by P. D. Smith
1
A Black Day
If sunbeams were weapons of war, we would have had solar energy long ago.
George Porter
The football stadium at the University of Chicago had not been used for three years. In 1939 the university’s president, Robert Hutchins, had made the controversial decision that football was a distraction in the academic life of a proud institution whose coat of arms displayed a phoenix rising triumphantly from the ashes, together with the motto Crescat scientia, vita excolatur – ‘Let knowledge increase so that life may be enriched’. So the football team hung up its boots, and gradually the weeds took over the stadium.
Like the rest of the campus, the stadium had been built during the late nineteenth century in an English Gothic style. Even a progressive, New World university found it hard to shrug off the ghosts of the old world completely. With its gargoyles and crenellated walls, Stagg Field looked more like a medieval castle than a football stadium. It was certainly an unlikely setting for the most important scientific experiment of the twentieth century.
On a crisp December morning, a group of students were making their way through the fresh snow to the first lecture of the day. Their breath rose like smoke in the pale sunlight. A short, comfortably overweight man passed them, walking hurriedly towards the disused stadium. His stride was purposeful and his bearing dignified, an impression only slightly spoiled by his roly-poly gait. Near the west stands the snow was stained black as if ash had been scattered there. For the past few weeks military guards had been stationed outside the imposing stone portal that led beneath the stands. It was still an unusual sight on the campus, even in wartime, but no one would say what they were there to protect.
‘Good morning, Dr Szilard,’ said one of the guards. He pronounced it with a drawl, See-lard.
Leo Szilard smiled briefly at the soldier, whose nose had turned red in the subzero temperatures. Just the other day, he had taken pity on the man and had recommended a drink from his Budapest student days to combat the bitter cold: rum tea. But this morning there was no time for idle chat, and Szilard passed swiftly through the door and made his way down the gloomy corridor.
The previous night, restlessness had driven him out of his small and sparsely furnished room on campus. Szilard had called on a colleague and convinced him to brave the freezing night air and go for a late meal. Not that he was hungry; he had already eaten. But he had to talk to someone to ease the burden that was weighing on his mind. Over his second dinner that evening, Szilard confessed his fears about the next day’s experiment. The precise nature of their work had to remain a secret, he told his biologist friend mysteriously, but if the experiment ‘works too well’ there might be an explosion. A big explosion.1
The corridor took Szilard underneath the west stands of Stagg Field to a slate-walled room. It was a doubles squash court, about sixty feet by thirty and thirty feet high. Incongruously, spotlights on tripods had been set up as if on a movie set. Szilard stepped gingerly over the cables. He trod carefully because the surface was as slippery as a dancefloor. A fine layer of grey powder lay on every surface – graphite dust, the purest graphite on earth. He could even taste it in the cold, still air. Szilard hurried up a staircase to the small spectators’ balcony, about ten feet above the court and overlooking its north end. There were plenty of spectators already there, over thirty of them, and they were all scientists. But today there was no college final – today they would witness the beginning of a new age in science and warfare.
Szilard was breathing heavily. Despite the penetrating cold, he loosened his tie and unbuttoned his thick overcoat. The front of the balcony was packed with scientific monitoring equipment. Leona Woods, a shy 23-year-old graduate student and the only woman present, was making last-minute adjustments together with a short, dark-haired man. That was Enrico Fermi. He was wearing a grey lab coat smeared with the same graphite dust that coated everything, even the snow outside the stadium. Just three years earlier, the Nobel prizewinning physicist had been forced to flee his native Italy with his Jewish wife because of Mussolini’s anti-Semitic laws.
The world’s first nuclear reactor, CP-1, goes critical on 2 December 1942. No photographers were present, but Chicago Tribune artist Gary Sheahan imagined the scene in 1957. Leo Szilard stands in the middle of the group at the left, holding a briefcase. Enrico Fermi is standing next to Walter Zinn, who is leaning with his elbow on the rail.
