Hiroshima and Nagasaki Occupation Forces

Overview: Atomic bombs were detonated over Hiroshima and Nagasaki, Japan, on Aug.

6 and Aug. 9,1945, respectively. Following the surrender of Japan on Aug. 14, 1945,

U.S. forces began occupying the country.

The first occupation troops arrived in the vicinity of Hiroshima about 60 days after the

bombing. The main body of occupation troops entered Nagasaki about 45 days after the

bombing. In each city, a group of American scientists arrived three days before these

troops and performed a radiological survey. However, repatriation of former prisoners of

war (POWs) through Nagasaki began before the survey and actual occupation of the city.

U.S. troops were in the vicinity of Hiroshima between Oct. 6, 1945, and March 6, 1946,

and in the vicinity of Nagasaki principally between Sept. 11, 1945, and July 1, 1946.

The mission of the occupation was to establish control of the area, ensure compliance

with surrender terms, and demilitarize the Japanese war machine. The mission did not

include the cleanup or any radiological decontamination of Hiroshima, Nagasaki, any

other areas, or the rebuilding of Japan.

Initial units involved:

Hiroshima - 186th Infantry Regiment of the 41st Division, X Corps of the Sixth

Army; later replaced by the 34th Infantry Regiment of the 24th Division.

Nagasaki - 2nd Marine Division, which included the 2nd, 4th, and 8th Regimental

Combat Teams (RCTs) and an Artillery Group composed principally of the 10th.

Marine Regiment. Other units of the 2nd Marine Division involved were a

Headquarters Battalion, Service Troops, an Engineer Group, a Tank Battalion, an

Observation Squadron and some smaller organizations.

Troops were constantly on the move and changing assignments during the occupation,

and the duration of assignment for any unit in the occupation forces was quite short.

Men with the longest service periods were given priority for transfer home and whole

units were deactivated as it became apparent that large numbers of troops were not

necessary to fulfill the mission. The size of the occupation force dropped sharply every

month.

The total number of troops occupying Hiroshima was about 40,000. Approximately

27,000 troops occupied Nagasaki. About 12,000 troops occupied outlying areas within

10 miles of either city through July 1,1946. An additional 118,000 servicemen had

passed through these areas by July 1, 1946. These transient personnel included POWs,

troops disembarked for elsewhere in Japan and crews of ships docked nearby.

Refer to Title 38, Code of Federal Regulations (38 CFR), part 3.309(d)(3) for the context

of the following extracted formal VA definitions of occupation forces and POWs.

Occupation force. The occupation of Hiroshima or Nagasaki, Japan, by

United States forces during the period beginning on August 6, 1945, and

ending on July 1, 1946.

POW: Internment as a prisoner of war in Japan (or service on active duty in

Japan immediately following such internment) during World War II which

resulted in an opportunity for exposure to ionizing radiation comparable to

that of the United States occupation forces in Hiroshima or Nagasaki, Japan,

during the period beginning on August 6, 1945, and ending on July 1, 1946.

The term "occupation of Hiroshima or Nagasaki, Japan, by United States

forces" means official military duties within 10 miles of the city limits of

either Hiroshima or Nagasaki, Japan, which were required to perform or

support military occupation functions such as occupation of territory, control

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of the population, stabilization of the government, demilitarization of the

Japanese military, rehabilitation of the infrastructure or deactivation and

conversion of war plants or materials.

Former prisoners-of-war who had an opportunity for exposure to ionizing

radiation comparable to that of veterans who participated in the occupation

of Hiroshima or Nagasaki, Japan, by United States forces shall include those

who, at any time during the period August 6, 1945, through July 1, 1946:

(A) Were interned within 75 miles of the city limits of Hiroshima or within

150 miles of the city limits of Nagasaki, or

(B) Can affirmatively show they worked within the areas set forth in (A)

although not interned within those areas, or

(C) Served immediately following internment in a capacity which satisfies

the above definition of occupation forces, or

(D) Were repatriated through the port of Nagasaki.

Occupation scenario:

Hiroshima. Elements of the 41st Division landed at Hiro, approximately 10 miles

southeast of Hiroshima, on Oct. 6, 1945, and secured the Kure Naval Yard. On Oct. 7,

the 186th Infantry Regiment of the 41st Division landed, and the Regiment's 2nd

Battalion established headquarters and billets in Kaidaichi, about 5 miles southeast of

the center of Hiroshima. Since most of the city of Hiroshima had been destroyed by the

bomb (see Figure 1), no major units were stationed there throughout the occupation.

During the next two months, units of the 186th Infantry Regiment conducted

reconnaissance patrols and other missions in its area of responsibility, including the city

of Hiroshima. Records indicate that troops occasionally patrolled the destroyed area of

the city. Additionally, individuals from nearby units of the 41st could have made brief

sightseeing trips to view the destruction caused by the bomb. About 900 U.S. POWs

were repatriated through Hiroshima.

