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Underwater Tests
Underwater tests were conducted to assess the damages to Navy boats and materials. The location for these tests was chosen to be Enewetak. This location was chosen due to the uniformity of the bottom of the sea around Enewetak. This is critical for the tests so that proper moorings of the target ships can be secured on the sea floor. The underwater explosions create a bubble from the expended energy of the blast. This bubble is due to the vaporization of water that directly absorbs the heat of the blast. This bubble may break the surface depending on how much energy is dispersed and the depth of the nuclear device. The bubble displaces large amounts of water that then collapses in on the bubble after all the energy is expended. The water collapsing in on the cavity is call the “radioactive pool” and has the highest concentration of radioactive material.

Many different projects were put in place to test the two underwater blasts. Wave generation, hydrodynamic variables, and energy released were all studied using multiple sensors. These sensors were placed in multiple areas such as target ships and floating balloons. Oceanographic, seismographic and hydrographic surveys were completed after each nuclear blast was completed to further the data. A project that used a nuclear blast to clear minefields was studied using the Umbrella blast. 120 inert mines were placed at different distances ranging from 1,500 feet to 8,000. These mines were then picked out of the water to study the effects of the blast at each interval.

Radiation Preparations
Another reason for the underwater tests was to detect the radiation contamination of the ships after an underwater nuclear explosion. This meant that the contaminated ships required unique “radsafe” preparations so that the data could be quickly accounted for. After the data on the damage and radiation was recorded, the ships needed decontamination work completed before repairs could be completed. The Radiological and Decontamination Unit consisting of 200 enlisted and one officer was created for task of decontamination. After decontamination, the target ships were repaired for use in a second underwater nuclear weapons test. The amount of radiation exposure was debated for the safety of the crews. Roentgens (R) were used to gage the gamma radiation that affected the units working on the nuclear tests. For the underwater test crew, a limit of 5 R per blast along with 10 R per operation was referenced as the limit for exposure. These initial limits were rejected and maximum permissible exposure standards (MPE) were solidified by commanders for the crews working on the decontamination unit. Due to the fast recovery of ships, high contamination values were expected by commanders and set the standard to four roentgens per hour (R/hr). An excess of 4 R/hr was designated off limits of recovery units. Personnel were given special breathing equipment to work in the interior of ships with an R/hr higher than one. High numbers of personnel working the tests were used to keep individuals within the MPE. Times were calculated to keep the total exposure below two roentgens for the personnel working on the ships. A floating decontamination facility was created on board a transport ship for the Wahoo and Umbrella nuclear tests. This allowed important data analysis and an increase in recovery efficiency.

Preparation
The nuclear explosion was carried out in open ocean outside of Enewetak. This nuclear test codenamed Wahoo, was the first underwater test in the Operation Hardtack series. This test could be considered a continuation of the Wigwam nuclear blast, a deep water nuclear test 500 nautical miles off the coast of San Diego. Like its predecessor, operation Wahoo was a scientific program that studied the effects of an underwater nuclear blast on Navy s ystems. The Wahoo nuclear device was positioned 500 feet deep in the Pacific Ocean. This deep water test required for precise targeting arrays to be setup around the blast location. The open ocean presented unique problem for the arrays as the winds, sea currents and tides needed to be within certain limits for the data to be accurate. Testing arrays were placed at depths of 400 to 800 fathoms (2400 to 4800 feet) around the nuclear device. The deep water mooring systems needed for the test provided to be difficult to be put in proper placement leading many analysts to believe the data to be skewed. The target ships to be used for this program consisted of three destroyers, an active submarine, a submarine mockup used in Wigwam tests, and a merchant marine ship. It was predicted that no air blast or thermal affects would be presented from the underwater blast. The fallout from the blast was also predicted to stay within the target array due to the southwest surface winds.

