After the rejection of the demand for unconditional surrender by the Japanse government in July of 1945, the United States President, Harry S. Truman, decided to drop an atomic bomb on the Japanese city of Hiroshima. During World War II, Hiroshima on the Japanse island of Honshu was home to several military bases and industrial facilities that supplied the Japanese war effort. There were headquarters of commanders of the Japanese home island defense force, assembly areas for troops, communication centers and other military targets. Five antiaircraft installations protected the city. There were some concreted buildings in the middle of the city but most of the homes, municipal buildings, barracks and factories were constructed of wood. The population of Hiroshima was estimated to be around three hundred and fifty thousand.

          On August 6, 1945, the Enola Gay B-29 commanded by Paul Tibbets took off from an airbase on Tinian, one of the islands in the Northern Marians. The Enola Gay rendezvoused with two other B-29 bombers over Iwo Jima and they headed Japan. After hours of flight, the squadron arrived in over Japan and Captain William Parsons armed the uranium gun-type bomb known as Little Boy.  The Japanese radar picked up the small fleet of American planes but did not activate air defenses for just a few planes. One of the accompanying planes flew over Hiroshima, reporting that the weather was clear and then turning out to sea.

          Tibbets in the Enola Gay began his bombing run at 8 AM. The bomb was dropped at 8:15 from a height of about thirty thousand feet. After falling for about forty seconds, the bomb exploded at a height of about two thousand feet above the ground. The Enola Gay managed to travel about ten miles before the shock wave hit. The bomb missed that target by eight hundred feet and detonated over a clinic with a force of about sixteen tons of TNT with less than two percent of the uranium undergoing nuclear fission. The radius of total destruction was about 1 mile with fires spread over a radius of about four miles. The Japanese estimated that about seventy percent of the buildings in Hiroshima were destroyed. Around seventy thousand people or about twenty percent of the population were killed in the blast and firestorm with another twenty percent being injured. It is estimated that eventually about half the population of the city died from the immediate effects of the bomb or from radiation related illnesses in the years following the attack.

          When communications with Hiroshima suddenly failed, the Japanese military were confused but they knew that no major air raid had taken place. They dispatched a plane to fly over Hiroshima to assess the situation. They saw a huge cloud of smoke of the devastated ruins of Hiroshima and were astonished by the destruction.   

          After the detonation of the first atomic bomb ever used in warfare, President Truman broadcast an announcement about the terrible new weapon that the United States possessed. He warned the Japanese that if they did not surrender immediately, they could expect “a rain of ruin from the air, the likes of which has never  been seen on this earth.”

        The Japanese government was still not ready to accept unconditional surrender. They were holding out for the following conditions:

• The preservation of the existing government
• Disarmament and demobilization to be carried out by the existing Japanese military
• No occupation of the Japanese islands
• Punishment for war crimes to be carried out by the Japanese government.

 

Hiroshima after the bombing from japanfocus.org:

          After the Japanese rejected the U.S. demand for unconditional surrender in the spring on 1945, Harry Truman, the U.S. President authorized the use of atomic weapons in Japan. The Manhattan project had produced two nuclear weapons by that time, one based on uranium known as Little Boy and the other based on plutonium named Fat Man.

           The goal for dropping an atomic bomb on a Japanese city was to force surrender according to the Potsdam Declaration. The psychological impact on the Japanese was especially important. In addition, the United States wanted other nations to recognize the importance of this new weapon when news of the devastation was made public.

          The choice of targets had three constraints.

• The target had to be more than three miles in diameter and had to be an important target in a large urban area.
• The detonation of the bomb would result in significant damage to the target
• The target was unlikely to be attacked by August of 1945.

           Four potential targets were initially selected that met the required criteria including Kokura, Hiroshima, Niigata and Kyoto. Kyoto was dropped from the list of targets because of it importance to Japanese religion. Nagasaki was added to the list as a potential target to replace Kyoto. Leaflets had previously been dropped on Japanese cities warning of possible bombing raids.

           Hiroshima was ultimately selected as a target for Little Boy. It was an important army depot and port of embarkation in the middle of an urban industrial area. Adjacent hills were expected to create a focusing effect to increase the damage of the blast. Nearby rivers made it a poor target for firebombing. Little Boy was dropped on Hiroshima on August 6, 1945 and exploded.

