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Nuclear Fundamentals Part IPowerPoint Presentation

Nuclear Fundamentals Part I

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Presentation Transcript

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

Introduction

- Early/alternate naval boilers used oil, coal, or wood -> 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

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 about the same number as protons
- mass: 1.00866 amu

- Proton:
- Electron: orbits about the nucleus
- Negative (-) charge
- Mass: 0.0005485 amu (over 1000’s times smaller)
- Help determine how element reacts chemically

Atomic Structure

- Isotopes: atoms which have the same atomic number 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 (p’s and n’s)
- Z = atomic number (p’s only)

Mass to 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
- Einstein’s Theory: E = mc2 or DE = Dmc2
- Energy released if nucleus is formed from its components is binding energy (due to mass defect)

Mass to Energy

- Mass Defect = mass of reactants - mass of products
- Conversion to energy
- 1 amu = 931.48 MeV

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

Fission

- Def’n: splitting of an atom
- 23592U is fuel for reactor
- Relatively stable
- Likely to absorb a neutron (large sa)
- 23692U fissions readily (large sf)

- Basic Fission Equation
10n + 23592U 23692U FF1 + FF2 + 2.43 10n + Energy

Basic Fission Equation

10n + 23592U 23692U FF1 + FF2 + 2.43 10n + Energy

Fission

- Neutrons produced (2.43 avg.) will cause other fissions -> chain reaction
- Neutrons classified by energy levels
- Fast n’s: n’s produced by fission (>0.1 MeV)
- 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

Neutron Interactions & Fission

- Interaction described in terms of probability (called microscopic cross section)
- the larger the effective target area, the greater the probability of interaction
- measured in barns (10-24 cm)
- Represented by s (single neutron interacting with single nucleus)

Neutron Interactions & Fission

- Scattering (ss)
- Elastic type collision w/ nucleus (thermalized)

- Absorption (sa)
- Neutron absorbed by nucleus

- Fission (sf)
- IF absorbed, causes fission

- Capture (sc)
- IF absorbed, causes no fission

Neutron Life Cycle

23592U

FISSION

Capture

FAST

n’s

Thermal

Absorption

Thermal

Leakage

Fast

Leakage

THERMAL

n’s

Fast

Absorption

THERMALIZATION

Condition of Reaction Rate

- keff = # of neutrons in a given generation
# of neutrons in preceding generation

- Critical: fission rate just sustained by the minimum number of thermal fissions (keff = 1)
- Subcritical: fission rate is decreasing since not enough thermal neutrons are produced to maintain fission reactions (keff < 1)
- Supercritical: fission rate increasing since more than necessary thermal neutrons created (keff > 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
- Elements undergo radioactive decay to try to achieve stability
- All isotopes w/ atomic number > 83 are naturally radioactive

Radioactivity

- Decay occurs in 3 modes:
- Alpha (a)
- Beta (b)
- Gamma (g)

- Alpha (a)
- positively charged particle w/ 2 p’s & 2 n’s
- usually emitted from heavy unstable nuclei
- Virtually no threat: Easily absorbed by dead skin layer
- Ex: 23892U 23490Th + 42a

Radioactivity

- Beta (b)
- negatively or positively charged particle
- emitted from nucleus when n -> p or vice versa
- like an electron (p -> n) or positron (n -> p)
- Minimal threat: can be absorbed by clothing
- Ex: 23490Th 23491Pa + b-

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: 6027Co 6028Ni + 2g + b-

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
- Some short (msec), some long (billions of years)
- 5 t1/2’s to not be radioactive

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