REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

1

Initial process of atomic bomb cloud formation andradioactivity distribution
Tetsuji Imanaka
Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka, 590-0494 Japan
5

Abstract

10

15

In order to provide basic information for the HiSoF project, current knowledge isreviewed about the process of atomic bomb cloud formation and radioactivity
distribution. Based on the results of hydrodynamic simulation as well as observations
at Nevada test site in US, the time course of the atomic bomb cloud formation was
discussed. The amount of radioactivity that could be the origin of local fallout wascalculated both for fission products and neutron-induced radioactivity.
From the preliminary analysis using simple assumptions, 0.1 % deposition offission product radionuclides in the atomic bomb cloud could cause radiation
exposure more than 10 mGy, while radiation exposure was at most 1 mGy from thedeposition of neutron-induced radionuclides in dust or fire ash after the bombing.
More detailed and reliable evaluation is expected through the efforts of HiSoF.

Introduction
The atomic bomb, named Little Boy exploded over the Hiroshima city at 0815 on August 6,
1945. According to the report of radiation dosimetry system DS02 (Young and Kerr 2005), theheight of burst (HOB) and the energy yield of explosion are estimated to be 600 ± 20 m and 16± 2 kt TNT, respectively. Although official information is not yet available about the detailed
structure and composition of Little Boy, the amount of initially loaded uranium is reported to be
64.15 kg, the average 235U enrichment of which is 80 % (Coster-Mullen 2008). Using equivalent
25 values per yield of 1 kt TNT (Glasstone and Dolan 1977), the 16 kt TNT explosion is equivalent
to 2.32×1024 total fissions, corresponding to a 235U mass of 910 g and a total released energy of
1.6×1013 cal. If a spherical ball is made from 64.15 kg of uranium with a density of 19.05 g cm-3,
its diameter will be 18 cm, something like the size of a volleyball. The duration of the fission
chain reaction that continued in Little Boy is considered to be about 1 μsec.
The most outstanding feature of the atomic bomb explosion is that a huge amount of energy
30
is released within a very short time in a very small space, which produces a core with extremely
high temperature (> 107 °K) and pressure (> 106 atm). This core expands very rapidly,transmitting its energy to the surrounding materials mainly by low energy X-rays. The entire
bomb materials are engulfed and vaporized by the expanding core, which is called the fireball.
35 According to the description in The Effects of Nuclear Weapons (Glasstone and Dolan 1977), at
20

ISBN 978-4-9905935-0-6
Revisit the Hiroshima A-bomb with Database: 1-14
© Tetsuji Imanaka

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

2

0.1 msec after a 20 kt explosion, the temperature of the fireball decreases to 300,000 °K with aradius of 40 feet (about 12 m). At this temperature, as the expansion velocity of the fireballdecreases compared to the local acoustic velocity, a shock wave appears at the fireball surface
and its front moves ahead of the fireball expansion. At this stage, because of the opacity of
5 shock wave heated air, the internal fireball is not visible through the shock wave front. Then, at
0.1 – 0.3 sec after the detonation, as the air becomes less opaque with the temperature decrease,the luminous inner fireball can be seen with a surface temperature of 6,000 – 7,000 °K. The
fireball loses its luminosity in several seconds.
Updraft throughcenter of toroid

Toroidal circulationof hot gases
Cool air being drawn
up into hot cloud

Figure 1 Mushroom cloud formed after nuclear explosion in air at low altitude.

10

15

In case of the Hiroshima explosion, the shock wave blast was estimated to arrive at the
ground about 1 sec after the detonation, while the fireball began to ascend without touching the
ground. As the fireball rose up, a strong upstream of air followed it like a chimney. Then the socalled ‘mushroom cloud’ was formed. A simple conceptual scheme of the mushroom cloud
structure is drawn in Figure 1. The formation process and radioactivity distribution of theHiroshima atomic bomb are discussed in this paper.