The squash court was eerily silent, and the scientists were speaking in whispers. Szilard nodded a greeting to his friend Eugene Wigner, who was deep in conversation with Crawford H. Greenwalt, who would later become president of the Du Pont Chemical Company. Wigner and Szilard had been close friends since the 1920s. Both men had left their homes in Budapest to study science in Berlin, but as the tide of fascism engulfed Europe they had made their way to America, as had many of their scientific colleagues gathered on the balcony that cold December morning.
Szilard looked down at the squash court. In its place loomed a large wooden scaffolding draped with grey rubberized sheeting. Inside this frame squatted a huge structure built of black bricks. This was Chicago Pile Number One, or CP-1 for short.
The world’s first atomic pile, what we would now call a nuclear reactor, was as big as a house – about twenty feet high and twenty-five feet wide. It consisted of fifty-seven layers of pure graphite blocks, the layers alternating between solid blocks and ones which were hollowed out to hold slugs of uranium. The blocks containing the uranium formed a cube-like lattice within the pile. In all they had used 250 tons of graphite and six tons of uranium.
Each block had been cut by hand. That was the unenviable task of a young Canadian physicist, Walter Zinn. Today he stood with Fermi on the balcony, his fingernails still blackened by the graphite. Together with half a dozen colleagues and about thirty local Chicago lads, Zinn and another physicist, Herbert Anderson, had worked and cursed non-stop in twelve-hour shifts for six solid weeks – until last night, the evening of 1 December 1942, when their labour was finally complete. Now all that remained was to see whether theory could be turned into reality and the energy of the atom released.
The spark that ignites an atomic chain reaction is a neutral particle – one with no electric charge – called, reasonably enough, a neutron. The nucleus of an atom of uranium-235 can be split in half when struck by a neutron. This is fission, the reaction at the heart of a nuclear reactor – and of an atomic bomb. Changing the mass of an atomic nucleus, either by splitting it (fission) or combining it with another nucleus (fusion), creates energy. Albert Einstein showed just how much energy was locked up inside every atom. His equation E = mc2 states that the amount of energy liberated when matter is annihilated equals the mass of the matter multiplied by the speed of light squared. The speed of light is 186,000 miles per second, so there is a vast reservoir of potential energy in matter. On 6 August 1945, when the first atomic bomb exploded above Hiroshima, just 1 per cent of the bomb’s uranium was transformed into the energy that devastated the Japanese city.
Every time a uranium nucleus splits, two or three spare neutrons are expelled, and each of these freed neutrons can split another nucleus. The neutrons this frees can in turn split between four and nine more nuclei, and so on in a succession of reactions involving an exponentially increasing number of atomic nuclei. This is what physicists call a supercritical chain reaction – a potentially explosive atomic wildfire spreading through the fabric of matter, turning it into pure seething energy.
That morning on the squash court, however, Fermi and Szilard did not want an explosive reaction, but a controlled one – a critical reaction in which just enough neutrons are produced to keep the chain reaction self-sustaining. They were also trying to make this reaction work using natural uranium, of which only 0.7 per cent was the highly fissile variety, uranium-235. To do this they needed to slow down the lightning-fast neutrons. This was the crucial task performed by the bricks of pure, black graphite: they acted a
s the moderator.
As an extra means of keeping the nuclear reaction under control, they had inserted cadmium rods into the pile. Cadmium is one of the most powerful absorbers of neutrons, and if there are no neutrons flying around in the pile, then there’s no chain reaction. Today, as an insurance policy, three young physicists stood on an elevator platform above the pile, ready to flood it with a cadmium-salt solution, just in case something went wrong and the rods didn’t work. These three were known, only half-jokingly, as the ‘suicide squad’.
Walter Zinn had designed the final cadmium rod to drop back automatically into the pile should the neutrons rise above a certain level. They christened this rod ‘ZIP’ in honour of its creator. If ZIP failed, then another rod could be released from the balcony by cutting a rope. A rather sheepish-looking physicist stood ready with an axe. If that failed to close down the pile, then there was the suicide squad, and after that… well, in 1942 no one had heard of the words ‘meltdown’ and ‘Chernobyl’.
