Operation HARDTACK I

Note: For information related to claims, call the Department of Veterans Affairs (VA) at 800-827-1000 or 

the Department of Justice (DOJ) at 800-729-7327. For all other information, call the Nuclear Test 

Personnel Review (NTPR) Program at 800-462-3683.

Operation HARDTACK was the designation given to the atmospheric nuclear test series conducted by the 

United States in the Pacific Ocean and Nevada from April 28 to October 31, 1958. Operation HARDTACK I 

was a series of 35 tests, all but 2 of which were detonated at Enewetak and Bikini Atolls in the Marshall 

Islands, part of the Atomic Energy Commission’s (AEC) Pacific Proving Ground (PPG). The other two were 

detonations at 43 and 77 kilometers (km) (26.7 and 47.8 miles) above Johnston Island, which lies about 

700 nautical miles (nmi) (1,296 km) west-southwest of the Hawaiian Islands. The tests were conducted by a 

joint military and civilian organization, designated Joint Task Force 7 (JTF 7), comprised of approximately 

19,100 personnel. JTF 7 was a military organization in form but was made up of military personnel, Federal 

civilian employees, and contractors of the Department of Defense (DOD) and the AEC. The commander of 

this force was the appointed representative of the AEC and reported also to the Joint Chiefs of Staff and the 

Commander-in-Chief, Pacific (CINCPAC).

Historical Background

During HARDTACK I, the United States fired as many nuclear devices (35) as had been fired in all prior Pacific 

Ocean tests. Not only was the total number of shots in HARDTACK I large, but the variety of types was great; 

land- and water-surface tests, underwater detonations, a balloon detonation, and rocket-borne high-altitude 

tests were conducted. The names, dates, and locations of the shots are listed in the accompanying table.

In a sense, HARDTACK I was divided into three parts. The first was aimed at the development of nuclear 

weapons, continuing the type of testing that had taken place at Enewetak and Bikini during the early and mid1950s. In these tests, the AEC weapon development laboratories (Los Alamos Scientific Laboratory and the 

University of California Radiation Laboratory) detonated their experimental devices, with the DOD providing 

support and conducting experiments that did not interfere with the AEC activities.

The second part, sponsored by DOD, consisted of the underwater test shots, WAHOO (open ocean) and 

UMBRELLA (lagoon). The purpose of these tests was to improve the understanding of the effects of 

underwater explosions on Navy ships and material. These tests could be considered as a continuation of the 

BAKER test of the CROSSROADS series at Bikini in 1946 and the WIGWAM test 500 nmi (926 km) off the 

U.S. west coast in 1955.
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Summary of Operation HARDTACK I Nuclear Weapons Tests (1958)a

Shot Local Date Location Type Yieldb

(kiloton)

YUCCA Apr 28 Between Enewetak 

and Bikini Balloonc 1.7

CACTUS May 6 Enewetak Surface 18

FIR May 12 Bikini Barge 1360

BUTTERNUT May 12 Enewetak Barge 81

KOA May 13 Enewetak Surface 1370

WAHOO May 16 Enewetak, in ocean Underwater 9

HOLLY May 21 Enewetak Barge 5.9

NUTMEG May 22 Bikini Barge 25.1

YELLOWWOOD May 26 Enewetak Barge 330

MAGNOLIA May 27 Enewetak Barge 57

TOBACCO May 30 Enewetak Barge 11.6

SYCAMORE May 31 Bikini Barge 92

ROSE Jun 3 Enewetak Barge 15

UMBRELLA Jun 9 Enewetak, lagoon Underwater 8

MAPLE Jun 11 Bikini Barge 213

ASPEN Jun 15 Bikini Barge 319

WALNUT Jun 15 Enewetak Barge 1450

LINDEN Jun 18 Enewetak Barge 11

REDWOOD Jun 28 Bikini Barge 412

ELDER Jun 28 Enewetak Barge 880

OAK Jun 29 Enewetak Barge 8900

HICKORY Jun 29 Bikini Barge 14

SEQUOIA Jul 2 Enewetak Barge 5.2

CEDAR Jul 3 Bikini Barge 220

DOGWOOD Jul 6 Enewetak Barge 397

POPLAR Jul 12 Bikini Barge 9300

SCAEVOLA (safety 

test) Jul 14 Enewetak Barge 0

PISONIA Jul 18 Enewetak Barge 255

JUNIPER Jul 22 Bikini Barge 65

OLIVE Jul 23 Enewetak Barge 202
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Summary of Operation HARDTACK I Nuclear Weapons Tests (1958)a (Continued)

Shot Local Date Location Type Yieldb

(kiloton)

PINE Jul 27 Enewetak Barge 2000

TEAK Jul 31 Johnston Island area High Altitude (rocket) 3800

QUINCE Aug 6 Enewetak Surface 0d

ORANGE Aug 11 Johnston Island area High Altitude (rocket) 3800

FIG Aug 18 Enewetak Surface 0.02

a Source: United States Nuclear Tests, July 1945 through September 1992, DOE/NV-209-Rev 15, Dec 2000.

b One kiloton equals the approximate energy release of the explosion of one thousand tons of TNT. c Listed as “High-altitude – 86,000 feet” in DOE/NV-209-Rev 15.

d Weapon-related; yield not up to expectation.

The third part, also sponsored by DOD, addressed a new military issue: nuclear weapons in air and ballistic 

missile defense. The HARDTACK I tests directed toward this issue were rocket borne high-altitude shots

TEAK and ORANGE (conducted at Johnston Island). A third test (YUCCA) was carried to 86,000 feet by a 

balloon over the ocean between Enewetak and Bikini. These tests used device placement techniques and 

data-recording operations that were new to nuclear weapons testing.