Atomic bomb cloud formation
Hydrodynamic simulation for Little Boy explosion

During the processes elaborating DS86/DS02, the US working group carried outhydrodynamic simulation of Little Boy and Fatman (Nagasaki bomb) for the purpose ofdetermining the position of the rising fireball as well as the air density disturbance by the
20 explosion, using the STLAMB code that was developed to simulate hydrodynamic processes for
low altitude nuclear explosions. An example of air density contour plot obtained by STLAMB
calculation in DS86 is shown in Figure 2 for the Little Boy explosion (height of burst: 580 m,
yield: 15 kt) (Roesch 1987). After the initial rapid growth up to a radius of about 260 m, theinternal pressure of the fireball reached equilibrium with the ambient air at 0.35 sec after the
25 detonation. At this moment the height of the fireball centre had not yet moved from the HOB at

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE①

0.354 sec

3

②1.034 sec④3.067 sec③2.028 sec⑤9.725 sec⑥20.004sec

Figure 2 STLAMB simulation of Little Boy explosion in DS86. Air density contour.
[Reproduced from US-Japan Joint Reassessment of Atomic Bomb Radiation Dosimetry in
Hiroshima and Nagasaki, Final Report (DS86), Vols. 1&2, Radiation Effect Research
Foundation (1987)]
5

8000

Height of fireball center, m

7000

STLAMB-1
STLAMB-2

6000
5000
4000
3000
2000
1000
0
0

60

120

Time after explosion, sec

Figure 3 Height of cloud centre after the bombing

180

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

4580 m. The air density of the fireball centre was 2.26×10-5 g cm-3, compared with 1.11×10-3 The
shock wave front had already gone ahead of the fireball, (120 m from the fireball surface), but it
cannot be seen in Figure 2. At 2.028 sec, the shock wave had been 580 m. The air density of thefireball centre was 2.26×10-5 g cm-3, compared with 1.11×10-3 already reflected at the ground5 surface. At 3.067 sec, the reflected wave passed through the fireball. The fireball began rising at
a speed of 50-60 m sec-1. It reached 1100 m at 10 sec, and 1600 m at 20 sec. During this time
the shape of the fireball was changing from spherical to toroidal.
Similar STLAMB simulation was carried out during the process developing DS02. The
author received the output lists of STLAMB calculation for Little Boy (HOB: 600 m, 16 kt) up
10 to 3 min after the explosion (Egbert 2010). Two sets of STLAMB results were obtained: one
580 m. The air density of the fireball centre was 2.26×10-5 g cm-3, compared with 1.11×10-3(STLAMB-1) is up to 30 sec with 18 time intervals and another (STLAMB-2) is up to 3 minwith 12 time intervals. From these data, the temporal change of the height of the fireball centeris plotted in Figure 3. At 3 min after the explosion, the fireball has risen up to 7,000 m.
15Comparison with observation in Nevada test site

A large report that compiled observation for all atmospheric nuclear tests in the Nevada testsite was obtained through the Internet (Hawthorne 1979). From this report, seven tests
comparable to the Little Boy explosion were chosen that were conducted by airdrop and
exploded at a height where the fireball did not touch the ground. Rising patterns of the atomic
20 bomb cloud for these seven tests were compared in Figure 4 with the STLAMB simulation for
Little Boy.
In Figure 4, HOB values for Nevada tests are adjusted to be the same HOB (600 m) as Little
Boy. The result of STLAMB simulation is similar to BJ Charlie（14kt）and BJ Dog（21kt）.
By extrapolating the tendency of STLAMB simulation to the later period, it can be roughly said
25 “The cloud height of Little Boy was about 8000 m at 4 min, and ascended to about 12000 at 12
min”.

16000

Height of fireball center, m

14000

STLAMB-1 16kt
STLAMB-2 16kt

12000

BJ Baker 3.5kt

10000

BJ Charlie 14kt
BJ Dog 21kt

8000

BJ Easy 31kt

6000

TS Charlie 31kt

4000

TS Dog 19kt

2000

UK Encone 27kt

0
0

1

2

3

4

5

6

7

8

9

10

11

12

Time after explosion, min
Figure 4 Comparison of cloud ascent data for STLAMB simulation and Nevada observations.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

5Size of the atomic bomb cloud

Values of the fireball radius obtained by STLAMB simulation are plotted in Figure 5. RFB
indicates the horizontal radius of the cloud, while RFBM is considered to be vertical radius of
the spheroidal cloud or toroidal cloud. Several observations are also reported for Nevada tests
5 about the cloud size increase. According to these observations, the horizontal diameter increased
almost linearly with time up to 20 – 30 min after explosion. The horizontal width (cloud top –cloud bottom), however, seemed to saturate at a constant value after the cloud stopped
ascending.

