Figure 5.4. Sketch of a binary-explosive implosion lens segment (not to scale). From [2].
Now imagine 32 such assemblies interlocked to form a complete spherical shell. Figure 5.5 shows a more detailed scale drawing of the implosion bomb; these assemblies are items B and C in this figure. Within this shell resided another shell of 32 blocks of Comp B (item D), which surrounded the tamper/core assembly. The purpose of this inner shell of Comp B blocks, which is detonated by the imploding Baratol lenses, was to achieve a high-speed symmetric crushing of the tamper and core in order to overcome the spontaneous-fission problem. A trap-door arrangement with a plug of tamper material (item E) allowed for insertion of the core when the bomb was being assembled. The tamper sphere next to the core (item M) was made of ‘depleted’ uranium, that is, essentially pure U-238, the waste product of Oak Ridge enrichment operations. The reason for this is that fast neutrons can fission U-238 nuclei; it has been estimated that some 20% of Fat Man’s yield was due to this effect.
Figure 5.5. Cross-section drawing of the Fat Man implosion bomb showing the major components. Only one set of 32 lenses (B, C), inner charges (D), and detonators (A) is depicted. Numbers in parentheses indicate the quantity of identical components. Drawing is to scale. Copyright by and used with kind permission of John Coster-Mullen.
(A) 1773 EBW detonator inserted into brass chimney sleeve (32)
(B) Comp B component of outer polygonal lens (32)
(C) Cone-shaped Baratol component of outer polygonal lens (32)
(D) Comp B inner polygonal charge (32)
(E) Removable aluminum pusher trap-door plug screwed into upper pusher hemisphere
(F) 18.5 inch diameter aluminum pusher hemispheres (2)
(G) 5 inch diameter Tuballoy (U-238) two-piece tamper plug
(H) 3.62 inch diameter Pu-239 hemisphere with 2.75 inch diameter jet ring
(I) 0.5 inch thick cork lining
(J) 7-piece Y-1561 Duralumin sphere
(K) Aluminum cup holding pusher hemispheres together (4)
(L) 0.8 inch diameter polonium-beryllium initiator
(M) 8.75 inch diameter Tuballoy tamper sphere
(N) 9 inch diameter boron plastic shell
(O) Felt padding layer under lenses and inner charges
Assembling the implosion bombs was difficult and time consuming; essentially, they were hand-assembled three-dimensional jigsaw puzzles with explosive pieces. The weight of the explosives alone was about 5300 pounds, just over half the bomb’s total weight of about 10 200 pounds. The entire assembly was housed within an inch-thick aluminum casing (item J) which weighed 1100 pounds. The choice of shells of 32 blocks was dictated by the fact that this is the number of pentagonal and hexagonal-shaped pieces that can be fitted together to give nearly regular outer faces; think of the patches on a soccer ball. The Trinity and Nagasaki weapons used 12 pentagonal and 20 hexagonal sections, which respectively weighed about 47 and 31 pounds each. The explosives were cast as molten slurries into molds set within water-cooled commercial candy-making machines. The level of activity of the implosion program can be judged by the fact that some 20 000 castings of acceptable quality were created over a period of 18 months, while many more than that were rejected. After being removed from the molds, all castings were checked for uniformity by x-raying them; those which contained air bubbles were repaired by drilling into them with non-conducting tools to get to the void, and then pouring in molten explosive to fill them–similar to how a dentist would fill a cavity. Thousands of such operations were conducted without a single accidental detonation.
One of the most problematic issues encountered with the Fat Man bomb was a piece of equipment known as the X-unit, which was responsible for triggering the network of spherically-distributed detonators within about a microsecond of each other to achieve symmetric implosion. With redundant detonators for each of 32 implosion lenses, 64 cables were involved, all of which had to be of the same length and have the same impedance.
Figure 5.6 shows the Fat Man device detonated at the Trinity test of July 16, 1945, and the combat weapon dropped over Nagasaki. In the latter, the spherical assembly has been encased within an armored ballistic shell, which provided for stable flight characteristics after the bomb was dropped. Combat plutonium bombs were 59 inches in diameter, a constraint set by the width of B-29 bomb bays. Many dozens of drop tests of both the Little Boy and Fat Man designs were carried out to ensure reproducible drops.
