Nuclear test

Question 1

How are mass and energy related?

Answer 1

E=mc^2,
energy and mass are interchangeable and are related through this equation which can be applied to all energy changes

Question 2

What is mass defect?

Answer 2

The difference between the mass of a nucleus and the sum of the masses of the individual protons and neutrons

Question 3

Why is there a mass defect?

Answer 3

Some of the mass of the nucleons is converted into energy and released when they fuse to form the nucleus

Question 4

What is binding energy?

Answer 4

The energy required to separate a nucleus into its individual nucleons

Question 5

What is an atomic mass unit?

Answer 5

1/12th the mass of a carbon 12 atom and is used as an estimate of the mass of a proton and neutron

Question 6

What is the energy equivalent for an atomic mass unit?

Answer 6

1 u = 931.5 MeV

Question 7

What is nuclear fission?

Answer 7

The splitting of a large nucleus to form two smaller daughter nuclei, neutrons while releasing energy

Question 8

Why is energy released during nuclear fission?

Answer 8

The smaller daughter nuclei have a greater binding energy per nucleon as they are more stable

Question 9

What is nuclear fusion?

Answer 9

The joining of two smaller nuclei to form a larger nucleus and release energy

Question 10

Why is energy released during nuclear fusion?

Answer 10

The larger nucleus has a much greater binding energy per nucleon

Question 11

What are the conditions required for nuclear fusion and why?

Answer 11

Extremely high temperatures are required as a lot of energy is required to overcome electrostatic repulsion between positive nuclei

Question 12

What is the shape of the binding energy per nucleon against nucleon number?

Answer 12

Increases very steeply from 1H to 16 O with spikes at 4He and 12C, peak at 56Fe then gradual decline until uranium 238

Question 13

How is nuclear fission induced?

Answer 13

A thermal neutron is fired into a uranium-235 nucleus causing it to be extremely unstable and split into two daughter nuclei, some neutrons and releasing energy

Question 14

What is a thermal neutron?

Answer 14

A neutron with low energy meaning it can induce fission as higher energy neutrons rebound off of nuclei after a collision and are not absorbed

Question 15

How can nuclear fission be a chain reaction?

Answer 15

The neutrons released during fission can be absorbed by other uranium nuclei causing more fission events, a chain reaction occurs where each fission causes at least one other fission event

Question 16

What is the critical mass?

Answer 16

The smallest mass of fissile material required to maintain a steady chain reaction

Question 17

What does the moderator do?

Answer 17

Slows down neutrons released in fission to thermal speeds through elastic collisions between the neutrons and moderator nuclei. The closer the nuclei mass to neutrons, the more momentum is transferred

Question 18

What is often used as a moderator and why?

Answer 18

Water as it is light, not very reactive, inexpensive
Graphite can also be used

Question 19

What do control rods do and what are they made of?

Answer 19

Absorb neutrons in the reactor to control chain reactions. Made of boron and cadmium which can absorb neutrons to form stable isotopes and so not become unstable themselves

Question 20

What does the coolant do?

Answer 20

Absorbs heat released in the reactor during fission reactions, which is used to make steam which drives turbines, generating electricity. Water can sometimes be used as the moderator and coolant, molten salt or helium can be used as coolant

Question 21

What fuel is used in nuclear reactors?

Answer 21

Enriched uranium which is 95% U-238, 5% U-235. 238 doesn’t undergo fission and absorbs neutrons, controlling rate

Question 22

How is radiation exposure for workers limited?

Answer 22

-Fuel rods inserted remotely into reactor
-Thick concrete shielding around reactor, can be radioactive after long use
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Question 23

How are nuclear reactors shut down in emergencies?

Answer 23

Control rods dropped entirely into the reactor by absorbing all free neutrons

Question 24

Why are spent fuel rods dangerous?

Answer 24

Daughter nuclei produced are very unstable and have high activity so they are high-level waste

Question 25

How is high-level waste (fuel rods) disposed of?

Answer 25

-Removed and handled remotely
-Placed in cooling ponds due to emissions causing heating
-Recyclable materials removed
-Waste encased in glass and stored in thick steel casks surrounded by lead in deep caverns in geologically stable places. Chosen places have minimum impact on environment and locals are consulted