History of the Ship (LSD-16) and Boatpool During Operation Greenhouse  (1951)

Operation GREENHOUSE was an atmospheric nuclear test series consisting of four separate nuclear detonations and occurring at Eniwetok Atoll in the spring of 1951.

USS Cabildo departed Mare Island Naval Shipyard, Vallejo, California on February 5,1951, and reached Eniwetok 18 days later where it remained for the duration of GREENHOUSE. Cabildo’s primary mission was to transport and support the GREENHOUSE Mobile Boatpool used for intra-atoll transportation at Eniwetok.

Except for USS Sproston (DDE-557) and USS Walker (DDE-517), who were maintaining security patrols, all ships participating in Operation GREENHOUSE remained anchored or moored within the lagoon during the test firings.

The anchorage was in the southeast quadrant of the lagoon waters while the bursts occurred on islands in the northeast quadrant of the atoll’s perimeter.

The table below shows the range and direction of each burst from the USS Cabildo.

Time/Date(1951)/Name/Location Range/Direction from Cabildo
0634/Apr 8./DOG/Runit (YVONNE) 9.0  miles north
0637/Apr 21/EASY/Engebi (JANET) 17.0 miles north
0930/May 9/GEORGE/Aomon (RUBY) 16.5 miles north
0617/May25/ITEM/Engebi (JANET) 13.5 miles north

Test DOG was detonated atop a 300-foot metal tower. An overcast and cloud formations were at low levels, approximately 4,000 to 8,000 feet. It was impossible to see the mushroom from the anchorage through these clouds.

The net effect of the prevailing wind condition subsequent to the DOG detonation was fallout being acted upon by winds blowing toward the east until it fell below the sheer point (at 17,000 feet), at which time it was carried over the atoll westward.

It seems obvious the radiological cloud fallout occurred as the result of this wind condition, although it was not expected to occur in the anchorage of the occupied islands of Japtan, Parry and Eniwetok.

The first indication of contamination due to the fallout of fission products on shipboard was noted on USS Curtiss (AV-4) at 0825. The meter readings showed levels from 1 to 4 mr/hr (milliroentgens per hour) increasing to a maximum of 100 mr/hr at 1100.

Soon after, reports of fallout started arriving from all units. Between 0900 and 1000 Cabildo’s average intensity topside during this time was 25 mr/hr, with a maximum reading of 40 mr/hr.

A second "wave" of radiation occurred from 1200 to 1400. During these periods, anti-contamination procedures were placed in effect. All unnecessary personnel were cleared from the weather decks. Outside ports, doors and hatches were closed and the ventilation systems were reduced to a minimum.

Radiological surveys done on Cabildo the next day showed an average intensity level topside of 5 mr/hr, with a maximum intensity of 20 mr/hr.

The intensity level on boats in the boatpool was reported to have been reduced to acceptable levels. Safe working levels for GREENHOUSE were established at 12.5 mr/hr and less.

The problem of radioactive fallout was much less serious for Shot EASY and Shot GEORGE, because of the wind conditions.

Following the ITEM blast, fallout on the ships and Eniwetok came in two waves, one four hours after the burst and the other ten hours. The first lasted lasted 90 minutes and sustained an intensity of 20mr/hr. The second lasted four hours, having an average intensity of 25 mr/hr.

The 135 film badges variously worn by 68 Cabildo crew members involved in recovery operations had a mean dose of 0.333 rem gamma, with a range of exposure on single badges from 2.0 rem gamma to zero rem gamma.

The 589 film badges issued to 62 Boatpool personnel had a mean does of 0.268 rem gamma, with a range of exposure on single badges from 3.0 rem gamma to zero rem gamma.

The assumption that ship’s personnel would receive the same fallout dose as island personnel is questionable. Recent research indicates that shipboard decontamination procedures, might result in lower fallout exposure.

This aspect is currently under investigation and should assist in developing a more accurate representation of exposure potential.

Personnel on Cabildo were assigned fallout doses based on the time they were known to be present in the area (determined from film badge issue dates).

Prior to the formal determination of assigned doses for Cabildo as well as Boatpool personnel, preliminary fallout doses were, in most cases, entered in the appropriate individual’s medical record.

In many cases these initial estimates were lower than the fallout dose finally assigned and rarely exceeded 2.0 em for Cabildo and Boatpool personnel.

For those personnel who were unabridged, subsequent fallout doses and the medical record entry is all that currently exists. Upon completion of the study of the effects of shipboard decontamination, shielding and weathering factors, such a dose will be assigned to all unabridged personnel on Cabildo or with the Boatpool.

As a worst case estimate the unabridged individual would not likely exceed the 3.955 em gamma dose which represents the maximum fallout dose possible for the period from April 8 to May 28, 1951.

Because of the short duration of time (approximately 12 to 24 hours) personnel on Cabildo -- or with the Boatpool -- were exposed to the fallout, it is not considered likely they received any significant amount of internal contamination by ingestion or inhalation.

Future efforts to reconstruct exposure parameters during this period may address this. At present, however, such information is not available. It may be some time before such a study for GREENHOUSE will be completed.

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USS Cabildo (LSD-16) Association Website - Last Revision April 1, 2006
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