Fat man
Fat Man ( English for fat man ) was the code name of the Mark 3 nuclear weapons design, which was developed as part of the Manhattan Project by American, British and Canadian scientists. The first nuclear explosion in history on July 16, 1945, the Trinity Test , was based on this design. A Fat Man bomb was on August 9, 1945 by a US B-29 - bombers of the 509th Composite Group dropped and exploded at 11:02 AM on the Japanese city of Nagasaki , which was largely destroyed. The bomb exploded around 550 meters above densely populated area and developed an explosive force of around 21 kilotons of TNT . It was the second - and last - nuclear weapon used in a war , after the Little Boy dropped three days earlier . The Mark 3 nuclear weapon was the first nuclear weapon that the United States added to its arsenal in large numbers after World War II . In 1946 and 1948, five more nuclear weapons tests of this type took place. The Soviet Union's first nuclear weapon , dubbed the RDS-1 , was a copy of the Fat Man draft spied into the Soviet Union.
History of Fat Man
Development until July 1945
When the element plutonium was discovered by American scientists in 1940/1941 , it was quickly recognized that the isotope 239 Pu in particular can be used for the construction of nuclear weapons just as well as the naturally occurring uranium isotope 235 U. When the United States developed Forced nuclear weapons and founded the Manhattan Project, it was therefore decided to use both isotopes for the new weapons. The construction of a large complex for the production of plutonium in Hanford , Washington State began in March 1943 , which included several production reactors and chemical plants for the extraction of plutonium from the irradiated fuel . As an intermediate step, however, a smaller reactor, called X-10, was built in Oak Ridge with a capacity of 1 MW, which was supposed to deliver smaller amounts of plutonium for research purposes. This became critical for the first time in November 1943 .
The plutonium was initially intended to be used in a cannon design like the Mark 1 ( Little Boy ). The data from pure 239 Pu, which has been obtained in very small amounts in cyclotrons since the element was discovered in 1940 , showed that this should be possible. Cyclotrons themselves can only produce tiny amounts of plutonium and are unsuitable for industrial production. When the first small amounts of reactor plutonium from the X-10 reactor in Oak Ridge were available for analysis in 1944, however, it became clear that plutonium was not suitable for the cannon principle. In July 1944 Emilio Segrè showed , as predicted the year before by the co-discoverer of the plutonium Glenn T. Seaborg in 1943, that reactor plutonium would be contaminated with the isotope 240 Pu. In contrast to cyclotrons, pure 239 Pu cannot be produced in nuclear reactors . Plutonium from reactors is always a mixture of different isotopes of the element, including the isotope 240 Pu. This has a high rate of spontaneous fission , that is, the isotope generates a relatively high background of neutron radiation . If this were to be used in a nuclear weapon based on the cannon principle, the chain reaction would start due to the neutrons generated by 240 Pu before the critical mass is completely united. This pre-ignition would cause the bomb to explode with very little explosive power. In short: the cannon principle is too slow for the use of plutonium.
Therefore, the scientists of the Manhattan project had to develop a new method with which plutonium could be transformed from a subcritical to a supercritical mass much faster before the high neutron background triggered a chain reaction. At that time, research was already being carried out on the implosion technique in Los Alamos. This was done in the Ordnance Division, which was headed by William Sterling Parsons . But he was mainly concerned with the development of the cannon principle and Seth Neddermeyer was responsible for the development of the implosion principle . His group made only little progress at this point, which is why Robert Oppenheimer brought the chemist George Bogdan Kistiakowsky to Los Alamos to support Neddermeyer. The idea at that time was to encase the plutonium core with a layer of highly explosive explosive, which should be ignited at many points at the same time and thus condense the plutonium sphere inside into a supercritical mass. However, with this technique, the shock waves emanating from the various ignition points overlap and generate interference so that no orderly inward-directed shock wave can arise. Implosion technology initially looked like a dead end in mid-1944, and the use of plutonium in nuclear weapons seemed dubious.