Figure 5.6. Left: The Trinity implosion device atop its test tower on July 15, 1945. Norris Bradbury (1909−1997), who served as Director of the Los Alamos Laboratory from 1945 to 1970, stands to the right. The cables feeding from the X-unit box halfway up the bomb go to implosion-lens detonators. Photo courtesy Alan Carr, Los Alamos National Laboratory. Right: assembled Nagasaki Fat Man bomb [3].
The origins of the names of the various bomb designs–Thin Man, Fat Man, and Little Boy–has been a matter of debate. One school of thought holds that Thin Man and Fat Man were coined by Air Force officials to make telephone conversations sound as if aircraft were being modified to carry President Roosevelt (Thin Man) and Prime Minister Churchill (Fat Man). However, in his autobiography, Robert Serber, who was a former student of Oppenheimer’s and one of his first recruits to Los Alamos, claims to have adopted Thin Man from the title of a 1934 detective novel by Dashiell Hammett, and Fat Man from the role played by actor Sydney Greenstreet in the 1941 movie The Maltese Falcon. Little Boy came along when the Thin Man design was abandoned.
5.4 Yield probability
In section 5.1, some of the mathematics of pre-detonation probabilities was explored. This section takes up a related issue, that of what fraction of a bomb’s design yield can be expected to be realized if a pre-detonation does occur. Equation (5.1) gives the probability that a pre-detonation will not occur during the time tcore that the assembling core could sustain a chain reaction before assembly is complete. However, it does not involve the question of when during the assembly process a pre-detonation occurs. This can in fact be at any time between when a chain reaction becomes possible (first criticality) and complete assembly. If it happens near the moment when assembly is complete, the effect on the yield might be small. On the other hand, if by bad luck a predetonation occurs just at the moment of first criticality, the result could be a very low-energy ‘fizzle’. Thus, there is a range of possible nuclear energy yields from essentially zero up to the full design yield of the weapon.
An analysis similar to the one that gives equation (5.1) can be used to develop an approximate expression for the probability of achieving at least the fraction Y of a weapon’s design yield. By formulating this calculation in terms of fractional yield means that we do not need to specify a design yield in advance, which during the Manhattan Project was difficult to predict in any event. This expression is
where the other symbols have the same meanings as in equation (5.1).
For the uranium bomb considered in section 5.1 with 9.18 kg U-238 contamination (v = 2.5, α = 0.78, ), equation (5.4) predicts 99.2% for Y = 0.9 if tcore = 180 μs. This means that there is less than a 1% chance that the device would explode with less than 90% of its intended yield−presuming that all other systems in the bomb work as intended! For tcore = 100 μs, the probability rises to 99.6%. For the Pu-239 core of section 5.1 contaminated with 1% Pu-240, the probability of achieving at least 90% yield is about 91% for tcore = 1 μs.
Beyond the disaster that a very expensive weapon would have been wasted, an issue related to the possibility of a low-yield ‘fizzle’ is that if a bomb fails to destroy itself, an enemy could recover it partially intact, reverse-engineer it, and possibly even recover some of the fissile material. Thus, it is important to make some estimate of the probability of a fizzle. We can use equation (5.4) to get an estimate of this. Suppose that a bomb is designed to have a yield of 10 kilotons TNT equivalent (the Hiroshima bomb released about 13 kilotons). If because of a fizzle the energy equivalent of only 10 ton
s TNT is released, the yield fraction would be Y = 0.001. This is small, but ten tons would surely be ample to destroy the bomb and prevent recovery. For the two-critical-mass plutonium core modeled above, equation (5.4) shows that there is less than a 1% chance of not obtaining even this low yield; this would probably be considered worth the risk of possible enemy recovery.
