Nuclear energy

Question 1

What is the equation for Mass energy equivalence/

Answer 1

E = energy (J)
m = mass (kg)
c = the speed of light (m s-1)

Question 2

What some examples of mass-energy equivalence?

Answer 2

Some examples of mass-energy equivalence are:

The fusion of hydrogen into helium in the centre of the sun

The fission of uranium in nuclear power plants

Nuclear weapons

High-energy particle collisions in particle accelerators

Question 3

What is the definition for mass defect?

Answer 3

The difference between an atom’s mass and the sum of the masses of its protons and neutrons.

Question 4

What is the equation for calculating the mass defect of a nucleus?

Answer 4

Δm = Zmp + (A – Z)mn – mtotal

Z = proton number
A = nucleon number
mp = mass of a proton (kg)
mn = mass of a neutron (kg)
mtotal = measured mass of the nucleus (kg)

Question 5

What is the definition for binding energy?

Answer 5

The amount of energy required to separate a nucleus into its constituent protons and neutrons.

Question 6

How to calculate the energy from formation of a nucleus?

Answer 6

Energy and mass are proportional, so, the total energy of a nucleus is less than the sum of the energies of its constituent nucleons

The formation of a nucleus from a system of isolated protons and neutrons is, therefore, an exothermic reaction - meaning that it releases energy. Using the mass-energy equivalence equation.

Question 7

Why do nuclear reactions release more energy than chemical reactions?

Answer 7

In a typical nucleus, binding energies are usually measured in MeV

This is considerably larger than the few eV associated with the binding energy of electrons in the atom

Nuclear reactions involve changes in the nuclear binding energy whereas chemical reactions involve changes in the electron binding energy

This is why nuclear reactions produce much more energy than chemical reactions

Question 8

What is the definition for an atomic mass unit?

Answer 8

The mass of exactly one-twelfth of an atom of carbon-12

Question 9

What is the definition for nuclear fusion?

Answer 9

The joining together of two small nuclei to produce a larger nucleus.

Question 10

What is a common fusion reaction within Stars?

Answer 10

Low mass nuclei (such as hydrogen and helium) can undergo fusion and release energy

When two protons fuse, the element deuterium is produced

In the centre of stars, the deuterium combines with a tritium nucleus to form a helium nucleus, plus the release of energy, which provides fuel for the star to continue burning

Question 11

What are the condition for fusion?

Answer 11

For two nuclei to fuse, both nuclei must have high kinetic energy

This is because nuclei must be able to get close enough to fuse

However, two forces acting within the nuclei make this difficult to achieve

Electrostatic Repulsion

Protons inside the nuclei are positively charged, which means that they electrostatically repel one another

Strong Nuclear Force

The strong nuclear force, which binds nucleons together, acts at very short distances within nuclei

Therefore, nuclei must get very close together for the strong nuclear force to take effect

It takes a great deal of energy to overcome the electrostatic force, hence fusion can only be achieved in an extremely hot environment, such as the core of a star.

Question 12

What is the definition for nuclear fission?

Answer 12

The splitting of a large atomic nucleus into smaller nuclei.

Question 13

How does fission happen?

Answer 13

Fission must be induced by firing neutrons at a nucleus

When the nucleus is struck by a neutron, it splits into two, or more, daughter nuclei

During fission, neutrons are ejected from the nucleus, which in turn, can collide with other nuclei, which triggers a cascade effect

This leads to a chain reaction which lasts until all of the material has undergone fission, or the reaction is halted by a moderator.

Question 14

What happens with well controlled not controlled fission?

Answer 14

Nuclear fission is the process which produces energy in nuclear power stations, where it is well controlled.

When nuclear fission is not controlled, the chain reaction can cascade to produce the effects of a nuclear bomb.

Question 15

How to find the energy within the nuclear reactions?

Answer 15

Find the difference of binding energies or find the difference in mass defect and use mass energy equivalence to find the binding energy released.

Question 16

Why is there a difference in binding energy?

Answer 16

The daughter nuclei produced as a result of both fission and fusion have a higher binding energy per nucleon than the parent nuclei

Therefore, energy is released as a result of the mass difference between the parent nuclei and the daughter nuclei

Question 17

What is the definition for binding energy per nucleon?

Answer 17

The binding energy of a nucleus divided by the number of nucleons in the
nucleus.

