Nuclear Fundamentals Part I

1. Nuclear Fundamentals Part I Unleashing the Power of the Atom

2. 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

3. 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

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

5. Basic Atomic Structure

6. 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)

7. Standard Notation & the Periodic Table 23892U -> U: uranium 238: p’s + n’s 92: p’s 146 n’s

8. 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)

9. 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

11. 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

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

13. Fission Fragments 10n + 23592U 23692U FF1 + FF2 + 2.43 10n + Energy

14. 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

15. 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)

16. 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

17. Neutron Life Cycle 23592U FISSION Capture FAST n’s Thermal Absorption Thermal Leakage Fast Leakage THERMAL n’s Fast Absorption THERMALIZATION

18. 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)

20. 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

21. 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

22. 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-

23. 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-

24. 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

25. Questions?