Nuclear Energy

Question 1

State the form in which energy released from a decaying nucleus (alpha decay) initially appears as

Answer 1

Kinetic energy

or electrostatic potential energy

Question 2

Graph to show how the number of neutrons, N, varies with the number of protons, Z, for stable nuclei over the range Z=0 to Z=80
Draw a scale on the N axis

Indicate where alpha, beta minus and beta plus emission occurs

Answer 2

Straight line between (Z=0, N=0) to (Z=20, N=20) then curves upwards to Z=80, N=120

Alpha: any region below the line of stability but N>80 Z>60

Beta minus: any region above and close to the line of stability

Beta plus: any region below and close to the line of stability

Question 3

Changes in proton and neutron number after electron capture

Answer 3

-1 proton

+1 neutron

Question 4

State what is meant by the binding energy of a nucleus

Answer 4

Energy needed to separate a nucleus into its constituent nucleons

Question 5

Describe what happens to the majority of the alpha particles in the Rutherford scattering experiment

Answer 5

pass through with no or very small deflection

Question 6

Name two deductions that can be made from the results of alpha particle scattering

Answer 6

volume of nucleus &laquo_space;volume of atom / nucleus small and atom mostly empty space

Most of mass in nucleus

Nucleus has positive charge

Size of nucleus &laquo_space;separation

Question 7

What is meant by the random nature of radioactive decay

Answer 7

Which atoms decays at what time is chance

Question 8

Explain what is meant by electron capture

Answer 8

Inner orbiting electron captured by a proton in the nucleus and is converted into a neutron

Question 9

Give one reason why EM radiation is emitted following electron capture

Answer 9

Daughter nuclide/nucleus/atom might be excited and energy given up as EM radiation upon de-excitation.

Question 10

Radioactive source - short range, different lengths and angles

What type of radiation is emitted and what can be deduced about the energy of the emitted particlees

Answer 10

Alpha

Different track lengths, short range particles have lower energy than long range particles, particles in each range have the same energy

Question 11

What has a longer half life, Pu-239 or U-235

Answer 11

U-235 because of the inverse relationship between half-life and alpha particle energy

Question 12

Explain why 231-Th-90 is likely to be a beta minus emitter

Answer 12

Because it is neutron-rich compared with more stable elements

Question 13

Graph of intensity (y) against diffraction angle (x) for electron diffraction

Answer 13

Starts high up y axis
Curves down with positive gradient
Minimum
Curves up and peaks
Then curves down with negative gradient 

Doesn’t touch x axis

Question 14

State why high energy electrons are used in determining nuclear size

Answer 14

The de Broglie wavelength of high energy electrons is comparable to nuclear radii - not subject to SNF

Question 15

Give the main conclusions from electron diffraction experiments about:

a) nuclear density
b) average separation of particles

Answer 15

a) nuclear density is constant

b) separation of neighbouring nucleons is constant - nucleons are closely packed

Question 16

Sketch a graph of nuclear radius (y) against nucleon number (x)

Answer 16

-1/x
from origin
r shape

Question 17

Explain what is meant by induced fission

Answer 17

A neutron strikes the nucleus causing it to split into two fragments

Question 18

Explain, using the charged liquid drop model, the energy changes in the fission of a U-235 nucleus

Answer 18

Some electrostatic potential energy is converted to kinetic energy of fragments

Some electrostatic potential energy used to overcome strong interaction

Some electrostatic potential energy used to increase surface energy

Question 19

Describe and explain how the fission of the U-235 nuclei in a fuel rod causes the fuel rods and the moderator to become very hot

Answer 19

Fission fragments repel and collide with other atoms in fuel rod

High energy fission neutrons enter/collide with moderator

Atoms gain kinetic energy due to collisions and vibrate more

Temperature depends on the average kinetic energy of vibrating atoms

A chain reaction occurs

Question 20

State the nuclear energy that occurs during positron emission hence explain why no positrons are emitted in this sequence

Answer 20

Proton changes to a neutron and a positron and electron neutrino

New element has a lower neutron to proton ratio

Alpha emission raises the neutron to proton ratio slightly

Beta minus emission lowers the ratio more

Beta plus emission increases neutron to proton ratio

Positron emission competes with alpha emission but is energetically less favourable

Question 21

Explain what is meant by half-life

Answer 21

time for half of active nuclei of radioactive substance to decay

Question 22

Suggest two ways in which the accuracy of the experiment (counter next to radioactive substance) could be improved

Answer 22

Count over longer period than half minute/repeat experiment

Use stronger source

Use background count correctly

Question 23

One pro one con of using nuclear fuel to produce electricity

Answer 23

Pro:
less mass of fuel used because more energy per kilogram
less harm to the environment because does not generate green house gases

Con:
hazardous waste because fission products are radioactie
Long term responsibility because waste needs to be stored for many years

Question 24

State one reason why methods other than alpha particle scattering are used to determine nuclear radii

Answer 24

SNF complicates process

Scattering caused by distribution of protons not whole nucleon distribution

Alpha particles are massive causing recoil of nucleus complicating results

Question 25

Explain what is meant by enriched uranium

Answer 25

Proportion of U-235 is greater than in natural uranium

Question 26

Why is enriched uranium rather than natural uranium used in many nuclear reactors

Answer 26

Induced fission more probable with U-235 than U-238

Question 27

By considering the neutrons involved in the fission process, explain how the rate of production of heat in a nuclear reactor is controlled

Answer 27

For steady rate of fission, one neutron per fission required to go on to produce further fission
Each fission produces two or three neutrons on average
Some neutrons escape
Control rods absorb sufficient neutrons to maintain steady rate of fission

Question 28

Explain why all the fuel in a nuclear reactor is not placed in a single fuel rod

Answer 28

Neutrons need to pass through a moderator to slow them in order to cause further fissions or prevent U-238 absorbing them.
Neutrons that leave the fuel rod are unlikely to re-enter the same fuel rod making it easier to replace the fuel ins tages

Question 29

State two observations about the alpha particles detected coming from the foil of alpha scattering

Answer 29

Most alpha particles undeflected
Some deflected through small angles
Very small but significant number deflected through > 90 degrees

Question 30

State two features of the structure of the atom which can be deduced from these observations

Answer 30

Atoms mostly empty space
Positive charge concentrated
In a volume much less than the total volume

Question 31

Define and explain binding energy

Answer 31

Binding energy (MeV) is the energy needed to separate all of the nucleons in a nucleus. To separate the particles of the object, they must be pulled apart against the attractive SNF. They must therefore have potential energy when separated. When the nucleons come together and form nuclei, their potential energy decreases and so because energy is always conserved, energy must be put in to separate the nucleons of a nucleus – this energy is the binding energy.

Question 32

How does binding energy relate to gravitational potential energy of an object

Answer 32

Binding energy is like when an object is lifted up from the ground. It gains potential energy similarly to the nucleons because work is done separating the object from the ground when you are lifting it just like when lifting the nucleons from the nucleus.

Question 33

Why does the fusion of small nuclei produce energy

Answer 33

as the two elements have a nucleon number < 56 they appear before the peak of the binding energy per nucleon curve implying that they are relatively unstable. Upon fusion, they create a more stable element higher up the curve. Work is done separating the nucleons and so by fusing nucleons together you decrease their potential energy – energy is always conserved so this reduction in potential energy means that energy is released to the surroundings. The a higher binding energy per nucleon is achieved and so there is a higher mass defect meaning a greater reduction in mass. This reduction in mass is equivalent to a reduction in energy (E=mc^2) meaning the new element has less energy thus energy is released to the surroundings