Nuclear Physics (Reactors)

Question 1

Which number on the elemental symbol shows the relative atomic mass?

Answer 1

The top one (aka biggest number)

Question 1

What is the relative atomic mass unit equivalent to in MeV?

Answer 1

1 u = 931.5 MeV

Question 2

What is the mass of a proton in terms of u?

Answer 2

1.00728u

Question 3

What is the mass of a neutron in terms of u?

Answer 3

1.00867u

Question 4

What is the mass of an electron in terms of u?

Answer 4

0.000549u

Question 5

What is meant by the mass defect of a nucleus?

Answer 5

• The actual mass of a nucleus is NOT equal to the mass of the added constituents
• The difference between the two is the mass defect

== Note: mass of constituents > mass when a nucleus

Question 6

How do you calculate mass defect?

Answer 6

Calculated mass - Actual mass = Mass defect

Note: calculated mass is the added mass of constituents

Question 7

What is meant by binding energy?

Answer 7

• When constituents bind = mass is lost
• Lost mass must be turned into energy

(The mass lost is technically mass defect = binding energy)

If we want to separate nucleus into constituents = must put energy back in to regain mass

Energy needed is binding energy

Question 8

Definition of binding energy?

Answer 8

Energy required to separate a nucleus into its constituents

Question 9

How do you calculate average binding energy per nucleon?

Answer 9

B/A

Binding energy/ Nucleon number

Question 10

Describe the relationship between average binding energy and stability

Answer 10

• Higher average binding energy = more energy needed to separate nucleus
• Decay is less natural = more stable nuclei

Question 11

Describe the graph of average binding energy against nucleon number

Answer 11

• A sharp increase at beginning (For light nuclei)
• Levels out into maximum point
• Slow decrease (For heavy nuclei)

Question 12

At which point on the graph of average binding energy against nucleon number, does the most stable nuclei occur?

And at which element

Answer 12

The most stable nuclei occur around the maximum point on the graph

• since higher binding energy = more stable

Hence for Iron

Question 13

How do you calculate binding energy?(if given mass defect)

Answer 13

Mass defect (in terms of u) x 931.5

Question 14

How do you convert mass defect into standard units (Kg)

Answer 14

1 u = 1.661 x 10^-27

Question 15

What is meant by nuclear fission

Answer 15

Breaking apart a large nuclei into two smaller daughter cells, causing a release in energy due to an increase in average binding energy

Question 16

Describe how Uranium undergoes nuclear fission

Answer 16

• Fission is induced by firing a thermal neutron at Uranium atom causing it to be more unstable
• The nuclei breaks apart into two daughter cells, and three neutrons are also ejected from the nucleus
• Energy is released because the new smaller nuclei will have a higher average binding energy

Question 17

Explain why energy is released during nuclear fission

Talk about both binding energy and mass defect

Answer 17

• Total binding energy has increased
• Since the nuclei “decays” it becomes more stable
• Decay of this more stable daughter cell will take more energy
  == Smaller daughter cell has higher average binding energy

AND

The total mass of products is less than parent nucleus therefore the “lost” mass is converted to energy according to Einsteins equation E=mc…

This energy is released in the form kinetic energy of the fission fragments AND as gamma radiation

Question 18

Explain how induced nuclear fission can be used in nuclear thermal reactors to generate power

Answer 18

• Fuel rods contain pellets of Uranium 238
• Thermal Neutrons fired rod to rod inducing fission in adjacent rod == undergoes chain reaction
• Coolant absorbs the heat released during fission reactions in the core of the reactor
• This is heat is them used to make steam which powers electricity-generating turbines

Question 19

Explain the use of moderators in a thermal nuclear reactor

Answer 19

• When neutrons are traveling too fast to be absorbed by Uranium pellets
• A moderator is used to slow neutrons released in fission reaction
• Increases amount of fission as more neutrons can be absorbed to induce fission

Question 20

Explain the use of control rods in thermal nuclear reactors

Answer 20

MUST have control rods

• Absorb neutrons so that they stop fission when too much is occurring
• Allowing a chain reaction to be controlled in a safe manner

Question 21

What substance is usually used as a moderator and why?

