Posted on 12/13/2017 11:20:50 AM PST by Red Badger
The finding that fission releases huge amounts of energy launched a scientific and military race to understand and use this new atomic source of power
Over Christmas vacation in 1938, physicists Lise Meitner and Otto Frisch received puzzling scientific news in a private letter from nuclear chemist Otto Hahn. When bombarding uranium with neutrons, Hahn had made some surprising observations that went against everything known at the time about the dense cores of atoms—their nuclei.
Meitner and Frisch were able to provide an explanation for what he saw that would revolutionize the field of nuclear physics: A uranium nucleus could split in half—or fission, as they called it—producing two new nuclei, called fission fragments. More importantly, this fission process releases huge amounts of energy. This finding at the dawn of World War II was the start of a scientific and military race to understand and use this new atomic source of power.
The release of these findings to the academic community immediately inspired many nuclear scientists to investigate the nuclear fission process further. Physicist Leo Szilard made an important realization: if fission emits neutrons, and neutrons can induce fission, then neutrons from the fission of one nucleus could cause the fission of another nucleus. It could all cascade in a self-sustained “chain” process.
Thus began the quest to experimentally prove that a nuclear chain reaction was possible—and 75 years ago, researchers at the University of Chicago succeeded, opening the door to what would become the nuclear era. Harnessing fission
As part of the Manhattan Project effort to build an atomic bomb during World War II, Szilard worked together with physicist Enrico Fermi and other colleagues at the University of Chicago to create the world’s first experimental nuclear reactor.
For a sustained, controlled chain reaction, each fission must induce just one additional fission. Any more, and there’d be an explosion. Any fewer and the reaction would peter out.
In earlier studies, Fermi had found that uranium nuclei would absorb neutrons more easily if the neutrons were moving relatively slowly. But neutrons emitted from the fission of uranium are fast. So for the Chicago experiment, the physicists used graphite to slow down the emitted neutrons, via multiple scattering processes. The idea was to increase the neutrons’ chances of being absorbed by another uranium nucleus.
To make sure they could safely control the chain reaction, the team rigged together what they called “control rods.” These were simply sheets of the element cadmium, an excellent neutron absorber. The physicists interspersed control rods through the uranium-graphite pile. At every step of the process Fermi calculated the expected neutron emission, and slowly removed a control rod to confirm his expectations. As a safety mechanism, the cadmium control rods could quickly be inserted if something started going wrong, to shut down the chain reaction.
They called this 20x6x25-foot setup Chicago Pile Number One, or CP-1 for short—and it was here they obtained world’s the first controlled nuclear chain reaction on December 2, 1942. A single random neutron was enough to start the chain reaction process once the physicists assembled CP-1. The first neutron would induce fission on a uranium nucleus, emitting a set of new neutrons. These secondary neutrons hit carbon nuclei in the graphite and slowed down. Then they’d run into other uranium nuclei and induce a second round of fission reactions, emit even more neutrons, and on and on. The cadmium control rods made sure the process wouldn’t continue indefinitely, because Fermi and his team could choose exactly how and where to insert them to control the chain reaction.
Controlling the chain reaction was extremely important: If the balance between produced and absorbed neutrons was not exactly right, then the chain reactions either would not proceed at all, or in the other much more dangerous extreme, the chain reactions would multiply rapidly with the release of enormous amounts of energy.
Sometimes, a few seconds after the fission occurs in a nuclear chain reaction, additional neutrons are released. Fission fragments are typically radioactive, and can emit different types of radiation, among them neutrons. Right away, Enrico Fermi, Leo Szilard, Eugene Wigner and others recognized the importance of these so-called “delayed neutrons” in controlling the chain reaction.
If they weren’t taken into account, these additional neutrons would induce more fission reactions than anticipated. As a result, the nuclear chain reaction in their Chicago experiment could have spiraled out of control, with potentially devastating results. More importantly, however, this time delay between the fission and the release of more neutrons allows some time for human beings to react and make adjustments, controlling the power of the chain reaction so it doesn’t proceed too fast.
