Nuclear Flashcards

Question

How is radiation used in medicine?

Answer 1

- Radiation therapy - Gamma radiation is used to destroy cancerous cells. They are directed at the cancerous cells from different angles so only they receive the full radiation and healthy cells are less affected

Answer 2

- The target material needed to be extremely thin ~10^-6m thick - This is because a thicker foil would stop the alpha particles completely - Gold was chosen due to its malleability, meaning it could easily be hammered into thin sheets

Answer 3

- Applies to activity, count rate and intensity - ONLY applies to gamma rays, not alpha or beta radiation

Answer 4

- The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta or gamma particle

Answer 5

- Radioactive decay is a random process, which means: - There is an equal probability of any nucleus decaying - It cannot be known which particular nucleus will decay next - It cannot be known at what time a particular nucleus will decay - The rate of decay is unaffected by surrounding conditions - It is only possible to estimate the proportion of nuclei decaying in a given time period

Answer 6

- The random nature of radioactive decay can be demonstrated by observing the count rate of a Geiger-Muller (GM) tube - When a GM tube is placed near a radioactive source, the counts are found to be irregular and cannot be predicted - Each count represents a decay of an unstable nucleus - These fluctuations in count rate on the GM tube provide evidence for the randomness of radioactive decay

Answer 7

- Since radioactive decay is spontaneous and random, it is useful to consider the average number of nuclei that are expected to decay per unit of time - This is known as the average decay rate

Answer 8

- The decay constant λ is defined as the probability that an individual nucleus will decay per unit of time - Each radioactive element can be assigned its own decay constant

Answer 9

- The activity A of a radioactive sample is defined as the average number of nuclei that decay per unit time

Answer 10

- A = ΔN / Δt = -λN - ΔN = number of decayed nucleu - N = number of nuclei remaining in a sample - A = activity of sample (Bq)

Answer 11

- Becquerels (Bq) - An activity of 1 Bq is equal to one decay per second

Answer 12

- The greater the decay constant, the greater the activity of the sample - The activity depends on the number of undecayed nuclei remaining in the sample - The minus sign indicates that the number of nuclei remaining decreases with time

Answer 13

- In radioactive decay, the number of undecayed nuclei falls very rapidly, without ever reaching zero - This is known as exponential decay - Number of undecayed nuclei decreases with time at a decreasing rate

Answer 14

- The steeper the slope, the larger the decay constant - The shallower the slope, the smaller the decay constant

Answer 15

- N = N₀ e^(-λt) - N = number of undecayed nuclei at a certain time t - N₀ = initial number of undecayed nuclei (when time = 0) - λ = decay constant - t = time interval

Answer 16

- The number of nuclei can be substituted for other quantities, such a count rate and activity - A = A₀ e^(-λt) - C = C₀ e^(-λt)

Answer 17

- The molar mass, or molecular mass, of a substance is the mass of the substance (in grams) in one mole - Measured in g / mol

Answer 18

- mol = mass (g) / molar mass (g/mol)

Answer 19

- Avogadro's constant Nₐ is defined as the number of atoms in one mole of a substance, equal to 6.02 x 10^23

Answer 20

- The number of atoms (hence number of nuclei) can be found using: - number of nuclei = (mass x Nₐ) / molar mass

Answer 21

- Half-life is defined as the average time taken for a given number of nuclei of a particular isotope to halve - Since activity A is proportional to the number of undecayed nuclei N, the activity of the sample also halves

Answer 22

- We know that when t is equal to the half-life t₁⁄₂, the activity of N of the sample will be half of its original value, so N = 1/2N₀ - Therefore, 1/2N₀ = N₀ e^(-λt₁⁄₂) - Cancel out N₀: 1/2 = e^(-λt₁⁄₂) - Take logs: ln(1/2) = -λt₁⁄₂ - Make t₁⁄₂ the subject: t₁⁄₂ = ln2 / λ

Answer 23

- The half-life and the radioactive decay constant λ are inversely proportional - Therefore, the shorter the half-life, the larger the decay constant and the faster the decay

