Physics - 7 Radioactivity - 7.7 Nuclear Fission & 7.8 Nuclear Fusion & 7.9 Nuclear Issues

Nuclear fission

nuclei of isotopes split apart when struck by neutrons

Induced fission

a neutron is fired at an unstable nucleus, splitting it into two roughly equal fragments

Spontaneous fission

the spontaneous splitting of a heavy nucleus into two lighter nuclei

What is released in nuclear fission? [3]

- two fragment nuclei of roughly equal size
- 2/3 neutrons
- energy in form of kinetic and gamma radiation

Fission neutron

a neutron emitted as a result of fission

Chain reaction of fission events

the fission neutrons strike other unstable nuclei, causing them to split (etc.)

In a nuclear fission reactor, on average ... neutron(s) from each fission go on to produce more fission

one

Why is only one of the fission neutrons used?

to maintain a steady rate of energy release

Fissionable isotope

an isotope that will undergo fission with low-energy neutrons and create a self-sustaining reaction

Enriched uranium

Mostly non-fissionable U-238 and 2-3 % fissionable U-235

Most common type of nuclear reactor

U-235

Water as a moderator

slows down fission neutrons so they can cause more fission - high speed neutrons are not easily absorbed

Control rods

absorb surplus neutrons

Why are reactors encased in a concrete shield?

to absorb ionising radiation

Nuclear fusion

the process by which two or more small nuclei fuse to make a bigger nucleus

Some of the ... of the nuclei can be converted into ... and released as ...

mass, energy, radiation

Nuclear fusion happens only if the nucleus formed has a relative mass less than...

55

Nuclear fusion of protons into helium nuclei [3]

- two protons fuse to form a 'heavy hydrogen' nucleus
- two more protons collide separately with two 'heavy hydrogen' nuclei
- two heavier nuclei collide to form helium nucleus

Conditions for nuclear fusion [2]:

- very high temperature
- very high pressure

How is plasma handled in a fusion reactor? [2]

- heated by passing a large electric current through
- contained by a magnetic field to avoid touching the sides

Heavy hydrogen

a hydrogen atom that has a neutron in the nucleus giving it an atomic mass double that of a normal hydrogen atom

Advantages of nuclear fusion [3]:

- heavy hydrogen fuel abundantly available
- helium product is harmless
- energy released can generate electricity

Sieverts (Sv)

standard dose equivalent units for radiation exposure

Danger of radon

emits alpha particles - can seep into homes, and is dangerous if breathed in

Nuclear waste handling [3]

- stored in large tanks of water for up to a year
- opened by remote-control machines which separate used and unused fuel
- waste is radioactive and must be stored for many years to decay

Chernobyl

a city in Ukraine that was the site of a 1986 meltdown at a Soviet nuclear power plant

Fukushima

a city in Japan damaged by the tsunami in 2011; the nuclear power plant released radiation

New reactor designs [4]:

- standard design to reduce costs and construction time
- longer operating life (60 years)
- more safety features e.g. convection of outside air through cooling panels
- less effect on environment

Alpha radiation effect on humans [2]:

- inside: dangerous, affects all surrounding tissue
- outside: slightly dangerous to skin and retinal cells

Beta and gamma radiation effect on humans:

dangerous, reaches cells throughout body