Nuclear Physics Sem 2

Question 1

Bohr Model of An Atom

Answer 1

The atom consists of a central core called the nucleus which contains the mass of the atom and is positively charged. It is made up of positively charged protons with a relative mass of one, and neutrally charged neutrons (no charge) with a relative mass of one. Negatively charged electrons orbit the nucleus at high speeds in electron shells in the electron cloud, they have a relative mass of 1/1836.

Question 2

Atomic Number

Answer 2

The number of protons in the nucleus of an element (Z)

Question 3

Atomic Mass

Answer 3

The number of protons and neutrons in the nucleus (A)

Question 4

Proton

Answer 4

A positively charged particle in the nucleus with the mass of one

Question 5

Neutron

Answer 5

A neutrally charged (no charge) particles in the nucleus with the mass of one

Question 6

Nucleons

Answer 6

Any particle in the nucleus (protons/neutrons)

Question 7

Strong Nuclear Force

Answer 7

One of the four fundamental forces, it is very strong over short ranges and hold nuclei together by binding quarks (small particles within protons) together to overcome the electromagnetic force that makes protons want to repel each other.

Question 8

Electromagnetic force

Answer 8

One of the four fundamental forces, it acts between charged particles and makes those of opposite charges want to attract each other and those of similar charges to repel each other.

Question 9

Radioactive decay

Answer 9

The process by which unstable nuclei emit radiation in order to loose energy and become more stable. They can emit high energy alpha or beta particles or they can emit high energy electromagnetic photons such as gamma rays.

Question 10

Strong Nuclear Force, Electromagnetic Force and Stability of Nucleus

Answer 10

Within the very small nucleus (nanometres big), strong nuclear force takes over the repulsion between protons from electromagnetic and bings nucleons together forming a positively charged nucleus. The role of neutrons is essential as they increase the amount of strong nuclear force without adding extra repulsion hence more neutrons than protons allows a nucleus to be more stable (especially in larger atoms). However, if strong nuclear force cannot counterbalance the electromagnetic repulsion the nucleus becomes unstable and undergoes radioactive decay. This can occur for two reasons:
1) The nucleus is too big and have too many nucleons in it. This means that strong nuclear force cannot act over the whole nucleus (out of short range), and electromagnetic repulsion takes over part of the nucleus causing instability.
2) The neutron to proton ratio is unfavourable. If there are too many or too less neutrons the strong nuclear force becomes weak (not enough of it) and cannot balance out the electromagnetic repulsion causing instability in the atom.

Question 11

Binding Energy and the stability of a nucleus

Answer 11

binding energy is how strongly each nucleon is held in the nucleus. The stronger the strong nuclear force, the higher the binding energy per nucleon the more stable the nucleus is as the nucleus is held together more tightly. Fe-56 is the most stable atom, as it has the highest binding energy per nucleon and the most favourable proton to neutron ratio. Nuclei that have too many or too less nucleons, don’t have enough strong nuclear force and have a lower binding energy per nucleon as the nucleus is held together more weakly it is more unstable.

Question 12

Binding Energy, Stability and Radioactive Decay

Answer 12

In order to become more stable, large nuclei loose energy and/or mass through radioactive decay by emitted particles or photons. As these particles become smaller, strong nuclear force can act over more of the nucleus, increasing binding energy holding the nucleus together more tightly, allowing the atom to adopt a more stable configuration. Hence why large atoms such as U-238 emits an Alpha particle to decreases the size of its nucleus and become more stable.

Question 13

Radiation

Answer 13

The transfer of energy through space from a source. It can be emitted as particles or waves.

Question 14

Ionising radiation

Answer 14

Particles or electromagnetic waves that have enough energy to ionise an atom (the atom gains/looses electric charge and becomes an ion by gaining or loosing electrons). For example alpha, beta particles and indirectly gamma ray.

Question 15

Non ionising radiation

Answer 15

microwaves and infrared waves.

Question 16

Isotopes

Answer 16

a version of an element with the same number of protons but a different number of protons. They have identical chemical properties but ranging physical properties such as boiling points and some isotopes have unstable nuclei and undergo radioactive decay to become more stable.

Question 17

Types of Radiation

Answer 17

Alpha, Beta, Gamma

Question 18

Alpha Radiation Properties

Answer 18

Occurs when an atom is too heavy and the nucleus is too big causing instability and decay. An alpha particle is released from the nucleus that contains 2 protons and 2 neutrons. Atomic mass decreases by 4 and atomic number decreases by 2. An alpha particle has a mass of 4 amu, and hence it is big and heavy. This means it travels slowly through air giving it a low penetrating ability (<5cm in air) and it can be stopped by paper and skin. However its large size means it has a high ionising ability as it large size means it can knock electrons out of atoms easily, ionising them. The particle itself is positively charged (made up of 2 neutrons).
After alpha decay a nucleus not only has less mass but also has less energy as it looses energy to heat energy when the alpha particle flies out. The atom that is left is now negatively charged.