           Kokura was the first selection from the primary list as the target for the Fat Man bomb.  It was felt that the distance to Niigata was too great for a successful mission. Nagasaki had already been bombed a number of times and it was spread out over hills and valleys which could dissipate the force of a big blast. The bomber carrying the Fat Man bomb took off on August 9, 1945 with the intention of dropping the bomb on the city of Kokura. However, the day was cloudy and after three passes over the city without being able to sight the target, the mission to bomb Kokura was aborted. This was fortunate for a few thousand American and Dutch prisoners of war who were being held at Kokura.

            Nagasaki was chosen as a target only after the attack on Kokura was aborted. The bomber, low on fuel, turned south and headed for Nagasaki. The weather was clear and the Fat Man bomb was dropped and exploded. With the explosion of these two nuclear bombs in Japanese cities, the United States became the first and, to date, only country on earth to use nuclear weapons against another country.

              The Manhattan Project to create nuclear bomb during the early 1940s was successful. Two bombs were created, one based on the fission of uranium-235 called “Little Boy” and a second based on the fission of plutonium-239 called “Fat Man”. Upon completion of the bombs in the summer of 1945, the U.S. President, Harry Truman was faced with making the decision of whether such terribly destructive weapons should be deployed in World War II which had been raging for years in Europe and Asia. The war in Europe ended in May of 1945 but the war with Japan in the Pacific continued.

             The U.S. had driven the Japanese out of Okinawa and Iwo Jima but they still had an army of two million in their home islands. The U.S. had demanded an unconditional surrender from the Japanese government threatening “total destruction” if the Japanese refused. The Japanese did refuse but there were hints that they would consider a conditional surrender.

            Truman said that his decision to drop two atomic bombs on Japan was a purely military decision. It was estimated that if the United States staged an amphibious landing on the coasts of Japan over a million U.S. soldiers would be killed. Truman was dedicated to ending the war as quickly as possible to save both American and Japanese lives. Some of his advisors suggested a demonstration of the power of the atomic bomb to persuade the Japanese military of the futility of continuing the conflict. Truman decided against it because there was no guarantee that a successful test would end the war and an unsuccessful test might embolden the Japanese to keep fighting.

           Some critics claimed that Truman’s decision to drop the atomic bombs on Japan were motivated by racism and that he would never have dropped such bombs on Europeans. Other critics suggested that Truman wanted to end the war quickly so that the Russians would not have an opportunity to invade the heavily industrialized northern Japanese islands. Considering what happened in Eastern Europe after the Russians beat back the Germans, this may have been a realistic fear. Other say that Truman wanted to intimidate the Russians and that the explosions of the atomic bombs were really the opening shots in the Cold War.

          In August 6 of 1945, Little Boy was dropped on Hiroshima, Japan and Fat Man was dropped on Nagasaki, Japan on August 9. On August  14, the Supreme War Council of Japan surrendered unconditionally. Although great destruction had been wrought by firebombing raids over Tokyo, the sheer fury of the atomic explosions and the fact that all that destructive potential was delivered in a pair of relatively small bombs.

         Truman succeeded in ending the war in Asia with the use of the atomic bombs. He also hurled the human race into the nuclear arms race and the threat of the annihilation of human civilization that continues to this day.

President Harry S. Truman:

             On July 26, the United States, Britain and China produced the Potsdam Declaration which contained the terms that they demanded for the surrender of Japan. The Declaration required that Japan surrender immediately and without conditions. The surrender terms included:

• the elimination “for all time [of] the authority and influence of those who have deceived and misled the people of Japan into embarking on world conquest”

 

• the occupation of “points in Japanese territory to be designated by the Allies”

 

• “Japanese sovereignty shall be limited to the islands of Honshu, Hokkaido, Kyushu, Shikoku and such minor islands as we determine.” As had been announced in the Cairo Declaration in 1943.

