Nuclear Power

Question 1

What are nuclear power stations?

Answer 1

1. Nuclear power stations generate about 20% of the UK’s energy using controlled nuclear fission reactions to produce heat used to generate electricity
2. Nuclear fission can be induced in some isotopes, including those of uranium and plutonium, by making the nucleus unstable when it absorbs a neutron

Question 2

What is the nuclear fuel used?

Answer 2

1. The nuclear fuel used in most nuclear power stations contains an isotope of uranium, U-235
2. The nucleus of the U-235 contains 92 protons and 143 neutrons
3. One problem with using uranium is that U-235 makes up only 0.7% of mined uranium, and most natural uranium is U-238, which does not undergo fission

Question 3

How can you enrich the U-238?

Answer 3

1. The mined uranium must be enriched until the U-235 content is about 3% before it can be used as a nuclear fuel
2. Although enriched fuel rods still contain a high proportion of U-238, which is not involved in fission
3. Some reactors use isotopes of plutonium or thorium instead

Question 4

How is nuclear fission used to generate power?

Answer 4

1. Fission reactions are established in the nuclear fuel using neutrons travelling slowly enough to be captured when they are fired at U-235 nuclei
2. A U-235 nucleus that captures a neutron becomes very unstable and splits into two or more smaller pieces, and releases energy in the form of heat
3. When it has absorbed the neutron, some people think of the nucleus as being like a wobbly jelly, which splits if it is wobbled too much

Question 5

What are the products of each fission reaction?

Answer 5

1. Each fission reaction produce, two, three or sometimes four neutrons, which may be absorbed by other U-235 nuclei if the neutrons are made to travel slowly enough and there are several possible reactions

Question 6

How is nuclear fission a chain reaction?

Answer 6

1. Nuclear fission reactions can only continue in a reactor if the number of nuclei involved in the fission reaction stays constant or increases
2. This occurs if, on average, one or more neutrons is produced and absorbed per fission reaction
3. This type of self-sustaining reaction is called a chain reactions
4. Chain reactions are only sustainable with a minimum amount of fuel, called the critical mass
5. This is because neutrons lost from the surface are no longer involved in the chain reactions
6. The shape as well as the mass of the sample affect the critical mass

Question 7

What are neutrons that induce fission reactions called?

Answer 7

• Neutrons that induce fission reactions in nuclear reactors are called thermal neutrons
• Their mean kinetic energy is equivalent to 3/2kT, where k is the Boltzmann constant and T is the absolute temperature of the reactor core
• Typically, thermal neutrons travel at between 2.5 and 3.0 kms-1, relating to a reactor core temperatures of about 290-350K

Question 8

How are neutrons moderated?

Answer 8

1. Neutrons produced by nuclear fission move so fast that they are unlikely to be absorbed in uranium nuclei, so they must be slowed down
2. The role of the moderator is to slow down fast neutrons as they pass through materials like graphite or water
3. Fast neutrons repeatedly collide with nuclei in the moderator, exciting the nuclei to higher energy levels
4. The fast neutrons lose energy during these collisions, and further collisions between neutrons and nuclei are elastic, slowing the neutron down even more
5. The slower neutrons are called thermal neutrons
6. The excited nuclei lose their surplus energy as gamma radiation when they return to ground level

Question 9

What sort of materials are the moderators made out of and why?

Answer 9

1. Graphite and heavy water are suitable materials for the moderator because they do not absorb neutrons
2. Also energy is transferred more efficiently during elastic collisions if the mass of the nucleus is close to the mass of a neutron

Question 10

What are the control rods and what are they purpose?

Answer 10

1. Control rods control the rate of reactions in the reactor
2. Materials such as born steel cadmium absorb neutrons without undergoing fission
3. Other materials such as silver, are also suitable but are rare and expensive
4. Boron is particularly useful because about 20% of the boron in control rods is boron-10, which absorbs neutrons to become boron-11

Question 11

What happens when a control rod is lowered into reactor?

Answer 11

• When a control rod is lowered into the reactor, the control rods absorb neutrons, so the rate of the reaction slows down because fewer neutrons are available to trigger fission reactions
• The position of the control rods can be adjusted to maintain the chain reaction at a steady rate, or to shut the reactor down completely

Question 12

What are the coolants in a nuclear reactor?

Answer 12

1. Coolants are fluids that absorb heat from the reactor, and transfer this heat away to drive the turbines that generate the electricity and to prevent the reactor from overheating
2. Most of the UK’s nuclear reactors use carbon dioxide as a coolant, but some use pressurised water
3. The coolant circulates through tubes inside the reactor core, absorbing heat from the reactor
4. This hot coolant then passes through a heat exchanger or boiler where its heat is transferred to water in secondary cooling systems

Question 13

What happens in a secondary cooling system?