1200
STLAB-1 RFB

Radius of fireball, m

1000

STLAB-2 RFB
STLAMB-2 RFBM

800
600
400
200
0
0

60

120

180

Time after explosion, sec

Figure 5 Cloud radius by STLAMB simulation: RFB and RFBM.

Center height or radius of cloud, m

10

15000
14000
13000
12000
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0

Cloud center height
Horizomtal radius
Vertical radius

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Time after explosion, min

Figure 6 A-bomb cloud formation when there are the same parameters as Little Boy: HOB;
600 m, 16 kt.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

6

Taking into account the results of STLAMB simulation (Figure 5) as well as observations of
nuclear tests in Nevada, an ideal case of the atomic bomb cloud formation with the same bomb
parameters as Little Boy is plotted in Figure 6 up to 20 min after the explosion. The cloud
ascends until 12 min after the explosion and its center height reaches 12 km. The horizontal
5 radius increases linearly with the elapsed time and it becomes 5 km at 20 min, while the verticalradius (half value of the cloud thickness) saturates at 1 km at 12 min.
Temperature decrease and particle formation

As the fireball rises up, its temperature decreases by various mechanisms of thermalradiation, adiabatic expansion and mixing with cool air. With decrease of temperature,
10 vaporized materials of the bomb components begin to condense and solidify, forming small
particles that absorb and/or adsorb fission product nuclides. In order to consider the process ofparticle formation, the time sequence for temperature decrease of the fireball is estimated basedon STLABM simulation as well as literature data.
100000

STLAMB-1
STLAMB-2

Tempearature, K

Extrapolation
Krasilov's equation

10000

Izrael's equation
Std atmosphere

1,643 K
1000

296 K

100
0

1

2

3

4

5

6

7

8

9

10

11

12

Elapsed time after explosion, min

Figure 7 Temperature change of the fireball/atomic bomb cloud.
Although temperature data are not included in the output of the STLAMB simulation, the air
density at the fireball centre is provided in the list. Considering that “pressure equilibrium”
between the fireball and the ambient atmosphere is attained at 0.4 sec after explosion, the air
density can be related to temperature based on the ideal gas equation PV = nRT . For example,
when compared with 1.16 × 10-3 g cm-3 air density (ambient air of about 300 °K), air density of-5
20 2.26 × 10
g cm-3 at 1 sec corresponds to 15,400 °K. The temperature decrease obtained by thisway from STLAMB calculation is plotted in Figure 7 together with temperature curves
proposed from Russian scientists. The black solid line is the equation;15⎛ 1 20 ⎞T (t ) = 7500° K exp⎜⎜ −
t ⎟⎟
3
q
⎝
⎠
given by Krasilov (2008), while the red broken line is taken from Izrael’ s book (1995); T(t)＝-0.58825 4000t（t<40sec） or 2183t-0.374 (t>40 sec) for 20 kt air burst. The green dotted line is an

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

7

extrapolation of STLAMB-2 up to the point of ambient temperature at 12 min using Izrael’s
equation of T(t)＝a･t-b. The blue line indicates ambient air temperature at the height of thefireball (Figure 6). Values of 1,643 and 296 °K are the melting point of iron oxide (FeO) anddew point for air of 27 °C and relative humidity of 80 %, respectively. The STLAMB plot in
5 Figure 7 indicates that droplets of FeO begin to solidify at about 20 sec after the explosion.
Size distribution of particles in the atomic bomb cloud

According to The Effects of Nuclear Weapons (Glasstone and Dolan 1977), in the case of air
explosion by which no appreciable quantities of surface materials are taken up into the fireball,small particles with a range of 0.01 to 20 μm are formed from condensed residues of the bomb
10 materials. Although it is difficult to say definitely what was the real situation in Hiroshima, thephoto taken from the B28 bomber at 2 – 3 min after the bombing (Figure 8) supports the ideathat particles of dirt and dust raised by the bomb
blast almost remained at low altitude and were
not substantially sucked into the fireball or the
15 cap part of mushroom cloud.
The process of particle formation fromvaporized materials is discussed in detail byStorebo (1974) dividing the process into
nucleation, condensation and coagulation. The
20 size distribution of particles is considered tofollow a log-normal law;dn(r )=
d ln r()⎛ − ln r − μ ) 2 ⎞
⎟⎟
exp⎜⎜
2
2
σ
2π σ
⎠
⎝
1

