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Surface Blast Tests

Of the 35 nuclear tests in Operation Hardtack I, four were surface burst shots: Cactus, Koa, Quince and Fig. These tests took place from May to August of 1958, all at the Enewetak Atoll. Surface tests inherently present the potential for more radioactive exposure issues than the high-altitude or underwater detonations. This is because there is more material present to be converted to radioactive debris by excess neutrons due to the proximity to the Earth’s surface, and due to the soil and other minerals excavated from the craters created by these blasts. The existence of this extra material allows for larger radioactive particles to be created and lifted into the blast cloud, falling back to the surface as fallout. Though surface and near-surface tests have a higher probability of radioactive exposure problems, the radioactive elements have significantly shorter residence times when injected into the atmosphere. As radioactive clouds from surface-type tests reach heights of around 20 kilometers at maximum, and thus cannot extend higher than the lower stratosphere, the residence times can be up to 13 years less than the high-altitude blasts. During original concept planning in 1954, Enewetak was supposed to be the location of the smaller tests conducted during Operation Hardtack I. Due to poor weather conditions and policy changes in 1958, five of the UCRL tests which were planned to be conducted at the Bikini Atoll were moved to Enewetak. This included the later two surface blast devices in the Quince and Fig tests.

Cactus

Operation Hardtack I Cactus shot Crater

The Cactus test took place May 6th 1958 at approximately 0615. A 18 Kiloton land-surface type shot was detonated on a platform at the northern tip of Runit, Enewetak in the second of the 35 tests for Operation Hardtack I. The initial cloud from the explosion reached as high as 19,000 feet (5.8 km) within the first ten minutes, and settled at around 15,000 feet (4.6 km) by 20 minutes after detonation. The nuclear fallout prediction map proved to be accurate in determining the span and the intensity of the resultant fallout. Measured peak intensity of fallout reached 440 R at hour three directly above blast site on the North end of the Atoll. At mid-island the radiation was measured to be 1.7 R. The southern tip received a very small amount of radiation, 0.005 R due to the easterly winds.

Out of the eight Programs listed, DOD-affiliated Projects 1.4, 1.7, 1.8, 1.9, 1.12, 2.8, 3.2, 5.2, 5.3, 6.4, 6.5 and 6.6 involved the Cactus test.

One of the goals of this test was to study the physical characteristics of the crater and the surrounding area pre- and post-detonation associated with Project 1.4. A camera was mounted to a RB-50 aircraft and the resultant crater was mapped using photogrammetry. Measurements were taken from 500 feet (152 meters) down to ground zero for both pre- and post detonation. Survey measurements of the Cactus test could not be made until the resultant radiation amounts had lowered to safer levels. Along with photogrammetric radial measurements, Airblast measurements were also recorded near surface zero for Cactus in accordance with Project 1.7. Similar to Project 1.8, Project 1.9 sought to determine transmission of blast pressure through the ground soil. Forty-three drums were buried at varying depths 600 feet (183 meters) from ground zero of the Cactus detonation. Individuals from the Air Force Ballistic Missile Division – TRW Space Technologies Laboratory sought to the Shock Spectra Studies of Project 1.12. Measurement gauges were located between 625 to 965 feet (191 to 294 meters) from the blast site. Fallout measurements and samples were taken by aircraft in the aftermath of the detonation, as part of Project 2.8. These surveys were designed in part to determine the impact of specific radionuclides to overall nuclear fallout. Samples were taken early post-blast using a new rocket sampler, the UCRL, which were then followed by B-57D and WB-50 aircraft. Between four and twenty-four hours post-detonation, the WB-50 collected several samples at 1,000 feet (305 meters) in altitude. Project 3.2 involved structural tests of corrugated steel arches. One of these arches was placed 980 feet (299 meters) away from the Cactus blast site. Eight days after the test, a crew of 13 men were allowed near the surface zero in order to extract the arch. The extraction took around twelve hours, and the radioactivity levels reached a maximum at 0.420 R/h during this time. Project 5.2 sought to determine the effects of the blast on two A4D-1 aircraft, both in thermal radiation and pressure. Nine film badges were placed throughout each aircraft in order to measure radiation. The Gamma dosimeter worn by the pilot in aircraft 827 indicated a neutron exposure level of 0.105 rem. There is not information available for the dosimeter worn by the pilot in aircraft 831. Radiation recorded by six of the nine film badges ranged from 0.49-1.74R. Information from the remaining three film badges, located at the Pilot’s right leg, left sleeve, and left vest is not available. The pilot’s exposures were both greater than 3R. Project 5.3 was very similar to 5.2, focusing on the effects of the blast on the structure of two test FJ-4 aircraft. Radiation from film badges on aircraft 467 ranged from 0.52-3.71 R, and 1.23-5.06 R for aircraft number 310. In Project 6.4, the Army Signal Research and Development Laboratory (SRDL) investigated the electromagnetic pulses post-detonation, utilizing two instruments, one at Wotho (roughly 240 nautical miles from Enewetak) and one at Kusaie (around 440 nautical miles from Enewetak). Though Cactus is mentioned in Project 6.5, whose goal was to study the radar echoes from the fireball, it is not clear what destroyers were involved if any. Destroyers for the Teak, Fir and Yucca tests are explicitly stated. Project 6.6, the lasts of the projects that involved the Cactus test, sought to measure the physical properties of the stabilized radioactive cloud following the blast.

