Nuclear Nuclear FundamentalFundamentals Part Is Part I

Unleashing the Unleashing the

Power of the Power of the AtomAtom

ObjectivesObjectives

• Purpose/advantages of nuclear Purpose/advantages of nuclear powerpower

• Atomic structure, notation, and Atomic structure, notation, and vocabularyvocabulary

• Mass-to-energy conversions (how Mass-to-energy conversions (how to get blood from a turnip)to get blood from a turnip)

• Basics of nuclear fissionBasics of nuclear fission• Controlling fission and nuclear Controlling fission and nuclear

reaction ratesreaction rates

IntroductionIntroduction

• Early/alternate naval boilers used oil, Early/alternate naval boilers used oil, coal, or wood -> nuclear fission is coal, or wood -> nuclear fission is viable optionviable option

• Advantages:Advantages:• Long life of nuclear coreLong life of nuclear core• Unlimited endurance/rangeUnlimited endurance/range• No need for outside material (air)No need for outside material (air)• Less logistical supportLess logistical support• Carrier carries more weapons, aircraft, Carrier carries more weapons, aircraft,

fuelfuel

Basic Atomic StructureBasic Atomic Structure• Nucleus: the core of an atomNucleus: the core of an atom

• Proton: Proton: • positive (+) chargepositive (+) charge• primary identifier of an element primary identifier of an element • mass: 1.00728 amumass: 1.00728 amu

• Neutron: Neutron: • no chargeno charge• usually usually aboutabout the same number as protons the same number as protons • mass: 1.00866 amumass: 1.00866 amu

• Electron: orbits about the nucleusElectron: orbits about the nucleus• Negative (-) charge Negative (-) charge • Mass: 0.0005485 amu (over 1000’s times Mass: 0.0005485 amu (over 1000’s times

smaller)smaller)• Help determine how element reacts Help determine how element reacts

chemicallychemically

Basic Atomic StructureBasic Atomic Structure

electron

neutron

proton

Atomic StructureAtomic Structure

• Isotopes: atoms which have the same Isotopes: atoms which have the same atomic number but a different atomic atomic number but a different atomic massmass number (ie: different number of number (ie: different number of neutrons)neutrons)

• Standard Notation: Standard Notation: AAZZXX

• 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 & Standard Notation & the Periodic Tablethe Periodic Table

2382389292U -> U:U -> U: uraniumuranium

238: 238: p’s + n’s p’s + n’s

92:92: p’sp’s

146146 n’sn’s

Mass to EnergyMass to Energy

• Remember conservation of mass & Remember conservation of mass & energyenergy

• Mass of an element/isotope is less Mass of an element/isotope is less than individual masses of p’s, n’s, and than individual masses of p’s, n’s, and e’s -> difference is called e’s -> difference is called mass defectmass defect

• Einstein’s Theory: Einstein’s Theory: E = mcE = mc2 2 or or E = E = mcmc22

• Energy released if nucleus is formed Energy released if nucleus is formed from its components is from its components is binding energybinding energy (due to mass defect)(due to mass defect)

Mass to EnergyMass to Energy

• Mass Defect = mass of Mass Defect = mass of reactants - mass of productsreactants - mass of products

• Conversion to energyConversion to energy• 1 amu = 931.48 MeV1 amu = 931.48 MeV

• Fission uses this principle -> Fission uses this principle -> large isotopes break into large isotopes break into pieces releasing energy which pieces releasing energy which can be harnessedcan be harnessed

FissionFission

• Def’n: splitting of an atomDef’n: splitting of an atom• 235235

9292U is fuel for reactorU is fuel for reactor• Relatively stableRelatively stable• Likely to absorb a neutron (large Likely to absorb a neutron (large aa))• 236236

9292U fissions readily (large U fissions readily (large ff))

• Basic Fission EquationBasic Fission Equation

1100n + n + 235235

9292U U 2362369292U FFU FF1 1 + FF+ FF2 2 + 2.43 + 2.43 11

00n + n + EnergyEnergy

Basic Fission EquationBasic Fission Equation

1100n + n + 235235

9292U U 2362369292U FFU FF1 1 + FF+ FF2 2 + 2.43 + 2.43

1100n + Energyn + Energy

Fission FragmentsFission Fragments

1100n + n + 235235

9292U U 236236

9292U U FFFF1 1 + FF+ FF2 2 + 2.43 + 2.43 1100n + Energyn + Energy

FissionFission

• Neutrons produced (2.43 avg.) will Neutrons produced (2.43 avg.) will cause other fissions -> chain reactioncause other fissions -> chain reaction

• Neutrons classified by energy levelsNeutrons classified by energy levels• Fast n’s: n’s produced by fission (>0.1 Fast n’s: n’s produced by fission (>0.1