At 9.45 a.m. Enrico Fermi and his team began the painfully slow process of withdrawing the cadmium rods from the pile, thus increasing the flux of neutrons. As they did so, final checks were made on the measuring equipment and the safety mechanisms. Once this was completed, everyone’s eyes turned to the man from Rome. He glanced down at his watch; it was 11.30. Fermi looked round at the expectant faces and smiled.
‘I’m hungry,’ he said. ‘Let’s go to lunch.’
Enrico Fermi was the captain of the team of forty-two scientists who had worked on the project. Unlike the Italian, Leo Szilard wasn’t a hands-on kind of scientist. Fermi had been annoyed when Szilard declined the opportunity of helping to build the graphite pile. Some said he didn’t like getting his hands dirty, but Szilard knew his strengths, and sawing through graphite blocks was not one of them. He was an ideas man, someone who could see solutions before most people had even grasped the problem. A friend once memorably described the portly physicist as an ‘intellectual bumblebee’, a footloose fertilizer of ideas.2
When Leo Szilard had first suggested in 1939 that atomic bombs were a real possibility, Fermi’s incredulous response had been ‘Nuts!’3 Since then, he had learned to treat the unconventional Hungarian’s insights with greater respect, although Fermi was never completely comfortable with his mercurial colleague. ‘He is extremely brilliant’, admitted Fermi in 1954, ‘and… he seems to enjoy startling people.’4
Although he was no graphite cutter, Szilard had provided many of the key theoretical insights during the building of the atomic pile. He suggested the pile’s lattice structure, the geometrical arrangement of uranium spheres within the hollow graphite blocks designed to maximize the effect of the neutrons. He also realized that it was essential to use pure graphite as a neutron moderator. Impurities simply absorbed neutrons, working against a chain reaction. (This was a subtlety which Hitler’s best atomic physicists – including quantum theorist Werner Heisenberg – failed to grasp. As a result, their bomb project remained largely wishful thinking.)
Most importantly it was Leo Szilard who, in 1933, had first seen how to unlock the fearsome forces in the heart of every atom. It came to him in a flash of insight while he was crossing a road near Russell Square, in London’s Bloomsbury. The key was a neutron chain reaction, a domino effect rippling through matter and releasing an ever-greater flood of neutrons. Uncontrolled, it would cause an explosion more powerful than any yet created by humankind; controlled, it could supply the world with an unlimited supply of cheap energy. Since this scientific epiphany nine years earlier, the prospect of atomic energy had dominated Szilard’s every dream and nightmare. And now his bold idea was about to be put to the test.
After an uneasy lunch, during which they discussed everything apart from the day’s experiment, the physicists returned to the squash court. At 2.20 p.m. they again began withdrawing the thirteen cadmium rods, little by little. Enrico Fermi kept a sharp eye on the dials of the neutron counters. At 3.25, he was ready to remove the final rod.
‘Pull it out another foot,’ he called to George Weil, who was down on the squash court operating the control rod.5 Everyone’s eyes were fixed on that rod. It was marked in feet and inches, showing how much of the cadmium remained inside the pile absorbing neutrons.
‘This is going to do it,’ said Fermi to Arthur Compton, the physicist in overall charge of the Chicago project. ‘Now it will become self-sustaining. The trace will climb and continue to climb. It will not level off.’6
The forty-two scientists scarcely breathed as they faced the implacable black mass of graphite and radioactive uranium. According to Herbert Anderson, ‘At first you could hear the sound of the neutron counter, clickety-clack, clickety-clack. Then the clicks came more and more rapidly, and after a while they began to merge into a roar.’
The number of neutrons was so high that the counters could no longer cope. Fermi, his voice steady, asked for the chart recorder to be switched on. Now there was just the faint scratching of the pen as it moved across the paper. The graph showed a steadily increasing level of neutrons. ‘It was an awesome silence,’ recalled Anderson with real emotion.7
‘I couldn’t see the instruments,’ said George Weil. ‘I had to watch Fermi every second, waiting for orders. His face was motionless. His eyes darted from one dial to another. His expression was so calm it was hard.’8
Fermi studied the rising graph, glancing away only to make calculations with his slide rule. ‘His gray eyes betrayed his intense thinking, and his hands moved along with his thoughts,’ his wife Laura wrote later, imagining the scene.9
Suddenly, the Italian’s face broke into a broad smile and he closed his slide rule. ‘The reaction is self-sustaining,’ he said quietly, looking round at his colleagues on the balcony. ‘The curve is exponential.’10
It was what everyone had hoped for, but no one had dared believe would happen: the pile had gone critical. But instead of ordering Zinn to drop the emergency rod, Fermi waited. For what to his fellow scientists seemed a lifetime, he stared at the inexorably rising line of the graph. It was as if the sceptical physicist could scarcely believe the evidence of his own scientific instruments.