Answer 24

- Draw a line to the curve at the point where the number of nuclei (or activity) has dropped to half of its original value - Draw a line from the curve to the time axis, this is the half-life

Answer 25

- N = N₀ e^(-λt₁⁄₂) - Take logs of both sides: lnN = lnN₀ - λt₁⁄₂ - Rearrange in y = mx + c form: lnN = - λt₁⁄₂ + lnN₀ - Gradient (m) = -λ - x-axis = t (time) - y-axis = lnN - y-intercept = lnN₀

Answer 26

- Nuclear power - In medicing (radiotherapy, tracers) - Radiocarbon dating for archeological artefacts - Uranium-lead dating of rock samples - Radioisotope power stations

Answer 27

- All living organisms absorb carbon-14, but after they die they do not absorb any more - The proportion of carbon-14 is constant in living organisms as carbon is constantly being replaced during the period they are alive

Answer 28

- The isotope carbon-14 is commonly used in radioactive dating - It forms as a result of cosmic rays knocking out neutrons from nuclei, which then collide with nitrogen nuclei in the air to form carbon-14 + proton

Answer 29

- All living organisms constantly absorb carbon-14 - When they die, the activity of carbon-14 in the organic matter starts to fall, with a half-life of around 5730 years - Samples of living materials can be tested by comparing the current amount of carbon-14 in them and compared to the initial amount (based on the ratio of carbon-14 to carbon-12) - Hence, they can be dated

Answer 30

- Carbon dating is highly is highly reliable for estimating the ages of samples between 500 and 60000 years old

Answer 31

- The activity of the sample will be too high to measure small changes accurately - Therfore the ratio of carbon-14 to carbon-12 will be too high to determine an accurate age

Answer 32

- The activity will be too low to distinguish between changes in the sample and background radiation - Therefore, the ratio of carbon-14 to carbon-12 will be too small to determine an accurate age

Answer 33

- For many years, scientists could not agree on the age of the Earth - Until recently, the Earth was believed to be only millions of years old - Over the last century, radiometric dating methods have enabled scientists to discover the age of the Earth is many billions of years old - The most critical of these methods is uranium-lead dating

Answer 34

- Initially, there is only uranium in a rock, but over time, the uranium decays via a decay chain which ends with lead-206, which is a stable isotope - Uranium-238 has a half-life of 4.5 billion years - Over time, the raio of lead-206 atoms to uranium-238 atoms increases - The ratio of uranium to lead in a sample of rock can then be used to determine its age

Answer 35

- The decay of an isotope may release energy as heat - Radioisotope power systems are designed to transform this heat into electrical power - Such devices can be used to power space probes and satelites - Typically, plutonium-238 is used as fuel, with 1g generating a power output of about 500 mW

Answer 36

- The most common elements in the universe all tend to have values of N and Z less than 20 (and iron which has Z=26, N=30) - This is because lighter elements (with less protons) tend to be much more stable than heavier ones

Answer 37

- N = number of neutrons - Z = number of protons - A = number of nucleons

Answer 38

- Plot N (number of neutrons - y-axis) against Z (number of protons - x-axis) - Line of stability: increases with a gradient of 1 until N and Z = 20, then increases with a steeper gradient (N increases faster than Z) - Above the line of stability: β- emitters - Below the line of stability & below N=100: β+ emitters - Below the line of stability & above Z=82: α emitters

Answer 39

- If it has: - Too many neutrons - Too many protons - Too many nucleons (too heavy) - Too much energy

Answer 40

- When Z < 20: - All nuclei tend to be very stable - They follow the straight line N=Z

Answer 41

- When Z > 20: - The neutron - proton ratio increases - Stable nuclei must have more neutrons than protons

Answer 42

- At short ranges (1-3fm), nucleons are bound by the strong nuclear force - Below 1fm, the strong nuclear force is repulsive in order to prevent the nucleus from collapsing - At longer ranges, the electromagnetic force acts between protons, so more protons cause more instability - Therefore, as more protons are added to the nucleus, more neutrons are needed to add distance between protons to reduce the electrostatic repulsion - Also, the extra neutrons increase the amount of binding force which helps to bind the nucleons together