The solution to the problem came from the physicist James Tuck , who was part of the British delegation in Los Alamos. Tuck previously worked on the development of shaped charges for armor-piercing ammunition. He therefore had experience with explosive devices that should emit their explosive power in one direction. He therefore suggested surrounding the plutonium core with an array of explosive lenses, which should consist of explosives of different explosiveness . By appropriately designing the lenses, one could control the propagation of the shock wave and thus create a perfect, inwardly directed, spherical shock wave. Initially, however, this was not viewed as desirable, since an implosion arrangement with explosive lenses is much more complicated than the simple explosive casing that was striven for up to that point. Tuck received support from the British hydrodynamicist G. I. Taylor , who was able to show through calculations that a simple arrangement would not work.
In August 1944, Robert Oppenheimer finally dissolved the Ordnance Division and replaced it with two new departments, the G Division (G for Gadget) under the direction of Robert Bacher and the X Division (x for eXplosives) under the direction of Kistiakowsky. The G Division should work out the physics for implosion and design the Fat Man bomb, and the X Division the necessary explosive lenses.
In contrast to the elongated, cylindrical shape of the Mark 1 bomb based on the cannon principle based on uranium, the implosion arrangement with the explosive lenses resulted in a thick, spherical shape of the bomb. Therefore the Mark-3 was called Fat Man (in German "Dicker Mann"), the design based on the cannon principle had the name Thin Man ("Thin Man").
In the fall of 1944, work on the production reactors at Hanford was nearing its end. Three reactors were built, named B, D and F. The Hanford facilities with the reactors were operated by the chemical company DuPont at the time . On September 15, 1944, the B reactor was loaded with fuel and started up under the direction of Enrico Fermi. At first the reactor behaved as planned and on September 26th they wanted to bring it to full capacity. The physicist John Wheeler was overseeing the reactor at the time. However, the reactor could only be brought up to about 9 MW of power, then it reduced this again. Attempts were made to counteract this by further extending the control rods made of cadmium , but on the evening of September 27 the reactor shut down completely with the control rods fully extended. It started up again the next day, but switched itself off again after reaching 9 MW. Wheeler finally identified the problem as self-poisoning of the reactor by the isotope 135 Xe. This arises from the fission of 235 U, which supplies the neutrons for the breeding of 239 Pu from 238 U. However, the xenon isotope absorbed so many neutrons that not enough were left to sustain the chain reaction. However, the design of the Hanford reactors made it possible to solve this problem by adding more fuel to the reactor. This enabled the poisoning effect to be overcome. The D reactor was started up on December 17, 1944 and the repaired B reactor on December 28, the F reactor followed in February 1945. On February 4, the reactors reached their full output of 250 MW each and could from now on Produce 19 to 21 kg of plutonium per month - enough for two to three bombs.
The X Division, under Kistiakowsky's direction, worked hard on the development of the implosion lenses in the winter of 1944/45 and used around a ton of explosives a day. The mathematician John von Neumann finally developed a lens consisting of an outer layer of explosives with a high detonation speed and an inner layer with a lower detonation speed. The inner layer focused the shock wave of the outer layer and made a perfect implosion possible. In February 1945, at a meeting of Oppenheimer, General Leslie R. Groves (military director of the Manhattan Project), James Bryant Conant , Hans Bethe and Kistiakowski, the final composition of the explosive lenses and the general design of the Fat Man bomb were decided. On March 1, Oppenheimer founded the Cowherd Committee under the leadership of Samuel Allison , which also included Bacher and Kistiakowski. This should propel the herd of Fat Man developers through the final stages. In the same month Oppenheimer had all further developments to improve explosive lenses stopped. Analyzes of the recent experiments with the explosive lenses showed that a spherical implosion had been achieved in accordance with the theoretical models. Oppenheimer reported to Groves that the first plutonium bomb was ready on August 1, 1945.
Trinity test
Due to the complicated structure of the Fat Man design, in contrast to the cannon principle of the Mark 1 bomb Little Boy , a test was considered indispensable. In 1944 part of the bombing area of the Alamogordo Test Range in New Mexico was selected for this. Oppenheimer named the test site Trinity , inspired by a sonnet by John Donne . He transferred the responsibility for preparing the test to the physicist Kenneth Bainbridge and put his brother Frank Oppenheimer at his side for support.