 

• “The Japanese military forces shall be completely disarmed”

 

• “stern justice shall be meted out to all war criminals, including those who have visited cruelties upon our prisoners”

The Declaration also included some clauses intended to reassure the Japanese people:

• “We do not intend that the Japanese shall be enslaved as a race or destroyed as a nation, … The Japanese Government shall remove all obstacles to the revival and strengthening of democratic tendencies among the Japanese people. Freedom of speech, of religion, and of thought, as well as respect for the fundamental human rights shall be established.”

 

• “Japan shall be permitted to maintain such industries as will sustain her economy and permit the exaction of just reparations in kind, but not those which would enable her to rearm for war. To this end, access to, as distinguished from control of, raw materials shall be permitted. Eventual Japanese participation in world trade relations shall be permitted.”

 

• “The occupying forces of the Allies shall be withdrawn from Japan as soon as these objectives have been accomplished and there has been established, in accordance with the freely expressed will of the Japanese people, a peacefully inclined and responsible government.”

 

At the end of the Declaration, came the demand for unconditional surrender and the penalty for refusal.

• “We call upon the government of Japan to proclaim now the unconditional surrender of all Japanese armed forces, and to provide proper and adequate assurances of their good faith in such action. The alternative for Japan is prompt and utter destruction.”

One thing that was left out of the Declaration was the status and ultimate of the Emperor who the Japanese people considered to be divine. The question of whether he would be declared a war criminal and prosecuted or would be left free to become part of the future government was left open.

The United States made radio broadcasts to the Japanese home islands and dropped leaflets over population centers in Japan. Despite laws against listening to foreign radio broadcasts or reading dropped leaflets, most of the Japanese people became aware of the terms of the Potsdam Declaration.

The Japanese Supreme War Council rejected the Potsdam Declaration and a government spokesman met with the Japanese press to tell the Japanese people that the Declaration had been rejected and that Japan would fight on.

Potsdam Conference:

            After the successful test of a plutonium implosion device in July of 1945 at the Trinity site near Alamogordo, New Mexico, the Manhattan Project proceeded with the design and construction of a plutonium bomb to be used as a weapon. The code name for the device was “Fat Man”, homage to one of the characters in the movie “The Maltese Falcon.” The name came to be used for the whole class of nuclear bombs based on the same design.

           The original Fat Man bomb was about ten feet long and about five feet in diameter. It weighed over ten thousand pounds. As with the Gadget, the device tested at Trinity, a subcritical sphere of plutonium was surrounded by both fast and slow explosives. When the explosives were triggered, they would compress the plutonium into a supercritical mass which would then explode in a nuclear blast of enormous power.

           The bomb was designed so that the plutonium pit could be inserted as late as possible. A Duralumin sphere of about one and one half feet was constructed with a hole left to insert the pit. Inside the aluminum sphere was a nine inch sphere of U-238 with a thin boron shell. The U-238 sphere had a five inch cylindrical hole running through it. When it was time to arm the bomb, a cylinder of U-238 with a three and one half inch sphere of plutonium was inserted into the spherical assembly. At the center of the plutonium sphere was a polonium-beryllium initiator that would give off a burst of neutrons when triggered. An outer shell of specially shaped charges of fast conventional explosives surrounded an inner shell of slow conventional explosives which contained the Duralumin sphere. On the outside of the shell of charges were arranged detonators in a precise configuration. The charges had to be triggered in just the right way to compress the plutonium correctly.

           The completed bomb was housed in a steel container that also contained four contact fuzes in the nose. This type of fuse triggers the bomb when it hits a hard solid surface. There was also a radar altimeter, batteries to power the detonators for the conventional explosives, baroswitches that trigger based on air pressure and timers. The whole assembly was to be housed in a steel teardrop shaped container that had a boxy tail piece to assure that the bomb would fall nose down when released. In August of 1945, all the components were sent to Tinian Island in the South Pacific and assemble for deployment.

            This was the third nuclear bomb to be constructed and the second bomb to be used in warfare. Later generations of bombs were based on the Fat Man design with additional features to make them more reliable, safer, more efficient, etc. When the Soviet Union built and tested its first nuclear bomb on August 29, 1949, the design was  based on the plans for the Fat Man bomb which had been stolen by spies from the United States.

