Lecture 5

Requirements for a Fission explosion

1. Trigger U-235 fission through neutron bombardment.
2. 1. Each fission process generates 3 neutrons
   2. High neutron-induced fission cross section (measurements with neutron beams on fissionable material)
   3. Maintaining high neutron flux (measurement of neutron production reactions).

1/v law of neutron capture

1. Neutrons have no charge!
   1. Neutron capture cross sections are proportional to the inverse neutron velocity.  (Particle-wave duality)
   2. The lower the velocity, the higher the reaction probability!
      
   3. 
      
2. This introduces the need for “moderating” neutrons to lower "thermal” velocities. The best neutron moderators are light mass materials because there is large energy transfer in scattering:
3. 1. Best Moderators: (low absorption cross section is crucial!)
   2. 1. Graphite: easy to make from carbon, obvious first choice
      2. Heavy Water

German Choices

1. Walter Bothe, the leading experimental nuclear physicist in Germany, did the crucial experiment and concluded that carbon in the form of graphite would not work. In America, Enrico Fermi did a similar experiment and concluded that graphite was marginal. He suspected that an impurity in the graphite was responsible for the problem. Leo Szilard, who was working alongside Fermi, had studied chemical engineering before going into physics. He remembered that electrodes of boron carbide were commonly used in the manufacture of graphite. It was known that one atom of boron absorbs about as many slow neutrons as 100,000 atoms of carbon. Very small boron impurities would "poison" the graphite for use as a nuclear reaction moderator. Szilard therefore went around to the American graphite manufacturers and convinced one of them to make boron-free graphite. Using this pure graphite as the moderator, the American group achieved a chain reaction on December 2, 1942.
2. The German team, however, needed to use heavy water. Ordinary water contains heavy water in the amount of about 1 part in 10,000. The two can be separated by repeated electrolysis, which requires large amounts of electric power in close proximity to a water source. The Germans had this at a hydroelectric plant in occupied Norway, and they set up a separation facility there  (Hans Bethe in Physics Today Vol 53, 2001).
   

The Manhattan Project

1. In response to the perceived German threat, the United States initiated a  program for the development of an “Atomic Bomb” under the Army Corps of Engineers in June 1942.  Lieutenant General Leslie Groves (the US Army Engineer officer who oversaw the Manhattan Project) projected three sites for the development of nuclear weapon production with the goal of:
   
   1. Enrichment of U-235
   2. Generating Pu-239
   3. Bomb assembly and testing.
2. J. Robert Oppenheimer (in picture on right along with Albert Einstein)
   
3. 1. After graduating from Harvard in 1925 and studying (unsuccessfully) at Cambridge under Ernest Rutherford, he obtained his PhD in Göttingen, Germany. In 1929 he returned to the United States to positions at Berkeley and Cal Tech. He was appointed by General Groves in 1942 as the Scientific Director of the Manhattan Project. Groves once said of Oppenheimer, "He's a genius. A real genius...Why, Oppenheimer knows about everything. He can talk to you about anything you bring up. Well not exactly. I guess there are a few things he doesn't know about. He doesn't know anything about sports."

 Nuclear Weapons and Nuclear Reactors

1. The first operating Reactor 
   The basic research for understanding fission properties was performed at the University of Chicago. For this purpose Enrico Fermi built the first nuclear reactor, CP-1, in a squash court under the football stadium. The first sustained nuclear reaction occurred on Dec. 2, 1942.
2. CP-1 Nuclear Reactor
   1. The  CP-1 used 235 enriched uranium metal from Iowa State.  The reactor used high purity graphite as a moderator for slowing down the neutrons to thermal velocities.
   2. CP-1 used cadmium control rods for absorbing neutrons and preventing the reactor from becoming critical. (Other neutron absorbing materials are e.g. Boron). Moderators need a high neutron scattering cross section.  Absorbers require a high neutron capture cross section.
3. Accelerator based radiation and material test facilities
   1. Wisconsin: neutron production to test material fissibility
   2. Notre Dame: high energy electron beam to test radiation hardness.
4. Oak Ridge
   
   1. In a remote area near Knoxville, Tennessee, a secret city was built. The main reason for choice of site was the abundant availability of  Tennessee water power.
   2. Primary purpose of the Oak Ridge facility was to enrich U-235.
   3. They also built a graphite reactor at site X-10 to study the production of plutonium.  Today, this is the site of the Oak Ridge National Laboratory (ORNL).
      
5. Y-12
   The purpose of Y-12 was the magnetic separation of U-235 from U-238.
6. Hanford
   
   1. Constructed in 1943 as follow up on X-10 in Oak Ridge as the main site for industrial plutonium production.  Hanford continued production until it was shut-down in 1963.
   2. Even today, Hanford stands as a major nuclear waste problem.

Graham’s Law of Effusion