Fission and Fusion

Question 1

nuclear reactions can be…

Answer 1

• a source of energy.

- including fission, fusion and radioactive decay.

Question 2

what is nuclear fission?

Answer 2

• the splitting of an atom, which releases energy.

- it can be spontaneous, but in a nuclear reactor, its made to happen, e.g. to uranium-235.

Question 3

how does nuclear fission occur?

Answer 3

• if a slow-moving neutron is absorbed by a uranium-235 nucleus, the nucleus can SPLIT.
• each time this happens, it spits out a small number of neutrons, these might go on to hit other uranium-235 nuclei, causing them to split…and so on. this is a chain reaction.
• when uranium-235 splits in two it will form TWO NEW DAUGHTER NUCLEI, both lighter elements than uranium.
• these new nuclei are usually radioactive. this is the big problem with nuclear power = radioactive waste.
• each nucleus splitting gives out a lot of energy, this energy is in the KE stores of the fission products (the daughter nuclei and the neutrons)
• in a reactor, this energy is transferred to the thermal energy stores to produces steam to drive a turbine.

Question 4

what is a chain reaction?

Answer 4

• if a slow-moving neutron is absorbed by a uranium-235 nucleus, the nucleus can SPLIT.
• each time this happens, it spits out a small number of neutrons, these might go on to hit other uranium-235 nuclei, causing them to split…and so on. this is a chain reaction.

Question 5

what is the role of the moderator in a nuclear reactor?

Answer 5

• the neutrons released by fission reactions in a nuclear reactor have a lot of energy. in order to be absorbed by uranium nuclei and sustain the chain reaction, they need to be slowed down.
• the moderator, USUALLY GRAPHITE OR WATER, slows down neutrons.

Question 6

what is the role of the control rods in a nuclear reactor?

Answer 6

• control rods, often made of BORON, limit the rate of fission by barbing excess neutrons.

Question 7

what is the radiation released in fission and how is it dealt with?

Answer 7

• the high-energy neutrons and gamma rays (energy) released in fission are highly penetrating ionising radiation.
• shielding has to be used to absorb the ionising radiation.
• the shielding is usually a thick concrete structure, which may also contain lead or other metals

Question 8

how does a nuclear reactor work?

Answer 8

• the neutrons released by fission reactions in a nuclear reactor have a lot of energy. in order to be absorbed by uranium nuclei and sustain the chain reaction, they need to be slowed down.
• the moderator, USUALLY GRAPHITE OR WATER, slows down neutrons.
• control rods, often made of BORON, limit the rate of fission by barbing excess neutrons.
• the high-energy neutrons and gamma rays (energy) released in fission are highly penetrating ionising radiation.
• shielding has to be used to absorb the ionising radiation.
• the shielding is usually a thick concrete structure, which may also contain lead or other metals
• a substance (e.g. CO2) pumped round the reactor transfers the energy (by heating) to the water in the heat exchanger. the water turns to steam, which turns a turbine, which turns a generator and generates electricity.

Question 9

what is nuclear fusion?

Answer 9

• nuclear fusion is the opposite of nuclear fission.
• in nuclear fusion, two light nuclei collide at high speed and join (fuse) to create a larger, heavier nucleus.
• e.g. hydrogen nuclei can fuse to produce a helium nucleus.

Question 10

what is the difference between nuclear fission and fusion?

Answer 10

• fission = splitting of an atom to form 2 daughter nuclei and a small amount of neutrons
• fusion = two light nuclei collide at high speed and join (fuse) to create a larger, heavier nucleus

Question 11

how does nuclear fusion occur?

Answer 11

• two light nuclei collide at high speed and join (fuse) to create a larger, heavier nucleus
• this heavier nucleus doesn’t have as much mass as the two separate, light nuclei did.
• some of the mass of the lighter nuclei is converted to energy.
• this energy is then released as RADIATION.

Question 12

where do stars get their energy from?

Answer 12

• fusion releases a lot of energy (more than fission for a given mass of fuel)
• all the energy released in stars comes from fusion

Question 13

what is the big problem with fusion?

Answer 13

• fusion only happens at REALLY HIGH PRESSURES AND TEMPERATURES (about 10 000 000°C)
• this is because the positively charged nuclei have to get very close to fuse, so they need to be moving very fast to overcome to strong force due to electrostatic repulsion of protons.
• so far, scientist haven’t found a way of using fusion to generate energy for us to use
• the temperatures and pressures needed for fusion are so high that fusion reactor are really hard and expensive to build.