Here, n(r): particle density function for
diameter, r,
25 μ, σ; geometric mean and geometric standard
deviation, respectively. In Storebo’s paper (1974)
Figure 8 Photo of the Hiroshima bomb
μ and σ values are evaluated as parameters
cloud taken from the B29, Enola Gay atdepending on the initial condition of vapour-toseveral min after the bombing.
air mass ratio in the fireball. Assuming that 4 ton
30 of iron was vaporized by the Little Boy explosion
and mixed with the spherical air mass of 260 m radius and 2.26×10-5 g cm-3 density, a value of
0.002 was obtained as a vapour-to-air mass ratio. Then, the values of the geometric mean and
GSD (geometric standard deviation) were calculated to be 0.2 μm and 1.35, respectively.
Histograms of log-normal particle-size distribution are shown in Figure 9 together with the case
35 for a surface explosion for which the particle distribution was arbitrarily chosen as having ageometric mean of 20 μm and GSD of 3.
To consider the possibility of local fallout by gravitational deposition on the ground, theterminal velocity of descending particles is calculated based on Stokes’ equation;

D 2 ρgVt ( D ) =18η
40

Here, Vt(D); terminal velocity of particle with diameter, D,
ρ, g and η are density, gravitational acceleration and viscosity.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

8

Density function n(r), arbitary unit

4

Air burst
3

Surface burst

2

1

0
0.01

0.1

1

10

100

1000

Particle radius r, μm

Figure 9 Examples of particle-size distribution for air burst and surface burst. Air burst;
mean 0.2 μm and GSD 1.35, Surface burst; mean 20 μm and GSD 3.
The results are shown in Figure 10 as a function of particle radius for three values of particle
density. Terminal velocity for particle radius more than 200 μm is not plotted in the figure
because deviations from the simple Stokes’s equation become significant and velocities are
5 lower in this region than those based on Stokes’s equation. As can be seen in Figure 10,
deposition velocity drastically changes with particle size. Considering that the fireball createdby Little Boy could rise up to about 10,000 m at 10 min after the explosion and the deposition
velocity for the supposed particle-size distribution (Air burst in Figure 9) would be less than 0.1
cm s-1, the possibility of local fallout due to gravitational deposition, the so-called ‘dry
10 deposition’ can be excluded. The only realistic path of local fallout is considered to bedeposition on the ground with rainfall, so-called ‘wet deposition’. In case, however, particles
more than 100 μm were somehow included in the atomic bomb cloud, they could descend to theground by gravity several hours after the explosion.
15

20

25

Terminal deposition velosity, cm s

-1

1.E+04

Density: 3 g cm^-3

1.E+03

Density: 2 g cm^-3

1.E+02

Density: 1 g cm^-3

1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
0.1

1

10

100

1000

Particle radius, μm

Figure 10 Particle-size dependency of terminal descent velocity for particle density of 1, 2
and 3 g cm-3.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

9Radioactivity distribution
Local radioactive fallout related with the Hiroshima/Nagasaki bombings are divided into two
origins of radioactivity: fission products (FP) from nuclear materials - uranium or plutonium and neutron-induced radionuclides in exposed materials.
5

Fission products in the atomic bomb cloud

The amount of radioactivity of fission product nuclides produced by the 16 kt explosion canbe calculated based on physical data such as fission yields, decay modes, branching ratio etc.
For the purpose of calculating fission product composition after nuclear fission reaction, ahandy program, named FPCOMP.xls was developed using Visual Basic installed in Microsoft
Excel (Imanaka and Kurosawa 2008). Nuclear data for 1,227 FP nuclides ranging from mass
10
number of 66 to 172 were incorporated from the JNDC Nuclear Data Library (Ihara 1989).
The results of the FPCOMP calculation for a 16 kt 235U explosion are shown in Figures 11
and 12. Temporal change of total radioactivity is shown in Figure 11 together with some ofindividual radionuclide. As can be seen in Figure 11, radionuclides contributing to the total
15 radioactivity change with elapsed time after explosion, while the amount of total radioactivity
steadily decreases along a rather straight line.
1E+22
Total
Y-92