Question 18

What does a higher binding energy per nucleon indicate?

Answer 18

A higher binding energy per nucleon indicates a higher stability
In other words, it requires more energy to pull the nucleus apart.

Question 19

What does binding energy per nucleon tell you?

Answer 19

In order to compare nuclear stability, it is more useful to look at the binding energy per
nucleon

Question 20

What element has the highest binding energy per nucleon and what does that mean?

Answer 20

Iron (A = 56) has the highest binding energy per nucleon, which makes it the most stable of
all the elements.

Question 21

Key features of the average binding energy per nucleon against number of nucleons in the nucleus

Answer 21

At low values of A:
Nuclei tend to have a lower binding energy per nucleon, hence, they are generally
less stable
This means the lightest elements have weaker electrostatic forces and are the most
likely to undergo fusion
Helium (4He), carbon (12C) and oxygen (16O) do not fit the trend
Helium-4 is a particularly stable nucleus hence it has a high binding energy per
nucleon
Carbon-12 and oxygen-16 can be considered to be three and four helium nuclei,
respectively, bound together
At high values of A:
The general binding energy per nucleon is high and gradually decreases with A
This means the heaviest elements are the most unstable and likely to undergo
fission.

Question 22

What value of A does fusion occur?

Answer 22

Fusion occurs at low values of A because:
Attractive nuclear forces between nucleons dominate over repulsive electrostatic
forces between protons
In fusion, the mass of the nucleus that is created is slightly less than the total mass of the
original nuclei
The mass defect is equal to the binding energy that is released since the nucleus that
is formed is more stable

Question 23

What values of A does fission occurs?

Answer 23

Fission occurs at high values of A because:
Repulsive electrostatic forces between forces begin to dominate, and these forces
tend to break apart the nucleus rather than hold it together
In fission, an unstable nucleus is converted into more stable nuclei with a smaller total mass
This difference in mass, the mass defect, is equal to the binding energy that is
released

Question 24

Why does fusion release more energy than fission?

Answer 24

Fusion releases much more energy per kg than fission
The energy released is the difference in binding energy caused by the difference in mass
between the reactant and products
Hence, the greater the increase in binding energy, the greater the energy released
At small values of A (fusion region), the gradient is much steeper
compared to the gradient at large values of A (fission region)
This corresponds to a larger binding energy per nucleon being released

Question 25

What is the definition of induced fission?

Answer 25

When a stable nucleus splits into small nuclei from the bombardment of a slow-moving neutron. For example, when a uranium-235 nucleus absorbs a neutron, it becomes a uranium-236 nucleus This uranium-236 nucleus is highly unstable and will decay almost immediately, which is why it is not usually shown in nuclear decay equations This isotope can then decay into smaller nuclei

Question 26

What are thermal neutrons?

Answer 26

Neutrons involved in induced fission are known as thermal neutrons Thermal neutrons have low energy and speed meaning they can induce fission This is important as neutrons with too much energy will rebound away from the uranium-235 nucleus and fission will not take place

Question 27

What is a chain reaction?

Answer 27

The products of fission are two daughter nuclei and at least one neutron The neutrons released during fission go on to cause more fission reactions leading to a chain reaction, where each fission goes on to cause at least one more fission

Question 28

What is the definition of a critical mass?

Answer 28

The minimum mass of fuel required to maintain a steady chain reaction

Question 29

What happens when you use less or more than the critical mass in a chain reaction?

Answer 29

Using exactly the critical mass of fuel will mean that a single fission reaction follows the last Using less than the critical mass (subcritical mass) would lead the reaction to eventually stop Using more than the critical mass (supercritical mass) would lead to a runaway reaction and eventually an explosion

Question 30

What is a moderator and its purpose?

Answer 30

The purpose of a moderator: To slow down neutrons The moderator is a material that surrounds the fuel rods and control rods inside the reactor core The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’ This ensures neutrons can react efficiently with the uranium fuel

Question 31

What are control rods and their purpose?

Answer 31

Purpose of a control rod: To absorb neutrons The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods Lowering the rods further decreases the rate of fission, as more neutrons are absorbed Raising the rods increases the rate of fission, as fewer neutrons are absorbed This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way so no reaction can take place

Question 32

What is a coolant and its purpose?

