Nuclear Energy

Question 1

Atomic mass unit, u

Answer 1

1/12 mass of Carbon-12 atom which is equal to 1.661x10^-27 kg.

Question 2

Conversion of mass units

Answer 2

Using Einstein’s equation, E=mc2, then the equivalent energy of 1u can be determined, which is 931.3 MeV.

Question 3

Calculate energy released in nuclear changes (using u)

Answer 3

• add up total mass of nuclei before change (in terms of u)
• add up total mass of nuclei after change (in terms of u)
• calculate the mass difference (in terms of u)
• convert mass difference into equivalent energy using conversion factor from formula sheet 1u=931MeV)

Question 4

Mass difference

Answer 4

difference in mass between a nucleus and the sum of the mass of its nucleons.
Total mass of individual nucleons is greater than mass of nucleus

Question 5

Binding energy

Answer 5

the energy required to separate the nucleus up into its constituent protons and neutrons
Using E=mc^2, binding energy = mass difference x c^2

Question 6

Binding energy per nucleon

Answer 6

the average energy per nucleon to remove all of the nucleons from a nucleus

Question 7

What produces binding energy

Answer 7

the attractive strong force which holds the nucleons together

Question 8

Graph of average binding energy per nucleon against nucleon number

Answer 8

Peak value at about 8.7 MeV at A ~ 55
Drops to about 7.5 MeV at A=240

You need to be able to put the appropriate scales on both x and y axes

Question 9

Explain how energy is released in fission and fusion

Answer 9

• energy is released/made available when binding energy per nucleon is increased
• in fission a (large) nucleus splits and in fusion (small) nuclei join
• the most stable nuclei are at a peak
• fusion occurs to the left of peak binding energy per nucleon and fission to the right

Question 10

fission process

Answer 10

• a large or heavy nucleus splits into two smaller nuclei
• neutrons are released
• fission is usually brought about by neutron bombardment

Question 11

fission equation

Answer 11

Note the specific “daughter isotopes” (fission fragments) can be a range of different elements. Important point is that total nucleon number (and atomic number) on left of equation equals the total nucleon number (and atomic number) on the right.

Question 12

fusion process

Answer 12

• two small or light nuclei combine
• electrostatic repulsion has to be overcome
• nuclei have to be given kinetic energy for them to meet

Question 13

Advantages of fusion over fission

Answer 13

• supply of fuel is almost unlimited (deuterium from sea water)
• fewer waste or radioactivity or environmental problems
• energy released per unit mass is (generally) greater

Question 14

What is enriched uranium?

Answer 14

proportion of Uranium-235 is greater than is found in naturally occurring Uranium.

Question 15

Definition of thermal neutrons

Answer 15

Neutrons that have low energies or speeds (eg 0.03eV)

Question 16

induced fission by thermal neutrons

Answer 16

splitting of nucleus into two smaller nuclei, brought about by bombardment with (usually thermal) neutrons.

Question 17

Definition of a chain reaction

Answer 17

• fission reaction is induced by neutron bombardment (or neutron absorption)
• fission releases neutrons
• released neutrons cause more fissions

Question 18

Definition of critical mass

Answer 18

• minimum mass of fissile material

* for a self-sustaining reaction to be maintained.

Question 19

What do control rods do in the reactor?

Answer 19

• Control involves limiting the number of neutrons, released from the fission of a nucleus, that can go on and cause fissions in other nuclei.
• Excess neutrons are absorbed by control rods.
• Control rods inserted into reactor slows reaction rate.
• for a steady rate of fission, only one neutron per fission is required to go on to produce further fission.
• each fission produces two or three neutrons on average.
• some neutrons escape [or some absorbed by U-238 without fission].

Question 20

Examples of suitable control rods and their properties

Answer 20

Suitable control rod material is boron or cadmium.

• Control rod materials must be good at absorbing neutrons.

Question 21

What do moderators do in the reactor?

Answer 21

• Neutrons from fission are fast (high energy) ( eg 2 MeV)
• Fission most favourable with low energy neutrons
• Moderation involves slowing down neutrons by collision with moderator atoms
• Large number of collisions required (eg 50)
• Collision are elastic so KE is transferred to the atoms

Question 22

Examples of suitable moderators and their properties

Answer 22

Suitable moderator material is graphite or water

• Moderator must not absorb neutrons and the moderator atoms should have (relatively) low mass

Question 23

What does coolant do in the reactor?

Answer 23

• Transfers thermal energy from core to a heat exchanger where water (in a secondary cooling system) is turned into steam

Question 24

Examples of coolants and their properties

Answer 24

Suitable coolants are water or carbon dioxide (gas)
• Coolants need to flow easily so that they can be pumped around the reactor core
• Coolants need to have large specific heat capacities so a lot of thermal energy can be transferred with a smaller volume of coolant.

Question 25

How is thermal energy obtained from nuclear fission?

Answer 25

• fission fragments (which are both positive nuclei) repel each other and collide with other atoms in the fuel rod.
• high energy fission neutrons enter moderator [or collide with moderator atoms].
• atoms in moderator and fuel rods gain kinetic energy due to collisions (and vibrate more).
• temperature depends on the average kinetic energy of (vibrating) atoms.

Question 26

What are fuel rods?

Answer 26

They are the rods that contain the enriched Uranium fuel.

The fuel rods are then inserted into the reactor.

Question 27

Why do fuel rods become less effective for power production after they have been used for a while?

Answer 27

• amount of (fissionable) Uranium (235) in fuel (rod) decreases
• fission fragments absorb neutrons

Question 28

Why are spent fuel rods more dangerous than unused fuel rods?

Answer 28

• Uranium (in fuel rods) is an alpha emitter
• it is easy to stay out of range or easy to contain an  source
• fission fragments are (more) radioactive with short half lives or high activities
• emitting mostly B- and y radiation
• B and y have greater range/are more difficult to screen

Question 29

Why are B- emitting isotopes produced when fuel rods are in the reactor?

Answer 29

• fission nuclei (or fragments) are neutron-rich and therefore unstable (or radioactive)
• neutron-proton ratio is much higher than for a stable nucleus (of the same charge or mass)
• - particle is emitted when a neutron changes to a proton (in a neutron-rich nucleus)

Question 30

How is reactor shielded?

Answer 30

The reactor is shielded with concrete in order to contain gamma radiation and neutrons.

Question 31

How are spent fuel rods handled and processed?

Answer 31

• Spent fuel rods are removed and handled by remote control.
• They are placed in cooling ponds for several months (to allow short half life isotopes to decay).
• They are transported in specially designed flasks that are resistant to impact.
• Any Uranium-235 that hasn’t fissioned is separated from the active waste.

Question 32

How are radioactive waste materials stored?

Answer 32

• The high level waste is stored (as liquid – usually dissolved in nitric acid).
• Spent fuel rods are buried deep underground at a geologically stable site.
• Special storage precautions are required eg shielded tanks or monitoring
• Some of the waste can be stored within relatively inert glass a process referred to as vitrification.

Question 33

What is an emergency shutdown?

Answer 33

Where control rods are dropped into reactor to slow down and stop rate of fission as quickly as possible.