The making of the atomic bomb was one of the biggest scientific projects in history. Some of the brightest minds of the world worked against exceedingly demanding deadlines to produce a nuclear weapon in record time. To do this, every kind of problem imaginable in physics, chemistry, metallurgy, ordnance and engineering had to be surmounted. Many of the problems had never been encountered before and challenged the ingenuity and perseverance of even the best of the brightest.
To accomplish this feat, human, material and monetary resources were poured in on a scale unsurpassed till then. Factories were constructed at Oak Ridge, Los Alamos and Hanford that were bigger than anything built until then. The resources required were staggering; at one point the Manhattan Project was using 70% of the silver produced in the United States. Steel production in the entire nation had to be ramped up to fulfill the needs of the secret laboratories. Extra electricity on a national scale had to be generated to power the hungry reactors and electromagnetic separators. The factories at Oak Ridge were giant structures; one of them was a whole mile under one roof. The gargantuan factories and the resulting employment increased the population of the small town from 3000 to about 75,000. At the end of the war, hundreds of thousands of people and an estimated 2 billion 1945 dollars had been spent on the biggest technical project in history. The entire country had had to be mobilized for it. In just three years, the scale of the project was consuming about as many resources as the US automobile industry, an astonishing achievement. Only the United States could have done something like that at the time.
Of all the myriad and complex problems involved in the project, two stand out for their formidable complexity and difficulty. One was the separation of uranium-235 from its much more abundant cousin uranium-238. The differences between the masses of the two isotopes is so small that at the beginning, nobody believed that it could be done. Indeed, the atomic bomb effort in Germany largely stalled because its leaders could not think of any way this could be done in any reasonable time. An entire town had to be constructed at Oak Ridge to surmount this problem. Even today this is probably the single-hardest problem for anyone wanting to construct an atomic bomb from scratch.
However, the uranium separation problem was at least anticipated at the very beginning. Compared to this, the second problem was completely unexpected. It involved a material from hell that nobody had seen before. This material was highly unstable and difficult to work with, intensely radioactive, and its discovery was one of the most closely-kept secrets of all time. The material would play a decisive role in the project and in the nuclear arms race that was to ensue. Today, its shadow looms large over the world. This material is plutonium.
Now in a succinct and readable book, well-known physicist and historian of science Jeremy Bernstein tracks the history of a diabolical fiend. Bernstein has earlier written biographies of Oppenheimer and Hans Bethe and a recent book on nuclear weapons. He is an accomplished veteran physicist who has known some of the big names in physics of the century, Oppenheimer and Bethe included. Bernstein is a fine writer who recounts many interesting anecdotes and bits of trivia. But he does have one annoying habit; his constant tendency to digress from the matter under consideration. He could be talking about one event and then suddenly digress into a four page life history of a person involved in that event. One gets the feeling that Bernstein wants to put his opinion of every small and sundry event from the life of every scientist he has met or heard of on record. At times, the connections he unravels are rather tenuous and long-winded. Readers could be forgiven for finding Bernstein's digressions too many in number. But at the same time, those interested in the history of physics and atomic energy will be rewarded if they persevere; most of Bernstein's forays, though exasperating, are also quite interesting. In this particular case, they weave a complex story around a singular element.
Plutonium was discovered by the chemist Glenn Seaborg and his associates at Berkeley in 1940. In a breathtakingly productive career, Seaborg would go on to discover nine more transuranic elements, advise four US presidents, win the Nobel prize, win enough other awards and honors to have an entry in the Guinness Book, and have an element and asteroid named after him while still alive. After fission was discovered, it was hypothesized that elements with atomic numbers 93 and 94 might also behave like uranium. In 1939 Seaborg was a young scientist working at Berkeley when he heard about the discovery of fission. In the next year he performed many experiments on fission at Chicago and Berkeley. In 1940, another future Nobel laureate named Edwin McMillan discovered a radioactive element past uranium with a postdoc, Philip Abelson. In logical sequence they named it neptunium. Abelson and McMillan's June 1940 paper on neptunium was the last paper to come out of the United States on fission and related issues; the need for secrecy in such matters had already been realised by senior scientists. There matters stood until December 1941- a decisive time due to Pearl Harbor- when Seaborg, McMillan and their associates Joseph Kennedy and Arthur Wahl discovered element 94 by using tedious and clever chemical techniques. After uranium and neptunium, Seaborg decided to name the new element after Pluto- the god of fertility but also the god of the underworld.
