C.7 Nuclear Fusion and Fission (HL)

Question 1

What is the mass defect?

Answer 1

The difference between the mass of a nucleus and the sum of the masses of the individual nucleons (neutrons and protons).

Question 2

What is the nuclear binding energy?

Answer 2

The energy required to break apart a nucleus into individual protons and neutrons.

Question 3

What is the unified atomic mass unit (u)?

Answer 3

Equivalent to 1/12 of the mass of a carbon 12 atom.

Question 4

How is the binding energy per nucleon worked out?

Answer 4

By dividing the nuclear binding energy by the number of nucleons.

Question 5

What does binding energy show about a nucleus?

Answer 5

It provides a measure of the stability of a nucleus the higher the binding energy per nucleon the more stable the nucleus.

Question 6

What happens to the mass of the nuclei of the products in fusion and fission reactions?

Answer 6

In fusion and fission reaction the mass of the nuclei of the products will be lower than that of the reactants. The lost mass is converted to energy according to E=mc^2

Question 7

What order of rate is radioactive decay?

Answer 7

Radioactive decay is a first order kinetic process, it has a constant half life t1/2.

Question 8

How is half life related to the decay constant?

Answer 8

Decay constant (Lambda) = ln2 / t1/2

Question 9

How can the number of undecided nuclei at any time be worked out?

Answer 9

N = N0e^(-lambda x t)

N is the number of undecided nuclei
N0 is the initial number of undecided nuclei
Lambda is the decay constant
t is the time

Question 10

What is effusion?

Answer 10

Effusion is the process by which a gas escapes through a very small hole in a container

Question 11

What is Graham’s law of effusion?

Answer 11

The rate of effusion of a gas is inversely proportional to the share root of the relative molecular mass of the gas

Question 12

How are gases affected by temperature?

Answer 12

At the same temperature the average kinetic energies of two gases are the same. given kinetic energy = 1/2mv^2 the particles of the lighter gas will on average be moving faster and therefore collide more often with the hole and escape more quickly.

Question 13

What is the issue with the uranium isotopes?

Answer 13

Uranium has two main isotopes U238 (99.27%) and U235 (0.72%). Only U235 is suitable for fission reactions in a nuclear reactor. Uranium fuel rods require at least 3% of U235 to achieve a critical mass.

Question 14

How does uranium enrichment take place?

Answer 14

Increasing the percentage of U235 in a sample of uranium relies on the conversion of uranium ores contains UO2 to the volatile UF6. The UF6 gas enriched in U235 is then converted back into UO2 to be turned into fuel rods for the reactors.

Question 15

What are the two methods of uranium enrichment?

Answer 15

Gas diffusion and Gas centrifugation

Question 16

What does the method of gas diffusion involve?

Answer 16

UF6 gas is forced at high pressure through a container with walls made of a porous membrane
235UF6 effuses through the pores slightly faster than 238UF6 due to having a lower relative molecular mass
Effused gas is slightly richer in 235UF6 than before
Process is repeated hundreds of times with effused gas to achieve enrichment of a few percent

Question 17

What does the method of gas centrifugation involve?

Answer 17

UF6 is pumped into a large centrifuge spinning at high speed
The slightly heavier 238UF6 molecules move towards the outside more than 235UF6
Gas slightly richer in 235UF6 is found in the centre of the centrifuge and pumped out into a second centrifuge
The process is repeated many times more to produce gas enriched in 235U by a few percent.

Question 18

What are the properties of UF6?

Answer 18

White crystalline solid
Octahedral shaped molecule
Non polar
Sublimes at 56 degrees Celsius
Strong covalent bonds between uranium and fluorine atoms
Weak London forces between molecules
Little energy required to separate molecules from each other

Question 19

What are the properties of UO2?

Answer 19

Dark brown crystalline solid
Giant ionic lattice
Very high melting point (>2800 degrees Celsius)
Strong electrostatic forces of attraction between U4+ and O2- ions
A lot of energy is required to break lattice apart

Question 20

What does nuclear fuel and nuclear waste produce?

Answer 20

They are radioactive and contain isotopes that can undergo radioactive decay by the emissions of alpha and beta particles and gamma rays, these are called ionising radiation as they cause the formation of ions when they interact with matter

Question 21

What are some issues with ionising radiation?

Answer 21

Ionising radiation can kill living cells by damaging DNA and enzymes directly or by the production of highly reactive oxygen containing free radicals