The events of December 2, 1942 marked a huge milestone. Figuring out how to create and control the nuclear chain reaction was the foundation for the 448 nuclear reactors producing energy worldwide today. At present, 30 countries include nuclear reactors in their power portfolio. Within these countries, nuclear energy contributes on average 24 percent of their total electrical power, ranging as high as 72 percent in France.
CP-1’s success was also essential for the continuation of the Manhattan Project and the creation of the two atomic bombs used during World War II. Physicists’ remaining questions
The quest to understand delayed neutron emission and nuclear fission continues in modern nuclear physics laboratories. The race today is not for building atomic bombs or even nuclear reactors; it’s for understanding of basic properties of nuclei through close collaboration between experiment and theory.
Researchers have observed fission experimentally only for a small number of isotopes – the various versions of an element based on how many neutrons each has—and the details of this complex process are not yet well-understood. State-of-the-art theoretical models try to explain the observed fission properties, like how much energy is released, the number of neutrons emitted and the masses of the fission fragments.
Delayed neutron emission happens only for nuclei that are not naturally occurring, and these nuclei live for only a short amount of time. While experiments have revealed some of the nuclei that emit delayed neutrons, we are not yet able to reliably predict which isotopes should have this property. We also don’t know exact probabilities for delayed neutron emission or the amount of energy released—properties that are very important for understanding the details of energy production in nuclear reactors.
In addition, researchers are trying to predict new nuclei where nuclear fission might be possible. They’re building new experiments and powerful new facilities which will provide access to nuclei that have never before been studied, in an attempt to measure all these properties directly. Together, the new experimental and theoretical studies will give us a much better understanding of nuclear fission, which can help improve the performance and safety of nuclear reactors.
Both fission and delayed neutron emission are processes that also happen within stars. The creation of heavy elements, like silver and gold, in particular can depend on the fission and delayed neutron emission properties of exotic nuclei. Fission breaks the heaviest elements and replaces them with lighter ones (fission fragments), completely changing the element composition of a star. Delayed neutron emission adds more neutrons to the stellar environment, that can then induce new nuclear reactions. For example, nuclear properties played a vital role in the neutron-star merger event that was recently discovered by gravitational-wave and electromagnetic observatories around the world.
The science has come a long way since Szilard’s vision and Fermi’s proof of a controlled nuclear chain reaction. At the same time, new questions have emerged, and there’s still a lot to learn about the basic nuclear properties that drive the chain reaction and its impact on energy production here on Earth and elsewhere in our universe.
“The 20th Century saw more technological advancements than the previous ten centuries combined........................”
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Very true,but I think that the printing press and electricity had more of a direct effect in the improvement of peoples’ lives.
“Well, I have a problem with the dangerous fission nuclear power plants but once they get the clean fusion process figured out (haven’t figured out how to deal with the heat requirement), that will be great I think.”
Think there is any possibility they’ll make an error and create a black hole in which we will all disappear ?
Maybe that explains why we can’t find anybody else...they are all gone because they were a more advanced civilization than we are and got to fusion first? LOL...
At that acceleration rate, the next 10 years are gonna be...fun.
Hence my pet theory on why we haven’t seen any [sophisticated] extraterrestrial life: all go through roughly the same process of scientific discovery, reaching a point where a sentient being goes “what happens if I try _this_?” and the entire planet disappears.
If I remember correctly, Disney had a program every Sunday night. They would introduce us to science. A man had 1000 spring type mouse traps in an enclosed room each with 2 ping pong balls resting on them. The man tossed a ping pong ball into the room and Pop! two ping balls were launched, then four, and a second later, the room exploded with ping pong balls. Hence, a simulated nuclear explosion.
Many of the great nuclear physicists in Europe were Jewish. Mrs. Enrico Fermi was also Jewish. They fled Europe, Germany in particular, in the 1930s.