In advance of this, on May 7, 1945, an explosion of 108 tons of conventional explosives (composite B) mixed with waste from spent fuel from Hanford was carried out at the Trinity test site. This was used to calibrate test instruments and to investigate the spread of radioactive material ( fallout ) due to the explosion. The results were worrying as they suggested that neighboring inhabited areas may be affected by the Trinity Test fallout. It was hoped that the higher height of the Trinity bomb would prevent this. The test bomb called the gadget was supposed to explode on a 30 m high steel tower.
The plutonium for the test bomb reached Los Alamos on June 24, 1945. A group led by Otto Frisch confirmed that the core that had arrived was sufficient for the bomb. It weighed around 6.1 kg and was around 9.2 cm in diameter with a 2.5 cm cavity in the middle, which was supposed to contain the polonium - beryllium neutron source.
The test was originally scheduled for July 4, 1945, but the first mass-produced explosive lenses had many defects. The new date was set on July 16. Before the real Trinity test, Oppenheimer insisted on a dry test of the bomb without a plutonium core in order to prove the function of the implosion mechanism. However, in early July there weren't even enough correct lenses available for a test. Kistiakowsky worked at full speed for several nights, repairing defective lenses by hand using a dentist's drill and molten explosives. On July 14th, the dry test was carried out near Los Alamos and initially considered a failure. This led to a bitter argument between Oppenheimer and Kistiakowsky, but Bethe revealed the next day that the measurement methods used were unable to determine whether a successful implosion had taken place. That means you didn't know whether the test was successful or not.
The test bomb Gadget was finally assembled in a tent next to the steel tower on which it was supposed to explode. It was finally lifted to the top of the tower with a crane and then wired to the ignition electronics.
The weather before the test, which was scheduled for 5:30 a.m. local time on July 16, was bad and it was feared that the test would have to be postponed. However, conditions improved from 2 a.m., Bainbridge set the bomb at 4 a.m. and went to the control bunker with the others who had remained with the bomb until the end. The 20-minute countdown began at 5:10 a.m. The first Fat Man bomb exploded at 5:30 a.m. as planned. Frisch later described the explosion as follows:
“And then, without a sound, the sun shone - or that's what it looked like. The sand hills on the edge of the desert shone in a very bright light, almost colorless and shapeless. I turned around, but the object on the horizon, which looked like a small sun, was still too bright to look at. I winked and tried to look more at it, and after about another 10 seconds it continued to grow and darken to what looked more like a large oil fire. ... It was a breathtaking spectacle; No one who has ever seen a nuclear explosion will ever forget it. And all in complete silence; the bang came minutes later, quite loud even though I had my ears plugged, and followed by a long rumble like heavy traffic at a very great distance. I can still hear it. "
Fat Man worked. Before the test, however, one was unsure how strong the explosion would be. There was a wager among the scientists on strength. Oppenheimer estimated a pessimistic 300 t, Edward Teller 45,000 t. Isidor Isaac Rabi won with his estimate of 18,000 t, as the explosion strength was initially given as 18.6 kt based on radiochemical measurements. However, later evaluations showed that the explosion was in the range of 20 to 22 kt.
The explosion cloud rose to a height of about 11 km. On the bottom, the bomb created a shallow crater measuring about 80 m with a maximum depth of 2 m, which was surrounded at the edge by molten sand ( trinitite ). In the center there was an area measuring about 10 m with neutron-induced radioactivity. The bomb generated significant fallout locally. The zone with the greatest pollution was about 32 km north of the explosion site on Highway 380. Some houses near Bingham were evacuated, but individual isolated houses with increased pollution were not.
Use over Japan
The use of nuclear weapons against Japan was preceded by major discussions in the United States about whether and how to use the weapons against Japan. One of the most determined opponents was the physicist Leó Szilárd , who lobbied politicians and scientists strongly against the use. There was also resistance in the military, so Dwight D. Eisenhower spoke out against the deployment, because he feared, among other things, an arms race with the Soviet Union. Even Curtis LeMay was not convinced of the use, but mainly because he could reach with its conventional bombing campaigns the same goal and therefore saw no military necessity. Many of the critics spoke out in favor of a demonstration of the new weapon in front of the world public, including representatives from Japan.
