01/31/2006
Missing Isotope Mystery
ON VACATION: If all goes as planned, I will be in transit to or on Marco Island in Florida when most of you read this message. If you have already read last week’s column below but still want more of old Bortrum, click on the archives at the bottom of this column. Next column on February 8.
Greetings from windy New Jersey. Although we didn’t suffer as badly as Connecticut from the near hurricane force winds last week, my wife and I could have had an unpleasant experience. While driving to our walk at the mall, ours was the first car turned back by firemen to avoid hitting a live power line hanging down across the street. After such an experience, there’s nothing like that orange juice blended with a Cavendish banana we talked about in last week’s column. Our good friend Dan in Honolulu wrote that he enjoyed a Hawaiian-grown “apple” banana while reading that column. Dan said the apple is so much better than the Cavendish, he pays the 60 cents more a pound for the former.
My morning routine of blending banana and orange juice has a regularity to it resembling Old Faithful geyser in Yellowstone. Such geysers operate like clockwork, erupting on schedule for years. What happens is the water heats up, and then boils off in the eruption. More water fills the geyser and the cycle is repeated. Which brings me to nuclear fission and uranium enrichment, in the headlines thanks to Iran’s apparent nuclear weapons program. (Incidentally, do I detect that President Bush is gradually approaching the correct pronunciation of the word “nuclear”? If so, I’m somewhat encouraged that perhaps the world’s more serious nuclear problems can be solved.)
Instead of weapons, let’s look at a peaceful use of nuclear fission and evidence that Nature beat us to its use for generating power by a couple billion years! Here in the U.S.A., there’s been a moratorium on building more nuclear power reactors. Not so in France, where you may see more than one on a train ride through the countryside. So it’s not surprising that it was in France that a routine analysis of uranium for use in a reactor led to the discovery of Nature’s own foray into nuclear power.
We’ve talked before about uranium having a number of isotopes, one being U235. Here on Earth, as well as on the moon and in meteorites, uranium contains 0.720 percent U235. U235 is the active ingredient in the atomic bomb and in most nuclear power reactors. An article by Alex Meshik in the November 2005 Scientific American, tells of a French worker who, in 1972, found that a sample of uranium contained not 0.720 but 0.717 percent U235. If I had been that worker, I would have probably congratulated myself on getting a result so close to the normal value. However, some French scientists took the result seriously and tracked down the source of that sample to ore that came from a place known as the Oklo uranium deposit in the African nation of Gabon.
What they found there was that ore from one section of the Oklo mine really was low in U235. Actually, enough was missing to make a handful of nuclear bombs! What happened? The French were mystified but the answer had been proposed back in 1953. To understand the proposal, let’s review briefly what goes on in nuclear fission of uranium. A slow moving neutron comes along and hits an atom of U235. The neutron is captured in the nucleus of the atom. Adding a neutron makes the U235 heavier – it’s now U236. But U236 is very unstable and quickly breaks up into pieces (fission), emitting 2 or 3 neutrons and a lot of heat in the process. If you slow down these neutrons with a “moderator” such as water, they can be captured by other U235 atoms, each splitting apart and releasing 2 or 3 more neutrons and more heat.
If you have enough U235, you can see that pretty soon you would have a lot of neutrons and a lot of heat. Too many neutrons and too much heat and you have meltdown, not a good thing. In a nuclear power reactor, you have control rods of materials such as boron, which soaks up excess neutrons and keeps things running under controlled conditions.
Now we’re ready to understand what caused the missing U235. What George Wetherill and Mark Inghram suggested in 1953 was, that if conditions were just right, there could have been spontaneous forms of nuclear reactors in uranium ore deposits. A fellow named Paul Kuroda calculated what the conditions would have to be for this to occur naturally. One condition is that the ore deposit had to be large enough; specifically, larger than the few feet that a slow neutron typically travels before being captured by U235 or otherwise absorbed.
Kuroda also concluded that the ore had to contain more U235 than the 0.720 percent found today. You may remember that U235 itself is unstable and decays to other elements. If we calculate back 2 billion years the uranium ore would have had 3 percent U235. Three percent is about the same amount of U235 that’s in the enriched uranium we use in today’s nuclear power reactors. To have a natural spontaneous reactor, you also need a moderator to slow down the neutrons. There was water in the Oklo deposit 2 billion years ago. Finally, there can’t be any boron or other metals that would absorb neutrons as in the control rods of a reactor.
Researchers found 16 sites in the Oklo and a nearby uranium deposit where all these conditions prevailed 2 billion years ago. Analyses of the ores showed the expected decay products that would have resulted from nuclear fission and the scientists concluded that there were indeed sustained spontaneous nuclear reactors at that time. They even came up with an estimate of the power level. Their estimate was a mere 100 kilowatts, which is only enough energy to light about a thousand 100-watt light bulbs. Not much power.
These spontaneous reactors obviously consumed U235 while fission was taking place. The mystery of the missing U235 in certain ores was solved. All this work was back in the 1970s. More recently, Alex Meshik, author of the Scientific American article, and his colleagues looked in detail at one of the decay products from fission and other sources in Oklo rock. Their attention was directed at xenon, a heavy gaseous element.
Xenon has nine different isotopes and different isotopes form at different stages of decay of products from fission and other normal decay reactions. By looking at the amounts of each xenon isotope and also noting which expected isotopes were missing (washed away by water, for example), Meshik and co- workers came to a startling conclusion. I won’t attempt to follow their complicated analysis but they concluded that from their xenon results they could reconstruct the actual cycle that particular spontaneous reactor followed 2 billion years ago.
Here’s where the geyser analogy comes into play. The xenon results indicate that the Oklo reactor operated in a cycle of about a half hour “on” (fission taking place) and at least two and a half hours “off” (no fission). In the on condition, water was present and slowed down the neutrons; fission took place. The heat generated by the fission heated the water until it boiled off, as in a geyser. Without the water, the neutrons were not slowed down and the fission stopped. The ore cooled, water flowed back in and slowed down the neutrons, and fission resumed. I picture little “nuclear” geysers spouting up every few hours with nobody there to see them.
One of the consequences of the water heating and cooling was the growth of aluminum phosphate, formed from the other minerals in the neighborhood. The aluminum phosphate has a structure that’s like a cage, which trapped the xenon and held it there for these billions of years. The Oklo area is actually a place where Nature has stored the waste from nuclear reactors. Could a study of what happened at Oklo help to solve the waste problem we face today? After decades of scientific and political controversy, we still must decide what to do with all the nuclear waste from our power reactors and nuclear weapons programs. How and where can we store these radioactive wastes and assure that they stay contained for tens of thousands or even hundreds of thousands of years? Will humans even be around that long?
Hey, these questions are taxing my feeble brain. I think I’ll whip up a Cavendish and orange juice. Allen F. Bortrum
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