Answer 43

- It is useful in determining which isotopes will decay via alpha emission, beta-plus emission, beta-minus emission and electron capture

Answer 44

- Alpha emitters are found below the line of stability when Z>82, where there are too many nucleons in the nucleus (too heavy) - These nuclei have more protons than neutrons, but they are too large to be stable - This is because the strong nuclear force between the nucleons is unable to overcome the electrostatic force of repulsion between the protons - Hence, the nucleus emits an alpha particle (2 protons and 2 neutrons) to become lighter

Answer 45

- Beta-minus emitters are found to the left of the stability line, where the isotopes are neutron-rich compared to stable isotopes (have too many neutrons) - A neutron is converted to a proton and emits a β- particle (electron) and an anti-electron neutrino

Answer 46

- Beta-plus emitters are found to the right of the stability line, where the isotopes are proton-rich compared to stable isotopes (have too many protons) - A proton is converted to a neutron and emits a β+ particle (positron) and an electron neutrino

Answer 47

- Electron capture occurs when a nucleus captures one of its own orbiting electrons - As with β+ decay, a proton in the nucleus is converted into a neutron, releasing a gamma-ray (and an electron neutrino) - Hence, this also occurs to the right of the stability line, where the isotopes are proton-rich compared to stable isotopes

Answer 48

ᴬZX → ᴬ(Z+1)Y + ⁰₋₁e + ν̅ₑ

Answer 49

- Beta-minus decay occurs - One of the neutrons in the nucleus changes into a proton and a β- particle (electron), and an anti-electron neutrino is released - The nucleon number remains constant: neutron number decreases by 1 and proton number increases by 1

Answer 50

ᴬZX → ᴬ(Z−1)Y + ⁰₊₁e + νₑ

Answer 51

- β+ emission or electron capture occurs - In β+ emission, a proton changes into a neutron and a β+ particle positron) and an electron neutrino is released - In electron capture, an orbiting electron is taken in by the nucleus and combined with the proton, causing the formation of a neutron and an electron neutrino - In both decays, the nucleon number stays constant: the neutron number increases by 1, and the proton number decreases by 1

Answer 52

ᴬZX + ⁰₋₁e → ᴬ(Z−1)Y + νₑ

Answer 53

- The nucleus is too heavy - Alpha (α) emission occurs - An α particle is a helium nucleus - The nucleon number decreases by 4, and the proton number decreases by 2

Answer 54

ᴬZX → ᴬ⁻⁴(Z−2)Y + ⁴₂α

Answer 55

- Gamma (γ) emission occurs - The gamma particle is a high-energy electromagnetic radiation - This usually occurs after a different type of decay, such as alpha or beta decay - This is because the nucleus becomes excited and has excess energy

Answer 56

- A daughter nucleus is in an excited state after a decay - This excited state is usually very short-lived, and the nucleus quickly moves to its ground state - It does this either directly or via one or more lower-energy excited states

Answer 57

- Once an unstable nucleus has decayed, it may enter an excited state - Here, it will emit any remaining energy in the form of a gamma photon (γ) - The emission of a γ photon does not change the number of protons or neutrons in the nucleus, it only allows the nucleus to lose energy

Answer 58

- Technetium-99m is used as a γ source in medical diagnosis - The "m" stands for metastable which means the nucleus exists in a particularly stable excited state

Answer 59

- Technetium-99m is the decay product of molybdenum-99 which can be found as a product in nuclear reactors

Answer 60

- ⁹⁹₄₂Mo → ⁹⁹₄₃Tc + ⁰₋₁e + ν̅ₑ - ⁹⁹₄₃ᵐTc → ⁹⁹₄₃Tc + γ

Answer 61

- Molybdenum-99 has a half-life of 66 hours. This is long enough for the sample to be transported to hospitals. Subsequently, the technetium-99m can be separated at the hospital - Technetium-99m has a half-life of 6 hours. This is an adequate timeframe for examining a patient and not too long to cause any damage

