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Objectives
Purpose/advantages of nuclear power
Atomic structure, notation, and
vocabulary Mass-to-energy conversions (how to get
blood from a turnip)
Basics of nuclear fission Controlling fission and nuclear reaction
rates
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Introduction
Early/alternate naval boilers used oil, coal, orwood -> nuclear fission is viable option
Advantages: Long life of nuclear core
Unlimited endurance/range
No need for outside material (air)
Less logistical support
Carrier carries more weapons, aircraft, fuel
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Basic Atomic Structure Nucleus: the core of an atom
Proton: positive (+) charge
primary identifier of an element
mass: 1.00728 amu Neutron:
no charge
usually aboutthe same number as protons
mass: 1.00866 amu
Electron: orbits about the nucleus Negative (-) charge
Mass: 0.0005485 amu (over 1000s times smaller)
Help determine how element reacts chemically
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Atomic Structure
Isotopes: atoms which have the same atomicnumber but a different atomic mass number (ie:
different number of neutrons) Standard Notation: AZX
where:
X = element symbol (ie: H for hydrogen)
A = atomic mass number (ps and ns)
Z = atomic number (ps only)
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Standard Notation &
the Periodic Table
23892U -> U: uranium238: ps + ns
92: ps
146 ns
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Mass to Energy
Remember conservation of mass & energy
Mass of an element/isotope is less than
individual masses of ps, ns, and es ->difference is called mass defect
Einsteins Theory: E = mc2 or DE = Dmc2
Energy released if nucleus is formed from itscomponents is binding energy(due to mass defect)
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Mass to Energy
Mass Defect = mass of reactants - mass ofproducts
Conversion to energy 1 amu = 931.48 MeV
Fission uses this principle -> large
isotopes break into pieces releasingenergy which can be harnessed
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Fission
Defn: splitting of an atom
235
92U is fuel for reactor
Relatively stable Likely to absorb a neutron (large sa)
236
92U fissions readily (large sf)
Basic Fission Equation
10n +
23592U
23692U FF1 + FF2 + 2.43
10n + Energy
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Basic Fission Equation
10n +
23592U
23692U FF1 + FF2 + 2.43
10n + Energy
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Fission Fragments
10n +
23592U
23692U
FF1 + FF2 + 2.4310n +
Energy
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Fission
Neutrons produced (2.43 avg.) will causeother fissions -> chain reaction
Neutrons classified by energy levels Fast ns: ns produced by fission (>0.1 MeV)
Thermal/slow ns: these cause fission (
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Neutron Interactions & Fission
Interaction described in terms ofprobability (called microscopic cross section)
the larger the effective target area, thegreater the probability of interaction
measured in barns (10-24 cm)
Represented by s(single neutroninteracting with single nucleus)
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Neutron Life Cycle
THERMALIZATION
23592U FISSION
FAST
ns
THERMALns
Thermal
Absorption
Fast
Absorption
Capture
Fast
Leakage
Thermal
Leakage
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Condition of Reaction Rate
keff= # of neutrons in a given generation
#of neutrons in preceding generation
Critical: fission rate just sustained by theminimum number of thermal fissions (keff= 1)
Subcritical: fission rate is decreasing since notenough thermal neutrons are produced tomaintain fission reactions (keff < 1)
Supercritical: fission rate increasing since morethan necessary thermal neutrons created (keff> 1)
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Stability & Nuclear Force
As the number of particles w/in a nucleusincreases, the energy which binds nucleus
together becomes weaker -> unstable isotopes -> more likely to give off particles
Elements undergo radioactive decay to try toachieve stability
All isotopes w/ atomic number > 83 arenaturally radioactive
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Radioactivity Decay occurs in 3 modes:
Alpha (a)
Beta (b)
Gamma (g)
Alpha (a)
positively charged particle w/ 2 ps & 2 ns
usually emitted from heavy unstable nuclei
Virtually no threat: Easily absorbed by dead skinlayer
Ex: 23892U234
90Th +4
2a
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Radioactivity
Gamma (g) electromagnetic wave of high freq/ high
energy
Not a particle: thus no charge
lowers energy level of parent nuclei (no
change in A or Z) Potential threat to operators (must be
shielded)
Ex: 6027Co60
28Ni + 2g + b-
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Radioactivity
Half life : time required for 1/2 of anygiven number of radioactive atoms to
disintegrate, thus reducing radiationintensity by of initial radiation
Some short (msec), some long (billions of
years) 5 t1/2s to not be radioactive
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Questions?