Operation Sandstone


Operation Sandstone was a series of nuclear weapon tests in 1948. It was the third series of American tests, following Trinity in 1945 and Crossroads in 1946, and preceding Ranger. Like the Crossroads tests, the Sandstone tests were carried out at the Pacific Proving Grounds, although at Enewetak Atoll rather than Bikini Atoll. They differed from Crossroads in that they were conducted by the Atomic Energy Commission, with the armed forces having only a supporting role. The purpose of the Sandstone tests was also different: they were primarily tests of new bomb designs, especially the more efficient levitated pits, rather than of the effects of nuclear weapons. Three tests were carried out in April and May 1948 by Joint Task Force 7, with a work force of 10,366 personnel, of whom 9,890 were military.
The successful testing of the new cores in the Operation Sandstone tests rendered every component of the old weapons obsolete. Even before the third test had been carried out, production of the old cores was halted, and all effort concentrated on the new Mark 4 nuclear bomb, which would become the first mass-produced nuclear weapon. More efficient use of fissionable material as a result of Operation Sandstone would increase the U.S. nuclear stockpile from 56 bombs in June 1948 to 169 in June 1949.

Origins

s were developed during World War II by the Manhattan Project, which created a network of production facilities, and the weapons research and design laboratory at the Los Alamos National Laboratory. Two types of bombs were developed: the Mark 1 Little Boy, a gun-type fission weapon using uranium-235, and the Mark 3 Fat Man, an implosion-type nuclear weapon using plutonium.
These weapons were not far removed from their laboratory origins. A great deal of work remained to improve ease of assembly, safety, reliability and storage before they were ready for production. There were also many improvements to their performance that had been suggested or recommended during the war that had not been possible under the pressure of wartime development. Norris Bradbury, who replaced Robert Oppenheimer as director at Los Alamos, felt that "we had, to put it bluntly, lousy bombs."
Plutonium was produced by irradiating uranium-238 in three 250 MW nuclear reactors at the Hanford site. In theory they could produce of plutonium per megawatt-day, or about per month. In practice, production never approached such a level in 1945, when only between was produced per month. A Fat Man core required about of plutonium, of which 21% fissioned. Plutonium production fell off during 1946 due to swelling of the reactors' graphite neutron moderators. This is known as the Wigner effect, after its discoverer, the Manhattan Project scientist Eugene Wigner.
These reactors were also required for the production of polonium-210, which was used in the initiators, a critical component of the nuclear weapons. Some of bismuth-209 had to be irradiated for 100 days to produce 600 curies of polonium-210, a little over. Because polonium-210 has a half-life of only 138 days, at least one reactor had to be kept running. The oldest unit, B pile, was therefore closed down so that it would be available in the future. Investigation of the problem would take most of 1946 before a fix was found.
Uranium-235 was derived from enrichment of natural uranium at the Y-12 plant and K-25 site in Oak Ridge, Tennessee. Improvements in the processes and procedures of the electromagnetic and gaseous isotope separation between October 1945 and June 1946 led to an increase in production to around of uranium-235 per month, which was only enough for one of the very wasteful Little Boys. A Fat Man was 17.5 times as efficient as a Little Boy, but a ton of uranium ore could yield eight times as much uranium-235 as plutonium, and on a per-gram basis, plutonium cost somewhere between four and eight times as much to produce as uranium-235, which at this time cost around $26 per gram.

