҉� What is nuclear fission? �

• Nuclear fission is when a nucleus breaks apart due to its divorce.

• It is the main process used in nuclear power plants to create energy.

• It may take place after any heavy nucleus has captured a neutron.

• However it doesn’t always follow this, it is dependant on the velocity of the neutron and the particular nucleus involved.

• The nucleus may decay into a new nuclide.

҉� The Process �

• The most common nuclei used are U-235, U-233, Pu-239 and Pu-241. This is due to the fact they’re fissionable by low-energy neutrons.

• After gaining an extra neutron, the new nuclides are unstable and therefore decay again, giving off energy and more neutrons.

• Due to this, the process of creating energy in nuclear reactions is a constant chain reaction.

҉� Decay Equations �

҉� Moderators�

• To maintain a controlled nuclear reaction, only 1 out of 3 neutrons can be allowed to strike another uranium nucleus. This must be maintained; as less than 1 uranium collision would cause the reaction to die out, and more than one would cause a NUCLEAR EXPLOSION.

• We can use: – Water.– Hydrogen.– Deuterium– Carbon– Beryllium– Lithium-7

҉� Moderators, what do? �

• The moderators change the momentum of the neutrons emitted by decay.

• This occurs as the kinetic energy of the neutron is transferred to the moderator each time they collide.

• Neutrons eventually slow down enough to reach speeds which can cause fission.

• The nuclei of the moderator can become excited by these collisions and can emit electromagnetic radiation to de-excite themselves.

҉� Control Rods �

• To stop the nuclear reaction increasing to unCONTROLable levels, movable CONTROL rods are placed in between the fuel rods of a reactor. Moderators such as these are needed due to how quickly a unmoderated reaction could spiral out of CONTROL.

• CONTROL rods help CONTROL the reactor by absorbing the extra neutrons emitted by the decay.

҉� Generation Time �• To calculate the time taken for the reaction rate to double we use:

• Where is the reproduction constant and N is the number of generations required to double the rate.

• The maximum reproduction constant for uranium-235 is 2.4, therefore it would take:

– N

– 0.792• If this were left without moderators then the reaction would double in

rate every 0.792 x seconds.• This is bad.

҉� The reproduction konstant �

• If the reaction is subcritical, which means it cannot sustain a chain reaction. Therefore any beginning chain reaction dies out over time.

• If the reaction is critical, then every fission causes one more fission, thus the reaction and power level remain constant. This is the level that power stations use.

• If the reaction is supercritical, the fissions will increase exponentially and create nuclear explosions.

҉� SaFeTy �

• There are many mechanisms in place to increase the safety of a nuclear reactor, such as control rods and moderators.

• Also most reactors have a flooding mechanism which cools down the core if it overheats. These often use water.

• They also use physical barriers to stop radiation leaks, such as big ass concrete walls and vacuum buildings.

҉� Nuclear Waste �

• This is what fuel rods become after all of their useful fuel has gone, but it is still dangerously radioactive.

• It is so radioactive that if you were stood next to it unshielded, you would absorb a lethal dose of radiation within a few seconds.

• If we were to share out the amount of nuclear waste produced in the US per year between everyone in the US, each person would have 39.5 grams of the stuff. That’s the weight of these sterling silver salt and pepper shakers.

҉� Types of nuclear waste �

• High Level Waste (HLW): this waste is like the Daddy Bear of waste, its very radioactive and would burn your mouth if you tried to eat it.

• Intermediate Level Waste (ILW: slightly better than HLW but still quite dangerous.

• Low Level Waste (LLW): the easiest to deal with and the least radioactive.

҉� You’ve got nuclear waste, what do? �

• The three types of waste are dealt with in different ways.• LLW is easy to deal with and just compressed and sealed

into steel drums, eventually transported to a repository for disposal.

• ILW is mixed with sand/cement and then poured into concrete drums. This is also sent to repository.

• HLW gets to live out 10 years of retirement in a swimming pool with all its old spent fuel buddies. It is then buried very deep in special underground repositories (tombs).

҉� Pressurised Water Reactor �

• This is the most common type of reactor worldwide.• They use water as both a coolant and a moderator.• The coolant is pressurised top stop it from turning

into steam instantly. • The heated pressurised water is used to turn normal

water into steam to turn a turbine, which creates electricity.

҉� Boiling Water Reactors �

• These make up 15% of reactors worldwide.• Also use water as both a coolant and

moderator.• The coolant is kept at a lower pressure than in

a PWR, allowing it to boil.• Steam powers a turbine, this creates

electricity.

҉� CANDU Reactors �

• 12% of worldwide reactors and only reactor type used in Canada.

• They use heavy water for a moderator and a coolant.

• As in PWR the heated pressurised water is used to turn normal water into steam to turn a turbine, which creates electricity.

҉� Gas Cooled Reactors �

• Only used in the UK• Two types: Magnox and Advanced Gas-cooled

Reactors (AGR)• Both types use as a coolant and graphite as their

moderator.• However the Magnox uses natural uranium for fuel,

whereas AGRs use enriched uranium.

҉� Light Water Graphite Reactors �

• LWGRs are used in Russia.• They use ordinary water for coolant and

graphite as a moderator.• Use the same method as the BWRs.• Chernobyl was this type of reactor.

҉� Fast Breeder Reactor �

• They operate mainly in Russia.• These don’t use slow neutrons unlike the other

reactor types shown.• They use fast neutrons, meaning they’re able to

convert materials such as thorium-232 and uranium-238 into fissile materials.

• This means they increase the potential of our nuclear resources.

҉� Summary �

The future

• Currently there are 16 nuclear reactors in the UK, which provide about 18% of its electricity.

• However all but one of these stations will be shut down by 2023.

• The government aims to have at least 16GWe of new nuclear capacity in place by 2023.

• They are expecting to have some of the first 19GWe stations built by 2023.

҉� Small Modular Reactors �

• These are approximately 1/3 the size of current nuclear power plants.

• They produce less energy then a regular station, but are becoming increasingly popular to produce power for a small electricity grid, or to provide heat in resource industries.

• The IAEA predicts there will be 96 operational worldwide by 2030.

• They can also be operated and built underground.

҉� ҉�

҉� Bibliography �• http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Introduction/Physics-of-Nuclear-Energy

/• http://www.atomicarchive.com/Fission/Fission2.shtml• http://www.docbrown.info/page03/3_54radio09.htm• http://www.atomicarchive.com/Fission/Fission4.shtml• http://en.wikipedia.org/wiki/Neutron_moderator#Nuclear_weapon_design• http://

www.bbc.co.uk/schools/gcsebitesize/science/add_gateway_pre_2011/radiation/fissionrev2.shtml

• http://undergroundbombshelter.com/bombshelter/understanding-radiation-scientific-basis-nuclear-explosion-time-scale-nuclear-explosion.htm

• http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/control.html#c3• http://nuclearinfo.net/Nuclearpower/WebHomeSafetyOfNuclearPowerPlants• http://www.whatisnuclear.com/articles/waste.html• http://www.hko.gov.hk/education/dbcp/pow_stat/eng/r10.htm• http://en.wikipedia.org/wiki/Nuclear_chain_reaction• http://teachnuclear.ca/contents/cna_nuc_tech/reactor_types/• http://www.world-nuclear.org/info/Country-Profiles/Countries-T-Z/United-Kingdom/