E4 nuclear fission

nuclear fission

definition:

• heavy unstable nuclei disintegrate into two or more smaller, lighter stable nuclei with higher binding energy

• in the process, releases 3 more neutrons which initiate other reactions → self-sustainable

process

1. 235U absorbs neutron, temporarily becomes 236U

2. overall BE increased due to the added energy from neutron BUT BE per nucleon decreased since there are more nucleons → lower BE per nucleon, less stable, more likely to undergo radioactive decay

3. 236U decays into daughter nuclei krypton-92 and barium-141 + 3 neutrons

   • sum of BE of krypton and barium is less than 236U → excess energy released via the 3 neutrons so that law of conservation of energy applies. (due to mass-energy equivalence)

mass difference

• nuclear fission releases energy equivalent to difference between energy needed to deconstruct a large nucleus and energy emitted when two smaller nuclei are constructed from its components

• loss of mass is emitted as KE of the fission products → use mass difference to find energy released

binding energy graph

• BE per nucleon against nucleon number

• to find energy released: read BE per nucleon off the graph for the parent and daughter nuclei → calculate BE of parent and daughter nuclei → calculate difference between parent and daughters

nuclear fuel enrichment

process by which the % composition of 235U in fuel rods is increased → makes nuclear fission more probable, since naturally occuring uranium contains less than 1% of 235U

reprocessing

• treating depleted fuel rods to recover uranium and plutonium → for use as fuel in fast-breeder reactors and nuclear weapons

why does nuclear fission cause a chain reaction?

• energy released by nuclear fission is very large (exponential)

• since smaller nuclei are stable with fewer neutrons, more neutrons emerge from each fission

• neutrons produced from fission will initiate more fission reactions → chain reaction, self-sustaining

critical mass

minimum mass needed to sustain a chain reaction. depends on fuel used and the shape of the assembly.

minimum because lower-energy neutrons (around 1eV) favour fission

control rods

• neutron-absorbing materials: boron/cadmium

• more control rods lowered → more neutrons absorbed → rate of fission reduced

• prevents uncontrolled fission: huge amount of energy in short amount of time → nuclear weapons destructive

  • controlled fission: limit rate at which fission takes place by limiting the no. of neutrons → energy released at slow rate, can use for power production

parts of nuclear reactor

• moderator

• control rods

• heat exchanger

moderator

• ideal KE of neutrons is about 1eV → need slow down neutrons for effective fission to occur

• nuclei with small atomic mass (eg H, O, C) collide with neutrons to slow them down to an appropriate speed

types of moderators:

• light water: used as moderator and coolant

• heavy water (D is isotope of hydrogen)

• graphite: also reflects neutrons back to nucleus

• combination of light water and graphite

heat exchanger

• heat produced by nuclear fission will vaporise the water in the heat exchanger into steam

• steam channeled to turn turbine to generate electricity

advantages/disadvantages of nuclear power plant

• advantages

  • high energy density: large amount of energy from small mass of uranium

  • reserves of uranium are larger than oil

  • no greenhouse gases emitted, environmentally friendly

• disadvantages

  • if something goes wrong, large scale and impact → high risk

  • non-renewable but can last for a long time

  • no ideal way to handle nuclear waste

health and safety issues of nuclear power plant

• nuclear meltdown:

  • fission was not controlled → overheating and melting of fuel rods

  • increased temperature → increases pressure built up in reactor

  • high pressure causes malfunction and explosion → radioactive material sent into atmosphere

  • prevention: containment building, made up of airtight steel and thick concrete

    • prevents radioactive material from leaking out (shielding)

    • prevent impact from outside (eg tsunamis, missile attacks)

• handling of nuclear waste

  • low level waste: waste from uranium extraction from earth, fuel enrichment, transfer of heat from fuel rods etc have small dosage of radiation but must be kept far away from humans for 100 to 500 years

  • high level waste: waste from used fuel rods → only safe after 240 000 years. current approach is keep used fuel rods under water for years then seal up in steel containers.

problems with dealing with water from nuclear fission reactors

Waste is very hot → has to be placed in cooling ponds to transfer the (thermal) energy away

Waste is very radioactive → has to be placed in cooling ponds to absorb this radiation

Waste will be radioactive for thousands of years → storage needs to be in geologically stable area

discuss whether all energy released during radioactive decay can be transferred to a thermal form

no. neutrinos/antineutrinos released in decay carry away energy because they interact poorly with matter

products of nuclear fission

• daughter nuclei are highly radioactive → beta decay over long periods, creating a large variety of radioactive isotopes.

• products

  • Have short to medium half-lives,

  • Exist in large quantities,

  • And are the main source of radioactive waste in reactors.