However, the supporters of the mission prevailed. They thought it was nonsensical to spend 2 billion US dollars on developing a weapon in order not to use it. Furthermore, it was hoped to be able to end the war as quickly as possible in order to spare the Allied troops the landing on the Japanese main islands, which was scheduled for autumn 1945, in which high losses among their own troops were feared. Another reason was that the Soviet Union had promised to declare war on Japan in August 1945. The aim was to prevent large Soviet territorial claims in East Asia and to strengthen their own position by quickly ending the war.
In August 1943, the United States began modifying B-29 bombers for use with the nuclear weapons under development. Production of the new bomber had just started, but the standard variant could not carry the heavy bombs. The curb weight of the modified bombers was reduced and a special bomber group ( 509th Composite Group ) was set up under the command of Paul Tibbets , which should work out the necessary flight maneuvers for the atomic bombs.
In order to gain experience with the ballistics of the Fat-Man bomb body and to give pilots the opportunity to practice with the unusually shaped bomb, the so-called Pumpkin Bomb ("pumpkin bomb") was developed. It was a conventional bomb with the ballistic properties of the Fat-Man design and was also developed as part of the Manhattan Project. In the course of the Second World War 49 of the 486 pumpkin bombs built were dropped on Japan.
A special target selection committee headed by Groves was formed. There was also a committee ( Interim Committee ) headed by US Secretary of War Henry L. Stimson , which dealt with nuclear policy after the war, but also discussed the use of Japan. Stimson summarized the outcome of a meeting on May 31, 1945 as follows: The Minister made the final statement, on which there was general agreement, that we cannot give any warning to the Japanese; that we cannot focus on a civil goal; but that we should try to make as deep a psychological impression as possible on as many residents as possible. At the suggestion of Dr. Conant agreed with the minister that the best possible goal was a war-essential factory, which employs a large number of workers and is surrounded by workers' houses.
Hiroshima , Niigata , Kokura and Kyoto were initially discussed as goals . At Stimson's insistence, however, Kyoto was dropped because of its cultural importance (he had visited Kyoto on his honeymoon).
Unlike Little Boy, whose components were transported on the USS Indianapolis , the parts for Fat Man reached the Tinian airfield by plane. The plutonium core and neutron source left Kirtland Field in the United States on July 26, 1945 and reached Tinian on July 28, 1945. On the same day, three specially converted B-29s left Kirtland carrying three Fat-Man bombs (F31, F32 and F33) according to Tinian. Unit F33 was the bomb intended for use. On Tinian, the bomb was assembled from the individual parts and was ready on August 5, 1945. The deployment was initially planned for August 11, 1945, but due to the announced bad weather, the dropping was brought forward to August 9, three days after the deployment of Little Boy against Hiroshima. Therefore, during assembly, some tests on the bomb were omitted in order to get it ready for use as quickly as possible.
The Great Artiste bomber was actually supposed to fly the mission. Because this aircraft was still equipped with measuring devices from the previous mission over Hiroshima and a complicated conversion should be avoided, the crew was simply swapped with that of the bomber Bock's Car . So the team flew The Great Artiste , the Bock's Car with the bomb and the crew of the Bock's Car , the The Great Artiste with the measuring devices.
On August 8, 1945, the day the USSR declared war on Japan, the Fat Man bomb was loaded into the bomb bay of the Bock's Car . On the morning of August 9, the bomber took off at 3:47 a.m. local time for the primary target Kokura . Shortly after take-off, the crew of the Bock's Car, under the command of Major Charles Sweeney , noticed that the aircraft's 600-gallon reserve tank was not available. The bomber reached Kokura at 10:47 a.m. local time. However, the city was covered by the thick haze of conventional bombing in the city's neighborhood. Therefore, the crew could not see the planned target point. Flak continued to flare up and Japanese fighters soared, so Sweeney decided to head for the secondary target Nagasaki. Nagasaki was also under clouds and the crew were instructed to bomb only on sight. But because the fuel for the return flight was running low, the crew had to drop the bomb. A gap in the cloud finally made it possible to drop it on sight, but several kilometers away from the actual target point.