1E+21
20

Radioactivity, Bq

25

Zr-97
Tc-101

1E+20

Xe-133
Xe-135

1E+19

I-131
I-133

1E+18

I-134
Cs-138

1E+17

Cs-139Ba-140

1E+16
30

0.1

1

10

100

1000

Time after explosion, hr

Figure 11 Radioactivity after 16 kt explosion of 235U: Contribution of major radionuclides.
In Figure 12, the total radioactivity is divided into ‘Refractory’, ‘Volatile’ and ‘Rare gas’
35 according to characteristics of elements that FP radionuclides belong to. The reason for thisclassification is that the deposition behaviour of FP radionuclides varies a good deal depending
on characteristics of the element. Rare gas nuclides do not deposit on the ground. It is known
that refractory elements tend to distribute inside FeO particles because they are absorbed whenparticles are still liquid droplets, while volatile elements tend to be adsorbed on the surface after
40 solidification of FeO particles. So, particle-size radioactivity distribution can be different
depending on the FP radionuclides, which can lead to a different type of deposition on the
ground. This phenomenon is called ‘fractionation’ of radioactive fallout (Imanaka et al 2010).
The dotted line is the equation of A(t ) = A0 t −1.21 that is often used to approximate the decay of
total FP radioactivity.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

10

1E+21

Total
Refractory
Volatile
Rare gast^-1.21

Radioactivity, Bq

1E+20

1E+19

1E+18

1E+17

1E+16

1E+15
1

10

100

1000

Time after explosion, hr

Figure 12 Radioactivity after 16 kt explosion of 235U: Refractory, Volatile and Rare gas.
Radiation exposure from FP fallout

As a sample problem to consider the radiological consequences of local fallout in Hiroshima,radiation exposure rate was evaluated on an assumption that 0.1 % of the total FP radionuclides
produced by the explosion uniformly were deposited at 1 hr after the explosion on the area of 66
2
5 km corresponding to the heavy black rain area reported by Uda (1953). For this case the initialdeposition density of 137Cs amounted to 1.6 kBq m-2. Following a retrospective dosimetry
method developed by Imanaka et al (2010), 31 nuclides were chosen as representatives of FP
radionuclides that could contribute to external gamma-ray exposure when they deposited on the
ground. The deposition amount of individual FP nuclides was determined using the theoretical137
10 activity ratio to
Cs. The total density of FP deposition at 1 hr after explosion became 1.2×106
kBq m-2, about 8×105 times larger than 137Cs itself. Then, summing up gamma-ray exposure
from the 31 FP nuclides, the temporal change of radiation exposure at 1 m above the ground
was calculated and shown in Figure 13 for a case without any fractionation effect.

Dose rate in air, mGy h-1

1.E+01

1.E+00Initial 137Cs deposition
１.6 kBq/m2（no fractionation）

I-134

La-142

Total
I-135

1.E-01
Zr-97

I-132

1.E-02
La-140

1.E-03
0.1

1

10

100

Time after explosion, day

Figure 13 Gamma-ray exposure rate at 1 m above the ground from deposited FP nuclides on
the ground.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

11

A radiation exposure rate of 10 mGy h-1 was calculated at the time of deposition of 1 hr after
explosion. Then, as seen in Figure 13, the exposure rate decreased rapidly with time to 0.6, 0.04
and 0.007 mGy h-1 at 1 day, 1 week and 1 month after, respectively. The corresponding
cumulative radiation exposure in air was calculated to be 12, 30, 36 and 40 mGy at 1 day, 1
5 week, 1 month and 1 year, respectively.
Neutron-induced radioactivity in exposed material

At the moment of the explosion, gamma-rays and neutrons are emitted from the atomic bomb
as components of the initial radiation. When materials are exposed to neutrons, some elementsin them absorb neutrons and become radioactive. This is another origin of residual radiation,
10 called ‘neutron-induced radioactivity’. If significant amounts of dust/soil that contain neutroninduced radioactivity are blown up by the bomb blast in the air, they also can be sources ofradioactive fallout.
Through the work to develop DS02 dosimetry system, the neutron energy spectrum released
from the Little Boy explosion was calculated at Los Alamos National Laboratory (Young and
15 Kerr 2005). Values of neutron fluence on the ground from Little Boy are given as a function of
the distance from the hypocenter (Egbert et al 2007). Based on these data the amount of neutron
activation reaction in exposed materials can be calculated (Imanaka et al 2008, Tanaka et al2008). Manganese-56 (half life: 2.58 h) and sodium-24 (14.96 h) are considered the two main
nuclides that can contribute to radiation exposure as neutron-induced radioactivity.
The specific radioactivity of these two nuclides is calculated based on the DS02 fluence values
20
and shown in Figure 14 as a function of distance from the hypocenter. Activation values at 2.5
cm in the ground are about 50 % higher than those at 1 m above the ground. This is because
thermal neutron fluence increases with depth of soil to some extent as fast neutrons change tothermal neutrons by loosing their energy in the soil layer. The thermal neutron fluence becomes
1.E+06