           While the Manhattan Project was developing plutonium production facilities and producing plutonium at Hanford, Washington in the early 1940s, the Project was also working on the design of a bomb that would utilize the plutonium. It turned out that gun-type design being worked on for a uranium bomb would not work for a plutonium bomb. Plutonium-239 was being produced in reactors but the reactors were also creating plutonium-240 as well. P-240 spontaneously fissions and produces neutrons. In a gun-type bomb, these extra neutrons would cause the bomb to explode early before a full critical mass of plutonium was formed. The resulting fizzle blast would be much weaker than a full nuclear explosion.

           After ruling out the gun-type bomb design, work was begun in 1944 at Los Alamos, New Mexico n a new implosion bomb design. The design was based on a sphere of plutonium with a neutron initiator at its core. The sphere was to be surrounded by conventional explosives that had different burn rates. When arranged properly and triggered in the right time sequence, the explosives would create a compression wave focused inward. This compression wave would compact the plutonium sphere into a smaller sphere that was much denser than the original sphere. Because the critical mass is a function of density in a volume, the amount of plutonium would be a critical mass in the smaller sphere. The neutron initiator was included to insure that the reaction triggered properly.

          On benefit of this design was that it required much less plutonium than the amount of uranium needed for a gun-type bomb. The implosion design only required about fourteen pounds of plutonium. The new design was very complex and pushed the state of the art for creating a compression effect. While the gun-type design was simple and reliable enough that it was not felt that a test was needed, the implosion design was so new and difficult that it was decided to create and test such a bomb before deploying it as a weapon. The test bomb was code named “the Gadget.”

          The Gadget was constructed and a new test site was created in New Mexico near Alamogordo during the first half of 1945. Laboratory leader J. Robert Oppenheimer named the site Trinity in reference to a poem by John Dunne. A one hundred foot steel tower was constructed for the test to simulate the air burst of an actual bomb to maximize effect. The components were assembled in July of 1945.

          Early on July 14^th, 1945, the Gadget was detonated in a blast equivalent to twenty kilotons of TNT. The blast created a crater of radioactive glass below the tower. The shock wave was felt over one hundred miles away. The mushroom cloud towered to about eight miles in the sky. The very first nuclear blast ever created by the human race lit up the surround mountains brighter than the sun and awed everyone who directly witnessed it. There were a number of reports in the area of a huge bright explosion which was explained as the explosion of an ammunition magazine to the media. Oppenheimer later remarked that he was reminded of a passage in the Hindu Bhagavad Gita; “Now I am become Death, the destroyer of worlds.”

This is a photograph of the Trinity fireball sixteen milliseconds after detonation:

         In the summer of 1945, the United States Manhattan Project to create an atomic bomb delivered  one hundred and ten pounds of uranium enriched to 89% U-235 too the Los Alamos testing grounds in New Mexico.

            A design had been developed for a uranium bomb. The bomb consisted of a tube that was ten feet long and about two feet in diameter. At one end of the tube there were bags of cordite explosive behind a hollow cylinder of stacked uranium rings. At the other end of the cylinder, there was another smaller cylinder of uranium rings stacked on a steel rod. The smaller cylinder would fit snugly inside the hollow cylinder and it had a polonium-beryllium neutron initiator behind it.  Both cylinders of uranium were surrounded by tungsten carbide to reflect neutrons.  

           When the explosive was fired, it would propel the hollow uranium cylinder along the tube to engulf the smaller uranium cylinder at the other end of the tube, creating a critical mass of uranium that could undergo fission. The neutron initiator release a burst of neutron when the bomb was triggered. The neutron burst would cause a runaway chain reaction in the uranium and an explosion would result. The actual explosive power of the bomb was highly dependent on the way that the critical mass was configured. If only one percent of the uranium in t he bomb fissioned, it would create an explosion equal to thousands of tons of TNT. On the other hand, if the configuration was off or the impact of the two pieces of uranium too slow, the bomb would just explode with the power of a few tons of TNT.

          The components of the bomb were assembled in the winter of 1945 before the uranium was available. It was decided that the design was so reliable that it was not necessary to actual blow up such a bomb to test it. There was a lot of work in the laboratory however to verify that the design concepts were correct. When the uranium arrived in the summer of 1945, the bomb was assembled and ready for use. It was named “Little Boy”. The name was in contrast to a “Thin Man” alternative design which would have been seventeen feet long. The “Thin Man” name came from the detective novels of Dashiell Hammett. It turned out that it was not necessary to make such a long bomb.