The Fat-Man bomb exploded at 11:02 a.m. local time at a height of about 503 m, directly above the Mitsubishi weapons factory, on the edge of an unpopulated area. The spread of the shock wave was slowed down by mounds near the drop point, so that the effects of the explosion were dampened. Nevertheless, at least 70,000 people died as a result of the operation. More recent analyzes suggest an explosive force of 21 kt, compared to the 16 kt from Little Boy over Hiroshima. About 60% of the released energy was given off in the shock wave, 35% as thermal radiation and 5% as ionizing radiation . Due to the high altitude of the explosion, there was hardly any radioactive fallout from the explosion over Japan. However, as in Hiroshima, many people fell ill with radiation sickness , which was caused by the neutron and gamma radiation given off by the explosion.
Different information is available about the exact number of deaths; in 1953 the US Strategic Bombing Survey came to the numbers of 35,000 dead, 60,000 injured and 5,000 missing. In 1960 the Japanese government published the figures of 20,000 dead and 50,000 injured, but this was later corrected to 87,000 deaths. Other sources speak of around 35,000 to 40,000 dead.
Many people died or were disfigured as a result of radiation sickness . Such people are called Hibakusha (explosive person) in Japan . (Estimates 1946: ~ 75,000 - 1950: ~ 140,000)
The next core for a Fat Man bomb was ready to be shipped to Tinian on August 13th. Other bombs were already on the island. The next bomb would have been operational between August 17th and 20th. After this third bomb, there would have been a short break in availability of about three weeks, after which a bomb core could have been delivered about every ten days. After the use against Nagasaki, however, President Truman had already decided not to use any further nuclear weapons against Japanese cities. At first it was unsure whether the bombings had the desired psychological effect. If Japan should not surrender after the two missions, it was decided to use the bombs in the tactical sense in the upcoming invasion against Japanese troops. However, Japan finally capitulated on August 17, 1945.
construction
Fat Man's implosion design had a bowl-shaped structure: In the middle was the neutron source ( 4 ) with a diameter of 2 cm, which was called the Urchin . The neutron source consisted of an inner solid beryllium sphere 0.8 cm in diameter and an outer beryllium shell 0.6 cm thick. Both together weighed about 7 g. On the inside of the beryllium shell, 15 wedge-shaped grooves, each 2.09 mm deep, were engraved. The surfaces of the inner beryllium ball and the outer beryllium shell were vapor-coated with nickel and then covered with a layer of gold. 11 mg 210 Po were applied to the 15 grooves . When the bomb was ignited, the inwardly directed shock wave ( 6 ) would also compress the composite beryllium sphere. The polonium in the grooves would be pressed into the beryllium and mixed with it. The alpha particles emitted by the polonium then hit the atomic nuclei of the beryllium, from which they beat neutrons, releasing a neutron every 5 to 10 ns, which triggers the chain reaction in the surrounding plutonium. The nickel-gold metal coating protected the beryllium from the alpha radiation of the polonium until the weapon was ignited, so that no neutrons are generated. The neutron source was placed in a frame in the surrounding plutonium core so that there was an approximately 0.25 cm gap between the two.
The plutonium core was a hollow sphere ( 5 ) with an outer diameter of 9.2 cm and an inner diameter of 2.5 cm. She weighed about 6.2 kg. The core consisted of two half-shells made of a δ-phase plutonium- gallium alloy. The half-shells were produced by hot pressing the plutonium-gallium alloy at 400 degrees Celsius and 200 MPa. The plutonium core, including the surrounding neutron reflector, corresponded to about 78% of the critical mass. After the implosion, the core reached more than double the original density, which corresponds to three to four critical masses.
Due to the high reactivity of the metallic plutonium, the half-shells had to be protected from corrosion. In the case of the core of the Trinity test, this was done with silver and later with gold. Between the two plutonium hemispheres was a thin foil made of corrugated gold. Later versions of the Mark 3 bomb, which were tested and introduced in late 1948, also had mixed cores of plutonium and highly enriched uranium or pure uranium cores.