Exercise
Terrorists claim to have stolen a 25 kg Pu-239 core which is contaminated with 1.5% Pu-240. They have configured a crude (untamped) gun bomb, and claim that they can achieve tcore = 15 μs. Adopting v = 2.5, what value of α does equation (5.2) predict? What is the probability that their device will not suffer pre-detonation? What is the probability that they will achieve at least 50% of the design yield?
Answer
α ∼ 0.354; probabilities 30% and 47%.
5.5 Trinity
Even though implosion research at Los Alamos was of lower priority than the gun bomb, planning for a full-scale test of an implosion bomb began in March, 1944, months before the plutonium spontaneous fission crisis emerged. If use of a Fat Man weapon proved necessary, no one wanted first use, given the uncertainties surrounding implosion, to be over enemy territory. Oppenheimer appointed Harvard University physicist Kenneth Bainbridge to oversee the test, which Oppenheimer himself named Trinity.
The site chosen for the test was a remote 18 by 24-mile tract in the northern portion of the Alamogordo Army Air Field in the Jornada del Muerto (‘Journey of Death’) desert about 160 miles south of Los Alamos (figure 1.3). Before the war, the land supported some cattle grazing, but in 1942 the Army appropriated the house of the family of George McDonald to serve as a part of the Alamogordo Bombing and Gunnery Range. The house was used as the assembly station for the Trinity bomb; while it was somewhat damaged by the explosion, it still stands about two miles southeast of Ground Zero, the site of the test (figure 5.7).
Figure 5.7. The author at the McDonald Ranch House, October 2004.
Except for the ranch house, the site was completely undeveloped. A Base Camp of barracks, warehouses, shops, a mess hall, and other support facilities was built about 10 miles south of Ground Zero. The bomb was mounted atop a 100 foot high steel tower whose footings were sunk some 20 feet into the Earth. General Groves witnessed the explosion from Base Camp, along with a group of distinguished visitors which included Vannevar Bush and Enrico Fermi. Within an area of about 100 square miles centered on Ground Zero were placed three instrument stations, roughly to the north, west, and south, all 10 000 yards from the test site (figure 5.8). The south station also served as the control center where final switches were thrown to activate an automatic firing sequence; Oppenheimer witnessed the test from the South shelter. Personnel who had participated in the development of the bomb but who were not needed at the control station during the test witnessed the event from Campaña Hill, some 20 miles to the northwest. This group included future Nobel laureate Hans Bethe, James Chadwick (discoverer of the neutron), cyclotron inventor Ernest Lawrence, and Robert Serber.
Figure 5.8. Detail map of Ground Zero area [4].
To test procedures and calibrate instruments in advance of the full-scale test, a rehearsal test was conducted on the morning of May 7, 1945. This involved detonating 108 tons of high explosive mounted atop a 20 foot-high tower located about 800 yards southeast of where the Trinity tower was erected. This detonation height was not random. At that time, the best prediction for the Trinity yield was about 5000 tons TNT equivalent. Theoretical analysis indicated that for an observer at distance d from a nuclear explosion of yield Y, the air pressure behind the initial shock wave would be proportional to Y2/3/d2, so the center of the 108 ton stack was placed at 28 feet above the ground to scale to Trinity’s planned 100 foot high detonation and anticipated yield. Trinity’s yield far surpassed 5 kilotons, which resulted in many recording instruments being overwhelmed in the real test.
A particularly unique aspect of the Trinity test was the ‘Jumbo’ program. When the chances for efficient implosion looked poor, the idea was conceived of setting off the device within a containment structure, so that, in the event of a nuclear fizzle, the plutonium could be recovered. This led to the design and construction of Jumbo, a massive steel cylinder within which the bomb would be placed; the ends would then be closed off. In its final form, Jumbo weighed 214 tons, was 28 feet long, with an inside diameter of 10 feet, and a wall thickness of 14 inches. By the time of the test, however, confidence in achieving symmetric successful implosion was much greater, and experimenters were concerned that the vessel would interfere with monitoring instruments. Jumbo was erected on a tower about 800 yards northwest of the explosion, which it survived. The remaining 100 ton central body of Jumbo now lies where it was on the morning of July 16, 1945 (figure 5.9).
