Nuclear energy]

Question 1

In radioactive decay, why is there a mass defect?

Answer 1

Some of the mass is converted into energy.

Question 2

What is the binding energy of the nucleus?

Answer 2

The work that must be done to separate a nucleus into its constituent neutrons and protons.

Question 3

If mass of each atom is given instead of the mass of the nucleus, what should be done first?

Answer 3

Calculate mass of nucleus by subtracting the mass of the electrons.

Question 4

Why is energy released when a nucleus is separated into its neutrons and protons?

Answer 4

Strong nuclear force does work pulling the nucleons together. Energy released is equal to the binding energy of the nucleus.

Question 5

Why is the mass of the individual nucleons greater than the mass of the nucleus?

Answer 5

Energy is released when a nucleus forms from separate neutrons and protons. This energy released corresponds to lost mass.

Question 6

What is the mass defect?

Answer 6

Defined as the difference between the total mass of the separated nucleons (protons and neutrons) and the mass of the nucleus.

Question 7

What causes a mass defect?

Answer 7

Due to energy released when a nucleus forms from individual nucleons, so it’s due to the binding energy.

Question 8

What is the binding energy per nucleon?

Answer 8

The average work done per nucleon to remove all the nucleons from a nucleus.

Question 9

What leads to a more stable nucleus?

Answer 9

The greater the binding energy per nucleon, the greater the stability of the nucleus.

Question 10

What is one atomic mass unit defined as?

Answer 10

It is defined as 1/12th of the mass of an atom of the carbon 12 isotope.

Question 11

What is the most stable isotope and what is it’s binding energy per nucleon?

Answer 11

Iron - 56. 8.8 MeV.

Question 12

What is nuclear fission?

Answer 12

The process by which a large nucleus splits into two fragment nuclei which are more stable than the original nucleus, usually by absorbing a neutron. Binding energy per nucleon is greater for fragment nuclei than for the large nucleus.

Question 13

What is nuclear fusion?

Answer 13

The process of making two smaller nuclei fuse together to form a larger nucleus. The product nuclei has a much greater binding energy per nucleon than the smaller nuclei.

Question 14

What is the range for the most stable nuclei?

Answer 14

A = 50 to A = 60.

Question 15

When is the change of binding energy per nucleon most observable?

Answer 15

During fusion, as graph is much steeper than during fission. So, fusion releases much more energy.

Question 16

What is the energy released in nuclear fission?

Answer 16

The change in the binding energy per nucleon.

Question 17

What are the two ways in which fission can occur?

Answer 17

By spontaneous reaction or by bombardment of neutrons.

Question 18

Which two isotopes are fissionable?

Answer 18

U-235 and Pu-239.

Question 19

How does a chain reaction occur in fission reactions?

Answer 19

Fission neutrons are released as by product which go on to bombard more large nuclei for them to undergo fission again.

Question 20

Why is energy released from fission?

Answer 20

Two smaller fragment nuclei repel each other as they are positively charged with sufficient force to overcome strong nuclear force trying to hold them together. Therefore they gain kinetic energy. Also fragments are smaller than the original nucleus, meaning they are more tightly bound, so they have a greater binding energy.

Question 21

What is required for nuclear fusion to occur?

Answer 21

The 2 smaller nuclei have to be travelling at a great enough velocity so that they can overcome the electrostatic forces of repulsion and interact via the strong nuclear force.

Question 22

Why does binding energy increase in fusion reactions?

Answer 22

Nucleons become more trapped in the nucleus when fusion occurs. So, energy is released which is equal to the increase of binding energy.

Question 23

What is the main purpose of a nuclear reactor?

Answer 23

To generate electricity by nuclear fission.

Question 24

What is the structure of a nuclear reactor?

Answer 24

Fuel rods spaced evenly inside a reactor core. Reactor core also contains control rods and the coolant (water). A pump forces coolant through reactor core where it’s heated. Finally, the coolant passes through a heat exchanger where it is used to raise stream to drive a turbine. All is contained in a concrete shield.

Question 25

What is purpose of the fuel rods?

Answer 25

They contain enriched uranium. 97% U-238 and 2-3% fissionable U-235. Fission occurs in fuel rods, energy released is used to heat up the coolant.

Question 26

What is purpose of control rods?

Answer 26

Used to absorb the neutrons. Exactly one neutron per fission event should go on to produce further fission. Depth at which the control rods are pushed in controls how many free neutrons there are and controls the rate at which fusion occurs.

Question 27

What property must control rods have?

Answer 27

Be able to absorb neutrons without undergoing fission. Boron is usually used.

Question 28

What is the use of the moderator?

Answer 28

Fission neutrons need to be slowed down before causing further fission otherwise they would be travelling too fast to cause further fission. The neutrons collide with the moderator atoms and this slows them down.

Question 29

What is the moderator used and why is it chosen?

Answer 29

Water is used. Moderator atoms should be to a similar size as the neutrons, as transfer of kinetic energy is most effective this way. Fission neutrons are slowed down to kinetic energies comparable to the kinetic energies of the moderator molecules.

Question 30

What are the desirable features of a coolant? What is it's purpose?

Answer 30

Shouldn't absorb fission neutrons, have a high boiling point and have a low viscosity. Used to remove the heat created by fission.

Question 31

What else is required for a chain reaction to occur?

Answer 31

The mass of the fissile material (U-235) must be greater than a minimum mass called the critical mass. This is because some fission neutrons escape from fuel rods without causing fission and some are absorbed by other nuclei.

Question 32

What happens if mass used is less than the critical mass?

Answer 32

Too many of the fission neutrons will escape because the surface area to mass ratio of the material is too high. So, critical mass also depends on surface area used.

Question 33

State 4 safety features of nuclear reactors?

Answer 33

1) Reactor core is a thick steel vessel designed to withstand pressure/temp of core. 2) Core is in a building with very thick concrete walls which absorb the neutrons and gamma radiation which escape. 3) Emergency shut down system where fuel rods can be pushed completely into the reactor core to stop fission completely.

Question 34

Why are the fuel rods more radioactive than before?

Answer 34

They are removed from reactor core by a remote handling device. This is because after absorbing many neutrons, they now emit beta and gamma radiation due to the many neutron rich fission products that form. Before, only alpha radiation was emitted.

Question 35

What is radioactive waste categorised as?

Answer 35

High level, intermediate level or low level radioactive waste.

Question 36

What is high level waste?

Answer 36

Used fuel rods which contain many neutron rich fission fragments. They have to be stored underwater in cooling ponds for up to a year, as they continue to release heat due to radioactive decay. Fuel cans are then stored in sealed containers in deep trenches so it doesn't contaminate food/water.

Question 37

What is intermediate level waste?

Answer 37

Radioactive materials with low activity are sealed in drums which are encased in concrete and stored in buildings made of reinforced concrete.

Question 38

What is low level waste?

Answer 38

Lab equipment and protective clothing which are sealed in metal drums and buried in large trenches.