Question 19

Beta Radiation

Answer 19

When a nucleus has too many neutrons it undergoes Beta decay where a neutron is converted to a proton and an electron to become more stable. The proton stays in the nucleus and the electron is ejected out of the atom and this is the beta particle. This beta particle is very small and has the mass of an electron which is 0.00055u. This small size means it has medium penetrating ability (<1m in air) and it can be stopped by a considerably thick piece of aluminium. The beta particles travels very fast at 98% of the speed of light (3 x 10^8 ms-1), and they have a medium ionising ability due to their small size meaning they have a lower chance of knocking electrons out and ionising an atom. The particle itself is negatively charged. It leaves the atom positively charged and with less energy and less neutrons.

Question 20

Gamma Radiation

Answer 20

When a nucleus is left excited after going through alpha or beta decay it has to release a gamma ray in order to loose energy and become de-excited and maintain stability. A gamma ray is an electromagnetic photon (a waves of particles) and has no mass and no charge. As it has no mass and is just a wave it has a high penetrating ability (< 1km in air) and it can only be stopped by lead and other heavy metals or concrete. It travels at the speed of light and cannot directly ionise other atoms. However it can indirectly, because interactions with the gamma ray produced can cause an atom to undergo alpha or beta decay which then releases alpha/beta particles that can ionise. The atom left behind is the same isotope but with less energy and more stable.

Question 21

Important Note about stopping materials

Answer 21

the particles stop at different distances within the materials, some might even make it all the way through. Sometimes some beta particles are stopped by paper or gamma rays by aluminium, but the materials are a general guide.

Question 22

Alpha Decay Equation

Answer 22

A X Z -> A-4 Y Z-2 + 4 a 2

Question 23

Beta Decay Equation

Answer 23

A X Z -> A Y Z+1 + 0 B -1

Question 24

Gamma Decay Equation

Answer 24

Am X Z -> A X Z + 0 y 0

Question 25

Mass of Proton

Answer 25

1.007276 u

Question 26

Mass of Neutron

Answer 26

1.008665 u

Question 27

Mass of Electron

Answer 27

0.00055 u

Question 28

Half life

Answer 28

the time it takes for one half of the atomic nuclei to decay

Question 29

Why are some isotopes unstable?

Answer 29

Some isotopes are unstable because they have differing number of neutrons. When certain isotopes have too many or too little number of neutrons they become unstable and have to undergo nuclear decay to become stable. When they have too many neutrons, the nucleus becomes too big for the strong nuclear force to act over the whole nucleus and electromagnetic force takes over part of the nucleus causing repulsion and instability. When there are not enough neutrons there is not enough total strong nuclear force (neutrons add strong nuclear force without adding repulsion) to balance the electromagnetic force that causes repulsion of protons and hence there is nuclear instability.

Question 30

Half Life Graphs

Answer 30

1) Title 2) It is a curved line of best fit that should resemble exponential decay. Plot points then draw line of best fit 3) X axis = time/ number of half lives passed (units) 4) Y axis = mass (Kg), percentage left (%), number of nuclei (billions undecayed), activity (counts per sec or becquerel - number of nuclei decay in one second) 5) for working out, you go halfway down your axis and corresponding x value is half life and draw dotted lines

Question 31

Half life calculations

Answer 31

A = Ao x (1/2)^n. n = t/T-half if its the number of half lives passed then n should be rounded down always answer with units and 2 decimal points to solve for n which isn't a nice indice use: n = ln(A/Ao) / ln(1/2)

Question 32

Fusion

Answer 32

A nuclear reaction in which two or more light nuclei combine to form one or more heavier atomic nuclei and subatomic particles. It produces a heavier atomic nuclei, a neutron and energy primarily in the form of heat.

Question 33

Fission

Answer 33

When an unstable heavy nucleus, split to form two or more smaller more stable nuclei, releasing fast neutrons and energy in the form of heat. The process of fission: 1) An unstable heavy nuclei absorbs a slow neutron (incident neutron if fission doesn't occur spontaneously), causing the nuclei to split into two or more smaller and more stable nuclei, releasing energy and fast neutrons. 2) These neutrons then collide with other fissile nuclei (have the ability to undergo fission) causing them to split and undergo fission starting a chain reaction of fission products and binding energy released increases exponentially.

