Nuclear Fundamentals Part I Unleashing the Power of the Atom

Objectives Purpose/advantages of nuclear power Purpose/advantages of nuclear power Atomic structure, notation, and vocabulary Atomic structure, notation, and vocabulary Mass-to-energy conversions (how to get blood from a turnip) Mass-to-energy conversions (how to get blood from a turnip) Basics of nuclear fission Basics of nuclear fission Controlling fission and nuclear reaction rates Controlling fission and nuclear reaction rates

Introduction Early/alternate naval boilers used oil, coal, or wood -> nuclear fission is viable option Early/alternate naval boilers used oil, coal, or wood -> nuclear fission is viable option Advantages: Advantages: Long life of nuclear core Long life of nuclear core Unlimited endurance/range Unlimited endurance/range No need for outside material (air) No need for outside material (air) Less logistical support Less logistical support Carrier carries more weapons, aircraft, fuel Carrier carries more weapons, aircraft, fuel

Basic Atomic Structure Nucleus: the core of an atom Nucleus: the core of an atom Proton: Proton: positive (+) charge positive (+) charge primary identifier of an element primary identifier of an element mass: amu mass: amu Neutron: Neutron: no charge no charge usually about the same number as protons usually about the same number as protons mass: amu mass: amu Electron: orbits about the nucleus Electron: orbits about the nucleus Negative (-) charge Negative (-) charge Mass: amu (over 1000’s times smaller) Mass: amu (over 1000’s times smaller) Help determine how element reacts chemically Help determine how element reacts chemically

Basic Atomic Structure

Atomic Structure Isotopes: atoms which have the same atomic number but a different atomic mass number (ie: different number of neutrons) Isotopes: atoms which have the same atomic number but a different atomic mass number (ie: different number of neutrons) Standard Notation: A Z X Standard Notation: A Z X where: where: X = element symbol (ie: H for hydrogen) X = element symbol (ie: H for hydrogen) A = atomic mass number (p’s and n’s) A = atomic mass number (p’s and n’s) Z = atomic number (p’s only) Z = atomic number (p’s only)

Standard Notation & the Periodic Table U -> U:uranium 238: p’s + n’s 238: p’s + n’s 92: p’s 92: p’s 146n’s 146n’s

Mass to Energy Remember conservation of mass & energy Remember conservation of mass & energy Mass of an element/isotope is less than individual masses of p’s, n’s, and e’s -> difference is called mass defect Mass of an element/isotope is less than individual masses of p’s, n’s, and e’s -> difference is called mass defect Einstein’s Theory: E = mc 2 or  E =  mc 2 Einstein’s Theory: E = mc 2 or  E =  mc 2 Energy released if nucleus is formed from its components is binding energy (due to mass defect) Energy released if nucleus is formed from its components is binding energy (due to mass defect)

Blood from a Turnip

Mass to Energy Mass Defect = mass of reactants - mass of products Mass Defect = mass of reactants - mass of products Conversion to energy Conversion to energy 1 amu = MeV 1 amu = MeV Fission uses this principle -> large isotopes break into pieces releasing energy which can be harnessed Fission uses this principle -> large isotopes break into pieces releasing energy which can be harnessed

Fission Def’n: splitting of an atom Def’n: splitting of an atom U is fuel for reactor U is fuel for reactor Relatively stable Relatively stable Likely to absorb a neutron (large  a ) Likely to absorb a neutron (large  a ) U fissions readily (large  f ) U fissions readily (large  f ) Basic Fission Equation Basic Fission Equation 1 0 n U U FF 1 + FF n + Energy

Basic Fission Equation 1 0 n U U FF 1 + FF n + Energy 1 0 n U U FF 1 + FF n + Energy

Fission Fragments 1 0 n U U 1 0 n U U FF 1 + FF n + Energy

Fission Neutrons produced (2.43 avg.) will cause other fissions -> chain reaction Neutrons produced (2.43 avg.) will cause other fissions -> chain reaction Neutrons classified by energy levels Neutrons classified by energy levels Fast n’s: n’s produced by fission (>0.1 MeV) Fast n’s: n’s produced by fission (>0.1 MeV) Thermal/slow n’s: these cause fission (<0.1 eV) Thermal/slow n’s: these cause fission (<0.1 eV) So, if chain reaction is to be sustained, n’s must slow down to thermal energy levels So, if chain reaction is to be sustained, n’s must slow down to thermal energy levels