Answer 62

- The decay mode (usually alpha or beta) is shown by a diagonal line - The excited state, or states, are generally stacked in descending energy order to the right of the decay - Gamma decay is shown by a vertical downwards arrow

Answer 63

- Can be calculated experimentally using: - Rutherford scattering: the closest approach method - Electron scattering

Answer 64

- The Rutherford scattering experiment indicates that there must be an electrostatic repulsion between the alpha particles and the gold nucleus, as some of the alpha particles are found to rebound from the gold foil by 180° - The alpha particle is fired with an initial kinetic energy Ek. At the point of closest approach r, the repulsive force reduces the speed of the alpha particles to zero momentarily - At this point, the initial kinetic energy of the alpha particle is equal to the electric potential energy of the gold nucleus - Ek = Ep (kinetic energy = potential energy) - The closest approach method gives an estimate of the upper limit of the radius of a nucleus

Answer 65

- At the point of closest approach r, the initial kinetic energy of the alpha particle is equal to the electric potential energy of the nucleus - Ek = Ep - ½mv² = Qq / (4πε₀r) - Q = 2e (charge of an alpha particle) - q = Ze (charge of target nucleus) - Rearrange to find: - r = Ze² / (πε₀mv²) - Z = proton number of target nucleus - e = elementary charge (e) - m = mass of alpha particle - v = initial velocity of alpha particle

Answer 66

- Gives a good estimate of the upper limit for a nuclear radius - Alpha particles are scattered only by the protons and not all the nucleons that make up the nucleus

Answer 67

- Alpha scattering does not give an accurate value for nuclear radius as it will always be an overestimate. This is because it measures the smallest separation between the alpha particle and the nucleus, not its radius - The nucleus will recoil as the alpha particle approaches - Very few alpha particles will rebound at exactly 180° - Alpha particles are hadrons, hence they will be affected by the strong nuclear force, which is not accounted for in the calculations

Answer 68

- Electrons accelerated to close to the speed of light are found to have wave-like properties, such as the ability to diffract - When a beam of electrons is directed at a thin target, each electron will diffract around a nucleus. This happens because the de Broglie wavelength of a high-speed electron is similar to the size of a nucleus - The resulting diffraction pattern that forms is a central bright spot with dimmer concentric circles around it - The size of the nucleus can be determined using the angle of the first minimum intensity

Answer 69

- For the electrons to diffract around a nucleus, the de Broglie wavelength must be similar to the size of a nucleus - The de Broglie wavelength of an electron is equal to: λ = h / mv - Therefore, to decrease the de Broglie wavelength, we must increase the speed of the electron

Answer 70

- Much more accurate than the closest approach method - Gives a direct measurement of the radius of a nucleus - Electrons are leptons, so they are not affected by the strong nuclear force

Answer 71

- Electrons must be accelerated to very high speeds to maximise the resolution - This is because significant diffraction takes place when the electron wavelength is similar in size to the nucleus. Higher speeds are needed to shorten the de Broglie wavelength since λ = h / mv - Electrons can be scattered by both protons and neutrons. If there is an excessive amount of scattering, then the first minimum of the electron diffraction can be difficult to determine

Answer 72

- We can plot the graph of intensity against angle obtained through electron diffraction - The pattern formed by this diffraction has a predictable minimum which forms at an angle θ to the original direction - To determine an accurate value for nuclear radius R, a multiplication factor of 1.22 is required - sinθ = 1.22λ / 2R - θ = angle of the first minimum

Answer 73

- Nuclear radii are measured by the order of 10^-15m of 1fm

Answer 74

- The graph starts with a steep gradient at the origin, and the gradient gradually decreases to almost zero - This shows that as more nucleons are added to the nucleus, the nucleus gets bigger however, the number of nucleons A is NOT proportional to the size R

Answer 75

- R = R₀A^(1/3) - R = nuclear radius - R₀ = constant of proportionality = 1.05fm - A = nucleon / mass number

