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  < Back to Table Of Contents  < Back to Topic: Create & Innovate Plus Home Made Gifts & Games

article number 594
article date 09-22-2016
copyright 2016 by Author else SaltOfAmerica
We Make an Atom Bomb, Part 2: How can we produce Uranium U-235 or Plutonium?
by Wesley Stout, Chrysler Corporation
   

From the 1947 Chrysler Corporation book, Secret.

* * *

. . . it would take twelve million years by these methods to produce one pound of U-235. Much more than a pound would be needed for one bomb.

Hitler was about to invade Poland. World War II would bring on a silent, deadly contest to find a means of making U-235 in quantities. The winner would win the war almost surely, for there could be no doubt about the paralyzing effect, physically and morally, of even one bomb. That this was true, however was known in the United States only to a number of scientists.

In the terms of a detective story, the murderer now was identified and the case against him completed by the police. But the facts and the alleged motive were fantastic, beyond the experience of normal people, and a jury remained to be convinced.

Before the war began, Dr. Pegram sent Dr. Fermi to Washington to warn our Government of the military possibilities of the discovery. Everyone with whom he talked was interested, the Navy especially so.

No one scoffed at his warning, yet nothing tangible came of his visit and so in August, 1939, Dr. Einstein wrote a letter to the President:

“In the course of the past four months,” he wrote, “it has been made probable that a nuclear chain reaction can be set up in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.

“This new phenomenon also would lead to the construction of bombs, and it is conceivable—though much less certain—that extremely powerful bombs of a new type, carried by boat or exploded in a port, might well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transport by air.”

Note Dr. Einstein’s doubt in 1939 that any airplane could carry an atomic bomb. How large were the atomic bombs dropped on Japan still is a secret, but London newspapers have said, whether true or not, that they weighed four tons each and were so long that only a B-29 Superfortress, not yet in existence in 1939, could carry one.

The President called in one of his aides, naming him as liaison officer for the White House and appointed a committee which soon met.

Dissatisfied with the results, Dr. Einstein wrote the President a second letter in the spring of 1940. When France fell that May, President Roosevelt created the Office of Scientific Research and Development under Dr. Vannevar Bush, which took over the atomic project.

By that summer Germany commonly was supposed to be as much as a year ahead of the British and us in this pursuit, and the British spent many lives in two Commando raids in Norway with the single purpose of destroying a “heavy water” plant being operated there by the axis, and discovering the results.

   
The British and Norwegians spent many lives in raids on this Nazi atomic bomb plant at Rjukan. Norway.

To scientists, “heavy water” spelled atomic bomb; it is a compound of hydrogen, the moderator used by Fermi to slow his neutrons.

In this country a voluntary censorship was imposed almost a year before Pearl Harbor, and atomic fission was never spoken of publicly.

But laboratory research had gone so far by June of 1942, when an atomic bomb still was a comic strip phantasy to most of us, that Dr. Bush was able to report to the White house that an explosion equal to “many thousand tons of TNT” could be “caused at a desired moment by the fission or explosion of U-235, and that enough U-235 for a bomb could he made by any one of four different methods.

It could be done by turning uranium into a gas and filtering the lighter U-235 isotope away from the heavier U-238. Or the separation could be done electromagnetically or by heat or with centrifuges. (The dairyman’s cream separator is a form of centrifuge).

   
FOUR WAYS TO SEPARATE U235 FROM U238: 1 –Thermal Diffusion Method. MCGRAW-HILL PHOTO
   
FOUR WAYS TO SEPARATE U235 FROM U238: 2 – Gaseous Diffusion Through Barriers. MCGRAW-HILL PHOTO
   
FOUR WAYS TO SEPARATE U235 FROM U238: 3 - Centrifugal. MCGRAW-HILL PHOTO
   
FOUR WAYS TO SEPARATE U235 FROM U238: 4 – Electro-Magnetic. MCGRAW-HILL PHOTO

The Hiroshima bomb first became a definite possibility on December 6, 1942, nearly a year after Pearl Harbor, when the first chain reaction in history was set off under controlled conditions in a “pile” built on a squash court beneath the stands of Stagg Field, University of Chicago’s stadium. That is to say that more neutrons were released than were used up in starling the explosion, and the process perpetuated itself.