Answer 21

• Water
• Pure water
• Contains hydrogen (You want moderator atom nuclei to be same size almost as neutron)
• Inexpensive
• Not very reactive

Question 22

What substance is usually used for control rods and why

Answer 22

• Boron
• As this does not undergo fission

Question 23

Explain how fission can be induced by a thermal neutron specifically

Answer 23

• Firing thermal neutron into the Uranium nucleus causing it to become extremely unstable
• Thermal neutrons have a low energy meaning they can induce fission

Oppose to neutrons with high energy = as they would just rebound away from nucleus (not causing fission)

Question 24

What is meant by critical mass?

Answer 24

- Minimum mass of fuel required to maintain a steady chain reaction Example: Using EXACT critical mass would give one single fission reaction as a chain (one after other) But using a mass lower than critical mass would lead to the reaction eventually stopping

Question 25

Explain how a moderator is able to perform its function

Answer 25

- Slows down neutrons to thermal speeds through elastic collisions (between the nuclei of the moderator atoms and the fission neutrons) - Closer the moderator atoms are in size to a neutron == Larger proportion of momentum is transferred (therefore lowing number of collisions needed until begins to slow neutrons speed)

Question 26

Explain why water is a good substance for a coolant?

Answer 26

- High specific heat capacity therefore can transfer large amounts of thermal energy (Can also be used as moderator for same reason)

Question 27

Explain how shielding is used as a safety aspect in nuclear reactors and the problem associated with it.

Answer 27

- Around the nuclear reactor is a very thick concrete shielding - This blocks radiation from escaping from the reactor and affecting workers at power stations However may become radioactive after some time, as neutrons that escape reactor enter the nuclei of concrete Thus they become unstable and emit beta minus decay

Question 28

Explain how control rods are used in emergencies

Answer 28

- Dropped into the reactor core entirely - In order to stop fission reactions from occurring by absorbing all the free neutrons "emergency shut down"

Question 29

What is high level waste and give two examples

Answer 29

Most dangerous type of waste remains radioactive for thousands of years - Spent fuel rods - Unusable fission products from fission of Uranium 235

Question 30

What is enriched Uranium and how does it help with safety

Answer 30

U-238 enriched with U-235 U-235 is the isotope that actual undergoes fission U-238 helps absorb fission neutrons thus helps control reaction

Question 31

What is intermediate level waste, how is it treated and give an example

Answer 31

- Daily used items - Example fuel rods themselves - Waste will have a longer half life than the low level waste - encased in cement in steel drums and stored underground

Question 32

What is low level waste, give example and how is it treated?

Answer 32

- Lightly contaminated ex: clothing and tools - will be radioactive for a few years - encase in concrete and store few metres underground

Question 33

How can high level waste be treated?

Answer 33

- Initially placed in cooling ponds of water close to the reactor for a number of years - Isotopes of plutonium and uranium are harvested to be used again - Waste is mixed with molten glass and made solid (vitrification) - This is encased in containers made from steel, lead or concrete - Stored VERY deep underground

Question 34

Benefits of nuclear power

Answer 34

- Nuclear power stations produce no polluting gases - Highly reliable - Require less fuel as uranium provides far more energy per kg compared to coal

Question 35

Risks of nuclear power

Answer 35

- Production of radioactive waste is very dangerous - Expensive to dispose of waste - Nuclear meltdown such as Chernobyl could have catastrophic consequences to environment and people

Question 36

What is meant by a thermal neutron

Answer 36

- Slow moving neutrons (low kinetic energy) (They need to be slow to induce fission hence moderators) - In thermal equilibrium with moderator/other material - Have energies of order 0.025 eV

Question 37

What is meant by a fission fragment?

Answer 37

- Particles released during fission - Two daughter nuclei and neutrons

Question 38

Difference between normal elastic collisions and elastic collisions due to moderator

Answer 38

- KE loss is variable - Collisions not always elastic - Collisions not always head on