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One of the most practical nuclear reactions results from the compound nucleus that results from A>230 nuclei absorbing neutrons.
Often split into two medium mass nuclear fragments plus additional neutrons.
Alpha particle energy
Cro
ss S
ect
ion
,n
,2n
,3n,4n
Total
NUCLEAR FISSION
1930 Bothe & BeckerStudying -rays bombarding berylliumproduced a very penetrating non-ionizing form of radiation
-rays?
Irène and Frédéric Joliot-Curie
knocked protons free from paraffin targetsthe proton energy range revealed the uncharged radiation from Be to carry 5.3 MeV
1932 James Chadwickin discussions with Rutherford
became convinced could not be s
Assuming Compton Scattering to be the mechanism, E>52 MeV!
Neutron chamber
ionization(cloud)
chamber
Replacing the paraffin with other light substances, even beryllium, the protons were still produced.
Nature, February 27, 1932
Chadwick developed the theory explaining the phenomena as due to a 5.3 MeV neutral particle
with mass identical to the protonundergoing head-on collisions with nucleons in the target.
1935 Nobel Prize in Physics
9Be has a loosely bound neutron (1.7 MeV binding energy)
above a closed shell:
nCBeHe 1294
5-6 MeV from someother decay
Q=5.7 MeV
Neutrons produced by many nuclear reactions (but can’t be steered, focused or accelerated!)
Natural sources of neutrons
Mixtures of 226Ra ( source) and 9Be ~constant rate if neutron production
also strong source
so often replaced by 210Po, 230Pu or 241Am
Spontaneous fission, e.g. 252Cf ( ½ = 2.65 yr)
only 3% of its decays are through fission 97% -decays
Yield is still 2.31012 neutrons/gramsec !
PdU 119
46
238
922
A possible (and observed) spontaneous fission reaction
238U119Pd
8.5 MeV/A
7.5 MeV/A
Gains ~1 MeV per nucleon!2119 MeV = 238 MeV
released by splitting
PdU 119
46
238
922
Yet
is a rare decay: ½ = 1016 yr
not as probable as the much more common -decay
½ = 4.510 9 yr
Atomic (chemical) processes ~few eV
Fission involves 108 as much energy as chemical reactions!
From the curve of binding energy per nucleon the most stable form of nuclear matter is as medium mass nuclei.
),(),(),(2211
ZAMZAMZAM Consider:
),(),(),(2211
ZABZABZABQ The Q value (energy release) of this process is
The mass differences cancel since the total number of constituents remains unchanged.
For simplicity, if we assume the protons and neutrons divide in the same ratio as the total nucleons:
111// yZZAA
222// yZZAA
121 yy
))()(1())()(1( 3/5
2
3/5
1
3/2
2
3/2
1yyEyyEQ
CS
The difference in binding energy comes from the surface and coulomb terms
so the energy released can then be expressed in terms of the surface energy Es and the coulomb energy Ec
of the original nucleus (A,Z).
maximum Q is found by setting dQ/dy1 = 0
121 yyNote: 1/
12dydy
maximum occurs when y1 = y2 = 1/2.
SCEEQ 26.037.0
Fission into two equal nuclei (symmetric fission) produces the largest energy output or Q value
The process is exothermic (Q > 0) if Ec/Es > 0.7.
in terms of the fission parameter, x
50
)/()/2)(/()2/(
2
2 AZaaAZEEx
CSSC >0.35
Suggesting all nuclei with (Z2/A) > 18 (ie heavier than 90Zr) should spontaneously release energy
by undergoing symmetric fission.
However
Half-life of spontaneous fission as a function of x
where
criticalAZ
AZx
)/(
/2
2
and
49)/( 2 criticalAZ
R.Vandenbosch and
J.R.Huizenga.Nuclear Fusion,Academic Press,New York, 1973.
There isa competition between
the nuclear force binding the nucleus together
and the coulomb repulsion
trying to tear it apart
Induced fission as nuclear reaction
nBrLaUUn 295
35
139
57
236
92
235
92
nCsRbUUn 2141
55
93
37
236
92
235
92
suggests the absorption of the neutron (and its energy)may induce such distortions/vibrations in the nucleus.