MeV)MeV)• Thermal/slow n’s: these cause fission Thermal/slow n’s: these cause fission

(<0.1 eV)(<0.1 eV)• So, if chain reaction is to be So, if chain reaction is to be

sustained, n’s must slow down to sustained, n’s must slow down to thermal energy levelsthermal energy levels

Neutron Interactions & Neutron Interactions & FissionFission

• Interaction described in terms of Interaction described in terms of probability (called probability (called microscopic microscopic cross sectioncross section))

• the larger the effective target the larger the effective target area, the greater the probability area, the greater the probability of interactionof interaction

• measured in barns (10measured in barns (10-24 -24 cm)cm)• Represented by Represented by (single neutron (single neutron

interacting with single nucleus)interacting with single nucleus)

Neutron Interactions & Neutron Interactions & FissionFission

• Scattering (Scattering (ss))• Elastic type collision w/ nucleus Elastic type collision w/ nucleus

(thermalized)(thermalized)

• Absorption (Absorption (aa))• Neutron absorbed by nucleusNeutron absorbed by nucleus

• Fission (Fission (ff))• IF absorbed, causes fissionIF absorbed, causes fission

• Capture (Capture (cc))• IF absorbed, causes no fissionIF absorbed, causes no fission

Neutron Life CycleNeutron Life Cycle

THERMALIZATION

23592U FISSION

FASTn’s

THERMALn’s

ThermalAbsorption

FastAbsorption

Capture

FastLeakage

ThermalLeakage

Condition of Reaction Condition of Reaction RateRate

• kkeffeff = = # of neutrons in a given generation# of neutrons in a given generation

## of neutrons in preceding generation of neutrons in preceding generation• CriticalCritical: fission rate just sustained by the : fission rate just sustained by the

minimum number of thermal fissions (kminimum number of thermal fissions (keffeff = 1) = 1) • SubcriticalSubcritical: fission rate is decreasing since : fission rate is decreasing since

not enough thermal neutrons are produced to not enough thermal neutrons are produced to maintain fission reactions (kmaintain fission reactions (keff eff < 1)< 1)

• SupercriticalSupercritical: fission rate increasing since : fission rate increasing since more than necessary thermal neutrons more than necessary thermal neutrons created (kcreated (keffeff > 1) > 1)

Stability & Nuclear Stability & Nuclear ForceForce

• As the number of particles w/in a As the number of particles w/in a nucleus increases, the energy which nucleus increases, the energy which binds nucleus together becomes binds nucleus together becomes weaker -> unstable isotopes -> weaker -> unstable isotopes -> more likely to give off particlesmore likely to give off particles

• Elements undergo radioactive Elements undergo radioactive decay to try to achieve stabilitydecay to try to achieve stability

• All isotopes w/ atomic number > 83 All isotopes w/ atomic number > 83 are naturally radioactive are naturally radioactive

RadioactivityRadioactivity• 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’spositively charged particle w/ 2 p’s & 2 n’s• usually emitted from heavy unstable usually emitted from heavy unstable

nucleinuclei• Virtually no threat: Easily absorbed by Virtually no threat: Easily absorbed by

dead skin layerdead skin layer• Ex: Ex: 238 238

9292U U 2342349090Th + Th + 44

22

RadioactivityRadioactivity

• BetaBeta• negatively or positively charged negatively or positively charged

particleparticle• emitted from nucleus when n -> p or emitted from nucleus when n -> p or

vice versavice versa• like an electron (p -> n) or positron (n -like an electron (p -> n) or positron (n -

> p)> p)• Minimal threat: can be absorbed by Minimal threat: can be absorbed by

clothingclothing• Ex: Ex: 234234

9090Th Th 2342349191Pa + Pa + --

RadioactivityRadioactivity

• Gamma (Gamma (• electromagnetic wave of high electromagnetic wave of high

freq/ high energyfreq/ high energy• Not a particle: thus no chargeNot a particle: thus no charge• lowers energy level of parent lowers energy level of parent

nuclei (no change in A or Z)nuclei (no change in A or Z)• Potential threat to operators Potential threat to operators

(must be shielded)(must be shielded)• Ex: Ex: 6060

2727Co Co 60602828Ni + 2Ni + 2 + + - -

RadioactivityRadioactivity

• Half life : time required for 1/2 of Half life : time required for 1/2 of any given number of radioactive any given number of radioactive atoms to disintegrate, thus atoms to disintegrate, thus reducing radiation intensity by ½ reducing radiation intensity by ½ of initial radiationof initial radiation• Some short (Some short (sec), some long sec), some long

(billions of years)(billions of years)• 5 t5 t1/21/2’s to not be radioactive’s to not be radioactive

QuestioQuestions?ns?