   
Official artist’s sketch of where, beneath the stands of Stagg Field stadium, University of Chicago, atomic fission first was accomplished in a chain reaction Dec. 6, 1942. Courtesy University of Chicago.

The government at once ordered plants built for all four processes on the great scale necessary to collect enough U-235 for a bomb, and had spent more than a billion and a half dollars on this top war secret before Hiroshima was blasted as if hit by a comet.

There were no provisions for cooling in the Chicago pile and so it was operated at low energy, but it provided the technical information needed to set up a larger U-235 pile at Oak Ridge, Tennessee, and the far greater ones at Hanford, Washington, which made the plutonium which went into one or more bombs.

Plutonium? What is that? Even before December, 1942, it had been discovered at the University of California that uranium when bombarded with slow neutrons was transmuted in part into a new manmade element, christened plutonium, with similar properties to U-235.

But all the plutonium the Berkeley cyclotron could produce in the year 1942 was 500 micrograms, equal in size to the head of a pin. Plutonium may exist in other worlds, but it never had been found in ours.

It isn’t that plutonium or U-235 are the only fissionable elements. The nucleus of the atom of any element heavier than silver can be broken down, releasing not much less hidden energy than does uranium.

The special quality of 235 and plutonium is a hair trigger. It would be so difficult to burst the atom of any element but uranium, thorium, plutonium and protoactinium that scientists can not yet foresee a time when an atomic bomb may be made from other elements.

   
HOW PLUTONIUM IS MADE FROM URANIUM. MCGRAW-HILL PHOTO.

What is called a “pile” for lack of a better word is a mass of graphite. Graphite is one of several possible moderators, and a moderator is, of course, something that will show a neutron down, a brake, increasing its probability of hitting a nucleus squarely on the button.

Oddly, while the Germans used an American discovery, heavy water, we chose pure carbon, which is graphite.

Just the getting of enough pure carbon and uranium for a small pile was a critical problem in 1941, one on which the American chemical industry concentrated. A cube pile of uranium oxide and graphite completed at Columbia University in July, 1941, failed to work. There were enough impurities in the uranium oxide to block the reaction.

There were only a few grams of pure uranium metal in all the world then, yet by 1942 we had produced the several tons which went into the successful University of Chicago pile.

The Eldorado pitchblende mines north of the Arctic Circle in the Canadian wilds are the principal American source of uranium, and they are frozen in except for a few weeks of midsummer. In the nick of time, the Manhattan District by telephone contracted for the mines’ entire output for a year.

The last boat was about to steam up the Mackenzie river before it closed for navigation, and the Eldorado Company barely had time to recruit a force of miners and load a winter’s stores of food aboard the river steamer.

A little later the Manhattan District learned of a large stock of uranium ore already mined and stored above ground, deep in the interior of the Belgian Congo. The Nazis were in North Africa and feinting toward Dakar, making the west coast unsafe. The uranium ore stockpile as bought by radio, shipped by rail to Africa’s east coast and thence by steamer around the southern tips of Africa and South America and to San Francisco.

   
All the pure uranium metal to be had in 1941 came from the laboratory of a Westinghouse lamp plant at Bloomfield, N. J. WESTINGHOUSE PHOTO.

In the University of Chicago pile the few tons of uranium metal then to be had was mixed with uranium oxide and scattered at carefully calculated distances throughout the graphite matrix.

What happened then? Given an outside trigger source, supplied by a trifle of pure U-235, some of the 235 nuclei in the uranium in the pile were split in the next fraction of a second. One million split 235 nuclei would release between one and three million more neutron bullets.

Some of these neutron bullets escaped from the pile and were lost, more were captured and disarmed by the many 238 nuclei, others by impurities. But on the average, a million of these new neutrons smashed another million 235 nuclei in the next fraction of a second to maintain the production rate.

These explosions were throttled down with bars of cadmium or of boron steel passed completely through the pile in slots, these neutron-resistant metals holding the produced energy down to a weak one half of one watt.

The energy appeared first in the 6,000-miles-a-second speed of the neutrons thrown off, then was converted into sensible heat as collisions slowed down these projectiles to the pace of a strolling man.

   
Removing dangerous radio-active material produced in the University of Chicago pile; instrument in foreground is a Geiger counter. COURTESY UNIVERSITY OF CHICAGO.