         The Little Boy gun design was only used once as a weapon. It was a very reliable design from the point of view of detonation but was not very safe. If the plane crashed, the two uranium charges could be driven together and explode. An electrical short circuit could trigger it. If a plane crashed in water, it could be triggered by water getting into the detonator. And, in water, the uranium charges would be subjected to a moderator effect leading to fission. Later nuclear bombs incorporated more safety features.

           While the U.S. Manhattan project refined and enriched uranium for an atomic bomb in the early 1940, a parallel project was carried out to create plutonium. Plutonium is very rare in nature so it was necessary to create nuclear reactor in which plutonium could be generated by injecting neutrons into a mass of uranium. Most naturally occurring uranium is the U-238 isotope. When U-238 is bombarded with neutrons, some of it is converted to U-239 by absorption of a neutron. The U-239 immediately decays into neptunium-239 by a neutron emitting an electron which leaves behind a new proton. This process occurs again to create plutonium-239. A very small amount of plutonium results from this process and it must be chemically separated from the unconverted uranium and purified.

          In March of 1943, the air cooled X-10 Graphite Reactor was built at the Oak Ridge facility in Tennessee. It consisted of a huge block of graphite that measured twenty four feet on each side. That cube was encased in seven feet of dense concrete as a radiation shield. There were initial problems with finding a way to encase the uranium slugs with a sealed metal shell to prevent corrosion and release of fission products. Several different approaches were tried and ultimately aluminum cans were welded with new techniques were developed. About thirty six tons of uranium were fed into the new reactor and half a gram of plutonium was created within the first month of operation. The reactor continued to produce plutonium for the Manhattan Project for the next year and was retired early in 1945.

          While the X-10 was in production, work proceeded at the Hanford facility in Washington on the more advanced water cooled Reactor B that would be water cooled. Six reactors were planned altogether. They were housed in buildings that were one hundred and twenty eight feet high. A total of eight hundred and thirty eight uranium slugs were inserted into Reactor B in mid-September of 1944. In late September, after the reactor had gone critical and fission had begun, the control rods were withdrawn to begin plutonium production. The reactor ran for a while but then the power level dropped and the reactor stopped. It turned out that Xenon-135 being produced during the reactor operation was poisoning the reaction by absorbing neutrons. By loading all two thousand tubes in the reactor, proper functioning was achieved and plutonium could be produced.

         Chemists had been working on the problem of how to separate the plutonium from the uranium. Little was known about the chemical properties of plutonium so a lot of basic research was necessary. A process was developed that involved bismuth phosphate that allowed precipitation of plutonium or precipitation of the uranium and impurities from solution. The separation plants consisted of four different buildings which housed a process cell, a concentration building, a purification building and a magazine store. Construction began in April of 1944 at Hanford before final choice of a processing method. New methods of remote control had to be developed to deal with the radioactive materials going through the sequence of buildings. In February of 1945, the first shipment of about two and one half ounces of ninety five percent pure plutonium was sent to the Los Alamos facility in New Mexico.

X-10 Graphite Reactor:

           After the Japanese attacked Pearl Harbor in December of 1942, the Manhattan project ramped up with millions of dollar and thousands of staff. Four major deposits of uranium ore had already been identified and efforts were being made to obtain ore from the three that were in Allied hands. In November of 1942 it had been determined that there should be sufficient ore available to produce an atomic weapon. Ore from the Belgian Congo, Ontario, Canada and a mine in Colorado was being collected during 1942.

          The ore was dissolved in nitric acid to produce uranyl nitrate. Ether was added to the solution to remove impurities. The solution was then heated to produce uranium trioxide which was ultimate reduced to pure uranium dioxide. The Ames process was developed to produce pure uranium metal after other methods failed. Uranium dioxide was converted to uranium tetraflouride which was mixed with powdered magnesium and heated in a sealed metal tube to produce pure uranium metal.