Answer 13

1. As the water in a. secondary cooling system heats up, it changes to high-pressure steam and is used to drive the turbines and generator
2. Any steam remaining in the secondary cooling system is condensed back into water before it circulates through the heat exchanger again
3. To achieve this, the steam passes through pipes in a condensing unit, which is another heat exchanger that used cold water-filled pipes
4. The water in the condensing unit is usually taken from a nearby sea or river

Question 14

Why are there safety aspects of nuclear fuel?

Answer 14

1. Nuclear fuel in particular the spent fuel rods and the nuclear reactor are highly radioactive
2. Workers and the community must be protected from exposure to radioactive materials to reduce the damage caused by ionising radiation
3. Exposure to ionising radiation can damage DNA in cells, and increase the long-term risk of cancer
4. The risk of harms higher if people are exposed to higher doses of radiation, or if the time or intensity of exposure increase
5. Workers involved in a nuclear accident may revive very high dose causing radiation sickness which can be fatal in a few days

Question 15

What steps are taken to reduce or prevent exposure to radioactive material?

Answer 15

1. The reactor is surrounded by shielding, which protects worker from exposure to radiation
2. In many nuclear power stations, this is a steel pressure container that also contains the high-pressure coolant
3. This container is surrounded by 5m of concrete to absorb neutrons and gamma radiation and this is surrounded by a steel and concrete building designed to contain radiation even if there is an accident

Question 16

How does cost and effectiveness come into play in nuclear power stations?

Answer 16

1. Cost and effectiveness are important factors when choosing a metal for the shield
2. Common materials are lead, concrete, steel and water
3. Concrete is one of the most cost-effective materials used in nuclear power stations

Question 17

What happens in an emergency in a nuclear power station?

Answer 17

1. In an emergency, nuclear power stations are designed our shut down automatically
2. During the shutdown, the control rods drip into the reactor core, absorb the neutrons and slow down or stop the nuclear fission reactions
3. In many nuclear power stations, the control rods are held vertically above the reactor core using electromagnets
4. If there is power failure, the rods drop automatically into the reactor

Question 18

Why is nuclear waste bad?

Answer 18

1. Nuclear waste is produced from nuclear power stations
2. It is grouped into low, intermediate and high level wastes
3. Nuclear waste is handled remotely ht protect workers from exposure to radiation
4. This included tele-operation, where workers manipulate equipment remotely and the use of robotic machinery

Question 19

Describe low-level waste

Answer 19

1. Low-level waste including clothing worn by workers, paper and rags account for 90% of the volume of nuclear waste, but only 1% of the radioactivity
2. Low level waste is compacted and encased I cement and stored on licensed tires until the radioactivity decays awe and it can be disposed of in a normal waste
   3, Isotopes in low-level waste have different half-lives and activities, so their exact disposal procedures vary

Question 20

Describe intermediate-level waste

Answer 20

1. Intermediate level waste is mainly produced when. nuclear power station is decommissioned, and occurs in chemical sludges and resins
2. Intermediate level waste accounts for 7% of the volume of nuclear waste, and 4% of the radioactivity
3. IL waste with long half-lives is encase in cement in steel drums and stored secure t underground, for example in caverns or in near-surface facilities
4. A near-surface facility holds drums containing isotopes with half lives of less than a few years, which are placed in deep trenches and then covered by several meters of soil

Question 21

Describe high level waste

Answer 21

1. The main source of HL waste is spent fuel rods
2. HL waste accounts for 3% of the volume of the nuclear waste but 95% of its radioactivity
3. The spent fuel rods are so radioactive that they continue to emit heat and have to be cooled as well as stored
4. Initially spent fuel rods are stored under water which acts as a coolant as well as a shield from ionising radiation
5. For long term storage, HL waste is missed with molten glass then solidified inside stainless steel containers and this process sis called vitrification
6. These stainless steel cases are stored in specifically designed facilities, either above or below ground
7. The half-life of high-level radioactive waste depends on the isotopes present, but several fission products have half-lives of several thousand years

Question 22

What are spent fuel rods?