Mn56: 2.5cm in the groundMn56: 1 m above the ground

Bq per mg-Mn or -Na

1.E+05

Mn56: Endo/Tanaka
Na24: 2.5 cm in the ground

1.E+04

Na24: 1 m above the groundNa24: Endo/Tanaka

1.E+03
1.E+02
1.E+01
1.E+00
1.E-01

0

500

1000

1500

2000

Ground distance from the hypocenter, mFigure 14 Specific neutron-induced radioactivity for 56Mn and 24Na as a function of distancefrom the hypocenter. Solid and broken lines are at 2.5 cm in the ground and at 1 m above theground, respectively, based on DS02 neutron fluence values. Endo/Tanaka data are a resultof Monte Carlo transport calculation using neutron leakage spectrum from Little Boy.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

12

maximum around 2.5 cm. As seen in Figure 14, the induced radioactivity decreases rapidly withthe distance from the hypocenter. So, here in this article, only activated radionuclides within
500 m from the hypocenter were considered as the origin of local fallout due to neutron-induced
radioactivity.
5

Dust and dirt

According to rough estimation (Aoyama 2011), as a result of the blast damage by the atomic
bombing, 1.5×104 ton of dust/dirt could be blown up into the air from the 500-m radius areaaround the hypocenter. The average concentrations of manganese and sodium elements in dust
are assumed to be 0.587 and 12.4 mg per g of dust, respectively, based on the Hiroshima soil
10 data in DS86 (Roesch 1987). Using the specific radioactivity data at 2.5 cm in the soil, the total
radioactivity contained in the blown-up dust can be calculated to be 5.0×1014 and 1.7×1014 Bq
for 56Mn and 24Na, respectively. If these radionuclides uniformly spread out over the area of 66
km2 of the strong black rain as given by Uda (1953), the radioactivity densities become 5.8 and
2.5 MBq m-2 for 56Mn and 24Na, respectively, at 1 hr after the explosion. Using conversion-1
15 coefficients of 6.7 and 15 μGy h
per MBq m-2 for 56Mn and 24Na, respectively, radiation
exposure rate of 76 μGy h-1 can be obtained at 1 h after the explosion: 39 μGy h-1 from 56Mn
and 37 μGy h-1 from 24Na. Assuming that these nuclides remain at the same place, a cumulative
radiation exposure becomes to be 960 μGy: 150 μGy from 56Mn and 810 μGy from 24Na.
Fire smoke and ash

As a result of fire after the bombing, Aoyama estimated about 1.4×104 ton of wooden material
were burnt out within the 500-m radius area around the hypocenter in Hiroshima (Aoyama
2011). Almost all ash, the weight of which was estimated 1 % of burnt wood, was considered tohave risen up into the sky during the fire. Considering the information that pine was the mainwood used as house construction at that time in Hiroshima as well as the experimental data on
25 ash composition for various wooden materials (Misra 1993), elemental concentrations in the fireash are assumed to be 4.04 and 0.06 wt-% for manganese and sodium, respectively.
Consequently, using the specific radioactivity data at 1 m above the ground, 2.0×1014 Bq of56
Mn and 6.2×1011 Bq of 24Na were supposed to be raised up by the fire from the 500-m radius
area around the hypocenter if radioactivity converted to values at 1 hr after the explosion.
Using the same method as dust/dirt, radiation exposure rate of 21 μGy h-1 was obtained at 1 hr
30
after the explosion: 21 and 0.1 μGy h-1 from 56Mn and 24Na, respectively. The total cumulative
exposure was 80 μGy: 77 and 3 μGy from 56Mn and 24Na, respectively.
20

Conclusions
Main points discussed in this article about the atomic bomb explosion in Hiroshima are
35 summarized as follows:• The atomic bomb cloud is supposed to have risen up to 12 km at 12 min after the
explosion. The radius of the cloud could be about 5 km (Figure 6).
• The temperature of the fireball/A-bomb cloud decreased to 1643 °K, the melting pointof FeO at about 20 sec after the explosion (Figure 7).
• Radial size distribution of particles that were formed during cooling process of the40
fireball and contain fission product nuclides is considered to have a mean less than 1 μm