As soon as more graphite and uranium metal were to be had, the duPont company built for the Government at Oak Ridge an air-cooled 1,000-kilowatt chain-reaction pile operated by University of Chicago physicists.

Instead of spotting the uranium in the graphite like raisins in a cookie, here for the first time uranium rods were sealed in aluminum cylinders and inserted in channels in the graphite.

Removal of some of the rods slowed down the process, the adding of rods speeded it up. Both the size of the rods and their spacing were delicate problems in mathematical physics. An error either would defeat the process or cause an uncontrolled explosion.

Though this was a sizeable plant, it would, operating continuously for one year, split only a few pounds of 235 and only about one thousandth of the mass of uranium nuclei would be converted into energy.

This plant at Oak Ridge and a plutonium plant built a few miles west of Chicago in Argonne Forest served as pilots for the Hanford works.

The huge Hanford plant, producing only plutonium, was begun early in 1943 on some 1,000 miles of sage brush and sand in Eastern Washington taken over by the Government. Hanford, a village of 436, ballooned to a population of 51,000 and now is entirely deserted. Richland, even smaller in 1941 than Hanford, now is a town of 15,000 and the permanent headquarters of the plutonium works.

   
Part of the huge Hanford, Washington, plutonium works, built in a sage brush desert. U.S. ARMY PHOTO.

The plutonium plant was to have been built at Oak Ridge like the others, but its dangers seemed so great that a more isolated spot was sought. Too, the process needed the abundant supply of cold water for cooling to be had from the Columbia river.

By the time the three piles were running there in the spring of 1945, the atomic heat generated had raised the temperature of the river fractionally, although the cooling water undergoes a long period of decontamination before being turned back into the Columbia.

The U-235 was supplied by the gaseous diffusion, electromagnetic and thermal diffusion plants in Tennessee. Though they were put in the Clinch river valley primarily in order to tap the nearby TVA power, one of the world’s greatest steam power plants also was erected alongside the gaseous diffusion works.

The thermal diffusion plant was closed down after the war, the centrifuge works dropped early; the other two continue to operate twenty-four hours a day.

Chrysler Corporation built the diffusers for the gas plant. It consists of 63 buildings at Oak Ridge costing half a billion dollars, the principal one a strange 4-story, windowless U-shaped structure, 2,500 feet long and 400 feet wide.

   
The U-shaped gaseous diffusion U-253 plant at Oak Ridge, Tenn., for which Chrysler built the diffusers. U.S. ARMY PHOTO.

It is the world’s greatest continuous chemcophysical process factory. The process was developed at Columbia University, the plant designed and built by the Kellex Corporation and operated by Union Carbide & Carbon.

A trailer moved onto the Oak Ridge site July 3, 1943, housing the original settlers of what in another year had become Tennessee’s fifth city.

Ground was broken for the first process buildings of the gas plant in September, and though producing 235 atoms well before, the entire plant had not been finished at the war’s end. It is an agonizingly slow process and there was a long, long search by a large and able group of experts for a feasible way of mass-producing the “barrier” material.

This was one of many instances in which General Groves gambled courageously on ultimate success. The Manhattan Project had spent a quarter of a billion dollars on the gas plant before the barrier production problem was whipped.

Without the barrier stuff, the plant would have been useless, yet had Manhattan held back until this problem was solved, Oak Ridge would have produced no 235 before 1946. As it was, the plant was waiting for the diffuser equipment as it began to arrive in quantities.

In this process, uranium is converted into a gas, the fiercely corrosive uranium hexafluoride.

The plant is operated under an extremely tight vacuum, the gas pumped through the filters or barrier material.

The molecules of the U-235 isotope being lighter than those of 238, travel faster and penetrate the faintly porous barrier more often.

So for a while the gas on the far side of the barrier will be richer in 235. This is drawn off and recycled.

After thousands of repetitions taking many days you get a rich concentration of U-235.

   
A control Center at Oak Ridge. U.S. ARMY PHOTO.

It is necessary to start tens of thousands times the quantity of gas finally delivered as the enriched product. Enormous quantities of the microscopically porous barrier stuff are needed. As finally worked out, this material resisted the devouring hexafluoride better than stainless steel resists ordinary air.