Answer 22

1. Spent fuel rods must be handled much more carefully than unused fuel rods because of the from of the ionising radiation that they emit
2. The fission reactions that occur inside the sober fuel rods initially emit beta radiation, then gamma and neutron radiation
3. These forms of ionising radiation are more penetrating than the alpha radiation emitted by unused fuel rods

Question 23

What are the benefits of using nuclear power stations? (4)

Answer 23

• Nucelar power stations generate electricity using fission reactions
  1. No smoke particles or GHGs are related so generating electricity by nuclear power does not contribute to acid rain or global warming
• By using nuclear power. many countries have reduced the amount of coal and oil brined to regenerate electricity which reduce their GHG emissions
  2. Coal mining, oil extraction, hydroelectricity kills people
  3. The quantity of waste produced during nuclear power generation is small in comparison to the amount from other methods of generating electricity, because the energy spruce, uranium, is very concentrated
  4. Nuclear power is a very reliable way of generating electricity, and the output from many nuclear power stations can be controlled to match changes in demand

Question 24

What are the risks of using nuclear power? (6)

Answer 24

1. As with any natural resource, there are limited supply as of uranium, a the supplies are likely to asks for thousands of years, especially if fast breeder reactor are used to change U-238 into Pu-239, another nuclear fuel
2. Uranium miners are at an increased risk of lung cancer from exposure to radon gas in mines
3. Radioactive waste, need to be stored securely for many decades or centuries even if the quantities of waste produced is small and some radioactive waste can be recycled
   - Only safe if geological conditions are suitable
4. New nuclear power stations are extremely expensive to build as a result of the safety features that need to be included
5. Decommissioning nuclear power stations is expensive, with the safe disposal of intermediate level waste adding to ongoing costs
6. Risks of nuclear accidents e.g. Fukushima and Chernobyl

Question 25

What is nuclear fission?

Answer 25

1. Large nuclei with at leat 83 protons, are unstable and can randomly split into two smaller nuclei, this is called nuclear fission
2. This process is called spontaneous, if it just happen by itself or induced if we encourage it to happen

Question 26

How can fission be induced? What is spontaneous fission?

Answer 26

1. Fission can be induced by making a neutron enter 235U nucleus, causing it to become very unstable
2. Only low energy neutrons (called thermal neutrons) can be captured in this way
3. The larger the nucleus, the more unstable it will be and so large nuclei are more likely to spontaneously fission
4. This means that spontaneous fission limits the number of nucleons that a nucleus can contain, in other words it limits the number of possible elements

Question 27

Why is energy released in nuclear fission?

Answer 27

Energy is released during nuclear fission because the new, smaller nuclei have a higher binding energy per nucleon

Question 28

What is fusion?

Answer 28

1. Two light nuclei can combine to create a larger nucleus, and this is called nuclear fusion
2. A lot of energy is released during nuclear fusion because the new heavier nuclei have much higher bonding energy per nucleon

Question 29

Why do nuclei need lots of energy to fuse?

Answer 29

1. All nuclei are positively charged, so there will be an electrostatic (or Coulomb) force of repulsion between them
2. Nuclei can only fused if they overcome this electrostatic force and get close enough for the attractive force of the strong interaction to hold them both together
3. About 1MeV of kinetic energy is needed to make nuclei fuse together and that is a lot of energy

Question 30

What happens in nuclear reactions?

Answer 30

1. Some nuclei can released energy from nuclear fission or fusion
2. Almost all nuclear reactions that occur naturally result in nuclei that are more stable
3. This increases the binding energy per nucleon compared with the original nuclei
4. The mass difference between the original nuclei and the nuclei of the products corresponds to the amount of energy related
5. In all nuclear reactions, total proton number z and mass number A are conserved and the reaction often results in more than one product

Question 31

What happens to increase the binding energy per nucleon?

Answer 31

1. Lighter elements fuse (to left of graph energy released by fusion)
2. Heavier elements tend to undergo radioactive decay or fission (to right of graph energy released by fission)

Question 32

What is nuclear fusion?

Answer 32

1. Light nuclei can join together by nuclear fusion to form a new element and released energy
2. Occurs naturally in star, so how can realise energy for billions of years
3. Large amounts of energy needed to create suitable conditions for fusion on Earth e.g. temp of core of stars

Question 33

Why is high temperature and high density needed for fusion?

Answer 33

1. High temperature of several million kelvin give nuclei enough kinetic energy to overcome electrostatic repulsion between protons in the nucleus
2. The high denser inside stars’s cores e.g 150,000kgm-3 forces nucleus so close together than the strong force becomes involved and this attractive force acts over very short distances

Question 34

What is an example of fusion?

Answer 34

1. One fusion react that releases energy ins tars like the sun is fusion of deuterium and tritium to form helium and a neutron
2. The fusion reacts that occur in different types of stars depend on stars mass, core temperature and density
3. A chain reaction can happens if conditions are suitable.g. the triple alpha cycle

Question 35

What is fission?

Answer 35

Nuclear fission is when a nucleus splits into two or more smaller parts, releasing energy

Question 36

What is not an application of radioactive sources?

Answer 36

Calcuatting the age of living treees

Question 37

In an experiment to investigate the absorption of alpha beta and gamma radiation by different material what must be kept constant?

Answer 37

The distances between the source absorber and GM tube

Question 38

What fundamental assumption does carbon dating rely on?