Concomitantly with the American effort, the Germans were also trying to understand the properties of plutonium and Bernstein devotes a chapter to their efforts and background. A resourceful German physicist named Carl Friedrich von Weiszacker had observantly noticed the dwindling and disappearance of papers from the United States after the paper by McMillan and Abelson appeared in mid 1940. He also realised the advantage of using plutonium in a nuclear weapon. But as the history of the German atomic project makes clear, Weiszacker's report was not taken too seriously, and in any case the Germans were too cash and resources-strapped to seriously pursue the production of plutonium. Notice was also taken by accomplished physicists in the Soviet Union but it was espionage that provided them with information about the real potential and importance of plutonium. The fascinating story of Soviet espionage is superbly narrated in Richard Rhodes's Dark Sun: The Making of the Hydrogen Bomb.
Plutonium was soon isolated in gram quantities by Seaborg's team and its enhanced fissile properties were investigated. After the enormous problems with separating U-235 were realised, the great advantage of plutonium became obvious; plutonium being a different element, it would be relatively easy to separate from its parent uranium, thus avoiding the difficulty of isotope separation. After plutonium was discovered, it was found that it is even more prone to fission than uranium. Compounded with its relative ease of separation, this property of plutonium made it a key material for a nuclear weapon. It was also realised however that many tons of uranium would have to be bombarded with neutrons to produce pounds of the precious element. By 1942, it was known that at least a few kilograms of both uranium and plutonium would be needed for the critical mass of a bomb. To this end enormous factories were constructed at Oak Ridge (for enriching uranium) and reactors at Hanford in Washington state (for producing plutonium) in 1943. The reactors at Hanford would keep on producing the material for thousands of nuclear warheads until the late 1980s. A secret lab at Los Alamos was concurrently established, headed by Robert Oppenheimer. He would bring a group of "luminaries" to the mesa high up in the mountains for working on the actual design of an atomic weapon.
At Los Alamos, initial designs of bombs with both uranium and plutonium involved the "gun method" wherein a plug of fissile material would be shot down at great speed along a large gun barrel into another mould of fissile material. When the two met a critical mass would suddenly materialize and fission would result in an explosive detonation. However, a fatal flaw was unexpectedly encountered in 1944. When the first few grams of plutonium arrived at Los Alamos from Hanford, it was observed that Pu-239 had a very high rate of "spontaneous" fission due to the copious presence of another isotope, Pu-240. Even today, the feature that distinguishes "reactor-grade" plutonium from "weapons-grade" plutonium is the higher presence of Pu-240 in reactor-grade material. Because of the presence of extra neutrons from spontaneous fission, a gun type bomb though it would work for U-235 would be worthless for Pu-239 since by the time the two pieces met, fission would have already started and the result would be a "fizzle", a suboptimal explosion. Because of this difficulty the whole lab was reorganised by Oppenheimer in August 1944 and experts were brought in to investigate new mechanisms for a plutonium bomb.
The result was one of the most ingenious concepts in nuclear weapons history and design- implosion. The idea was to suddenly squeeze a sub-critical ball of plutonium using high explosives into a highly compressed supercritical mass, causing fission and a massive explosion. The problem was that this microsecond compression had to be perfectly symmetrical, otherwise the Pu-239 would simply squirt out along the path of least resistance like dough squeezed within the cupped palms of our hands. To circumvent this problem would require the capabilities of some of the greatest scientists of the day. The Hungarian genius John von Neumann supplied the crucial idea of using "lenses" of explosives of differing densities to direct shock waves that would symmetrically converge onto a point, just like light through glass lenses. The concept required a paradigm shift- nobody had used explosives before as precision tools; they were generally used to blow things out, not in. Even after the idea was floated, the engineering and diagnostics obstacles were formidable. Chemist George Kistiakowsky from Harvard was put in charge of a division that would painstakingly develop the moulds for the lenses; machining had to be accurate to within microns as any air bubbles, cracks or irregularities would immediately impede the symmetrical shock wave. Another challenging device was the "initiator", a tiny ball of radioactive elements in the center of the sphere that would release neutrons right after the implosion, but not a moment before. Its design was so challenging that it is one of the few things that's still almost completely classified. One of the physicists who worked on both shock wave hydrodynamics and on initiator design was Soviet spy Klaus Fuchs. He was ironically brought in as part of a British team to replace Edward Teller, whose reluctance to pursue implosion and obsession with hydrogen bombs tested the patience of theoretical division leader Hans Bethe. Information obtained by Fuchs would prove invaluable to the Russians in building their own implosion bomb.
