Nuclear

Part 16 - Fermi's Pile

The Americans had finally decided to built the bomb when, without warning, on December 6, 1941, the Japanese navy attacked Pearl Harbour in Hawaii.

Soon afterward, on January 19, 1942, US President Roosevelt formally authorized the atomic bomb project. 

On 1942 August 16, the Chief of the US Army Corps of Engineers, Major General Eugene Reybold established The Manhattan Engineering District, the code name for the secret project.

Arthur Compton was put in charge and given little time to come up with a plan. His schedule showed that a bomb could be produced before the end of the war. He decided to concentrate the chain reaction development in Chicago. Responding to a report, on March 9, 1942, President Roosevelt stated that time was more important than money. All methods of 235-U92 separation and Plutonium production were to be pushed regardless of expense.

On April 19, 1942, Glen Seaborg arrived in Chicago, where Enrico Fermi and his group was setting up to convert 238-uranium92 to 239-plutonium93 in a controlled nuclear chain reaction.

Seaborg had been bombarding 300 pound of uranium nitrate hexafluoride with neutrons in the Washington University cyclotron for a month, and by August 20, 1942 he had extracted a trace quantity, about 1 microgram of 239-plutonium93, from 50 micrograms of uranium and fission products.

To do this he had to invent tiny instruments for the chemical processes needed to separate, concentrate and isolate the minuscule amount of plutonium. These new processes were later scaled up at the Clinton Engineering Works in Oak Ridge, and then were used for full-scale production at the Hanford Engineer Works, in Richland, Washington state.

In November 1943, Seaborg used plutonium trifluoride to produce the first sample of plutonium metal; a few micrograms of metallic beads. Enough plutonium to make it the first synthetically made element to be visible with the unaided eye.

The nuclear properties of 239-plutonium were reassessed to confirm that it would fission, releasing energy and more neutrons, to produce a chain reaction. A sufficient (critical) mass could cause an explosion large enough to destroy a city.

Edward Teller was becoming obsessed by the possibility of making a fusion bomb.  Theoretically, 26 pound of deuterium would have the explosive power of one million tons of TNT the equivalent of 500 fission bombs.  A fission bomb would provide the needed temperature, 400 million degrees.  And, separating deuterium was far easier than extracting 235-U92 or 239-Pu94.

(Chemically identical to hydrogen, deuterium has a proton and a neutron in the nucleus, if two deuterium atoms could be fused to make helium, this would release the energy of the Sun).  

Hans Bethe and Oppenheimer agreed but Bethe calculated that a deuterium mass would ignite too slowly.  A deuterium/tritium core would be faster but tritium (an isotope of hydrogen with one proton and two neutrons) would have to be created.  The combination would require an initiating temperature of 40 million degrees and would release 17.6 Mev per fission, four times more than a deuterium only fusion. 

A report predicted enough 235-uranium92 would be available for a testing by March 1944. If this was used to ignite 400 kg of liquid deuterium it would have the explosive power of ten million tons of TNT and devastate an area of 100 square miles. 

On September 17, 1942 Leslie Groves, Army Corps of Engineers was promoted to Brigadier General and put in charge of the entire project. He was horrified to find it in worse condition than he had feared. No one had secured a significant supply of uranium, but the Belgian Union Miniére had shipped 1250 tons of concentrated ore to New York in 1940 and Groves promptly bought it. He also bluffed Donald Nelson, Head of the War Production Board, into assigning an AAA first priority rating to the project and then took off for Tennessee to buy 52 thousand acres of land along the Clinch river, near Oak Ridge.

In Chicago, Enrico Fermi, with the experience of 16 previous piles, was assembling 45,000 graphite bricks, (with 19,000 holes containing uranium oxide 'eggs') into a roughly spherical pile. Test showed that some of the neutrons from fission were emitted slowly enough to permit the pile to be manually controlled. There was no risk of a run-away reaction resulting in a melt down (an expression not used at the time). 

Because of a strike at the planned site for the reactor, Compton allowed Fermi to build the pile under the west stands of the university stadium.

The workers began laying the bricks, alternating one layer of plain graphite with two layers of bricks each containing two five pound uranium 'eggs'. The workers carefully aligned the slots for the ten control rods, each a 13 foot long piece of wood with a cadmium strip nailed to it. Cadmium, with a gargantuan capture cross section for neutrons, kept the pile inactive. The control rods were locked in place and the only keys kept by Walter Zinn and Herbert Anderson.

 The Metallurgical Lab had devised a technique of smelting pure uranium metal and shaping it into 2.25 inch cylinders. These were much denser than the pressed oxide and Fermi calculated they would remove the need for another 20 layers and obviate the need to extract the air from the pile. As they began arriving, Fermi placed them around the centre of the pile. 

They stopped work at the 57th layer when the pile was 20 feet tall and 25 feet in diameter. The pile now contained 771,000 pound of graphite, 80,590 pound of uranium oxide and 12 400 pound of uranium metal. It was within 1% of a chain reaction, when more than one neutron was being produced for each one causing a fission. 

When he showed up in Chicago for the first time, Groves was impressed but he demanded a quick decision on the cooling method for the prototype production pile (reactor) at Oak Ridge. If there was more than one alternative they would test them all. Within the month Groves persuaded E.I. du Pont de Nemours, the Delaware chemical and explosives company, to take over the building and operation of the production piles (reactors) at Hanford in Washington State.

On December 2, 1942 Fermi was ready. As the control rods were gradually withdrawn, he checked the instrument readings against calculations made on his six inch slide rule. (Calculators and computers had yet to be invented). Each time, the neutron counters increased their activity and then levelled off. The slow careful checking went on all morning and then Fermi order a shut down for lunch. 

They resumed work at the same point and continued checking until Fermi ordered the last control rod to be pulled out 12 inches. 'Now,' Fermi said, 'it will not level off.' 

For four minutes he watched calmly as the count moved higher. The pile was past the critical limit and the count was doubling every two minutes. If left uncontrolled, within 90 minutes it would have killed anyone nearby and melted down. He calmly order the control rods replaced.

Every one drifted off until only Fermi and Szilard were left. They shook hands as Szilard said he thought this would go down as a black day in the history of mankind. 

Using information obtained from Fermi's work, DuPont constructed an air-cooled experimental production reactor (known as X-10) and a pilot chemical separation facility at Oak Ridge. Information from X10 was in turn used to design the water-cooled, plutonium production reactors at Hanford. Construction at the site began in mid-1943.