nuclear physics Flashcards
atomic mass unit
1/12 mass of Carbon-12 atom which is equal to 1.661x10-27 kg
conservation of mass units to energy
using Einstein’s equation, E = mc², the equivalent energy of 1u can be determined, which is 931.5 MeV.
calculate energy released in nuclear changes
- add up total mass of nuclei before change in terms of u and add up the total mass of nuclei after change in terms of u
- calculate the mass difference in terms of u
- convert the mass difference into equivalent energy using conversion factor from formula sheet
mass difference
difference in mass between a nucleus and the sum of the mass of its nucleons. total mass of individual nucleons is greater than mass of nucleus
mass defect
the difference between the mass of the nucleus and the mass of its constituent parts.
as nucleons join together, their total mass decreases because some of their mass is converted into energy and released
binding energy
the energy required to separate the nucleus up into its constituents protons and neutrons. It is equivalent to the mass defect
binding energy per nucleon
the average energy per nucleon to remove all of the nucleons from a nucleus
what produces binding energy
the attractive strong force which holds the nucleons together
graph of average binding energy per nucleon against nucleon number
- rapid increase up to iron
- peak at iron
- decreases gradually after iron
explain how energy is released in fission and fusion
- energy is released/ made available when binding energy per nucleon is increased
- in fission, a large nucleus splits and in fusion nuclei join
- the most stable nuclei are at a peak
- fusion occurs to the left of peak binding energy per nucleon and fission to the right
fission process
- neutrons are released
- fission is usually brought about by neutron bombardment
fission equation
the daughter isotopes can be a range of different elements. The total nucleon number on the left of the equation equals the total nucleon number on the right
fusion process
- two small or light nuclei combine
- electrostatic repulsion has to be overcome
- nuclei have to be given kinetic energy for them to meet
advantages of fusion over fission
- supply of fuel is almost unlimited
- fewer waste or radioactivity or environmental problems
- energy released per unit mass is generally greater
what is enriched uranium
proportion of uranium - 235 is greater than is found in naturally occurring Uranium
definition of thermal neutrons
neutrons that have low energies or speeds
induced fission by thermal neutrons
splitting of nucleus into two smaller nuclei, brought about by bombardment with (thermal) neutrons
definition of a chain reaction
- fission reaction is induced by neutron bombardment]
- fission releases neutrons
- released neutrons cause more fission
definition of critical mass
- minimum mass of fissile material
- for a self-sustaining reaction to be maintained
what do control rods do in the reactor
- control involves limiting the number of neutrons, released from the fission of a nucleus, that can go on and cause fission in other nuclei
- excess neutrons are observed by control rods
- control rods inserted into reactor slows reaction rate
- for a steady state of fission, only one neutron per fission is required to go on to produce further fission
- each fission produces two or three neutrons on average
- some neutrons escape
examples of suitable control rods and their properties
- suitable control rod material is boron or cadmium
- control rod materials must be good at absorbing neutrons
what do moderators do in the reactor
- neutrons from fission are fast (high energy)
- fission most favourable with low energy neutrons
- moderation involves slowing down neutrons by collision with moderator atoms
- large number of collisions required
- collision are elastic so KE is transferred to the atoms
Examples of suitable moderators and their properties
Suitable moderator material is graphite or water
* Moderator must not absorb neutrons and the moderator atoms should have (relatively) low mass
what does coolant do in the reactor
- transfers thermal energy from core to a heat exchanger where water in a secondary cooling system is turned into steam
examples of coolants and their properties
Suitable coolants are water or carbon dioxide
- coolants need to flow easily so they can be pumped around the reactor core
- coolants need to have large specific heat capacities so a lot of thermal energy can be transferred with a smaller volume of coolant
how is thermal energy obtained from nuclear fission
- fission fragments repel each other and collide with other atoms in the fuel rod
- high energy fission neutrons enter moderator [or collide with moderator atoms]
- atoms in moderator and fuel rods gain kinetic energy due to collisions
- temperature depends on the average kinetic energy of vibrating atoms
what are fuel rods
they are rods containing enriched uranium fuel which are inserted into the reactor