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Critical Assembly: A Technical History of Los Alamos during the Oppenheimer Years, 1943-1945

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Hoddeson, Henriksen, Meade & Westfal
Critical Assembly, ISBN 0-521-54117-4
Cambridge University Press, 2004

The title of the book has double meaning. It denotes the critical assembly of uranium 235, or plutonium 239 to start the chain reaction in an atomic bomb. But it also points to the »critical« assembly of numerous scientists, engineers, technicians and US Army personnel. The authors described how in a race with time all these experts were pursuing a single objective: to make an atomic bomb before the Nazi scientists could (supposedly) do it. Each of them was working in his or her special field, but only a handful of the privileged ones knew they were making an atomic bomb. The resolute, competent and mercilessly hard driving conductor of this huge orchestra was General Leslie R. Groves; its concertmaster was the physicist J. Robert Oppenheimer. To carry on this parable, the musicians - with rare exceptions - were obliged to play their scores with plugged ears. The conductor allowed them to know only their own score, because the whole composition named MED (Manhattan Engineering District) must not become known before the end of the war.

Though the fission was discovered in Germany (in the winter 1938/39) many Jewish scientists, being suppressed under Nazi-fascist reign, had left Europe as soon as they could. Among them A. Einstein, H. Bethe, R. Peierls, C. Fuchs (unfortunately also a soviet spy), N. Bohr, E. Teller, E. Wiegner, L. Szilard, E. Fermi and J. von Neumann, to name just the most important ones, arrived in the USA, where they contributed essentially to MED. When Groves began leading the project, it started advancing like an avalanche. What in 1939 was deemed to be a science fiction has become a real bomb within just six years.

To quench the thirst for information after the first bomb was dropped on Hiroshima in August 6, 1945, Henry de Wolf Smyth of MED had prepared the book "Atomic Energy for Military Purposes". In it the most basic knowledge of how an atomic bomb works, as well as the enormous effort of MED to make it, was made public. But in its preface Groves attached a latch, telling us this is all, which can be released at the time; take it, do not ask any further questions - or else! Though many books published after this date had disclosed this or that, the book "Critical Assembly" has definitely broken that latch by disclosing many minute details, which were classified almost up to present time. In the book we learned how the scientists and other personnel, forced to work under the circumstances as outlined above, starting with micrograms of highly enriched uranium 235 and (up to then non existent) plutonium, have gradually extended the production up to kilogram quantities, determined the critical masses, avoided the nuclear explosion, and had managed to build two combat-ready weapons, which ended the war. The details will certainly be interesting for physicists as well as for engineers of chemistry, electronics, metallurgy, mechanics, ordnance and some others. For the layman the minute descriptions would be mostly too difficult to understand and the same might be valid even for professionals, if the matter lies too far outside of their specialty. But as a whole, the book is a great work of reference, with an enormous collection of interesting data, not known so far. On top of all this the book has 74 pages of references.

Unfortunately, the Department of Energy was too thorough when removing many "sensitive data" from the original text. Initially numerous details became gradually scarce when the discussion advanced toward August 1945. However, some common sense and simple calculations, based on the data published in many other books, magazines and films, converge to the following conclusions: Approximately 10 lbs (4.53 kg) of plutonium was used in the Fat Man and 70 lbs (31.75 kg) of 88 % enriched uranium 235 in the Little Boy bomb. In the uranium bomb the active material of about 3 critical masses was divided into 4 projectiles and one equilateral cylindrical target, with holes to fit the projectiles, placed inside the massive tungsten steel tamper. When three critical masses are assembled, the chain reaction starts spontaneously within about 0.1 s; so an initiator (with dangerously radioactive polonium 210) is basically not needed. Why was such data not mentioned in the book, which is full of less important details? Today nobody would waste so much precious, highly enriched uranium by not resorting to implosion, which needs less than one critical mass. On the other hand, the metallurgy of plutonium is a science in itself and so is the implosion. So why be so scared?

Peter Staric, PhD, BSEE
Ljubljana, Slovenia

Depending of your technical proficiency.
Engineers or physicists will love it as an introduction to the Manhattan Project, and would subsequently read Rhodes excellent books to get a larger view.
Critical Assembly will allow non-technical people to understand the degree of complexity of this undertaking.
Being of the latter kind, I naturally read almost every other book about the subject, before resigning myself to buy this one. It's been a very pleasant surprise to find out that it's very readable. You will not get everything from the very detailled technical processes described but it's comforting to understand that at that time, they didn't either.
The main feeling throughout the book amounts to "oh, those people where in the dark most of the time, you could almost say it's been really incidental that they pulled it out in the end".
You precisely get how tedious this has been and how sparks of individual genius have made it possible at all.
The cover is pink, though.