Compounding all of these difficulties was the hideously diabolical nature of Pu-239 itself. Chemists and metallurgists had never faced the challenge before of working with such an unusual and dangerous material. Pu-239 exists as several allotropes, different physical forms of the same element, depending upon the conditions. When one investigates the use of plutonium in a bomb and then looks at its allotropic behavior, it's almost as if nature had conspired to keep humans from using it. The reason is that at room temperature, Pu-239 exists as an allotrope named the alpha phase allotrope. The problem with this is that while it is dense, it is brittle and won't do at all for an implosion. On the other hand the allotrope of Pu-239 that is suitable for a bomb, the delta phase, exists only at 315 degrees centigrade and above. This is a catch-22 situation; the useful and machinable allotrope exists only at high temperatures while the one at room temperature is worthless. A very clever solution to this was discovered by human ingenuity; Cyril Smith, head of the metallurgy division at Los Alamos found that adding a small amount of the metal gallium to Pu-239 stabilized the valuable delta phase at room temperature. This was found only a few months before the first test of the bomb.
In the end, while the uranium bomb was reliable enough to not require testing, the implosion bomb was too novel to use without testing. On July 16, 1945, the sky thundered and a new force surpassing human ability to contain it was unleashed in the cold desert sands of New Mexico at the Trinity test site. Plutonium tested on that ominous dawn would reincarnate into Fat Man, the bomb that leveled Nagasaki in less than ten seconds.
In addition to Pu-239's unusual chemistry, there were of course its radioactive properties that make its name so dreaded for laypersons. But we have to put things in perspective. I would easily be within a kilometer of Pu-239 than within a kilometer of anthrax or VX nerve gas. Plutonium decays by emitting alpha particles and simple laws of physics dictate that these particles have a very short range. You could hold Pu on a sheet of paper in the palm of your hand and live to talk about it. The real danger from Pu-239 comes from inhaling it; it can cause severe damage to lungs and bone and cause cancer. Its half-life is 24,000 years and another law of physics dictates that half-life and radioactive intensity are inversely related. To help understand Pu-239's true nature, Bernstein narrates a fascinating study of 37 technicians and scientists at Los Alamos who ended up getting Pu-239 into their system. This group was whimsically named the "UPPU" (U Pee Pu) group as Pu-239 could be detected in their urine. The group was tested periodically at Los Alamos for many years. The verdict is clear; none of these people suffered long-term damage from Pu-239. Many of them lived long and healthy lives and some of them are still alive. As with other aspects of nuclear power, the danger from plutonium has to be carefully reasoned and objectively assessed. As with other nuclear material, Pu needs to be handled with the utmost care, but that does not mean that fears about it should outweigh benefits that one could get from its potential for providing power. There is naturally a real proliferation danger with plutonium, but even there, risks are often inflated. Terrorists will have to steal a substantial amount of Pu using special equipment from facilities which are usually heavily guarded. Stealing Pu and using it is not as easy as robbing a bank and laundering the money.
However, there are sites in the former Soviet Union where plutonium is not that heavily guarded and these will have to be secured. 5 kilograms of Pu-239 if efficiently utilised can be used for a weapon that will easily destroy Manhattan. It is very difficult to keep track of such small quantities through inspection. International collaboration will be necessary to keep track of and contain every gram of plutonium at vulnerable facilities. At the same time, power-generating plutonium is indispensable for the future of humanity. Forged on earth by human brilliance, Pu outlived its initial use. Most of the warheads in the US arsenal including thermonuclear warheads use plutonium for the fission assembly. Several hundred tons of both weapons-grade and reactor-grade plutonium have been produced and are being produced. Hundreds more sit in fuel rods immersed in huge water pools, glowing eerily with a bluish light. Plutonium production sites in the United States are facing a heavy and expensive backlog of cleanups.
Plutonium seems to be a classic case of the "careful what you wish for" adage. Glenn Seaborg would not have imagined the consequences of his discovery that hazy morning in December 1941, when after an all-night session the angry element revealed itself to a warring world, kicking and screaming from its fiery radioactive cradle. But as Richard Feynman once so lucidly put it, science is a set of keys that open the gates to heaven. The same keys open the gates to hell. Plutonium constitutes one of the keys to heaven that's given to us. Which gate to approach is entirely our choice.