Nuclear Energy (Module 27)

Question 1

Why are most nuclei stable?

Answer 1

Because of an attractive force between the neutrons and protons, known as the strong nuclear force.

Question 2

Define binding energy of a nucleus

Answer 2

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

Question 3

Define mass defect of a nucleus

Answer 3

This is the difference between the mass of the separated nucleons and the mass of the nucleus.

Question 4

What is the range of the strong nuclear force?

Answer 4

3-4 fm

Question 5

Why must the strong nuclear force become repulsive at separations of about 0.5 fm between two nucleons?

Answer 5

Because if not, nucleons would pull each other close and closer together and the nucleus would be much smaller than it is.

Question 6

Define binding energy per nucleon

Answer 6

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

Question 7

Define nuclear fission

Answer 7

This is the process in which a large unstable nucleus splits into two fragments which are more stable than the original nucleus. The binding energy per nucleon increases in this process.

Question 8

Define nuclear fusion

Answer 8

The process of making small nuclei fuse together to form a larger nucleus. The product nucleus has more binding energy per nucleon than the smaller nuclei. So the binding energy per nucleon also increases in this process, provided the nucleon number of the product nucleus is no greater than about 50.

Question 9

What is the difference in the change of binding energy between fission and fusion?

Answer 9

Fission leads to a 0.5MeV change, whilst fusion can be more than 10 times greater.

Question 10

Define induced fission

Answer 10

This is the process of bombarding a uranium isotope with neutrons.

Question 11

What are fission neutrons?

Answer 11

These are neutrons that are released in a fission event as a result of a collision with another uranium nucleus.

Question 12

What is a chain reaction?

Answer 12

This is possible when fission neutrons continue to produce further fission events which also release fission neutrons and so on.

Question 13

What do control rods do?

Answer 13

They are made of a neutron absorbing substance such as cadmium/boron that is moved in or out of the core of a nuclear reactor to control the rate of fission events in the reactor.

Question 14

What do the fuel rods contain?

Answer 14

They contain enriched uranium which consists mostly of U-238 (non-fissionable uranium isotope) and about 2-3% of U-235 which is fissionable.

Question 15

What happens when control rods are pushed in further?

Answer 15

If they are pushed in further they will absorb more neutrons, therefore leading to a reduction in the rate of release of fission energy.

Question 16

How does a moderator help in a nuclear reactor?

Answer 16

It is used to slow down neutrons by colliding with the moderator repeatedly,

Question 17

Why is the reactor described as a thermal nuclear reactor?

Answer 17

Because the fission neutrons are slowed down to kinetic energies comparable to the kinetic energies of the moderator molecules.

Question 18

Define critical mass

Answer 18

For a chain reaction to occur, the mass of the fissile material must be greater than a minimum mass.

Question 19

Give a couple of examples of the nuclear reactor safety features.

Answer 19

1) The reactor core is a thick steel vessel designed to withstand high pressure and temperatures in the core.
2) The core is in the building with a very thick concrete wall surrounding it.
3) Every reactor has an emergency shut-down system designed to insert the control rods fully into the core (stop fission completely).
4) The sealed fuel rods are inserted/removed from the reactor by means of remote handling devices.

Question 20

What is the difference between high, intermediate and low-level radioactive waste?

Answer 20

High: Tends to come from nuclear power stations containing radioactive isotopes which must be stored underwater in cooling ponds for up to a year. These rods are then transferred in large steel casks to the THORP reprocessing plant where this material can be reused for further possible use. (If not, waste must be stored safely for centuries)
Intermediate: Such as radioactive materials with low activity and containers of radioactive materials are sealed in drums that are encased in concrete and stored in specially constructed buildings with walls of reinforced concrete.
Low: Such as laboratory equipment and protective clothing is sealed in metal drums and buried in large trenches.

Question 21

Why is storing radioactive waste such a problem in the UK?

Answer 21

Because people don’t want the storage to be local or even carried throughout their own locality.