Question 34

Compare and Contrast fission and fusion

Answer 34

Both reactions, generate energy primarily in the form of heat which can be utilised in order to generate electricity. Differences: - Fission splits an atom into smaller nuclei, fusion join nuclei into a heavier atomic nucleus - Fission rarely occurs naturally, fusion occurs naturally in the sun everyday - Fission is the principle of the atomic bomb, fusion is the principle of the hydrogen bomb. -Fission requires less energy to start and produces less energy compared to fusion which requires more to start and produces more - Fission uses uranium as a fuel in reactions and fusion uses deuterium and tritium,

Question 35

Predicting energy changes in fission and fusion reactions

Answer 35

Both fission and fusion are exothermic reactions because they form products that are more stable and tightly binned together which results in a mass defects which according to Einsteins equation is converted into energy that is released as heat/light/motion. Hence these reactions are exothermic. The energy release can be calculated by calculating mass defect and inputing it into the equation E =mc^2. Mass defect is calculated by mass of reactants - mass of products, and you use the masses on the data sheet which are exact.

Question 36

Fission Equation

Answer 36

1n0 + 235U92 -> 141Ba56 + 92Kr36+ 3-1n0 + energy

Question 37

Fusion Equation

Answer 37

2H1 + 3H1 -> 4He2 + 1n0 + energy

Question 38

Rules for fusion and fission equations

Answer 38

- the reactants products do not have to necessarily be what they are - top and bottom row should be equal - coefficient for neutrons is written separately -> spontaneous fission means no neutron is a reactant

Question 39

Fusion/Fission + Mass defect + energy released

Answer 39

Mass defect in fusion: The larger nucleus formed has less mass than the two original nuclei combined. This is because the larger nucleus is more stable and has a larger binding energy per nucleon (less electromagnetic force causing repulsion) meaning the nucleus is more tightly bound together and has a smaller mass. Hence the products have less mass than the reactants and this difference is called the mass defect. Mass Defect in fission: The smaller nuclei formed together have less mass than the original larger nuclei. This is because the smaller nuclei formed are more stable and have a higher binding energy per nucleon (more strong nuclear force less electromagnetic repulsion), which means these smaller nucleus are more tightly packed with less mass, and together the products have less mass than the reactants. This loss mass is known as the mass defect. the mass defect formed because of the difference in binding energies between products and reactants is converted into energy according to Einstein's equation e =mc^2 and then released as products are in a lower energy (more stable) state primarily as heat, light or kinetic energy.

Question 40

Einstein's equation

Answer 40

When isolated nucleons are combined to form a stable nucleus energy is released. The energy emitted is created by converting a small fraction of nuclear mass into energy. This occurs via the equation E =mc^2. E is energy in MeV or J, m is mass in kg or amu. c = 3 x 10^8 ms-1. If mass is in amu then use c^2 = 931.5 MeV.

Question 41

Mass Defect

Answer 41

delta m = m(reactants) - m(products). The mass of the products of the nucleus will always be less than the individual nucleons and reactants added together because the new nucleus is more stable and tightly packed hence having a smaller mass. This small fraction of the nuclear mass is converted into energy (binding energy) and released as the nucleus formed is more stable.

Question 42

Binding Energy

Answer 42

The energy required in order to split an atom into its component parts. It is the energy equivalent of the mass defect. And the same amount of energy is released to bind a nucleus and put into it to split it.

Question 43

What are 3 factors that need to be addressed for nuclear reactors?

Answer 43

1) Neutrons released by U-235 need to be slowed in order to enable radiation of other U-235 atoms efficiently and with control 2) if every neutron released by U-235 were to induce a chain reaction there would be an explosion 3) Heat released from the reaction has to be harnessed to generate electricity

Question 44

Main components of nuclear reactor

Answer 44

Reactor vessel, control rods, fuel rods, steam generator, moderator.

Question 45

Reactor vessel

Answer 45

A thick walled steel container that houses the reactor core which contains the moderator and fuel and control rods built into it. It encases and prevents radiation from escaping into the environment and harming workers. It is highly pressurised as it contains the high temperature liquid coolant which is pumped around the reactor vessel and reactor core not only to slow down fast moving neutrons but also to absorb and remove heat which can be transferred to the steam generator to produce electricity and also to prevent overheating of the reactor.

Question 46

Control rods

Answer 46

They control the number of neutrons involved in the fission reaction and products by being inserted and withdrawn from the reactor core to regulate the nuclear fission rate. They are made of materials such as Boron and Cadmium which absorb slow neutrons without undergoing fission. When they are inserted, there is more absorption of neutrons which slows down or stops the chain reaction, decreasing rate of reaction preventing a melt down or overheating. Whereas if they are withdrawn, there is less absorption, more chain reaction, more energy and high reaction rate.

