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Author Topic:   Spontaneous fission, decay rates, and critical mass
Member (Idle past 1626 days)
Posts: 166
From: Houston
Joined: 04-06-2009

Message 29 of 29 (648481)
01-16-2012 12:33 AM
Reply to: Message 23 by RAZD
01-12-2012 5:45 PM

Re: More thoughts about Gamow's equations or "When nuclides decay"
Initially the fission and resulting neutrons come from the fission of 235U. However, the presence of very high abundance of 238U absorbs some of the neutrons to become 239U. This in turn decays by beta decay to Neptunium 239 and the 239Pu. The Resulting 239Pu then fissions...

This is true of every nuclear fission reaction in the presence of 238U. More than 1/3 of the power from reactors in the U.S. is from the fission of Pu239 that results from neutron capture in 238U.
...but there is another twist to the story. The natural reactors operated for so long that the 239Pu had sufficient time to decay by alpha decay to 235U. Thus the natural reactors were true ‘Breeder’ reactors, fissioning in some cases more 235U than originally existed in the reactors.

To be honest, I had never thought of the alpha decay of Pu239 as a source of 235U. However, since the Pu239 is generally involved in the reaction almost as fast as it is produced, it doesn't seem possible that it could increase the depleted levels of U235 to levels above that of the surrounding ores.
Zen Deist writes:

(Hence resulting in the enriched ore that brought this site to international scientific attention)

What? The Oklo reactors were discovered, not because of higher levels of U235, but of lower levels of U235. The natural reactors were confirmed by the presence of fission products.

My source is my son's nuclear engineering text: Raymond L. Murray. Nuclear Energy. 1988. Pergamon Press. New York.

Of course, the time from when the reactors were active to the time when they were discovered has been sufficient for levels of most isotopes of Plutonium to have decayed below levels of detection. I have read that trace amounts of Pu244 have been detected in the Oklo reactors.

The concept of a breeder reactor is not that it produces fissile material from the reaction, but that it produces more fissile material than was consumed in the reaction. Modern breeder reactors accomplish this by surrounding the reactor core with a blanket of U238 that is not part of the core reaction. Neutrons that escape the core produce Pu239 in the blanket. The plutonium is then refined from the blanket and has potential energy more than that produced from the core reaction.

Please correct me if I am wrong. Sources please.

This message is a reply to:
 Message 23 by RAZD, posted 01-12-2012 5:45 PM RAZD has not yet responded

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