Instructor: Gordon D. CatesOffice: Physics 106a, Phone: (434) 924-4792

email: cates@virginia.edu

How Things Work II(Lecture #32)

April 11, 2008

Course web site available through COD and Toolkitor at http://people.virginia.edu/~gdc4k/phys106/spring08

Announcements

• Final exam will be Thursday, May 1st, from 9-12AM.

• Please read Chapter 16 in your text.

• Problem Set #5 will be posted within the next few days.

Question

A. 0.B. more than 1C. more than 1.5D. more than 2.0E. more than 3.0

On average, in order to sustain a chain reaction, how many neutrons must be captured by U-235 nuclei following the fission of a single U-235 nucleus?

Not all uranium is the same !

Both U-238 and U235 are unstable, but U-235 is more unstable than U-238 (half-lives of 4.5x109 and 7.0x108 years respectively).

This is why most of the naturally occurring uranium is U-238.

23892 U

23592 U

99.27% of all uranium is U-238When hit by a neutron it will sometimes undergo fission, but most of the time the neutron is just absorbed.

0.63% of all uranium is U-235When hit by a neutron it will almost always undergo fission.

This is the stuff out of

which you can make bombs and reactors.

It is very difficult to separate the two isotopes of uranium

Chemically they are essentially identical. You can only separate them by taking advantage of the fact

that their masses are different.

23892 U

23592 U

99.27%

0.63%

Secret efforts to develop the bomb

Article from NY Times the day after the bombing of Hiroshima (August 7th, 1945) describing the “hidden cities”

where the materials to make the bombs, and the bombs themselves, were produced.

The simplest atom bomb - using U-235

If a sphere of U-235 is big enough, on average, more than one neutron per fission reaction will cause another fission reaction. This happens with a sphere of roughly 52 kg, around 17 cm in diameter. The simplest nuclear weapon simply “assembles” this critical mass. Not shown is the “trigger” that releases neutrons

at the key moment.

Video of bomb tests and results

“Little Boy” - dropped on Hiroshima

No one will ever know for certain how many died as a result of the attack on Hiroshima.  Some 70,000 people probably died as a result of initial blast, heat, and radiation effects.  This included about twenty American airmen being held as prisoners in the city.  By the end of 1945, because of the lingering effects of radioactive fallout and other after effects, the Hiroshima death toll was probably over 100,000.  The five-year death total may have reached or even exceeded 200,000, as cancer and other long-term effects took hold.

Photos and text from U.S. Dept. of Energy website.

“Little Boy” - dropped on Hiroshima

Though already eleven and a half miles away, the Enola Gay was rocked by the blast.  At first, Tibbets [Captain and pilot of the Enola Gay] thought he was taking flak.  After a second shock wave (reflected from the ground) hit the plane, the crew looked back at Hiroshima.  "The city was hidden by that awful cloud . . . boiling up, mushrooming, terrible and incredibly tall," Tibbets recalled.  The yield of the explosion was later estimated at 15 kilotons (the equivalent of 15,000 tons of TNT).

Plutonium-239, the other commonly used material for making nuclear weapons

Plutonium is different from U-235 in that it undergoes spontaneous fission much more often than does U-235 (by a factor of just over 100).

Consequently, a “cannon-like” design would result in a core that would blow itself apart before undergoing an efficient explosion.

Plutonium-based weapons

With 239Pu a “cannon-like” design would result would blow apart prematurely, resulting in an efficient explosion. For this reason, and

implosion-based design was developed.

Cross-section view of “Fat Man”

This is the design that was tested near Alamogordo, New Mexico, on June 16, 1945. The critical mass for Pu-239 is around 10 kg, but Fat Man used a subcritical mass of around 6.1 kg, which was

sufficient after being compressed by the implosion.

“Fat Man” - dropped on Nagasaki

According to a Nagasaki Prefectural report "men and animals died almost instantly" within 1 kilometer (0.62 miles) of the point of detonation.  Almost all homes within a mile and a half were destroyed, and dry, combustible materials such as paper instantly burst into flames as far away as 10,000 feet from ground zero.  Of the 52,000 homes in Nagasaki, 14,000 were destroyed and 5,400 more seriously damaged.  Only 12 percent of the homes escaped unscathed. 

As with the estimates of deaths at Hiroshima, it will never be known for certain how many people died as a result of the atomic attack on Nagasaki.  The best estimate is 40,000 people died initially, with 60,000 more injured.  By January 1946, the number of deaths probably approached 70,000, with perhaps ultimately twice that number dead total within five years.

The explosion affected a total area of approximately 43 square miles.

Video of bomb tests and results

The “Davy Crockett”

The “Davy Crockett”

Breeding plutonium from U-238

When U-238 captures a neutron, after two beta decays, it turns into Pu-239, the fissionable isotope of plutonium.

Plutonium can be manufactured in a nuclear reactor

Thus, fuel rods in a nuclear reactor will, after time, contain 239Pu. Plutonium is chemically different from uranium and can be extracted chemically. The re-processing of the fuel rods to extract plutonium is complicated and difficult, but far easier than isotopic separation.

Proliferation Issues

• 235U - Requires isotopic separation. Major industrial effort.

• 239Pu - Can be obtained from re-processed fuel rods, which of course implies you already have nuclear reactors.