Never count on your enemy being a genocidal maniac and chasing away his best scientists ... but be grateful when it happens.
As the old saying goes, fusion is the energy of the future, and it always will be.
The drive of Nobel Laureate Enrico Fermi to produce a sustained nuclear fission reaction was more than just to produce electricity after the war.
Kept secret at the time was the fact that already in 1941, Art Wahl, a graduate student of Glenn Seaborg, had isolated plutonium from bombarded uranium, and they, along with others at the University of California had discovered that plutonium-239 was significantly more fissionable than uranium-235. Thus began the Manhattan Project, with Fermi and Wahl going to Los Alamos, Seaborg going to the Met Lab in Chicago, and others going to Hanford, WA, in the enormous Manhattan Project to build a plutonium bomb. (Oak Ridge was primarily responsible for working to enrich U-235 for a uranium bomb.)
Less than three years after the first controlled nuclear chain reaction under the University of Chicago football stadium, a plutonium bomb was tested near Alamagordo, NM, on July 16, 1945. Less than a month later, a uranium bomb had exploded over Hiroshima, Japan, another plutonium bomb had exploded over Nagasaki, and Japan had surrendered.
A little known fact about CP-1:
Brian Williams was standing right behind Enrico Fermi’s right shoulder double-checking Fermi’s slide rule calculations on his HP=35 hand calculator. He later confirmed that it was his approval that allowed Fermi to pull the rod out the last few millimeters to attain criticality.
#ThanksBrian
For all Nazi Germany’s technological prowess, they applied it poorly. A German officer was captured during the Normandy invasion and was allowed to watch part of the American supplies coming ashore. He asked, “Where are the horses?”
At that acceleration rate, the next 10 years are gonna be...fun.
And while we have landed on Mars, where are our flying cars?
They were forecast for the 1950s.
It’s my opinion that innovation has flattened out and struggles under the weight of federal regulation, which gets less and less onerous as you go back in time.
I was raised about 50 miles from there................
Thanks for posting. Great short history.
Regulation indeed limits our productivity & advances.
The other factor in not already being in the future imagined decades ago: we discovered much of “the future” actually was boring, costly, or otherwise undesirable. “2001”’s orbiting hotel offered nothing but a great view, or a stopover on the way to the Moon. Getting to the Moon is cool, but being there isn’t - it’s inert dead rocks amid radiation extremes. Flying cars (in the form predicted) are possible, but the space & cost & training needed are more trouble than worth; scaled-up human-carrying drones ARE looking quite possible, in part because of the computing power reaching the point where you tell it where to go and it takes care of piloting for you (autonomous flight being actually easier than autonomous ground-driving). In all of these, the energy costs are staggering relative to the benefits; you’ll get a lot more done by staying on the ground.
What wasn’t expected was the Information Age. Instant worldwide communication using a pocket-sized magic slate, coupled with staggering data storage capacities, opened up possibilities which the past’s futurists couldn’t predict.
With that context - the unexpected Information Age supplanting the predicted Space Age (because cat videos are more interesting than moon rocks) - consider what will happen when the _next_ unexpected development occurs.
“No tree grows to the sky.”
Correct, but I can make a tree into an airplane and fly there.
Good points, all.
My dad worked on the Manhattan project.
He told me that at the time, he was living in an apartment. Seems he became friends with a science fiction buff who lived in the same building. The friend knew that my dad was working on some top secret military project. One day, to my dad’s shock, he announced that he had figured out what the project was. It seems that all discussion of atomic bombs had abruptly disappeared from sci-fi literature, so he concluded that the a-bomb was, in fact, the secret project.
The Japanese came to same conclusions using the same reasoning. They started a nuclear program (One of their leading physicists had been a student of Oppenheimer!) it was roughly a year to 18 months behind ours when the war ended.) There is some evidence try actually tested “something” on a Korean (now North Korean!) island in the China Sea.
Could very well be. But I don’t think so. “Impossible” is the trigger mechanism for breakthrough.
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