July 25, 1946: First Underwater Atomic Explosion

Today marks the sixtieth anniversary of the world's first underwater atomic explosion. During the summer of 1946, the United States military conducted a series of nuclear tests near Bikini Atoll in the Marshall Islands to determine the effects of nuclear weapons on ships, military equipment, and ammunition. Operation Crossroads, the first atomic blasts since Hiroshima and Nagasaki (and the first atomic test since Trinity), detonated two 21-kiloton fission bombs near a fleet of abandoned naval vessels in Bikini Lagoon. The target fleet consisted of an aircraft carrier, cruisers, destroyers, submarines and auxiliary craft; as well as captured German and Japanese ships from World War II. After relocating the residents of Bikini Atoll to an adjacent island, the U.S. Navy moved a support fleet of more than 150 ships and 42,000 military personnel away from the target area by 10 nautical miles. According to Richard V. Fisher, a sailor who monitored the targeted ships, "The Army Air Force intended to sink them with bombs; the Navy expected the Air Force to fail".

Operation Crossroads began on July 1 with the ABLE shot, an atmospheric test in which an atomic bomb was dropped from a B-29 Superfortress and detonated at an altitude of 520 feet. Although the blast destroyed five ships, a problem with the bomb's tail stabilizer caused the weapon to miss its target, a battleship painted orange, by nearly 2,000 feet. According to the Naval Historical Center, the radioactivity from ABLE had "only a transient effect", and crews boarded all of the surviving target ships by the end of the following day. After inspecting and re-mooring the damaged ships, members of Joint Task Force 1 (JTF 1) evacuated the area to prepare for the next nuclear test. The BAKER shot would demonstrate the power of an underwater atomic explosion and ultimately cause the Navy to cancel the rest of Operation Crossroads. On the morning of July 25, 1946, landing craft LSM-60 lay anchored in Bikini Lagoon and tethered to an atomic bomb. Shortly after 8:30 A.M., the BAKER test detonated the weapon inside a concrete container submerged some 90 feet underwater. As a result of the blast, a foamy column of seawater streaked skyward, rising thousands of feet above the lagoon. A white wave moved outward from the column's base while a cloud of steam rose from the column's heights. According to naval observer Richard V. Fisher, the collapse of the column caused a "circle of mist" to move across the lagoon "like a strong wind", engulfing all of the target ships in a dense, radioactive fog. Fifteen minutes later, the fog dissipated; however, the mist that remained was so thick that members of JTF 1 could not see the target fleet. Later, the U.S. Navy determined that the BAKER test sank a total of eight ships – three more than the ALPHA shot.

Although the military was anxious to study the effects of BAKER, extensive radiological contamination prevented scientific crews from performing more than brief on-board activities. In early August, Col. Stafford Warren ordered sailors to wash the target ships' exteriors with water drawn from outside the lagoon; however, the support fleet's hulls soon became contaminated with low-level radioactivity. In mid-August, Stafford halted decontamination operations and received permission to tow the target fleet to Kwajalein Atoll, where work could continue in uncontaminated waters. Although 12 of the target ships were eventually re-manned, the remaining vessels were scuttled in the Pacific Ocean. All of the support ships were decontaminated and returned to the fleet. According to the Naval Historical Center, this process "required a great deal of experimentation and learning at naval shipyards."

Col. Stafford canceled Operation Crossroads before a third and final test, CHARLIE, could demonstrate the effects of using a nuclear weapon as a depth charge. Although the Navy sought to limit sailors' exposure to more than 0.1 roentgen (R) per day, the maximum accumulated exposure reached 3.72 R. According to Richard V. Fisher, the support fleet distilled drinking water from the lagoon soon after BAKER. "We even took seawater showers in it and were bathed in the spray when traveling in small boats from ship to ship." Ed Alton, a member of the Navy's Radiological Control Section, claimed that a Geiger counter of that era "could only measure gamma radiation with any degree of reliability; its measurement of beta radiation was misleading; and worst of all the X-263 or Geiger Counter, had no ability to measure plutonium's deadly alpha emissions which erupted with the Baker Test."

Resources:

http://en.wikipedia.org/wiki/Operation_Crossroads

http://en.wikipedia.org/wiki/Pacific_Proving_Grounds

http://www.history.navy.mil/faqs/faq76-1.htm

http://www.aracnet.com/~pdxavets/fisher1.htm

http://www.aracnet.com/~pdxavets/parson1.htm

http://www.bikiniatoll.com/divetour1.html

http://en.wikipedia.org/wiki/Nuclear_test