The day the construction teams left it, September 13, Fermi had inserted the first aluminum-canned uranium slug to begin the loading, the Pope conferring his blessing as he had on the piles at Chicago and Oak Ridge.
Slug canning had almost come to a crisis. Two years of trial-and-error effort had not produced canning technology adequate to seal the uranium slugs, which quickly oxidized upon exposure to air or water, away from corrosion. Only in August had the crucial step been devised, by a young research chemist who had followed the problem from Du Pont in Wilmington to Chicago and then to Hanford: putting aside elaborate dips and baths he tried soaking the bare slugs in molten solder, lowering the aluminum cans into the solder with tongs and canning the slugs submerged. The melting point of the aluminum was not much higher than the melting point of the solder, but with careful temperature control the canning technique worked.
Greenewalt then pushed production around the clock. Slugs accumulated in the reactor building faster than the loading crews could use them and Marshall and Fermi observed them there on one of their inspections:
Enrico and I went to the reactor building… to watch the loading.The slugs were brought to the floor in solid wooden blocks in which holes were drilled, each of a size to contain a slug, and the wooden blocks were stacked much as had been the slug-containing graphite bricks in CP-1. Idly I teased Fermi saying it looked like a chain- reacting pile. Fermi turned white, gasped, and reached for his slide rule. But after a couple of seconds he relaxed, realizing that under no circumstances could natural uranium and natural wood in any configuration cause a chain reaction.
Tuesday evening, September 26, 1944, the largest atomic pile yet assembled on earth was ready. It had reached dry criticality — the smaller loading at which it would have gone critical without cooling water if its operators had not restrained it with control rods — the previous Friday. Now the Columbia circulated through its 1,500 loaded aluminum tubes. “We arrived in the control room as the du Pont brass began to assemble,” Marshall remembers. “The operators were all in place, well-rehearsed, with their start-up manuals on their desks.” Some of the observers had celebrated with good whiskey; their exhalations braced the air. Marshall and Fermi strolled the room checking readings. The operators withdrew the control rods in stages just as Fermi had once directed for CP- 1; once again he calculated the neutron flux on his six-inch slide rule. Gradually gauges showed the cooling water warmed, flowing in at 50°F and out at
The pile went critical a few minutes past midnight; by 2 a.m. it was operating at a higher level of power than any previous chain reaction. For the space of an hour all was well. Then Marshall remembers the operating engineers whispering to each other, adjusting control rods, whispering more urgently. “Something was wrong. The pile reactivity was steadily decreasing with time; the control rods had to be withdrawn continuously from the pile to hold it at 100 megawatts. The time came when the rods were completely withdrawn. The reactor power began to drop, down and down.”
Early Wednesday evening B pile died. Marshall and Fermi had slept by then and returned. They talked over the mystery with the engineers, who first suspected a leaking tube or boron in the river water somehow plating out on the cladding. Fermi chose to remain open-minded. The charts, which seemed to show a straight-line failure, might be hiding the shallow curve of an exponential decline in reactivity, which would mean a fission product undetected in previous piles was poisoning the reaction.
Early Thursday morning the pile came back to life. By 7 a.m. it was running well above critical again. But twelve hours later it began another decline.
Princeton theoretician John A. Wheeler had counseled Crawford Greenewalt on pile physics since Du Pont first joined the project. He was stationed at Hanford now and he followed the second failure of the pile closely. He had been “concerned for months,” he writes, “about fission product poisons.” B pile's heavy breathing convinced him such a poisoning had occurred. The mechanism would be compound: “A non-[neutron-]absorbing mother fission product of some hours' half-life decays to a daughter dangerous to neutrons. This poison itself decays with a half- life of some hours into a third nuclear species, non-absorbing and possibly even stable.” So the pile would chain- react, making the mother product; the mother product would decay to the daughter; as the volume of daughter product increased, absorbing neutrons, the pile would decline; when sufficient daughter product was present, enough neutrons would be absorbed to starve the chain reaction and the pile would shut down. Then the daughter product would decay to a non-absorbing third element; as it decayed the pile would stir; eventually too little daughter product would remain to inhibit the chain reaction and the pile would go critical again.
Fermi had left for the night; Wheeler on watch calculated the likely half-lives based on the blooming and fading of the pile. By morning he thought he needed two radioactivities with half-lives totaling about fifteen hours:
If this explanation made sense, then an inspection of the chart of nuclei showed that the mother had to be 6.68 hr [iodine]135 and the daughter 9.13 hr [xenon]135. Within an hour Fermi arrived with detailed reactivity data which checked this assignment. Within three hours two additional conclusions were clear, (a) The cross section for absorption of thermal neutrons by Xe135 was roughly 150 times that of the most absorptive nucleus previously known, [cadmium]113, (b) Almost every Xe135 nucleus formed in a high flux reactor would take a neutron out of circulation. Xenon had thrust itself in as an unexpected and unwanted extra control rod. To override this poison more reactivity was needed.
Greenewalt called Samuel Allison in Chicago on Friday afternoon. Allison passed the bad news to Walter Zinn at Argonne, the laboratory in the forest south of Chicago where CP-1 was meant to be housed and where several piles now operated. Zinn had just shut down CP-3, a shielded six-foot tank filled with 6.5 tons of heavy water in which 121 aluminum-clad uranium rods were suspended. Disbelieving, Zinn started the 300-kilowatt reactor up again and ran it at full power for twelve hours. It was primarily a research instrument and it had never been run so long at full power before. He found the xenon effect. Laborious calculations at Hanford over the next three days confirmed it.
Groves received the news acidly. He had ordered Compton to run CP-3 at full power full time to look for just such trouble. Ever the optimist, Compton apologized in the name of pure science: the mistake was regrettable but it had led to “a fundamentally new discovery regarding neutron properties of matter.” He meant xenon's consuming appetite for neutrons. Groves would have preferred to blaze trails less flamboyantly.
If Du Pont had built the Hanford production reactors to Eugene Wigner's original specifications, which were elegantly economical, all three piles would have required complete rebuilding now. Fortunately Wheeler had fretted about fission-product poisoning. After the massive wooden shield blocks that formed the front and rear faces of the piles had been pressed, a year previously, he had advised the chemical company to increase the count of uranium channels for a margin of safety. Wigner's 1,500 channels were arranged cylindrically; the corners of the cubical graphite stacks could accommodate another 504. That necessitated drilling out the shield blocks, which delayed construction and added millions to the cost. Du Pont had accepted the delay and drilled the extra channels. They were in place now when they were needed, although not yet connected to the water supply.
D pile went critical with a full 2,004-tube loading on December 17, 1944; B pile followed on December 28. Plutonium production in quantity had finally begun. Groves was enthusiastic enough at year's end to report to George Marshall that he expected to have eighteen 5-kilogram pluto-nium bombs on hand in the second half of 1945. “Looks like a race,” Co-nant noted for his history file on January 6, 1945, “to see whether a fat man or a thin man will be dropped first and whether the month will be July, August or September.”
17
The Evils of This Time
The bombs James Bryant Conant speculated about early in 1945 were crude designs of uncertain yield. The previous October he had traveled out to Los Alamos to ascertain their prospects. To Vannevar Bush he reported that the gun method of detonation seemed “as nearly certain as any untried new procedure can be.” The availability
