches
High level waste may be made into solid pyrex glass (vitrification) then placed in stainless steel or lead barrels or concrete cylinders and stored deep underground
Must be stored safely for centuries since it contains long-lived radioactive isotopes which must be prevented from contaminating food and water supplies’

Question 39

Issue of transporting waste

Answer 39

Provides a potential danger to the public

So transported enclosed in crash resistant, extra thick and strong casing or processed onsite nearby

Question 40

Why is someone living near a nuclear reactor more of a danger to other people than someone living next to an X-ray

Answer 40

Nuclear reactor emits fast moving neutrons which the body cells absorb
Creating unstable isotope cells
Therefore emit alpha, beta and gamma radiation which can damage others

X-rays emit alpha and gamma and beta radiation which strip cells of electrons
Meaning you don’t emit radiation
So can’t damage others

Question 41

Nuclear fusion

Answer 41

Two light nuclei combining to form one heavier nucleus with the release of energy

Question 42

What reaction produces the biggest possible increase in binding energy

Answer 42

Deuterium + deuterium into a helium nucleus

Question 43

Explain the outcome of nuclear fusion in terms of the average binding energy per nucleon against number of nucleons graph

Answer 43

Heavy nucleus after the reaction has a greater binding energy per nucleon therefore more stable
So must be a loss of mass
Heavy nucleus at the end has a lower mass than the sum of the masses of the lighter products at the beginning of the reaction

Question 44

How does fusion work

Answer 44

For two nuclei to fuse they have to get close enough to each other to virtually come into contact (within 1fm)
So the strong nuclear force can act to hold the nuclei together
To get just the nuclei on their own they have to be isolated to form plasma
All nuclei have a positive charge so repel each other
So must have a high kinetic energy to overcome the repulsion
So plasma must be at a high temperature and pressure
Which is hard to achieve
At the moment more energy is put into the system to achieve these conditions than is gained from the fusion
Much easier to achieve in the centre of stars due to a high pressure and temperature caused by the large mass of the start

Question 45

Explain why the mass of all nuclei is less than the mass of its constituent parts in terms of gluons

Answer 45

Total energy of hadron = quarks plus gluons

When brought together, they share gluons, meaning they share energy and so the energy decreases and hence mass decreases