Group Work

1. Should nuclear power capacity be increased or reduced? Why?

Announcements

• Get 3 scraps paper per person in addition to whiteboard and marker

• Standards retakes 23-30 today 11–mn

• Last Exam 3 today–6 PM tomorrow

• Final (makeups) Tuesday 10:15–12:15

Nuclear Implications and Applications

Chapter 34

Nuclear Power

Source: Jim Zimmerlin, http://www.zimfamilycockers.com/DiabloCanyon.html

Diablo Canyon Power Plant, California

Nuclear Power

• Nuclear fission reactions release heat (kinetic energy of products)

• Heat boils water, powers turbine to generate electricity

• No greenhouse gas production

Fission Chain Reaction

Source: Griffith

Criticality

• Reaction self-sustaining if each fission on average induces another

• Critical mass depends on geometry, moderation, reflection, many other factors

Poll Question

Which shape of fission fuel will be the most likely to reach criticality?

A. A sphere.

B. A plate.

C. A long, thin cylinder.

Fissile Nuclei

• Required for reactors and bombs

• Fission started by absorption of thermal neutrons

• Only fissile nuclei are U-233, U-235, and Pu-239

• U-235 is less than 1% of natural uranium (rest is U-238)

Plutonium Breeding

Source: Griffith

Breeder Reactors

• Transmute U-238 to Pu-239 or Th-232 to U-233

• Increase amount of usable fuel

Nuclear Waste

Source: Savannah River Site

Nuclear Fuel Cycle

• Fission chain reaction produces fission products and neutron capture products

• Certain fission products absorb neutrons

• Fuel becomes unusable after ~1% burnup

• Must be reprocessed or discarded

Poll Question

The amount of nuclear waste generated would be reduced if

A. spent fuel were reprocessed and reused.

B. all reactors were shut down.

C. reactors with higher “burn-up” were used.

D. any of these.

Spent Fuel Hazards

Source: Cohen, B. L. Rev. Mod. Phys. 1977, 49, 1–20.

Reprocessing

• Recover U, Pu from spent fuel

• Discard fission product waste

• Possibility of U, Pu diversion

Advanced Reactor Designs

• Passive cooling systems

• Melt-proof fuel pellets

• Higher-burnup design

• On-site fast reprocessing

• Fast neutron fission of non-fissile nuclei

• Accelerator-based systems

Group Work

2. What should be done with nuclear waste?

Stellar Fusion

NASA photograph, Skylab, 10 December 1973

Think Question

What sort of nucleus releases energy by fusion?

A. Nuclei lighter than iron.

B. Nuclei about as heavy as iron.

C. Nuclei heavier than iron.

Fusion in the Sun

Source: Seeds, Horizons: Exploring the Universe

Net reaction: 4 p+ 4He + 2 e+ + 2 + 2

Fusion in Hotter Stars

Source: Seeds, Horizons: Exploring the Universe

Net reaction: 4 p+ 4He + 2 e+ + 2 + 3

Fusion Life of Massive Stars

• Massive star’s hot core fuses atoms to ever-higher masses

• Greater nuclear charges require higher temperatures, pressures

• Less energy per nucleon from fusing massive nuclei

• Fusion energy exhausted at iron

Fusion Death by Supernova

• Iron core does not produce energy to resist gravitational collapse of star

• High temperature and pressure in shock wave creates more massive nuclei

• Some products scattered by explosion

All Fe, Cu, W, I, Ag, Au, Pb, U, etc. were released by supernovas!

Nuclear Weapons

Fission Weapons

Grable 15-kT yield artillery shell, Nevada Test Site, 25 May 1953.

U.S. Department of Energy photo.

Gun Device (“Little Boy”)

Source: Griffith

Implosion Device (“Fat Man”)

Source: Griffith

Fusion Weapons

Dakota 1.1 MT shot, Enewetak, 25 June 1956. U.S Department of Energy photo.

Fusion in Weapons

• Require high temperatures to bring nuclei together (thermonuclear)

• Heated by fission bomb “pit”

2H + 3H 4He + n

Thermonuclear Device