Figure 5.9. Left: Jumbo, 1945 [5]. Right: the author (light-colored shirt and hat) inside the remains of Jumbo, 800 yards from Trinity Ground Zero.
Trinity was probably the most monitored scientific experiment in history to its time. Among other effects, instruments measured implosion diagnostics, energy release, shock waves, detonator simultaneity, fission-rate growth, gamma rays, neutrons, fission products, seismic disturbances, and ignition of structural materials. General Groves paid particular attention to obtaining seismic measurements as evidence in the event of any damage lawsuits that might arise from the test.
Weather forecasts predicted July 18−21 as the most promising window for the test, but other factors converged on favoring the 16th. This was not only the earliest date at which the bomb would be ready, but Groves was under intense pressure to carry out the test as soon as possible: President Harry Truman would be in Potsdam, Germany from the 16th until August 2, negotiating with Winston Churchill and Josef Stalin regarding post-war occupation arrangements in Europe and the war against Japan.
The test was scheduled for 4:00 a.m. on the morning of the 16th. As luck would have it, strong thunderstorms moved into the area about 2:00 a.m. that morning, forcing a postponement until 5:30. At Base Camp, Enrico Fermi occupied himself by offering to take wagers on whether the bomb would ignite the atmosphere and, if so, would it destroy only New Mexico or the entire world? (He estimated that if nitrogen in the air were to ignite, it would propagate for only about 35 miles.) By the time of the test, Oppenheimer was practically a nervous wreck. He had lost a significant amount of weight; despite standing at over six feet, he weight only about 115 pounds.
As the time of the test approached, the tension in the control bunker was extreme. In the words of Brigadier General Thomas Farrell, Groves’ deputy:
The scene inside the shelter was dramatic beyond words. In and around the shelter were some twenty-odd people concerned with last minute arrangements … For some hectic two hours preceding the blast, General Groves stayed with the Director, walking with him and steadying his tense excitement. Every time the Director would be about to explode because of some untoward happening, General Groves would take him off and walk with him in the rain, counseling with him and reassuring him that everything would be all right.
Trinity was detonated at 5:29 a.m., just before sunrise (figure 5.10). Farrell:
As the time interval grew smaller … the tension increased by leaps and bounds. Dr Oppenheimer, on whom had rested a very heavy burden, grew tenser as the last seconds ticked off. He scarcely breathed. He held on to a post to steady himself. For the last few seconds he stared directly ahead and then when the announcer shouted ‘Now!’ and there came this tremendous burst of light followed shortly thereafter by the deep growling roar of the explosion, his face relaxed into an expression of tremendous relief. Several of the observers standing back of the shelter to watch the lighting effects were knocked flat by the blast.
Figure 5.10. Left: The Trinity fireball 25 milliseconds after detonation [6]. Right: The Trinity mushroom cloud a few seconds later [7].
As Farrell later reported to Groves, who had left for Base Camp about 20 minutes before the test:
Th
e effects could well be called unprecedented, magnificent, beautiful, stupendous and terrifying. No man-made phenomenon of such tremendous power had ever occurred before. The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined. It was that beauty the great poets dream about but describe most poorly and inadequately. Thirty seconds after the explosion came, first, the air blast pressing hard against the people and things, to be followed almost immediately by the strong, sustained, awesome roar which warned of doomsday and made us feel that we puny things were blasphemous to dare tamper with the forces heretofore reserved to The Almighty. Words are inadequate tools for the job of acquainting those not present with the physical, mental, and psychological effects. It had to be witnessed to be realized.
Robert Oppenheimer’s reaction to the test has been a matter of debate. In a 1965 interview for a television documentary, The Decision to Drop the Bomb, Oppenheimer gave this reaction to Trinity:
We knew the world would not be the same. Few people laughed, few people cried, most people were silent. I remembered the line from the Hindu scripture, the Bhagavad-Gita. Vishnu is trying to persuade the Prince that he should do his duty and to impress him takes on his multi-armed form and says, ‘Now I am become Death, the destroyer of worlds.’ I suppose we all thought that, one way or another.
The Manhattan Project Page 8