CDS (II) 2020 General Knowledge · Question 55 of 120

Correct — A, The fission bomb dropped at Hiroshima. Little Boy was the code name of the gun-type weapon carried by the aircraft Enola Gay and released over Hiroshima on 6 August 1945. It worked by nuclear fission of highly enriched uranium: one sub-critical mass of uranium-235 was fired down a gun barrel into another, and the combined mass went supercritical. Its counterpart three days later was Fat Man, an implosion device using plutonium, dropped on Nagasaki on 9 August 1945 — implosion was needed there because the plutonium made in the Hanford reactors carried too much plutonium-240 for a gun assembly to work. Both were fission weapons; fusion weapons came several years later.

• (b)The fusion bomb dropped at Nagasaki — Wrong on both counts. The Nagasaki weapon was Fat Man, not Little Boy, and it was a plutonium implosion fission bomb. No fusion weapon existed in 1945.
• (c)The first nuclear bomb tested by America — That was the Trinity test at Alamogordo in New Mexico at 05:30 on 16 July 1945, a plutonium implosion device of about twenty kilotons. The gun-type uranium design was considered reliable enough to be used without a test.
• (d)The first nuclear bomb tested by North Korea — North Korea's first nuclear test came in October 2006, six decades later, and had no connection to the American programme or its code names.

Fission splits a heavy nucleus such as uranium-235 or plutonium-239 into lighter fragments, releasing energy and further neutrons that sustain a chain reaction. Fusion joins light nuclei, typically isotopes of hydrogen, and needs the temperatures a fission explosion supplies, which is why every thermonuclear weapon has a fission stage first. The Manhattan Project, run from 1942 to 1946 under Major General Leslie Groves with J. Robert Oppenheimer directing the Los Alamos laboratory, produced both of the 1945 designs.

The item is really a check on whether the fission and fusion labels are attached to the right names. Little Boy goes with uranium, gun-type, Hiroshima and 6 August; Fat Man with plutonium, implosion, Nagasaki and 9 August; Trinity with the first test, New Mexico, 16 July. Note that the four options are constructed so that each wrong one is wrong in a different way — wrong weapon and wrong physics, wrong event, wrong country. CDS came back to the same physics in 2023, asking which of the hydrogen bomb and the uranium bomb rests on fusion and which on fission.

• Little Boy was a gun-type weapon using highly enriched uranium-235, dropped on Hiroshima on 6 August 1945 by the Enola Gay.
• Fat Man was a plutonium implosion weapon, dropped on Nagasaki on 9 August 1945 by the aircraft Bockscar.
• Plutonium from the Hanford reactors carried too much plutonium-240 for a gun-type design, which is why implosion was developed.
• The Trinity test, on 16 July 1945 at Alamogordo, New Mexico, was the first nuclear test and used the implosion design.
• The Manhattan Project ran from 1942 to 1946; Leslie Groves directed it and Oppenheimer led the Los Alamos laboratory.

• Assuming the more destructive-sounding label must mean fusion; both 1945 weapons were fission devices.
• Swapping Little Boy and Fat Man, or their cities and dates.
• Treating Hiroshima as the first nuclear explosion; the Trinity test came three weeks earlier.

As a name-identification item like this one, as a fission-or-fusion pairing, or through the Manhattan Project and its scientists.

• practice — not a real PYQ

  The first nuclear device ever exploded, the Trinity test of 16 July 1945, used which fissile material?

  • (a)Uranium-235
  • (b)Plutonium-239
  • (c)Uranium-238
  • (d)Thorium-232

  Answer(b) Plutonium-239 — assembled by implosion, the same design later used in Fat Man.

• practice — not a real PYQ

  A thermonuclear or hydrogen bomb requires a fission device as its first stage because

  • (a)fission produces the neutrons that fuse
  • (b)fusion needs the extremely high temperature a fission explosion provides
  • (c)fusion fuels are radioactive and need shielding
  • (d)fission slows the reaction to a controllable rate

  Answer(b) fusion needs the extremely high temperature a fission explosion provides — light nuclei will not fuse until that barrier is overcome.