Lesson 3: Nuclear Basics

Dr. Andrew Ketsdever

Nuclear Reactor

Reactor Schematic

Basic Atomic Structure• Atoms are fundamental

particles of matter– Composed of three types of

sub-atomic particles• Protons• Neutrons • Electrons

– The nucleus contains protons and neutrons• Most of the atom’s mass• Small part of the atom’s volume

– Electron Cloud• Contains electrons• Most of the atom’s volume

Basic Atomic Structure

• Atomic Number– Number of protons in the nucleus

• Mass Number– Number of protons AND neutrons in the

nucleus• Mass of proton : 1.6726 x 10-27 kg• Mass of neutron: 1.6749 x 10-27 kg • Mass of electron: 0.00091x10-27 kg

Basic Atomic Structure

• Isotope– Same element implies two atoms have the

same atomic number– Isotopes of a given element have the same

atomic number but a different mass number• Same number of protons in the nucleus• Different number of neutrons in the nucleus

          

       

                 

 

       

               

 

Hydrogen Deuterium Tritium

Basic Nuclear Physics• An atom consists of a small, positively charged

nucleus surrounded by a negatively charged cloud of electrons

• Nucleus– Positive protons– Neutral neutrons– Bond together by the strong nuclear force

• Stronger than the electrostatic force binding electrons to the nucleus or repelling protons from one another

• Limited in range to a few x 10-15 m• Because neutrons are electrically neutral, they

are unaffected by Coloumbic or nuclear forces until they reach within 10-15 m of an atomic nucleus– Best particles to use for FISSION

Fission• Fission is a nuclear process in which a heavy

nucleus splits into two smaller nuclei– The Fission Products (FP) can be in any combination

(with a given probability) so long as the number of protons and neutrons in the products sum up to those in the initial fissioning nucleus

– The free neutrons produced go on to continue the fissioning cycle (chain reaction, criticality)

– A great amount of energy can be released in fission because for heavy nuclei, the summed masses of the lighter product nuclei is less than the mass of the fissioning nucleus

Fission Reaction Energy• The binding energy of the nucleus is directly

related to the amount of energy released in a fission reaction

• The energy associated with the difference in mass of the products and the fissioning atom is the binding energy

atomnep MmZAmmZ )()(

2cE

Radioactivity• In 1899, Ernest Rutheford discovered

Uranium produced three different kinds of radiation.– Separated the radiation by penetrating ability– Called them , , a b g

• a-Radiation stopped by paper (He nucleus, )• b-Radiation stopped by 6mm of Aluminum

(Electrons produced in the nucleus)• g-Radiation stopped by several mm of Lead

(Photons with wavelength shortward of 124 pm or energies greater than 10 keV)

He42

Half-Life• The half life is the

amount of time necessary for ½ of a radioactive material to decay

• Starting with 100g of Bismuth– Half life of 5 days– 50 g of bismuth

after 5 days– 50 g of thallium

a-Particle Decay

• The emission of an a particle, or 4He nucleus, is a process called a decay

• Since a particles contain protons and neutrons, they must come from the nucleus of an atom

-b Particle Decay• b particles are negatively charged electrons emitted by

the nucleus – Since the mass of an electron is a small fraction of an atomic

mass unit, the mass of a nucleus that undergoes b decay is changed by only a small amount.

– The mass number is unchanged. • The nucleus contains no electrons. Rather, b decay

occurs when a neutron is changed into a proton within the nucleus. – An unseen neutrino, n, accompanies each b decay. – The number of protons, and thus the atomic number, is

increased by one.

-g Radiation Decay

• Gamma rays are a type of electromagnetic radiation that results from a redistribution of electric charge within a nucleus.

• A g ray is a high energy photon. • For complex nuclei there are many different possible

ways in which the neutrons and protons can be arranged within the nucleus. – Gamma rays can be emitted when a nucleus undergoes a

transition from one quantum energy configuration to another. – Neither the mass number nor the atomic number is changed

when a nucleus emits a g ray in the reaction

152Dy* 152Dy + g

Fission

Fission Fragments and the Chain Reaction

Uranium Enrichment

Plutonium