Then Fermi gave the order they had all been waiting for: ‘ZIP in!’ It was 3.53 p.m. For 28 minutes they had watched the world’s first nuclear reactor in operation. The atomic age had begun.
There were no cheers that day, but the excitement and relief were felt by everyone. Fermi smiled across at Leo Szilard and then shook Compton by the hand. Eugene Wigner produced a bottle of straw-bound Chianti from a brown paper bag and presented it to the Italian physicist. It had been no mean feat tracking one down during wartime. They toasted their success and the new age of science with Chianti in paper cups. Wigner recalled that as they drank the bitter-sweet wine, ‘we sent up silent prayers that what we had done was the right thing’.11 Afterwards they all solemnly signed their names on the Chianti bottle for posterity.
That evening, Compton telephoned James B. Conant, who was leading the US Government project to turn atomic energy into a superweapon. Compton’s message was in code, but its meaning was crystal clear:
‘The Italian navigator has landed in the New World’.
‘How were the natives?’ asked Conant.
‘Very friendly.’12
Enrico Fermi and Leo Szilard stood alone on the balcony overlooking the now dormant atomic pile after the others had left. Both of them knew what their success meant. The world was at war. That very day the US State Department revealed that two million Jews had already been killed by Hitler and a further five million were now at risk. Perhaps Hitler’s physicists had already built an atomic pile like theirs and were even now creating an atomic bomb.
More than anyone, Leo Szilard had seen this moment coming. For almost a decade he had been warning of its consequences. Before the war, few would listen to his fears. Now, as Szilard had told his colleagues just two months earlier, they were entering a new and terrible age. ‘One has to
visualize a world’, he said, ‘in which a lone airplane could appear over a big city like Chicago, drop his bomb, and thereby destroy the city in a single flash. Not one house may be left standing and the radioactive substances scattered by the bomb may make the area uninhabitable for some time to come.’13
No wonder that as Szilard turned to Fermi and shook his hand, he told him: ‘This day will go down as a black day in the history of mankind.’14
2
The Gift of Destruction
He had in his hands the black complement to all those other gifts science was urging upon unregenerate mankind, the gift of destruction…
H. G. Wells, The World Set Free (1914)
On 2 December 1952, the University of Chicago held a celebration. On the squash court beneath the Stagg Field stadium, twenty-four of the original forty-two scientists, including Enrico Fermi and Leo Szilard, came together with leading politicians and businessmen to ‘mark the end of the first decade of the atomic age and the beginning of the second’.1 The straw-covered Chianti bottle they had all signed ten years earlier was displayed as the first sacred relic of the atomic age. The newspapers reported that its proud owner had insured the empty bottle for $1,000.
The New York Times devoted a series of articles to the anniversary. William L. Laurence, the only journalist to have been given access to the atomic bomb project, compared the scientists to the mythic heroes of antiquity. ‘That afternoon ten years ago’, wrote Laurence, ‘witnessed the lighting on earth of a new type of fire, the first of its kind since the legendary Prometheus taught man the use of fire and started him on the slow march to civilization.’
Laurence went on to say that their achievement ‘brought civilized mankind one of the greatest threats to its existence’.2 Within three years of the Chicago chain reaction, two Japanese cities had been destroyed by atomic weapons. Eugene Wigner wondered whether they had unlocked ‘a giant’ whom they could not control.3 It was a fear shared that December by people around the world, for the previous month America had exploded the world’s first hydrogen bomb – the ultimate weapon of mass destruction.