Upon deactivation of the 41st Division in December 1945, the 34th Infantry Regiment of

the 24th Division took over its mission and moved into the buildings in Kaidaichi

originally used by units of the 186th. The 34th Regiment was responsible for such a wide

geographic area that eventually only Company G of the 2nd Battalion was stationed in

the vicinity of Hiroshima. On March 6, 1946, the 34th Regiment was relieved by an

Australian Infantry Battalion, and the U.S. occupation in the vicinity of Hiroshima ended.

Nagasaki. Nagasaki was used to repatriate former POWs because the waterfront was

sufficiently far from the hypocenter (the spot on the ground directly under the

detonation, i.e., ground zero) to have escaped most of the destructive effects of the

bomb, and to have been completely free of radioactivity (see Figure 2). Over 9,000

allied (including 2,300 U.S.) POWs were processed at Nagasaki Sept. 11-23, 1945. A

POW recovery team and a detachment of Marine guards were ashore in Nagasaki to

support POW processing. Additionally, a small advance party of the occupation force

(about 12 personnel) arrived in Nagasaki on Sept. 16, 1945, and remained until the

main force arrived on Sept. 23, 1945.

Upon landing, the 8th RCT and the 10th Marines deployed immediately to Isahaya,

about 10 miles north of Nagasaki. The 8th RCT did not occupy Nagasaki, but the 10th

Marines did so two months later. The other elements of the 2nd Marine Division

debarked in the vicinity of Dejima Wharf and the Mitsubishi shipyard and established

command posts and billets in those vicinities. The 2nd RCT left Nagasaki in early

November, and the 6th RCT departed in December 1945 along with two-thirds of the

Engineer Group. The Headquarters Battalion and portions of the Service Troops left

Nagasaki in January 1946. The Tank Battalion, which had landed and remained in

Fukahori, about nine miles southeast of Nagasaki, arrived in Nagasaki in November 1945

and departed the next month. The 10th Marines took over the responsibilities of the 2nd

RCT in November, and later also those of the 6th RCT. The last units of the 2nd Marine

Division left Nagasaki on July 1, 1946.

The specific billet locations of all units have not been precisely determined, but they

were undoubtedly outside of the radiation survey contours surrounding the hypocenter

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in Figure 2. An area extending beyond those contours was uninhabitable because of

complete destruction, and historical documents confirm that the area was avoided. As

with Hiroshima, presumably patrols and sightseers occasionally entered the areas of

residual contamination in Nagasaki. The U.S. Navy transported Marines to Nagasaki and

evacuated POWs, but its role ashore was limited. Some Navy personnel, including

hospital corpsmen, medical and dental officers, chaplains, and a construction battalion,

were assigned to the 2nd Marine Division.

Radiation data:Analysis of the scientific data for the Hiroshima and Nagasaki airbursts

continues, resulting in revised statistics for the detonations. The Hiroshima bomb was a

uranium-235 weapon that detonated about 1,900 feet above the ground with a yield of

15 kilotons (kT). The Nagasaki bomb was a plutonium-239 weapon that detonated 1,650

feet above the ground with a yield of 21 kT. Figures 1 and 2 show the built-up areas of

the respective cities, the hypocenter of each burst, residual gamma radiation intensity

contours, and the approximate perimeters of total destruction from blast and fire.

The radiological effects of the detonation in each city were similar. Japanese citizens in

the vicinity at the time of the detonations were exposed to intense radiation produced

almost instantaneously. High doses of hundreds of rem from this initial neutron and

gamma radiation contributed to the lethality of Japanese citizens located beneath the

bursts. This initial radiation only occurs for about one minute after a nuclear detonation

and does not persist thereafter. In contrast, the earliest residual radiation levels

encountered by Japanese were survivable. Both burst altitudes were sufficiently high

that bomb debris did not reach the ground in the vicinity of the hypocenter. After the

detonations, strong updrafts were produced which lifted the radioactive bomb debris,

ground dust and smoke together in clouds. Most of the mixed debris settled to the

ground as radioactive fallout downwind of the cities. In each city, there was one area of

low-level residual radioactivity in a roughly circular area caused by neutron activation of

soil and building materials around the hypocenter. Additionally, there was a second area

of residual radioactivity located downwind and outside the city, caused by fallout carried

to the ground during rainshower activity within an hour after the detonation. Subsequent

heavy rainfall washed away some of the residual radioactivity. During the intervening

weeks before the occupation forces arrived, this rainfall, combined with radiological

decay, reduced the radiation levels from fallout and neutron-activated materials by a

factor of several thousand. This explains, in part, why the radiation doses of occupation

forces were at least a thousand times lower than those Japanese located near ground

zero at the time of the detonation.