Answer 76

- Assuming that the nucleus is spherical, its volume is equal to: V = 4/3 πR³ - We know R is the nuclear radius: R = R₀A^(1/3) - Combining the equations gives: V = 4/3 πR₀³A - Therefore, the nuclear volume V is proportional to the mass of the nucleus A (as π and R₀ are constant)

Answer 77

- We know that the volume of a nucleus is: V = 4/3 πR₀³A - We also know that ρ = m / V - Further, we know that the mass of a nucleus is equal to: m = Au where A is the mass number and u is 1 atomic mass unit - Combining the 3, we find that the density of a nucleus is equal to: ρ = 3u / (4πR₀³)

Answer 78

- Since the mass number A cancels out, the remaining quantities in the equation are all constant - Therefore, this shows the density of the nucleus is constant and independent of the radius - This shows that nucleons are spread evenly throughout the nucleus, regardless of their size

Answer 79

- Nuclear density is significantly larger than atomic density - This shows that the majority of the atom's mass is contained in the nucleus - The nucleus is very small compared to the atom - Atoms must be predominantly empty space

Answer 80

- Matter can be considered a form of energy - Therefore, mass can be converted into energy - Energy can be converted into mass

Answer 81

- E = mc² - E = energy (J) - m = mass (kg) - c = speed of light (m/s)

Answer 82

- 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

Answer 83

- The difference between a nucleus's mass and the sum of the masses of its protons and neutrons - The total mass of a nucleus is less than the sum of the masses of its constituent nucleons. This difference is known as mass defect

Answer 84

- Due to the equivalence of mass and energy, the mass defect implies that energy is released in the process - Since nuclei are made up of neutrons and protons, there are forces of repulsion between the positive protons - Therefore, it takes energy (binding energy) to hold the nucleons together as a nucleus

Answer 85

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

Answer 86

- Since we know that there is a mass defect, and that mass and energy are proportional, we also know that the total energy of a nucleus is less than the sum of the energies of its nucleons - Therefore, the formation of a nucleus is an exothermic reaction: it releases energy

Answer 87

- E = Δmc² - Δm = mass defect (kg)

Answer 88

- Binding energies are measured in MeV - This is much larger than the few eV associated with the binding energies of electrons - This means that nuclear reactions release much more energy than chemical reactions

Answer 89

- It is roughly equal to the mass of one proton or neutron - 1u = 1.66 x 10^-27 kg

Answer 90

- Since mass and energy are interchangeable, the atomic mass unit can be expressed in MeV - 1u = 931.5 MeV

Answer 91

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

Answer 92

- Low mass nuclei such as hydrogen and helium

Answer 93

- A deuterium nucleus is produced - A positron and electron neutrino are also produced as one of the protons converts into a neutron through beta-plus decay

Answer 94

- Four hydrogen nuclei fuse to produce a helium nucleus, plus the release of energy - This provides fuel for the star to continue burning

Answer 95

- Research is focused on achieving the deuterium-tritium reaction - This involves fusing a deuterium nucleus and a tritium nucleus to form a helium nucleus and a neutron

Answer 96

- It takes a great deal of energy to overcome the electrostatic force of repulsion between protons in two nuclei - Therefore, fusion can only be achieved in an extremely hot and dense environment, such as the core of a star

Answer 97

- Both nuclei must have high kinetic energy. This is because the nuclei must be able to get close enough to fuse; however, two forces acting within the nuclei make this difficult: - Electrostatic repulsion as the nuclei are positively charged, meaning they will repel each other - Strong nuclear force: the force binds nucleons together but only acts at very short distances. Therefore, nuclei must get very close together for the strong nuclear force to take effect

Answer 98

- The splitting of a large atomic nucleus into smaller nuclei

Answer 99

- High mass nuclei (such as uranium) can undergo fission and release energy

Answer 100

- Fission might be induced by firing neutrons at a nucleus - When a neutron strikes a nucleus, it splits into two or more daughter nuclei - During fission, neutrons are ejected from the nucleus, which in turn can collide with other nuclei, triggering a cascading 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

Answer 101

- Fission is the process which produces energy in nuclear power stations, where it is well controlled