Answer 32

The purpose of coolant: To remove the heat released by the fission reactions The coolant carries the heat to an external boiler to produce steam This steam then goes on to power electricity-generating turbine

Question 33

How does the moderation of fission reactors work?

Answer 33

During fission, neutrons are released with high energies and must be slowed down by water moderation to maintain the chain reaction The first few collisions of a neutron with the moderator transfer sufficient energy to excite nuclei in the moderator with the neutrons being absorbed The subsequent collisions of a neutron with the moderator are elastic In these subsequent collisions, momentum is transferred to the moderator atoms With each collision, the neutron slows down until the average kinetic energy of the neutrons corresponds to that of the moderator nuclei Eventually (after about 50 collisions), the neutrons reach speeds associated with thermal random motion (hence the name thermal neutron) At these speeds, neutrons can cause fission rather than rebound off of the uranium nuclei

Question 34

What materials are moderators made from and why?

Answer 34

Moderators must be made from light nuclei which are not fissionable and will not absorb neutrons but will absorb a large amount of energy from them Graphite and water are commonly used for moderators

Question 35

What material are control rods made from and why?

Answer 35

Control rods must be made with non-fissionable materials This is so that they can absorb excess neutrons without decaying themselves Boron and cadmium are commonly used for control rods

Question 36

What material is used as coolant and why?

Answer 36

Often water is used as both the coolant and moderator This is because it has a high specific heat capacity meaning it can transfer large amounts of thermal energy Other materials such as molten salt or inert gas (e.g helium) are sometimes used as a coolant

Question 37

What is shielding and what material is used for it and why?

Answer 37

Another important component of a nuclear reactor is shielding Alpha and beta radiation can be stopped by a few cm of material, however, gamma radiation is much more penetrating Therefore, lead or concrete is needed to ensure there are no radiation leakages

Question 38

What material is used as nuclear fuel and why?

Answer 38

The fuel used in nuclear reactors is called enriched uranium This is U-238 enriched with U-235 as U-235 is the isotope that undergoes fission The U-238 isotope absorbs fission neutrons which helps to control the rate of fission reactions

Question 39

What are the measures in place to ensure that the worker's exposure is reduced?

Answer 39

Several measures are in place to reduce the worker’s exposure to radiation The fuel rods are handled remotely ie. by machines The nuclear reactor is surrounded by a very thick lead or concrete shielding, which ensures radiation does not escape In an emergency, the control rods are fully lowered into the reactor core to stop fission reactions by absorbing all the free neutrons in the core, this is known as an emergency shut-down

Question 40

What are the three types of nuclear waste?

Answer 40

There are three main types of nuclear waste: Low level Intermediate level High level

Question 41

What is low level waste and how do you dispose of it?

Answer 41

Low-level waste This is waste such as clothing, gloves and tools which may be lightly contaminated This type of waste will be radioactive for a few years, so must be encased in concrete and stored a few metres underground until it can be disposed of with regular waste

Question 42

What is intermediate waste and how do you dispose of it?

Answer 42

Intermediate-level waste This is everything between daily used items and the fuel rods themselves Usually, this is the waste produced when a nuclear power station is decommissioned and taken apart This waste will have a longer half-life than the low-level waste, so must be encased in cement in steel drums and stored securely underground

Question 43

What are the benefits of nuclear power?

Answer 43

Benefits Nuclear power stations produce no polluting gases They are highly reliable for the production of electricity They require far less fuel as uranium provides far more energy per kg compared to coal and other fossil fuels

Question 43

What is high level waste and how do you dispose of it?

Answer 43

High-level waste This waste comprises of the unusable fission products from the fission of uranium-235 or from spent fuel rods This is by far the most dangerous type of waste as it will remain radioactive for thousands of years As well as being highly radioactive, the spent fuel roads are extremely hot and must be handled and stored much more carefully than the other types of waste How high-level waste is treated: The waste is initially placed in cooling ponds of water close to the reactor for a number of years Isotopes of plutonium and uranium are harvested to be used again Waste is mixed with molten glass and made solid (this is known as vitrification) Then it is encased in containers made from steel, lead, or concrete This type of waste must be stored very deep underground

Question 44

What are the risks of nuclear power?

Answer 44

Risks The production of radioactive waste is very dangerous and expensive to deal with A nuclear meltdown, such as at Chernobyl, could have catastrophic consequences on the environment and to the people living in the surrounding area