26 - Nuclear Physics

what are two conclusions drawn from E=mc²

- mass is a form of energy

- energy has mass

for a simple nuclear decay equation, how do you calculate the energy released, what are the steps?

mass of initial unstable nucleus - mass of nuclei produced = mass defect

△m x c² = △E

for annihilation or creation of particle and antiparticle pair, how do you calculate the energy of each photon emitted, or the mass of each particles

△m = final - inital mass

so initial is 2 x mass of one particle and final mass = 0

△m x c² = △E

divide △E by 2 as 2 photons are emitted to get the energy of each photon

define mass defect

difference between the mass of a nucleus and the mass of its individual nucleons

Define binding energy

The minimum energy required to completely separate a nucleus into its constituent nucleons

how do you get binding energy per nucleon?

why do we use it?

BE/num of nucleons

- useful to compare the stability of different nuclei

binding energy per nucleon graph

conclusions from the BE graph:

- what happens to BE per nucleon throughout the graph?

- what is the most stable isotope?

- what area is fusion, how does this graph show what the fusion process is?

- what area is fission, how does this graph show the fission process?

- for A < 56, BE per nucleon increases as A increases

- for A > 56, BE per nucleon decreases as A increases

- Iron-56 has the greatest BE per nucleon ∴ most stable isotope in nature

- in the fusion process, two low A num nuclei join to produce a higher A num nucleus that has much higher BE. Therefore, energy is released

- in fission process, high A num nucleus splits into two low A num nuclei, which have higher BE than parent nucleus so energy is released: process used in fission reactors

what is nuclear fission?

fission - a large nucleus splits into smaller nuclei causing a mass defect and energy to be released

explain the process of induced fission inside a nuclear reactor and its possible deathly dangers.

state what can be done to stop these deathly dangers (don't explain how)

- occurs when a thermal neutron is absorbed by a fissile nucleus

- the nucleus absorbs the nucleon, becoming an isotope in an excited state

- the excited state is highly unstable and quickly undergoes nuclear fission

- as well as producing 2 daughter nuclei, 3 fast-moving neutrons are also produced

- The total mass of the particles after fission is always less than the total mass of the particles before the reaction

- △m x c² = △E

- 3 fast-moving neutrons slowed down can lead to triggering more fission, causing a chain reaction leading to an explosion = danger

- to stop this in a nucelar reactor, we ensure that on avg one slow neutron survives between nuclear fission reactions

what is the moderator for in a fission reactor?

why do we need a moderator?

what are good moderators?

the moderator is to slow down the fast neutrons produced in fission reactions

- fast-moving neutrons bounce off uranium nuclei but when they collide with protons in water or with carbon nuclei, they transfer kinetic energy and slow down

- water or carbon = good moderator

what is the purpose of control rods in a fission reactor?

how would you stop fission in the reactor using the control rods?

how do the control rods carry out their purpose?

- control rods are there to ensure exactly one slow neutron survives per fission reaction by absorbing neutrons

- The position of the control rods is automatically adjusted

- to slow down or stop fission, rods are pushed into the reactor core

what are fuel rods inside a fission reactor ?

fuel rods contain fissile material

what is done with high level radioactive waste?

high-level radioactive waste, like spent fuel rods, has to be buried deep underground for many centuries because isotopes with long halflife must not contaminate our water and food supplies