Answer 102

- The chain reaction can cascade to produce the effects of a nuclear bomb

Answer 103

- The binding energy is equal to the amount of energy released in forming a nucleus. It can be calculated by: - E = Δmc² - E = binding energy released (J) - Δm = mass defect (kg)

Answer 104

- 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 parents and the daughter nuclei

Answer 105

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

Answer 106

- It is used to compare nuclear stability - A higher binding energy per nucleon indicates a higher stability - This is because more energy is required to separate the nucleons contained within the nucleus

Answer 107

- By plotting a graph of binding energy per nucleon against nucleon number (atomic mass number)

Answer 108

- When A (nucleon number) is low (A < 56): - Nuclei have lower binding energies per nucleon than at large values of A, but they tend to be stable when N=Z - This means light nuclei have weaker electrostatic forces and will undergo fusion - The gradient is much steeper compared to the gradient at large values of A. This means that fusion reactions release a greater binding energy than fission reactions

Answer 109

- When A (nucleon number) is high (A > 56): - Nuclei have generally higher binding energies per nucleon, but this gradually decreases with A - This means the heaviest elements are the most unstable and will undergo fission - The gradient is less steep compared to the gradient at low values of A - This means that fission reactions release less binding energy than fusion reactions

Answer 110

- Iron (A = 56) has the highest binding energy per nucleon, which makes it the most stable element

Answer 111

- In both fission and fusion, the total mass of the products is slightly less than the mass of the reactants - The mass defect is equivalent to the binding energy that is released - Both fusion and fission release energy

Answer 112

- In fusion, two smaller nuclei fuse to form a larger one. In fission, one larger nucleus splits to form two smaller nuclei - Fusion occurs between light nuclei (A<56), and fission occurs between heavy nuclei (A>56) - Fusion releases much more energy per kg than fission - Fusion requires a greater initial input of energy than fission

Answer 113

- When a stable nucleus splits into small nuclei due to the absorption of a slow-moving neutron

Answer 114

- When a nucleus of Uranium-235 absorbs a neutron, it becomes Uranium-236 - A Uranium-236 nucleus is highly unstable and will decay into two smaller nuclei almost immediately - This is why it is not usually shown in nuclear decay

Answer 115

- They have low kinetic energy - They are slow moving

Answer 116

- Only slow-moving neutrons can be absorbed by Uranium-235 - If a fast-moving neutron is incident on a Uranium-235 nucleus, it will rebound from it

Answer 117

- A neutron which is in thermal equilibrium with its surroundings

Answer 118

- In nuclear reactors, neutrons are slowed until they are in thermal equilibrium with the moderator - This corresponds to a core reactor temperature of about 300K

Answer 119

- Thermal neutrons have kinetic energies associated with 3/2KT

Answer 120

- The products of fission are two daughter nuclei and two or three neutrons - 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

Answer 121

- The minimum mass of fuel required to maintain a steady chain reaction - Rate of neutron loss = rate of neutron creation by fission

Answer 122

- This is achieved by using a precise amount of uranium fuel, known as the critical mass

Answer 123

- Using exactly the critical mass of fuel will mean that a single fission reaction will follow the last - This will allow the nuclear reaction to be self-sustaining and controlled

Answer 124

- Using less than the critical mass - This would lead the reaction to eventually stop

Answer 125

- Using more than the critical mass - This would lead to a runaway reaction and eventually an explosion

Answer 126

- When the reactor is producing energy at the required rate, two factors must be controlled: - The number of free neutrons in the reactor - The energy of the free neutrons

Answer 127

- A moderator - Control rods - Coolant

Answer 128

- To slow down neutrons

Answer 129

- 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 - Usually made from graphite or water

Answer 130

- A material that surrounds the fuel rods and control rods inside the reactor core - They are used to slow down neutrons without absorbing them

Answer 131

- 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 - This ensures the neutrons can react efficiently with the uranium fuel

Answer 132

- To absorb neutrons

Answer 133

- Control rods are made of a material that absorbs neutrons without becoming dangerously unstable or decaying themselves - Usually made of boron or cadmium