Neutron Interactions & Fission Interaction described in terms of probability (called microscopic cross section ) Interaction described in terms of probability (called microscopic cross section ) the larger the effective target area, the greater the probability of interaction the larger the effective target area, the greater the probability of interaction measured in barns ( cm) measured in barns ( cm) Represented by  (single neutron interacting with single nucleus) Represented by  (single neutron interacting with single nucleus)

Neutron Interactions & Fission Scattering (  s ) Scattering (  s ) Elastic type collision w/ nucleus (thermalized) Elastic type collision w/ nucleus (thermalized) Absorption (  a ) Absorption (  a ) Neutron absorbed by nucleus Neutron absorbed by nucleus Fission (  f ) Fission (  f ) IF absorbed, causes fission IF absorbed, causes fission Capture (  c ) Capture (  c ) IF absorbed, causes no fission IF absorbed, causes no fission

Neutron Life Cycle THERMALIZATION U FISSION FAST n’s THERMAL n’s Thermal Absorption Fast Absorption Capture Fast Leakage Thermal Leakage

Condition of Reaction Rate k eff = # of neutrons in a given generation k eff = # of neutrons in a given generation # of neutrons in preceding generation # of neutrons in preceding generation Critical: fission rate just sustained by the minimum number of thermal fissions (k eff = 1) Critical: fission rate just sustained by the minimum number of thermal fissions (k eff = 1) Subcritical: fission rate is decreasing since not enough thermal neutrons are produced to maintain fission reactions (k eff < 1) Subcritical: fission rate is decreasing since not enough thermal neutrons are produced to maintain fission reactions (k eff < 1) Supercritical: fission rate increasing since more than necessary thermal neutrons created (k eff > 1) Supercritical: fission rate increasing since more than necessary thermal neutrons created (k eff > 1)

Stability & Nuclear Force As the number of particles w/in a nucleus increases, the energy which binds nucleus together becomes weaker -> unstable isotopes - > more likely to give off particles As the number of particles w/in a nucleus increases, the energy which binds nucleus together becomes weaker -> unstable isotopes - > more likely to give off particles Elements undergo radioactive decay to try to achieve stability Elements undergo radioactive decay to try to achieve stability All isotopes w/ atomic number > 83 are naturally radioactive All isotopes w/ atomic number > 83 are naturally radioactive

Radioactivity Decay occurs in 3 modes: Decay occurs in 3 modes: Alpha (  ) Alpha (  ) Beta (  ) Beta (  ) Gamma (  ) Gamma (  ) Alpha (  Alpha (  positively charged particle w/ 2 p’s & 2 n’s positively charged particle w/ 2 p’s & 2 n’s usually emitted from heavy unstable nuclei usually emitted from heavy unstable nuclei Virtually no threat: Easily absorbed by dead skin layer Virtually no threat: Easily absorbed by dead skin layer Ex: U Th  Ex: U Th 

Radioactivity Beta  Beta  negatively or positively charged particle negatively or positively charged particle emitted from nucleus when n -> p or vice versa emitted from nucleus when n -> p or vice versa like an electron (p -> n) or positron (n -> p) like an electron (p -> n) or positron (n -> p) Minimal threat: can be absorbed by clothing Minimal threat: can be absorbed by clothing Ex: Th Pa +  - Ex: Th Pa +  -

Radioactivity Gamma (  Gamma (  electromagnetic wave of high freq/ high energy electromagnetic wave of high freq/ high energy Not a particle: thus no charge Not a particle: thus no charge lowers energy level of parent nuclei (no change in A or Z) lowers energy level of parent nuclei (no change in A or Z) Potential threat to operators (must be shielded) Potential threat to operators (must be shielded) Ex: Co Ni + 2  +  - Ex: Co Ni + 2  +  -

Radioactivity Half life : time required for 1/2 of any given number of radioactive atoms to disintegrate, thus reducing radiation intensity by ½ of initial radiation Half life : time required for 1/2 of any given number of radioactive atoms to disintegrate, thus reducing radiation intensity by ½ of initial radiation Some short (  sec), some long (billions of years) Some short (  sec), some long (billions of years) 5 t 1/2 ’s to not be radioactive 5 t 1/2 ’s to not be radioactive

Questions?