Flashcards in Nuclear Power Deck (24):
What are nuclear power stations?
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
What is the nuclear fuel used?
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
How can you enrich the U-238?
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
How is nuclear fission used to generate power?
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
What are the products of each fission reaction?
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
How is nuclear fission a chain reaction?
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
What are neutrons that induce fission reactions called?
-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
How are neutrons moderated?
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
What sort of materials are the moderators made out of and why?
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
What are the control rods and what are they purpose?
1. Control roade 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
What happens when a control rod is lowered into reactor?
-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
What are the coolants in a nuclear reactor?
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
What happens in a secondary cooling system?
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
Why are there safety aspects of nuclear fuel?
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
What steps are taken to reduce or prevent exposure to radioactive material?
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
How does cost and effectiveness come into play in nuclear power stations?
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
What happens in an emergency in a nuclear power station?
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
Why is nuclear waste bad?
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
Describe low-level waste
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
Describe intermediate-level waste
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
Describe high level waste
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
What are spent fuel rods?
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
What are the benefits of using nuclear power stations?
-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