Answer 38

That the ratio of carbon-14 atoms to carbon-12 atoms in the atmosphere has remained constant over time

Question 39

What is fission?

Answer 39

1. Large nuclei with at least 83 protons (e.g. uranium) are unstable and some can randomly split into two smaller nuclei and this is called nuclear fission
2. This process is spontaneous if it happens by itself or indicted if it is encouraged

Question 40

How can fission be induced?

Answer 40

Fission can be induced by making a neutron enter a 235U nucleus, causing it to become very unstable, and only low energy neutrons (thermal neutrons) can be captured in this way

Question 41

How is energy released in fission?

Answer 41

1. Energy is released during nuclear fission because the new, smaller nuclei have a higher binding energy per nucleon
2. The larger the nuclei, the more unstable it will be so large nuclei are more likely to spontaneously fission
3. This means that spontaneous fission limits the number of nucleons that a nucleus can contain and so it limits the number of possible elements
4. We can harness the energy related during nuclear fission reactions in a thermal nuclear reaction, but these reaction need to be carefully controlled

Question 42

What rods are used?

Answer 42

1. Nuclear reactors use rods of uranium that are rich in 235U as ‘fuel’ for fission reaction (the rods also contain a lot of 238U that does not undergo fission)
2. These are placed into he reactor remotely which keeps workers as far away from the radiation as possible

Question 43

Why is it a chain reaction? Why do the rods need to be placed in a moderator? What sort of moderator should you choose?

Answer 43

1. These fission reactions produce more neutrons which then induce other nuclei to fission, a chain reaction
2. The neutrons will only cause a chain reaction if they are slowed down, which allows them to be captured by the uranium nuclei
3. The 235U fuel rods need to be placed in a moderator (e.g. water) to further slow down and/or absorb neutrons
4. These slowed down neutrons are called thermal neutrons
5. This happens through elastic collision (KE is conserved) with nuclei of the moderator material
6. Choosing a moderator with a similar mass to the neutrons (e.g. water) is more efficient at slowing neutrons down

Question 44

What is the critical mass?

Answer 44

1. You want the chain reaction to continue on its own at a steady rate where one fission followed another
2. The amount of fuel you need to do this is called the critical mass and any less the the critical mass (sub critical mass) the reaction will just Peter out
3. Nuclear reactors usually use a supercritical mass of field (where several new fissions normally follow each fission) and control the rate of fission using control rods

Question 45

What are control rods?

Answer 45

1. Control rods control the chain reaction by limiting the number of neutrons in the reactor
2. These absorb neutrons so that the rate of fission is controlled
3. Control rods are made up of a material that absorb neutrons (e.g. boron) and they can be instead by varying amount to control the reaction rate
4. In an emergency the reactor will be shut down automatically by the release of the control rods into the reactor which will stop the reaction as quickly as possible

Question 46

What is a coolant?

Answer 46

1. Coolant is sent around the reactor to remove het produced in the fission, often the coolant is the same water that is being used int he reactor as a moderator
2. The heat from the reactor can then be used to make steam for powering electricity-generating turbines

Question 47

What is the nuclear reactor surrounded by?

Answer 47

• A thick concrete case, which acts as shielding

- This prevents radiation escaping and reaching the people working in the power station

Question 48

What happens if a chain reaction in a nuclear reactor is left to continue unchecked?

Answer 48

• Large amounts of energy are released in a very short time
• Many new fissions will follow each fission, causing a runaway reaction which could lead to an explosion and this is what happens in a fission (atomic) bomb

Question 49

How are waste products of fission stored?

Answer 49

• Produces lots of energy and less GHG
  1. Waste products of nuclear fission have a larger proportion of neutrons than nuclei of a similar atomic number and this makes them unstable and radioactive
  2. The products can be sued for practical application such as tracers in medical diagnosis
  3. They may be highly radioactive and so their handling and storage needs great care
  4. When material is removed form the reactor it is initially very hot, so is placed in cooling ponds until the temperature falls to a safe level
  5. This is done remotely just like the handling of fuel to limit the radiation workers are exposed to
  6. The radioactive waste is then stored in sealed container until its activity has fallen sufficiently and areas for storage are chose where there will be minimal impact on animals and the environment and any people that live nearby are consulted about the decision to store nuclear waste near them

Question 50

What is fusion?

Answer 50

1. Two light nuclei combine to create a larger nucleus
2. A lot of energy is related in ocular fusion because the new, heavier nuclei have a much higher binding energy per nucleon

Question 51

Why do nuclei need lots of energy to fuse?

Answer 51

1. All nuclei are positively charged so there will be an electrostatic force of repulsion between them
2. Nuclei can only combine if they overcome this electrostatic force and get close enough for the attractive force of the strong interaction to hold them both together
3. About 1MeV of KE is needed to make nuclei fuse together