          The big problem was separating the U-235 isotope from U-238. Only .7 % of uranium ore is U-235. It was estimated that the percentage of U-235 needed to create a bomb was around 90%. Research and development of separation methods proceeded during the early 1940s.

           Converting uranium to uranium hexafluoride gas was a necessary first step in isotope separation. In a centrifuge, gas with the lighter isotope would move further than the heavier isotope. Feeding the output of one centrifuge into the next would theoretically allow the needed enrichment. Unfortunately, attempts to use centrifuges for separation were unsuccessful due to technical problems with running the big centrifuges at very high speeds for extended periods of time.  

           Electromagnetic separation was a known technology which used magnetic fields to deflect charged particles based on mass. Copper was in short supply so tons of silver were used instead to build production systems. Despite mechanical problems and efficiency, this process was used to enrich uranium to 15% U-235.  

           The third process was based on the idea that gases of different molecular weight will pass through membranes at different rates. With one chamber feeding the next, a cascade of these cells could enrich uranium hexafluoride gas up to 7% U-235. This resultant product could be used to feed other processes such as the electromagnetic separation system.

           The final process developed was referred to as thermal diffusion. When there is a vertical thermal gradient in a chamber full of a mixture of gases, the heavier gas will concentrate in the lower cooler part of the chamber and the lighter gas will collect in the warmer upper part of the chamber. This was a new idea and was not part of the original attempts to separate isotopes of uranium.  Fifty foot columns with three tubes were constructed. Steam and water created the thermal gradient. This process was able to enrich uranium from .7% U-235 to .9% U-235.

          In 1945, all three of these processes were used in series to enrich uranium. The thermal diffusion plant enriched uranium from .7% U-235 to .9% U-235. This was fed to the gaseous diffusion plant where the enrichment reached 23% U-235. The gaseous diffusion plant fed the electromagnetic separator which enriched to 89% U-235 which was sufficient for weapons production.

Electromagnetic isotope separation “racetrack”:

       The Manhattan Project was started in 1939 by the US Government to explore the military potential of uranium. The knowledge that the Germans were working on nuclear weapons research at the same time spurred the creation of the program. It started with a modest budget and a small group of researchers. In the meantime, Brittan was also pursuing nuclear research and verified in 1939 that fifty pounds of uranium could be made into a bomb that could be carried in a conventional bomber.

           World War II began in 1939 with the German invasion of Poland. Brittan created their own atomic bomb project in 1940. Information on their research was forwarded to the United States. Having received no reaction to their information, a member of the project flew to the United States and visited physicists doing nuclear research.

           In 1941, the President moved forward to create a major project dedicated to creating an atomic bomb as quickly as possible. The attack on Pearl Harbor in December of 1941 and the declaration of war with Japan and Germany added urgency to the project. The Manhattan Project was charged in 1942 with the task of developing the infrastructure necessary to building an atomic bomb.

           Millions of dollars were allocated and the project was carried out at a number of different sites around the United States. The first major problem that had to be solved was how to separate the U-235 isotope from uranium ore which consisted mostly of U-238. Three different techniques were pursued. Research was also done on the newly discovered radioactive element plutonium as a possible alternative to U-235. Finally, graphite was investigated as a possible moderator to control nuclear chain reactions. The Germans were also working on graphite moderation but their graphite rods contained boron which reduced their effectiveness. U.S. researchers created rods without boron and they worked as expected to control the reaction.

            Many physicists worked on theoretical problems of neutron diffusion and possible designs for an atomic bomb. The main approach was to create a sphere of U-235 which would exceed the critical mass and result in an explosion. The problem was how to arrange the parts of the bomb so that the sphere was created when the bomb was triggered. One idea was to shoot a plug into a subcritical sphere. Another was to use shaped charges to slam segments of a sphere together. When the critical mass was achieved, a runaway chain reaction in the U-235 would cause it to fission, creating a huge explosion.

           Physicist Edward Teller proposed a more powerful nuclear bomb which he called a hydrogen bomb. His idea was to use an atomic bomb as a trigger to cause deuterium and tritium to undergo nuclear fusion. The resulting explosion would be much more powerful than that atomic trigger. Teller pushed hard to build such a bomb but all his proposals were turned down in favor of creating a fission bomb.