Physics - Nuclear Fission

Question 1

Define isotope

Answer 1

Elements with the same number of protons, but a different number of neutrons

Question 2

List isotopes of uranium from most abundant to least abundant giving percentages

Answer 2

Uranium-238 (99.2745%)
Uranium-235 (0.720%)
Uranium-234 (0.0055%)

Question 3

How can different isotopes be separated? (6 methods)

Answer 3

Centrifuge – heavier elements found at the edge and lighter in the middle

Diffusion – lighter isotopes travel faster and diffuse more readily

Mass spectrometry – heavier elements not bent as much

Laser – laser frequency tuned to ionise one isotope, but not the other

Chemical reaction – although isotopes have similar properties they can sometimes be separated due to the slight differences

Gravity – heavier isotopes of liquid carbon, oxygen and nitrogen sink to bottom

Question 4

What is meant by a stable nuclei?

Answer 4

One that does not undergo radioactive decay

Question 5

What is meant by ‘more stable’ nuclei?

Answer 5

Has a longer half-life

Question 6

Describe nuclear fission

Answer 6

A neutron is absorbed by a heavy parent nuclei. This causes it to become unstable and split into two daughter nuclei and other neutrons

Question 7

How does U-235 undergo nuclear fission?

Answer 7

U-235 absorbs a neutron to become U-236. This is unstable and splits into two lighter daughter nuclei and several neutrons

Question 8

Why does nuclear fission of U-235 produce energy and in what form does this energy come?

Answer 8

The mass of U-235 and the addition neutron is greater than the combined mass of the daughter nuclei and the several neutrons produced.
This extra mass is converted into energy in the form of KE of mainly the neutrons

Question 9

What do the terms in this symbol represent? (_Z^A)X

Answer 9

A=Mass Number
Z=Proton Number
X=Element symbol

Question 10

What rule is used to balance a nuclear equation?

Answer 10

Sum of the mass number and atomic number to the left of the arrow must equal that to the right of the arrow

Question 11

Explain how one fission reaction can lead to a chain reaction

Answer 11

During the process of fission several neutrons are released.
These neutrons can go on to cause subsequent fission reactions.
The process is then repeated

Question 12

Use inequalities to show how the number of neutrons released and also absorbed can lead to a nuclear bomb or a nuclear reactor

Answer 12

If number produced is <=1 then a nuclear reactor forms

If number produced is >1 then a nuclear bomb can be made

Question 13

What form does the energy in a nuclear reactor come in?

Answer 13

KE of daughter nuclei and neutrons

Question 14

What can the energy generated in a nuclear reactor be used for?

Answer 14

This can be used to heat water and generate steam to turn turbines and generate electricity

Question 15

Sketch a simple diagram of a nuclear reactor

Answer 15

(See diagram with: uranium fuel rods, boron control rods, core, concrete shield, graphite moderator, cold water)

Question 16

Describe the function of the control rods

Answer 16

Absorb excess neutrons

Question 17

Describe the function of the moderator

Answer 17

Slows down neutrons so they become thermal neutrons and are more easily absorbed by U-235

Question 18

What safety mechanisms are in place?

Answer 18

Control rods can be dropped to reduce number of neutrons

Concrete shield to absorb neutrons and other radiation

Question 19

How can a nuclear bomb be created?

Answer 19

Pure fission:
gun assembly - fires one piece of uranium into another therefore reaching critical mass;
implosion – fissile mass surrounded by explosives that compress the fissile material so they reach critical mass

Boosted fission weapon:
fission creates high temperature that fuses hydrogen together to form helium and several neutrons. Fusion creates little energy, but the additional neutrons speeds up the fission reaction

Pure fusion weapons: Not created yet

Question 20

How much energy does 1kg of nuclear fuel and fossil fuel produce?

