Content uploaded by Eiichiro Ochiai
Author content
All content in this area was uploaded by Eiichiro Ochiai on Sep 26, 2017
Content may be subject to copyright.
Chapter 5 Devastations caused by the Atomic
Bombs – Hiroshima and Nagasaki –
Now let’s go back to the Atomic bombs dropped on Japan. The following is a summary of
features of the atomic bomb (Little Boy) dropped on Hiroshima. It has been estimated that it
was equivalent to 16 KT of TNT (trinitrotoluene, explosive). Only about 860 g of the 70 kg of
U-235 contained in the bomb was supposed to have exploded. The energy released was 6.3 x
1013 J (63 tera joule) (Little Boy, Wikipedia; Los Alamos Report (1985)). About 50% of it
caused the windblast, 35% turned into heat, and 15% ware released as radiation. The
temperature just below the epicenter is believed to have reached as high as 6000 oC. Many
charred bodies were scattered around there. The high temperature caused fires. The pressure
created by the wind was about 35 atm at the epicenter, and 5 atm even at 2 km from it. This
caused very strong wind (shock wave) that destroyed most buildings, and caused death of
many people by the debris. Just as a comparison, the hurricane or typhoon’s very strong wind
is caused by only about 0.1 atm pressure difference. These two factors, blast and heat, caused
the devastating visible effects of the atomic bomb.
The Fatman, dropped on Nagasaki, contained 6.2 kg ton of highly enriched Pu-239, and
about 1.2 kg of it exploded and was converted to energy (Fatman, Wikipedia). The
Fig. 5.1. Nagasaki city before the atomic bomb (left) and after the bomb (right)
energy released was estimated to be about 8.3 x 1013 J and equivalent to 22 KT of TNT, 40 %
more powerful than the “Little Boy”. Fig. 5.1 shows the aerial photos of a part of Nagasaki
city taken before and after the dropping of the atomic bomb. This enormous change was
brought about by a single bomb and almost instantaneously. The area shown in the picture is
about 3.5 x 5 km, and does not represent the entirety of devastation.
All kinds of radioactive material were released, and in addition, a very high dose of
neutrons reached the land below, and it made many non-radioactive materials radioactive. In
Hiroshima immediately after the explosion, the so-called “Black Rain” fell over a large area of
25 x 19 km (oblong), and the amount of rain was recorded as 5-100 mm over three hours. It is
supposed to have contained radioactive material produced by explosion as well as the uranium
left unexploded. It killed most of fish in the rivers and ponds, and affected cattle that fed on
the contaminated weed, and human being (loss of hair, blood in feces, and other severe
symptoms). The black rain was recorded in a documentary film “The Effects of Atomic
Weapons” produced in 1945 (starting in September) by a Japanese filmmaker, but was
classified and was never made public. The film also indicated that some soil samples in the
area showed high radiation readings (p93 in Takahashi (2008).
Fig. 5.2. A charred body and people with skin hanging due to the burn
gathering near the epicenter (*Y. Yamahata, photograper)
Fig. 5.3. Burned bodies (*the right: Y. Yamahata photographer)
Fig. 5.4. A cathedral (Nagasaki) and the Hiroshima exhibition building destroyed
The overall number of immediate death (by heat and blast) and death by radiation in
Hiroshima is estimated to have been more than 200,000 by the end of 1945. It was about
80,000 in Nagasaki. Some of the damages on human bodies and the buildings and the entire
city are seen in Figs. 5.2~ 5.5. The radiation effects are difficult to visualize, but devastating
as well and will be discussed later.
Fig. 5.5. The instantly flattened Hiroshima