Central to the series was the testing program. This program and its requirements dictated the form of the test 

organization and the detail of personnel participation. The HARDTACK experimental program incorporated 

two aspects, the first of which was the development of the weapons themselves, and the second involved the 

measurement of the explosive and radiation effects. Unlike earlier nuclear test series, the HARDTACK test 

operations supporting each aspect were in large part separate.

These two aspects can serve as a rough measure of differentiation of interest between the major participants: 

the AEC interest in weapon development, and the DOD interest in the military application of the effects of the 

explosions. The several parts of the weapon development and effects studies each had particular features 

that led to the possibility of radiation exposure.

Radiation Protection Standards

For HARDTACK I, CJTF 7 was directed to “assume overall responsibility for the radiological safety of Task 

Force personnel and of populated islands.” To carry out this responsibility, the JTF 7 Operation Plan further 

directed that a Fallout Plotting Center be set up and that the capability be established to keep the task force 

and CINCPAC informed of the fallout situation at all times, including the announcement of safe reentry times. 

Fallout stations were to be set up and technical assistance given to personnel in the Trust Territory of the 

Pacific Islands. Monitors and couriers were to be provided for radioactive sample centers.

In addition, the Operation Plan specified that task group commanders establish radiation safety units within the 

task groups with adequate special clothing and radiological instrumentation. Task groups were also required 

to provide a roster of their personnel for film badge preparation.

The radiological safety program for HARDTACK I was divided into two parts: onsite and offsite. Onsite 

radiological safety activities were conducted by the various task groups, with the scientific task group given the 

responsibility for all radiological safety functions associated with diagnostic experimental programs and for 

dosimetry and other technical services to the entire task force. The operation of the offsite program and the 

coordination of the onsite activities were conducted by the Radiological Safety Office of HQ JTF 7.
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A maximum permissible exposure (MPE) for personnel was set at 3.75 roentgens (R) (gamma only) per 

consecutive 13-week period with a maximum of 5 R for the operation. Exceptions were made for emergency 

and other tactical situations. The operation was defined as the period from 15 days before the first ready date 

to 15 days after the last shot. A special MPE of 10 R was authorized for crewmembers of air-sampling aircraft. 

In the event of operational error or emergency, an additional exposure of 10 R would be accepted. Any 

exposure in excess of 20 R total would be considered as an overexposure for aircrew samplers.

The limit of 3.75 R per 13-week consecutive period was slightly greater than the limit of 3 R per 13-week 

period in effect at that time (as recommended by the National Council on Radiation Protection and 

Measurements and the International Commission on Radiation Protection). The limit of 5 R for the operation is 

equivalent to the annual dose currently permitted by Federal guidelines for radiation workers. Appropriate 

remarks were to be included in the medical records of personnel who exceeded the 3.75 and 5 R limits. 

Military personnel were to be advised that they should not be exposed to further radiation until sufficient time 

elapsed to bring their average radiation exposure down to 0.3 R/week. Civilian personnel in this category were 

to be informed that limitations of further radiation exposure were to be determined by the laboratory or agency 

having administrative jurisdiction over such personnel.

A film badge program provided an exposure-indicating device to all JTF 7 personnel to maintain complete 

exposure information on everyone entering the PPG during the operation. The commander of the scientific 

task group assigned overall badging responsibility to a special task unit. Beginning April 1, 1958, film badges 

were issued to all individuals upon their arrival at the PPG with instructions that the badge be worn at all times 

and turned in on recall, upon exit from any contaminated area, or upon departure from the PPG.

Radiation Doses at Operation HARDTACK I

The fallout from Shots FIR and KOA contributed most to the reconstructed total dose of 1.3 to 2.7 rem∗ for 

personnel who resided on Enewetak Atoll. The dose range reflects variations due to different residence sites 

and work activities. Corresponding dose levels are confirmed by the film badge readings that are available for 

nearly all personnel on the atoll. For personnel who resided on Bikini Atoll (Eneu Island), the reconstructed 

dose is 1.1 rem, mostly from Shot FIR fallout. Naval personnel aboard ships in the vicinity of Enewetak or 

Bikini also were exposed to fallout, and some were further exposed to contaminated target ships and seawater 

after WAHOO and UMBRELLA. The shots over Johnston Island, though visible from Hawaii, did not expose 

the Hawaiian Islands to fallout.

The mean dose of all DOD personnel at HARDTACK I was 0.8 rem. This was less than for the preceding 

operations with multiple thermonuclear tests and reflected improvements in weather and fallout predictions. 

About 140 DOD personnel, or fewer than 1 percent, exceeded the MPE of 3.75 rem. Fewer than 10 personnel 

exceeded the cloud sampler limit of 10 rem. The highest dose was 12.4 rem. The totals of reconstructed and 

film badge doses for HARDTACK I participants are depicted below.

 ∗ A rem is a radiation protection unit of measure that quantifies the risk of biological effects resulting from exposure to 

ionizing radiation. Ionizing radiation is any radiation (gamma, x-ray, beta, neutron, or alpha) capable of displacing 

electrons from atoms or molecules, thereby producing ions. According to the National Council on Radiation Protection 

and Measurements (NCRP, Report No. 160, Table 1.1), the general U.S. population receives about 0.62 rem per year 

from natural background radiation sources (radon, cosmic rays, and rocks) and man-made radiation sources (medical 

diagnostic x-rays and consumer products). As a basis of comparison, a standard diagnostic chest x-ray delivers a 

radiation dose of about 0.02 rem.
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For more information, see the report “Operation HARDTACK I 1958” (DNA 6038F), available online at 

https://www.dtra.mil/DTRA-Mission/Reference-Documents/NTPR-Info/.

September 2021