Nuclear

Part 17 - Oak Ridge & Hanford

Impressed with Fermi's work, in Chicago, General Groves immediately went to meet Robert Oppenheimer at Berkeley, California. When Oppenheimer proposed a new laboratory to work on bomb design, they chose a desert location at Los Alamos, New Mexico. Groves immediately ordered construction, and staff started arriving March 15, 1943.

In April 1943, concerned about German progress on secret weapons, a conference worried that Germany might poison food supplies with radio-active isotopes in bombs. Oppenheimer, Fermi and Edward Teller discussed the use of radioactive strontium90. Chemically similar to calcium, it would be irretrievably taken up in bones. The British were more concerned about stopping the Germans from obtaining Norwegian heavy water.

In November 1943, the pilot reactor at Oak Ridge produced the first five tons of irradiated uranium and by the next year small quantities of plutonium; both were shipped to Los Alamos for testing. 

In March 1944, a modified Airforce B-29 dropped the first dummy bomb.  Made of concrete and explosives,  it looked like the plutonium bomb known as Fat Man.  Engineers replaced the bomb release mechanism with a sturdier British design and 17 more B-29's,  of the new special Bombardment Squadron (the 393rd), were modified.   Aircrews wondered why they were required immediately dive away in a 155-degrees turn after releasing the bomb at 30 000 feet. This allowed the aircraft to be ten miles away before the bomb exploded.  

Late in 1944, Otto Frisch persuaded Fermi to drop a sub critical plug of uranium hydride  so that it passed though a hole in another sub critical mass. The hydride would slow the chain reaction. The plug passed through the hole in a fraction of a second but it produced a burst of neutrons and a temperature rise of several degrees. It was as close to an atomic explosion as they could get with out blowing anything up. 

This was the first direct confirmation of a super critical reaction with fast neutrons alone. Unknown to the Americans, in November 1944, constrained by shortages of uranium hexafluoride and parts for the cyclotron and by leaks and corrosion, the Japanese were making slow progress. 

The German program was more advanced, although far behind the Americans', but Hitler's threats of a super weapon maintained a real fear that a German atomic bomb was possible.

Meanwhile, at Oak Ridge in Tennessee, Groves had constructed three plants each using a different method of separating 235-uranium from 238-uranium. 

Developing porous membranes capable of withstanding the corrosive gas uranium hexafluoride for the gaseous-diffusion plant proved difficult so Groves was persuaded to build a thermal separation plant method, developed by a US navy research project. The plant was build at Oak Ridge in 90 days.

The third plant was based on Ernest Lawrence's mass spectrograph. It was an electromagnetic isotope separation machine derived from his California University Cyclotron, which he called the Calutron, and it would separated the isotopes one atom at a time. Lawrence estimated that 2000 Calutrons could produce enough 235-Uranium92 for one bomb every 300 days. Groves decided to start with 500 hoping that the efficiency would improve. 

The Calutrons required thousands of tons of copper, which was in desperately short supply so the engineers borrowed 14,700 short tons of silver from the West Point Bullion Depository. (The Treasury Department recorded this as 430,000,000 troy ounces). The 1,000-troy-ounce (31 kg) silver bars were extruded into strips and wound into the magnet coils by Allis-Chalmers in Milwaukee, Wisconsin. After the war, all but a tiny fraction, was returned to the U.S. treasury department.

Oppenheimer and Groves were disappointed with the low levels of enrichment from all the three plants, until they realized that the process would be improved by putting the plants in series where the output of one would be input into the next. The calutrons, last in the series, ultimately raised the enrichment to 95%.By July 1945, Oak Ridge had delivered about 50 kilograms (110 lb) of uranium enriched to 89% 235-uranium to Los Alamos together with some enriched to 50%, (the total averaging about 85% enriched). This was used in the first uranium bomb, Little Boy, that was dropped on Hiroshima.

Meanwhile, the three plutonium production plants (which the Du Pont engineers called reactors) at Hanford, in Washington state, were spaced out at six mile intervals along the Columbia river for safety.

Water had been selected as the cooling medium after Fermi had calculated that there would be an excess of neutrons sufficient to permit water instead of helium for cooling.

Eugene Wigner's team had devised an elegantly simple design. A 28 by 36 foot high-purity graphite cylinder lying on its side, horizontally pierced by over a thousand aluminum tubes. Two hundred tons of uranium metal slugs, in aluminum cans, filled the tubes within the 1,200 ton graphite cylinder. The chain reaction would generate 250,000 kilowatts of heat to be carried away by water pumped through the tubes. After 100 days, when about one atom in 4000 had been transmuted to plutonium, the uranium would be pushed out into a deep pool of pure water simply by loading more slugs. After 60 days in the pool, the short lived but intensely radio-active fission products would decay to safer levels and the slugs could be removed for processing.

The Du Pont chemical separation plants at Hanford were huge. The three building were each 800 feet by 65 feet by 80 feet tall. Within the solid concrete structures were cells with seven feet thick concrete walls to shield the operators from the highly radioactive output of the production reactors. Glen Seaborg's complex micro processing apparatus had been scaled up but the entire process and any maintenance had to be done remotely. The intense direct radiation and radioactive chemicals in the cells were lethal.

The first reactor at Hanford started up smoothly on September 26, 1944 . . . And a day later, it spontaneously shut down. The next day it start up again, but twelve hours later it shut down again. The reactor was producing a radioactive isotope of iodine that was decaying to a radioactive isotope of xenon that was absorbing neutrons until the process shut down, when it eventually decayed to a stable isotope the reactor would restart. More neutrons were need to overcome the 'poison'.

Luckily, one of the DuPont engineers, George Graves, had stubbornly insisted on adding spare tubes, as a contingency against an unforeseen failure, and the reactor had been build with an extra 504 tubes.  Modification delayed the restart to December 17 and the second reactor started up December 28.   Groves anticipated having enough plutonium for 18 five kilogram bombs ready by the second half of 1945.