Lesson 07a - Nuclear Fundamentals

8/10/2019 Lesson 07a - Nuclear Fundamentals

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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?

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Lesson 07a - Nuclear Fundamentals