Answer 20

Coal: 8kWh
Oil: 12 kWh
Nuclear: 24,000,000 kWh

Question 21

Describe the results of the alpha particle scattering experiment (Rutherford Experiment) and explain how it gave us the present structure of the atom

Answer 21

Most alpha particles went straight through the gold leaf – atoms are mostly empty space
Small number where deflected by large angles – small very dense nucleus with positive charge

Question 22

Sketch the structure of the atom and label the protons, neutrons and electrons

Answer 22

(Find a diagram)

Question 23

Define mass number/atomic mass number/nucleon number (A)

Answer 23

Number of protons and neutrons (nucleons) in an atom

Question 24

Define atomic number/proton number (Z)

Answer 24

Number of protons in a nucleus

Question 25

Define relative atomic mass

Answer 25

The ratio of the average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12

Question 26

Describe how electrons are held in the atom

Answer 26

Electrostatic force between the protons and electrons

Question 27

Describe how protons and neutrons are held in the nucleus

Answer 27

Strong nuclear force between protons and neutrons

Question 28

Explain why it takes energy to separate the protons and neutrons in the nucleus

Answer 28

You need to overcome the strong nuclear force

Question 29

Write down Einstein’s equation for energy*

Answer 29

E=mc^2

Question 30

By comparing the ranges of different forces, explain why large nuclei are radioactive

Answer 30

Strong nuclear force is short range, while electrostatic is long range. In large nuclei the electrostatic force builds as more protons are added and outer nucleons are repelled - and the strong nuclear force is less effective at keeping them together

Question 31

What is Alpha radiation? What is its charge/mass? How penetrating is it? How ionising is it?

Answer 31

Helium nucleus (2p+2n), atomic mass 4u, charge +2e, short range (10cm air and paper) and highly ionising

Question 32

What is Beta radiation? What is its charge/mass? How penetrating is it? How ionising is it?

Answer 32

Electron or positron (e­- or e+), atomic mass u/2000, charge -1e, medium range (few mm aluminium) and medium ionising

Question 33

What is Gamma radiation? What is its charge/mass? How penetrating is it? How ionising is it?

Answer 33

EM radiation, mass 0, charge 0, long range (few cm lead) and weakly ionising

Question 34

Explain why the different types of radiation are emitted?

Answer 34

Alpha – heavy nucleus Beta minus – too many neutrons (neutrons are unstable), so neutron converted into a proton and electron and electron is lost Beta plus* – too many protons (need to reduce electrostatic repulsion), so proton converted into a neutron and positron and positron is lost Gamma – nucleus is just unstable

Question 35

Define activity

Answer 35

The number of nuclear decays per second and measured in Becquerel (Bq)

Question 36

Define half-life

Answer 36

Time taken for half the nuclei to decay or for the activity to drop by a half

Question 37

Sketch the graph of binding energy per nucleon vs mass number*

Answer 37

(See image on OneNote)

Question 38

Use the graph of binding energy to explain why nuclear fission produces energy*

Answer 38

The elements closer to Fe have a greater binding energy, therefore less mass per nucleon than the heavier elements. During fission lighter elements are made and the excess mass is converted into energy

Question 39

Use the graph of binding energy to explain why nuclear fusion produces energy

Answer 39

The elements closer to Fe have a greater binding energy, therefore less mass per nucleon than the lighter elements. During fusion a heaver element is created and the excess mass is converted into energy

Question 40

What natural source of nuclear fusion do we use?

Answer 40

The sun

Question 41

Describe the conditions needed to produce nuclear fusion and explain why they are needed

Answer 41

High temperatures so the nuclei can get close enough to overcome the electrostatic repulsion and the strong nuclear force can take over High density so the probability of a collision is increased

Question 42

Why is boron used to make the control rods?

Answer 42

It can absorb neutrons

Question 43

Why is the moderator made of graphite?

Answer 43

``` It doesn't absorb neutrons It conducts heat well It is chemically stable It is cheap and readily available It doesn't disintegrate much ```

Question 44

Describe the role of shielding around a nuclear reactor and what it is made from

Answer 44

High energy neutrons and gamma rays are released during fission, which is absorbed by the shielding. Usually made from thick concrete and possibly lead

Question 45

What is a chain reaction?

Answer 45

When fission reactions lead to more ones, leading to a continuous chain of reactions.