Question 47

Fuel Rods

Answer 47

They are thin aluminium rods which contain 97.7% U-238 which is non fissile and absorbs slow moving neutrons without undergoing fission. The other 2.3% is U-235 or Pu-239 pellets which are fissile and undergo fission releasing heat energy when bombarded with neutrons. The fuel rods are housed in the reactor core and need to be changed approximately every 4 years.

Question 48

Steam Generator

Answer 48

Includes the heat exchanger, turbine and generator. The steam generator is a heat exchanged of sorts as in pressurised water reactors is transfer heat collected from the reactor core/vessel by the primary coolant loop (highly pressurised so liquid doesn't boil) and transfer the heat to the secondary water loop (not pressurised) which produces steam which is used to turn a turbine and generate electricity in the generator. In Boiling water reactors, steam is generator directly in the primary loop and then the steam is used to turn the turbine and produce electricity

Question 49

Moderator

Answer 49

Slows down fast moving neutrons. Made of materials with small nuclei such as graphite, normal water, heavy water (made with deuterium) - most effective but expensive, and carbon dioxide. These moderators decrease the kinetic energy of the neutrons, making them slow neutrons and allowing them to efficiently induce fission in other U-235 or Pu-239 atoms.

Question 50

Step of a Nuclear Fission Reactor

Answer 50

Nuclear Fission in the reactor core, heat transfer, steam to electricity, cooling and recycling

Question 51

Nuclear Fission In the Reactor Core

Answer 51

In the reactor core fuel rods containing U-235 and Pu-239 undergo fission by being bombarded by neutrons splitting the nucleus releasing heat energy and more neutrons. The chain reaction is controlled by control rods (made of boron and cadmium which are non fissile and absorb neutrons) and they are inserted and withdrawn to regulate the number of neutrons involved in the reaction to ensure a steady, controlled release of heat energy rather than an explosion. The moderators slows down neutrons to sustain the reaction efficiently.

Question 52

Heat Transfer

Answer 52

The heat produced by a fission reaction is absorbed by a coolant (high temperature water kept liquid by being highly pressurised). In pressure water reactors, the hot coolant (300-320 deg C) flows to the steam generator where is transfers and heats up a secondary loop of water which is unpressurised and boils to produce steam. In boiling water reactions the steam is produced in the reactor vessel, as the heat from the reactions boils the unpressurised water forming steam.

Question 53

Steam to Electricity

Answer 53

The high pressure steam turns a turbine which is connected to a generator. The generator converts the mechanical energy of the turbine to electrical energy through electromagnetic induction and the power is then sent to the grid.

Question 54

Cooling and Recycling

Answer 54

Once steam has passed through the turbine it is cooled in a condenser using water from a nearby source or in a cooling tower to turn steam back to liquid water which can be used in the steam generator again. The coolant is recycled back to the reactor vessel in order to absorb more heat and the cycle repeats.

Question 55

uncontrolled chain reaction

Answer 55

Occurs in reactors due to failures in the coolant or control system resulting in a meltdown or steam explosion. In fission reactions, once they begin neutrons multiply exponentially as every U-235 atoms releases 2-3 neutrons and if these neutrons aren't absorbed or slowed the amount of fission reactions increases rapidly, causing an exponential increase in the amount of energy produced. If nuclear energy and heat increases too quickly it can cause an explosion

Question 56

controlled chain reaction

Answer 56

In reactors, the reaction has to be carefully controlled. Neutrons are released at a rate that sustains the reaction but doesn't escalate it. In reactors control rods absorb extra neutrons so that there is a steady, predictable, controlled release of heat for energy production. If there is too much heat the reactor automatically shuts down.

Question 57

Advantages of nuclear energy

Answer 57

1) High energy density: a small amount of nuclear fuel can produce alot of energy. eg 1 kg u-235 produces energy equivalent of 270000 tones of coal. 2) Low greenhouse gas emissions: Other than building, in daily operations there are no greenhouse gas emissions 3) Reliable Power: Electricity can be produced all year round not dependant on seasons or weather like solar. 4) Low Operating Costs: Once running, they have relatively low operating cost. The cost of uranium is small and with proper maintenance plants can work for decades.

Question 58

Disadvantages of nuclear energy

Answer 58

1) Takes a long time to build and expensive: takes billions of dollars and 5-10 years. 2) Safety Concerns: Nuclear meltdowns and failures can be life threatening to workers and people in surrounding cities eg Chernobyl and Fukushima. 3) Radioactive Waste: Waste produced stays radioactive for thousands of years afterwards and needs adequate handling. Mishandling can be detrimental for the environment. 4) Decomissioning Challenges: after a plant is done being used decommissioning is expensive, time consuming (decade) and produces alot more radioactive waste that needs to be stored somewhere.