Koa

At 0630 on May 13, the Koa surface device was detonated at the western side of Dridrilbwij. The size of the blast was 1.37 MT, around 76 times larger than the yield of the previous surface test, Cactus. The test was conducted in a large water tank. Within 17 minutes of the blast, surface clouds reached approximately 60,000 feet (18.3 kilometers). Nuclear fallout predictions for Koa were larger and covered a broader range than Cactus, with significantly higher radiation levels, an order of magnitude larger, in the immediate region surrounding the blast. There had been a larger nuclear barge detonation, the 1.85 MT Apache test, off the island of Dridrilbwij two years prior in 1956, during which the island survived. The detonation of the Koa device, however, caused complete destruction of the island.

The Department of Defense (DOD) sponsored a series of experiments for Koa: Projects 1.4, 1.7, 1.8, 1.9, 1.12, 2.9, 3.2, 3.6, 5.1, 5.3. 6.4, 6.5, 6.6, 6.9. and 6.11. Project 1.4, which sought to study land craters post detonation for several shots, took ground survey measurements from surface zero out to 2,500 feet (762 meters). Detailed measurements could not be made until four days after the blast due to radiation levels. Using a boat, the majority of the crater was mapped, though some measurements could not be made until 1959 due to radioactivity levels around the Koa blast site. In Project 1.9, the blast-induced pressure through the soil was measured by burying 43 drums at depths ranging from 0 to 20 feet and around 3,000 feet from the Koa blast site. The early radioactive cloud samples, normally collected post-shot as a part of Project 2.8 using a UCRL rocket sampler, were not collected after the Koa test due to technical difficulties. Fallout at high and low altitudes utilizing B-57D and WB-50 aircraft were measured with no technical problems. Project 3.6, which was conducted only on Koa, tested the effects of the blast on reinforced-concrete slabs buried close to surface zero. One station was located at 1,830 feet (558 meters) from the blast site, and the other was located 3,100 feet (945 meters) away. Project 5.2 was designed to measure the radiation levels using film badges on different areas of the aircraft and on the pilot directly. Koa had the lowest average radiation measurements of the eight shots on this project, with aircraft 827 ranging from 0.01-0.02R and aircraft 831 reporting levels between 0.03-0.19R. Unlike in the Cactus test, all nine film badges from both aircraft were successfully recovered and reported.

Quince and Fig

Quince and Fig were a series of surface shots that occurred in early August at the center of Runit. They were the last of the Hardtack tests conducted at the Enewetak Atoll. Quince was detonated at 2:15pm on August 6^th 1958. Twelve days later, the Fig device was detonated on August 18^th 1958 at 4:00pm. Quince and Fig were UCRL devices with co-sponsorship through the DOD and the AEC, though the main focus of these tests was weapons development. One of the main differences between these two tests was that over 130 tons of soil from the Nevada Test Site had been shipped in and was placed at surface zero of the Fig test. The radioactive cloud from the Quince blast rose to 1,500 feet (457 meters).  As Runit was to also be the detonation site for Fig, the area had to be decontaminated after the Quince test. Around three to five inches of contaminated topsoil was removed from an area 75 feet by 25 feet upwind of ground zero. An area of 60 square feet at the blast site was scraped to remove the top three inches of soil as well. Even with these precautions, the measured alpha activity was around 20,000 counts per minute (CPM), and the area closest to surface zero was roped off to prevent personnel from entering. Upon detonation, the mushroom cloud produced by the Fig device rose to approximately 6,000 feet (1.8 kilometers), with a base of roughly 4,300 feet (1.3 kilometers). 30 minutes post detonation, radiation measurements at the blast site reached over 10,000 R/hr.

The DOD experiments for Quince and Fig included Projects 1.4, 1.7. 2.4, 2.9, 2.10, 2.11, 2.14. and 8.7.