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

13(Figure 9). Under this condition, the main path of local fallout becomes deposition with
rainfall because the possibility of gravitational deposition on the ground can be excluded.
• If 0.1 % of fission product nuclides are assumed to deposit uniformly on the ground of
66 km2 corresponding to the strong black rain area, cumulative radiation exposure of 36
mGy is estimated at 1 m above the ground for the period of one month after the5
deposition.
• As a possibility of local fallout due to neutron-induced radioactivity, very roughestimate for cumulative radiation exposure of 0.96 and 0.08 mGy are obtained due to
updraft of dust/dirt by the blast and smoke/ash by the fire, respectively.
10This manuscript is prepared to provide basic knowledge about the atomic bomb explosion in
Hiroshima, for hypothetical persons who would join HiSoF project as meteorological simulation
modellers, but are not familiar with physical process of atomic bomb explosion. The author asks
readers to think that all points mentioned in this manuscript are not fixed views of HiSoF, but
can be changed flexibly depending on views of person who are going to join our study. The
15 author is ready to provide more detailed information related to the contents of this manuscript
when he is asked.

Reference
Aoyama M., 2011 private communication from Aoyama M.
Coster-Mullen J. 2008. ATOM BOMBS: The top secret inside story of Little Boy and Fat man. John
Coster-Mullen (self-published).
20
Egbert S. D. et al., 2007. DS02 fluence spectra for neutrons and gamma rays at Hiroshima and Nagasaki
with fluence-to-kerma coefficients and transmission factors for sample measurements. Radiat Environ
Biophys 46, 311–325
Glasstone S., Dolan J. P., 1977. The effects of nuclear weapons, Third edition, US Department of
25
Defence and ERDA
Hawthorne H. A. ed., 1979. Compilation of local fallout data from test detonations 1945-1962 extracted
from DASA 1251, Vol.I- Continental U.S. Tests. DNA 1251-1-EX, 1979
Ihara H. ed., 1989. Tables and Figures from JNDC Nuclear Data Library of fission products, Version 2.
JAERI-M 89-204.
30 Imanaka et al. 2008. Gamma-ray exposure from neutron-induced radionuclides in soil in Hiroshima andNagasaki based on DS02 calculations. Radiat Environ Biophys 47 pp.331-336.
Imanaka T., Kurosawa N. 2009. Application and development of FPCOMP.xls for calculating FP
composition. Proc. Ninth Workshop on Environmental Radioactivity, Tsukuba, Mar 27-28, 2008,
KEK Proc 2008-9 pp51-60 (in Japanese)
35 Imanaka et al, 2010. Reconstruction of local fallout composition and gamma-ray exposure in a village
contaminated by the first USSR nuclear test in the Semipalatinsk nuclear test site in Kazakhstan.
Radiat Environ Biophys 49, 673-684.
Izrael Yu. A., 1996. Radioactive fallout after nuclear explosions and Accidents. Progress-pogoda, St.
Petersburg (in Russian).
40 Krasilov, 2008. Private communication.
Misra M. K. et al., 1993. Wood ash composition as a function of furnace temperature. Biomass andBioenergy 4, 103-116.
Roesch W. C. ad., 1987. Reassessment of atomic bomb radiation dosimetry – Dosimetry System 1986.
Vols. 1&2. Radiation Effects Research Foundation, Hiroshima
45 Storebo P. B., 1974. Formation of radioactive size distribution in nuclear bomb debris. Aerosol Science 5,557-577.

REVISIT THE HIROSHIMA A-BOMB WITH A DATABASE

14

Tanaka K. et al, 2008. Skin dose from neutron-activated soil for early entrants following the A-bombdetonation in Hiroshima: contribution from β and γ rays. Radiat Environ Biophys 47, 323-330.
Uda, M. et al., Kita I. Meteorological conditions related to the atomic bomb explosion in Hiroshima. In:
Collection of reports on investigations of the atomic bomb casualties. Science Council of Japan,
Tokyo, 1953, 98-136 (in Japanese).
5Young R. W., Kerr G. D. ed., 2005. Reassessment of the atomic bomb radiation dosimetry for Hiroshima
and Nagasaki: Dosimetry System 2002. Radiation Effects Research Foundation, Hiroshima