Nuclear fission and fusion

Question 1

What is a mass deficit/defect?

Answer 1

The difference between the measured mass of a nucleus and the total mass of it’s constituent nucleons.

Question 2

Why is there a mass deficit?

Answer 2

Because a small amount of the mass of the nucleons is converted into energy needed to hold the nucleus together.

This is called ‘binding energy’

Question 3

What are the two common systems of units for calculating binding energy?

Answer 3

SI units, giving the energy in Joules.

If in atomic mass units, then convert mass deficit into binding energy via 1u = 931.5MeV

Question 4

What is the ‘binding energy per nucleon?’

Answer 4

The amount of energy needed to remove one nucleon from a nucleus.

You would need to know the binding energy of the nucleus and the number of nucleons within it to calculate this.

Question 5

Which isotope has the highest binding energy per nucleon?

Answer 5

Iron - 56

Question 6

Explain the process of nuclear fusion.

Answer 6

Small nuclides combining together to make larger nuclei, with a greater binding energy per nucleon.

Any nuclear reaction which increases the binding energy per nucleon will give out energy.

Question 7

What is nuclear fission?

Answer 7

Larger nuclei breaking up into smaller pieces which have a greater binding energy per nucleon.

Question 8

How is energy released during nuclear fusion? As in, how does the binding energy per nucleon increase?

Answer 8

In forcing the light nuclei to join together, the mass of the new heavier nucleus will be less than the mass of the constituent parts, as some mass is converted into energy.

However, not all of this energy is used as binding energy for the new nucleus, so energy will be released from this reaction.

This is what provides energy to make stars shine.

Question 9

What problems do scientists face in maintaining a controlled nuclear fusion reaction?

Answer 9

Forcing two positively charged, mutually repelling protons to fuse together.

The kinetic energy required to overcome this electrostatic force requires temperatures of many million Kelvin.

A high density of protons are required to ensure enough collisions for the reaction to be sustained.

Question 10

What else is released along with the energy in nuclear fission?

Answer 10

Neutrons

Question 11

What is a moderator?

Answer 11

That which slows neutrons to speeds needed to sustain fission in controlled nuclear fission reactions.

Question 12

Name a reaction in which a moderator is not required.

Answer 12

Fission reactions such as those using plutonium.

Question 13

What is the concept behind the design of atomic bombs?

Answer 13

A plutonium fission reaction that is allowed to run uncontrolled. It in turn produces energy continuously at an ever increasing rate until all the fuel is used up.

A lump of plutonium-239 about the size of a cricket ball can completely react in less than a microsecond, producing the energy equivalent of 20 kilotonnes of TNT (about 90*10^12J)

Question 14

What is the most common nuclear fission reaction used in power stations?

Answer 14

Uranium-235

Question 15

What is a thermal neutron?

Answer 15

The slow moving neutron that hits the nucleus of the uranium isotope.

The isotope absorbs the thermal neutron to become Uranium 236 which is unstable.

Question 16

What are the products of the disintegration of the unstable U-236?

Answer 16

Two medium sized nuclei with roughly half the nucleons each, plus a number of high speed neutrons.

The actual composition of the 2 main products varies, the vast majority of these fissions will produce heavy daughter nuclei: mass numbers in the range 130-150.

Lighter products usually have a mass number in the range 85-105.

Question 17

How is the difference in the amount of binding energy in the U-236 and the daughter products given off?

Answer 17

As Kinetic energy of the various particles and this energy is harnessed by the nuclear reactor to drive an electricity generating system.

Question 18

How are the fast emitted neutrons slowed down in a nuclear reactor?

Answer 18

By the use of ‘moderators’ such as graphite or water. These are used so that the emitted electrons arrive at the next ‘uranium fuel rod’ at a slow enough sped to be absorbed.

Question 19

How is the chain reaction controlled in a nuclear reactor?

Answer 19

By the use of control rods, typically made of cadmium or boron. These absorb excess neutrons.

These control rods are lowered into the reactor on an electromagnetic support.

In case of a failure in the control rods system, gravity will pull down the rods into the reactor core where they will absorb all neutrons and shutdown the reaction.

Question 20

Name some countries in which U-235 is abundant.

Answer 20

Australia, Canada, Kazakhstan, SA, Brazil.

Uranium is about 500 times more abundant than goal.

And in order to supply a standard sized power station, such fuels are needed in very small quantities.

3 kg of U-235 per day will run a 1000MW power station.

Question 21

Can the material of the reactor core’s construction become radioactive?

Answer 21

Slightly, during the course of it’s lifetime. Mostly due to exposure to free neutrons which can be absorbed by a nucleus.

Question 22

What is the most dangerous radioactive waste generated in the process of nuclear reactors?

Answer 22

the spent fuel. Heavy metals and fuel rods, results of spent fuel and reprocessing products are cooled in a water pool for 1-20 years, then buried in a deep underground repository.

Question 23

how are resins, chemical sludges, reactor components which are waste from reactor decomissioning dealt with?

Answer 23

Solidified in concrete or bitumen and buried.

Question 24

How are low level wastes like paper, rags, tools and clothing dealt with?

Answer 24

Incinerated and buried.