Wahoo Blast
The test conditions were met on May 16th allowing for the nuclear device to be detonated. Within a second of detonation, a spray dome was created that reached a height of 840 feet after seven seconds. The overall shape of the spray dome resembled an inverted cone with 45 degree sloped sides. Plumes were seen breaking through the spray dome after six seconds in every direction. The vertical plume continued rising until 12 seconds after the blast while the lateral plumes traveled for 20 seconds before collapsing. The diameter of the spray dome was approximately 3,800 feet at the 20 second mark. The base surge reached a radius of 8,000 feet in the downwind direction after 1.7 seconds. The downwind surge aided by a 15 knot wind reached speeds of 21 knots (24.2 mph). This base surge could be seen for three and a half minutes and for longer from the air as it continued to move across the ocean. When the spray dome and base surge had dissipated, a foam patch could be seen spreading from the surface zero water to reach over 6,000 feet. The nuclear device had a blast that was calculated to be nine kilotons. All fallout stayed within the predicted fallout area with a maximum of 0.030 R/hr. The target ship at 5,900 yards was directly hit by the shockwave vibrating the entire ship and shaking it violently. The Moran, merchant marine ship moored at 2,346 feet away, was immobilized due to shock damage to its main and auxiliary equipment while also attaining minor hull damage. One hour and ten minutes after detonation, a five gallon water sample was taken directly above the blast location showing a 5 R/hr. The retrieval team entered a 3.8 R/hr field after an hour and thirty five minutes.

Preparation
The second underwater explosion of the Operation Hardtack series was code named Umbrella. This test was conducted in the lagoon inside Enewetak. This test could be considered a continuation of underwater test Baker, which was conducted in Bikini Lagoon. The Explosions in shallow water can create an underwater crater if close enough to the bottom. The Target ships from the Wahoo test were towed into the lagoon to be used again for the Umbrella test. The damage to the target ships was minor in all cases with the exception of the Moran which required extra refurbishing of the hull. These ships had to be washed down and have the measuring equipment replaced before being put into use. The mooring for Umbrella was considerably easier than for Wahoo due to the location and depth of the lagoon. Information gathered from the Wahoo shot directly affected the Umbrella plans in regard to target array placement and decontamination times. The device was placed with a buoy at a depth of 150 feet.

Umbrella Blast
The 9th of June held the second underwater nuclear test of the Hardtack I series. The detonation occurred at 1115 with clear skies and a 15 to 17 knot wind from the east-northeast. Within one-tenth of a second, the spray dome had broken through the surface. The overall shape of the spray dome resembled a vertical column with the plumes helping create the shape. Within 20 seconds detonation, the maximum height of the column reached 5,000 feet. The base surge reached a distance around 10,000 feet downwind and reaching out approximately 6,000 feet in all directions. This base surge could be seen from the air for about 25 minutes and the foam patch over the blast location lasting even longer. The blast yield of the nuclear device was measured to be eight kilotons. The Umbrella blast created a crater 3,000 feet in diameter and 20 feet in depth in the lagoon. The maximum radiation reading was found to be 0.350 R/hr by an aircraft flying over the blast location. Radiation was reported to be of small portions, and reentry was granted 30 minutes after detonation. Out of the major target ships, the highest radiation reading was logged at 0.0015 R/hr of the stern of the Moran. After all scientific data had been gathered from the target ships, they were restored to be seaworthy and towed back to Pearl Harbor. The Moran was concluded to be unseaworthy due to the blast and was sunk by naval gunfire near Ikuren Island.

The underwater nuclear tests Wahoo and Umbrella gathered scientific data that helped the Department of Defense understand the effects of nuclear weapons on Navy ships. Some of the key points found were that underwater nuclear blasts create less fallout due to the absorption of radioactive material in the water and vaporized droplets.(76) Direct gamma radiation was found to be extremely low, but the base surge was found to be highly radioactive as it passed measurement devices. Smaller radiation concentrations followed in waves mimicking the initial base surge. Almost all free-field radiation activity was found to disappear after fifteen minutes allowing for return to normal operations. (86) The impact of shockwaves was less than prior expected. The damage from the shockwave was found to be negligible on the destroyers at the ranges tested, but equipment failure from the blast could immobilize the ships. The preliminary tests of explosive tapered charges were proved to simulate the shockwaves of a nuclear device using the data gathered. (91)