Answer 134

- The number of neutrons absorbed is controlled by varying the depth of the control rods in the reactor core - 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

Answer 135

- Control rods are 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 be shut down, the control rods can be lowered all the way, so no reaction can take place

Answer 136

- To transfer thermal energy efficiently between the water systems of a nuclear power plant

Answer 137

- The coolant (usually water) used in the reactor vessel - The water and steam that drive the turbine - The condenser that cools the steam

Answer 138

- The heat exchanger mediates the thermal energy exchanges between the water systems of a nuclear reactor

Answer 139

- The coolant is a substance, such as water, that is pumped into the reactor at a cold temperature to extract the heat released by the fission reactions - In the heat exchanger, the coolant transfers the heat to water that is pumped in externally to produce steam - This steam then goes on to power electricity-generating turbines

Answer 140

- Water has a high specific heat capacity - This means it can transfer large amounts of thermal energy

Answer 141

- Water - Sometimes molten salt or inert gas

Answer 142

- The first few collisions transfer sufficient energy to excite nuclei in the moderator without being absorbed - Then they subsequently de-excite, this energy is released as gamma radiation

Answer 143

- After the first few collisions, the following collisions are elastic - In these collisions, momentum is transferred to the moderator atoms - With each elastic collision, the neutron slows down until the average kinetic energy of the neutrons corresponds to that of the moderator nuclei

Answer 144

- Eventually (after about 50 collisions), the neutrons reach speeds associated with thermal random motion - At these speeds, neutrons can cause fission rather than rebound off the uranium nuclei

Answer 145

- 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

Answer 146

- Some substances have a very long half-life - This means that they will be emitting harmful radiation well above background radiation for a very long time

Answer 147

- Common methods are water tanks and storage underground - This is to prevent damage to people and the environment now and for many years into the future - Sealing them underground means that they are less likely to be dislodged or released due to natural disasters

Answer 148

- The fuel used in nuclear reactors - 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

Answer 149

- The fuel rods are handled remotely (by machines) - The nuclear reactor is surrounded by a thick lead or concrete shielding, so radiation doesn't escape - In an emergency, the control rods are fully lowered into the reactor core to stop fission reactions by absorbing all of the free neutrons in the core

Answer 150

- Low-level - Intermediate-level - High-level

Answer 151

- This is waste such as clothing, gloves and tools which may be slightly contaminated

Answer 152

- This type of waste will be radioactive for a few years, so it must be encased in concrete and stored a few metres underground until it can be disposed of with regular waste

Answer 153

- This is everything between the daily used items and the fuel rods themselves - Usually, this is the waste produced when a nuclear power station is decommissioned and taken apart

Answer 154

- This waste will have a longer half-life than low-level waste, so it must be encased in cement in steel drums and stored securely underground

Answer 155

- This waste comprises 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

Answer 156

- It will remain radioactive for thousands of years - The spent fuel rods are extremely hot and require additional care when being handled and stored

Answer 157

- The waste is initially placed in cooling ponds near 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

Answer 158

- It is much more recent and controversial

Answer 159

- A significant proportion of the world's electricity is generated by nuclear fission reactors - The threat of nuclear weapons highlights the importance of international cooperation and peace

Answer 160

- People in society tend to have mixed feelings about nuclear power, with some viewing it positively, while others are fearful - With increased education on nuclear energy, people in society can use this knowledge to inform their own decisions and opinions

Answer 161

- Power stations produce very little pollution (no carbon dioxide or sulphur dioxide) - Highly reliable source - Can adjust output to meet higher demand - Nuclear fuel has the highest energy density of any fuel - Nuclear reactors produce some useful by-products (such as molybdenum-99)

Answer 162

- Nuclear fuels are non-renewable energy resources - The production of radioactive waste is very dangerous and expensive to deal with - Commissioning and decommissioning a nuclear power station is expensive and time-consuming - A nuclear meltdown could have catastrophic impacts on the environment and residants

Nuclear Flashcards

(186 cards)