Based on radiation surveys by American scientists from the Manhattan Engineer District,

the greatly decayed residual radioactivity levels in and around Hiroshima and Nagasaki

at the time the occupation forces arrived were such that military activities could proceed

as planned, unimpeded by radiological considerations.

Figure 1 depicts the results of the Naval Medical Research Institute survey taken in

Hiroshima on Nov. 1-2, 1945, showing a residual radiation level of 0.069 milliroentgen

per hour (mR/hr) at ground zero, and an average residual radiation level of 0.011 mR/hr

in the area of rainout to the west of the city.

Figure 2 depicts the results of the Naval Medical Research Institute survey taken in

Nagasaki on Oct. 15-27, 1945, showing the residual radiation level of 0.072 mR/hr

(maximum) at ground zero, and a residual radiation level of up to 1.08 mR/hr in the

rainout area at the Nishiyama Reservoir. Scientific analysis of these data indicated that

two radionuclides, scandium-46 and cobalt-60, which resulted from neutron activation of

surface soil and building materials, produced the radiation levels near ground zero in

each city. Fission product radionuclides produced the radiation levels in the rainout

areas.

The Nishiyama Reservoir had the highest radiation measurement recorded at the time of

the troops' arrival. However, this area was remote and rugged, with steep slopes and

heavy forests, few trails or roads, and even fewer buildings. The Japanese population in

the area was sparse, so there were no occupation forces stationed in the vicinity, and

little need for military patrols into the area.

Personnel doses: Dose reconstructions are based on (1) residual radiation

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measurements, documented and published shortly after the bombings, (2) extensive

review and analysis of the residual radioactivity in ensuing decades, and (3) the

documented arrival and departure dates of each military unit which operated in the

vicinity of Hiroshima and Nagasaki.

Using the "worst case" assumptions that lead to the highest radiation dose consistent

with a military unit's potential for exposure, the dose reconstructions show that the

maximum total* radiation dose any member of the U.S. occupation forces in Japan could

have received was less than 1 rem.** The average dose received by individuals in the

Hiroshima and Nagasaki occupations was less than 0.01 rem. More than 95 percent of all

Hiroshima and Nagasaki participants received a dose less than 0.1 rem, which is the

annual radiation dose limit for the U.S. general public currently in effect.

Similar dose reconstructions indicate that U.S. servicemen who survived imprisonment

in Japanese camps received virtually no radiation dose, with the exception of the POWs

held in the camp at Kumamoto. Fallout was detected in this city downwind of Nagasaki.

The range of total doses for occupation forces, POWs, and transient personnel is:

Category of Defined Participants Percentage of Defined

Participants

Probable

Dose

(rem)

Maximum

Dose

(rem)

Occupation forces - Nishiyama area

and POWS - Kumamoto Camp 4% <0.1 <1

Other Hiroshima/Nagasaki

occupation forces 36% <0.01 <0.1

Naval ship crews 43% 0.0 <0.01

Disembarked troops (for elsewhere

in Japan) 13% 0.0 <0.01

Transients on railroads 2% 0.0 <0.01

Repatriated POWs 2% 0.0 <0.01

These doses are in contrast to the reconstructed initial radiation doses, which ranged

between about 10 rem to hundreds of rem for the hundreds of thousands of Japanese

survivors whose health continues to be monitored by the Radiation Effects Research

Foundation (RERF). The RERF (formerly known as the Atomic Bomb Casualty

Commission) members are officials from the Japanese Department of Health and the

American National Academy of Sciences. For further information on these studies

contact:

Director, Board on Radiation Effects Research

National Academy of Sciences

(Room 342)

2101 Constitution Avenue, NW

Washington, D.C. 20418

For additional information on the RERF, contact the Director of the Board on Radiation

Effects Research, telephone (202) 334-2836, or visit the RERF's Internet site

(http://www.rerf.or.jp/eigo/experhp/rerfhome.htm).

* Sum of external and internal dose, where internal dose is the 50-year committed

effective dose equivalent.

** A rem is a unit that quantifies the biological effect of ionizing radiation (gamma,

x-ray, beta, neutron or alpha) on man. Ionizing radiation is any radiation capable of

displacing electrons from atoms or molecules, thereby producing ions. The general U.S.

population receives about 0.36 rem per year (National Council on Radiation Protection

and Measurements (NCRP), Report No. 93, Table 8.1) from natural background radiation

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

diagnostic x-rays and consumer products). The standard diagnostic chest x-ray delivers

a dose of about 0.02 rem. For more information about NCRP Report 93, contact the

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NCRP internet site (http://www.ncrp.com).

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Current As of January 2000

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