A layer of depleted uranium ( 238 U) about 6.665 cm thick or 108 kg weight followed around the plutonium core ; ( 3 ). This served as a neutron reflector, so it threw back neutrons that left the plutonium nucleus during the chain reaction so that they could trigger further fission. Furthermore, the uranium shell was supposed to delay the expansion of the plutonium core after the chain reaction began, so that more time was available for the reaction to proceed. In addition, around 40% of the neutrons produced by the fission of plutonium were energetic enough to fission 238 U. The uranium shell also contributed directly to the bomb's energy release. Of the 22 kt explosive force of the first Fat-Man models, around 80% was released by the chain reaction taking place in the plutonium and 20% by the fission of 238 U by fast neutrons.
The uranium jacket was coated with a 0.32 cm thick layer of boron -containing plastic. This should absorb stray neutrons and thus reduce the risk of pre-ignition.
The outermost and thickest part was the multilayer implosion assembly ( 1 ) and ( 2 ) with a thickness of about 45 cm and a weight of about 2,400 kg. It again consisted of three layers. The outer layer was formed by the 32 explosive lenses, which interlocked like the fields of a soccer ball. The lenses consisted of an outer layer of high explosive material ( Composit B ) and an inner layer of low explosive material ( Baratol ). The interface between the two types of explosives was slightly curved, so that the lens effect was created, which created the spherical, inwardly directed shock wave. Another layer of composite B followed under the explosive lenses, which was supposed to strengthen the shock wave. Between the explosive and the uranium jacket inside the bomb there was an approximately 12 cm thick and 130 kg heavy layer of aluminum , which served to improve the properties of the implosion.
The 32 lens blocks were each ignited by a detonator that had been newly developed for Fat Man. This consisted of a special wire that explosively vaporized through a brief, powerful flow of electrical current. Heavy batteries, a strong voltage source and a corresponding capacitor bank were required for this. The entire ignition system weighed about 180 kg.
The entire spherical structure was placed in the egg-shaped bomb body. At the rear end of the bomb there were four radar antennas that were responsible for igniting the bomb at the planned height above the explosion site.
For the Trinity test as well as for the use against Nagasaki, the respective bombs had to be assembled from individual parts on site, which each took about two days. Later models of the Mark 3 design made it possible to make prefabricated bombs. In these, a segment of the onion-shaped structure could be removed so that the neutron source could be relatively easily inserted in the center of the bomb before use.
Data
| Weight | 4,670 kg (10,300 lbs ) |
| length | 3.66 m (12 ft ) |
| diameter | 1.52 m (60 in ) |
| Fissile material | 6.2 kg delta phase plutonium alloy ( mainly 239 Pu, very small amount of gallium ) |
| Neutron reflector | depleted uranium (mostly 238 U) |
| Neutron source (initiator) | Polonium - beryllium |
| Chemical explosives | Composition-B (60% Hexogen , 39% TNT ) Baratol ( TNT and Barium Nitrate ) |
| Detonator | Air pressure detonator Remote measurement detonator ( radar ) |
| Explosion energy | 22 ± 2 kT / 92 ± 8 terajoules |
| Explosion height | 500 ± 10 m (1650 ± 33 ft) |
Web links
- historylearningsite.co.uk - article about the atomic bombing (English)
- wpafb.af.mil - US Air Force Museum (English)
Individual evidence
- ↑ a b c d e f g h i j k l m n o p q r s t u v w x J. Baggott: Atomic - The first war of physics and the secret history of the atom bomb. 1939-1949. Icon Books, UK 2009, ISBN 978-1-84831-082-7 .
- ↑ a b c d e f g h i j k l m n o p q r s t u Nuclear Weapon Archive - Section 8.0 The First Nuclear Weapons
- ↑ a b c d Nuclear Weapons Archive - Trinity
- ^ Nuclear Weapon Archive - Section 5.0 Effects of Nuclear Explosions
- ↑ Original document - recording of a conversation between General Hull and Colonel Seaman (PDF; 126 kB)
- ^ R. Rhodes: Dark Sun - The Making of the Hydrogen Bomb. Simon & Schuster, 2005, ISBN 0-684-82414-0 .
- ^ Nuclear Weapon Archive - Operation Sandstone