As any leak would halt the process, the plant was built to incredible tightness. In power-plant practice, a vacuum of 1-inch of mercury is very good. Here was a vacuum twenty-five million times greater. If the plant were to be shut down today, be sealed and allowed to stand in a non-corrosive atmosphere, few who are reading this would live to see the vacuum completely broken down.

To create such a void of air, unheard of quantities of pumps were needed. Pumps were operated here for the first time at a speed beyond that of sound. More than 25,000 man-hours of research went into the pump problem alone, revolutionizing pump technology.

An entirely new technique of cutting and welding glass pipe was developed, making it possible to duplicate in glass anything an expert pipefitter can do with steel, and even to add a trick impossible in metal.

In the nearby electromagnetic plant, a uranium gas is electrified by passing it through an electric arc. These ionized uranium atoms then are passed through an electric field which accelerates them to a speed of many thousand miles a second.

At this tremendous speed the mixed 238 arid 235 atoms enter an intensely strong magnetic field which curves them in to circular paths.

The lighter 235 atoms are deflected more than the 238 and half-way around their race track a splitter divides the two streams.

Again, many, many repetitions are necessary before nearly pure U-235 metal results.

The success of the separation depends upon the acute sharpness of the focus of the two magnetic beams and this hinges in turn upon the precision of the magnetic field. The specification limited the variation in the field to 1 part in 5,000, a uniformity believed impossible by many.

The electro-magnets are 250 feet long, containing thousands of tons of special steel; a hundred times bigger than that in the 184-inch cyclotron at tile University of California, until then the world’s greatest.

The electro-magnets’ pull on the nails in the shoes of the workers made walking difficult. A carpenter had to clutch a nail tightly to keep it from twisting out of his fingers.

An ordinary wrench either would be wrested from a man’s hand, or, if he clung to it, he would be banged against the magnetic face. This was avoided by the use of non-magnetic steels in tools and nearby equipment.

   
Man working at Oak Ridge. U.S. ARMY PHOTO.

At the Hanford works where the plutonium is recovered chemically from thirty or so radioactive by-products, all very dangerous, this is done by complex, remotely-controlled and nearly automatic machinery, much of it installed underground.

The radioactivity of any one of Hanford’s three piles is equal roughly to that given off by a million pounds of radium—and all the radium man has isolated so far amounts to about two pounds.

The plutonium process was developed and the Hanford works designed on the results of experiments using only one millionth of a gram of plutonium. One thousandth of a gram is invisible to the eye; a millionth of a gram weighs less than a baby’s breath. There chemical engineers had to deal with a brand new element, its chemistry, physical properties and constants all unknown.

Each of the Hanford piles contains many thousand blocks of graphite machined to such precision that the over-all structures are accurate to machine shop standards. The holes in the graphite holding the uranium slugs have the accuracy of rifle barrels.

The amount of plutonium recovered in the chemical separation is little more than the percentage of natural lime in the river water used to make the solution. These mountains produce rnolehills, hence their huge size.

* * *

Oak Ridge is served by spurs of the Southern and the Louisville & Nashville railroads. When the two roads had moved in some 50,000 car lots of materials and gotten back only empty cars, it is said that a freight agent for one road called at the headquarters building.

‘‘One of these days you will be moving out a lot of tonnage and we hope we are going to get our share of this traffic,’’ he said.

He was told to be patient. When 100,000 cars had been shunted in and 100,000 empties hauled away, the freight agent paid a second call. Again he was told to be patient. By the time the railroads had delivered 150,000 car loads and had gotten back nothing but rolling stock, the general freight agent is supposed to have investigated.

“You are getting the business whether you can see it or not,’’ they told him.

And so they were.

* * *

The product of more than a billion dollars spent at Oak Ridge alone was leaving there in nothing bigger than a brief case, each carried by a messenger.

The messengers had no idea what they were transporting. Each as sent by a new route to a new destination, though the powder was destined always either for the Hanford works or for Los Alamos, New Mexico.

You have heard less about Los Alamos, a New Mexican mesa some 30 miles from Santa Fe . . .

   
“Bubble, bubble, toil and trouble.’’ The seething blister of the trial atomic bomb, Alamogordo, N. M., July 16, 1945. INTERNATIONAL NEWS PHOTO.
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