Weapon development 1945–1948

The objectives of the Sandstone series of tests were to:
  1. Test nuclear cores and initiators;
  2. Improve the theory and knowledge of implosion type weapons;
  3. Test levitated cores;
  4. Test composite cores; and
  5. Determine the most economic designs in terms of efficient use of fissionable material.
Levitation meant that instead of being immediately inside the tamper, there would be an air gap between the tamper and the core, which would be suspended inside on wires. This would allow the tamper to gain more momentum before striking the core. The principle was similar to swinging a hammer at a nail versus putting the hammerhead directly on the nail and pushing as hard as possible. In order for this to work outside the laboratory, the wires had to be strong enough to withstand being dropped from an aircraft, but thin enough to not disturb the spherical symmetry of the implosion. The Theoretical Division at Los Alamos, known as T Division, had run computer calculations on the levitated core as early as March 1945. The use of the levitated core had been proposed during the planning for Operation Crossroads, but it had been decided instead to use the existing solid core "Christy" design. This was named after its designer, Robert Christy. For Sandstone, however, it was decided that at least two of the three tests would use levitated cores.
The motivation behind the composite core was to make better use of the available fissionable material. The use of uranium-235 in an implosion weapon instead of the inefficient gun type Little Boy was an obvious development. However, while plutonium was more expensive and harder to produce than uranium-235, it fissions faster, because it makes better use of the neutrons its fission produces. On the other hand, the slower reaction of uranium-235 permits the assembly of super-critical masses, making it theoretically possible to produce weapons with high yields. By July 1945, Oppenheimer and Groves were considering using both materials in a composite core containing of plutonium and of uranium-235. The composite cores became available in 1946. Los Alamos' priority then became the development of an all-uranium-235 core. By January 1948 the national stockpile contained 50 cores, of which 36 were composite Christy cores, nine were plutonium Christy cores, and five were composite levitated cores. Testing the new levitated, composite and uranium-235 cores would require at least three test firings.
More efficient weapons would require less efficient initiators. This meant that less polonium would be required. At the time of Sandstone, the national stockpile of polonium-beryllium initiators consisted of 50 A-Class initiators, with more than 25 curies of polonium, and 13 B-Class initiators with 12 to 25 curies. During Sandstone, at least one test would be conducted with a B-Class initiator.

Preparations

Organization

The tests were authorized by President Harry S. Truman on June 27, 1947. The Atomic Energy Commission's Director of Military Applications, Brigadier General James McCormack and his deputy, Captain James S. Russell, met with Bradbury and John Henry Manley at Los Alamos on July 9 to make arrangements for the tests. They readily agreed that they would be scientific in nature, with Los Alamos supplying the technical direction and the armed forces providing supplies and logistical support. The cost of the tests, around $20 million, was divided between the Department of Defense and the Atomic Energy Commission. Lieutenant General John E. Hull was designated as test commander. Rear Admiral William S. Parsons and Major General William E. Kepner reprised their Operation Crossroads roles as deputy commanders. Joint Task Force 7 was formally activated on October 18, 1947. As its commander, Hull was answerable to both the Joint Chiefs of Staff and the Atomic Energy Commission.
Joint Task Force 7 consisted of 10,366 personnel, 9,890 of them military. Its headquarters consisted of about 175 men, of whom 96 were on board the . The rest were accommodated on the, and. A special division of the Los Alamos National Laboratory, known as J Division, was created specifically to manage nuclear testing. An Atomic Energy Commission group was responsible for preparing and detonating the nuclear weapons, and conducting the experiments. It consisted of some 283 scientists and technicians responsible for nuclear tests from J Division, the Armed Forces Special Weapons Project, the Naval Research Laboratory, the Naval Ordnance Laboratory, Argonne National Laboratory, the Aberdeen Proving Ground, the Atomic Energy Commission, Edgerton, Germeshausen & Grier, and other agencies.
Each dealt with a different aspect of the tests. The Naval Ordnance Laboratory handled the blast measurement tests, while the Naval Research Laboratory conducted the radiation measurement experiments, and Argonne National Laboratory did gamma ray measurements. Edgerton, Germeshausen, and Grier were contractors hired to design and install the timing and firing systems. Seven experimental weapon assemblies and six cores were delivered to San Pedro, California, and loaded on the weapon assembly ship, in February 1948, but the Atomic Energy Commission only gave permission for the expenditure of three cores in the tests.

Ships

The naval forces were organized as Task Group 7.3. It consisted of:
;Task Unit 7.3.1
;Task Unit 7.3.2 Main Naval Task Unit
  • USS LST-45
  • USS LST-219
  • USS LST-611
; Task Unit 7.3.3 Offshore Patrol
; Task Unit 7.3.4 Helicopter Unit
; Task Unit 7.3.5 Services Unit
  • USS YOG-64
  • USS YW-94
;Task Unit 7.3.6 Cable Unit
  • USS LSM-250
  • USS LSM-378
  • Naval Signal Unit No. 1
; Task Unit 7.3.7 Boat Pool Unit
  • USS LCI-549
  • USS LCI-1054
  • USS LCI-1090
  • USS LCI-472
  • USS LCI-494
  • USS LCI-l194
  • USS LCI-1345
Source: Berkhouse et al, Operation Sandstone, p. 40