The surface if any arbitrary figure can be expanded as
00
]),(1[l
l
lm
m
lm lYRR
If lm time-independent: permanent deformation of the nucleus
If lm time-dependent: an oscillation of the nucleus
The Spherical Harmonics Y ,ℓ m(,)
ℓ = 0
ℓ = 1
ℓ = 2
ℓ = 34
100 Y
ieY sin
8
311
cos4
310
Y
ieY 2
2
sin2
15
4
122
ieY cossin
8
1521
2
12cos2
3
4
1520
Y
ieY 3
3
sin4
35
4
133
ieY 2
cos2
sin2
105
4
132
ieY 12cos5sin
4
21
4
131
cos
2
33cos2
5
4
730Y
ℓ = 0
ℓ = 1
sin1~R
cos1~R
z Nuclear Charge Density
ℓ = 2Lowest order to be considered:
quadrupole deformation
For which we write the nuclear radius
]),(1[2
2220
m
m
mYRR
The l=2, m=0 mode:
]1[)( )12
cos3(
2/1
16
5200
RtR
]1[)( )12
cos3(
2/1
16
5200
RtR
Z
Example of a vibrational spectrum (levels denoted by the number of phonons, N)O.Nathan and S.G.Nilsson, Alpha- Beta- and Gamma-Ray Spectroscopy,
Vol.1, (K. Siegbahn, ed.) North Holland, Amsterdam, 1965.
Nuclei do show spectra for such vibrational modes
We can approximate any small elongation from a spherical shape by
)1(2
10
Rb
)1(0
Ra
3/12123/2 )( AZaANZaAaAaBCsymSV
The semi-empirical mass formula
3/42
2
2
3/1 )(
AZaA
NZaAaa
A
BCsymSV
semi-major axis
semi-minor axis
)1( 23/1
52 AaE
SS
)1( 23/12
51 AZaE
CC
From which:
)()( 23/223/12
52
51 AaAZa
EEE
SC
SC
With the surface energy (strong nuclear binding force) proportional to area
2Ewhich we can write in the form
where ][ 3/23/12 2 51 AaAZa SC
Notice > 0
(so the Coulomb force wins out) for:
.492
2
C
S
a
a
A
Z Same fission parameterintroduced when estimating available Qin symmetric fission
Coulomb force deforming nucleus
surface tension holding spherical
shape
2E
r
comes from considering small perturbations from a sphere.
As long as these disturbances are slight, the Separation, r, of distinct fragments linearly follows
2r
for small r
2
04
)(
R
rQrV
separation r
V(r
)
At zero separation the potential just equals the release energy Q For Z2/A<49,
is negative.
r
for small r r
eZZrV
2
21)(
separation r
V(r
)
r r
While for large r, after the fragments have been scissioned
for large r
For such quadrupole distortions the figure shows the energy of
deformation (as a factorof the original sphere’s
surface energy Es)
plotted against for different values of
the fission parameter x.
When x > 1 (Z2/A>49)
the nuclei are completely unstable to such distortions.
The potential energy V(r) = constant-B as a function of the separation, r, between fragments.
Z2/A=36
Z2/A=49
such unstable statesdecay in characteristicnuclear times ~10-22 sec
Tunneling does allow spontaneous fission, but it must compete with
other decay mechanisms (-decay)
No stable stateswith Z2/A>49!
Tunnelingprobabilitydrops as
Z2/A drops(half-life
increases).
At smaller values of x, fission by barrier penetration can occur, However recall that the transmission factor (e.g., for -decay) is
eXwhere
drh
ErVm ])([22 m
while for particles (m~4u)this gave reasonable, observable probabilities for tunneling/decay
for the masses of the nuclear fragments we’re talking about, can become huge and X negligible.
nBrLaUUn 2* 95
35
139
57
236
92
235
92
nRbCsUUn 2* 93
37
141
55
236
92
235
92
Neutron absorption by heavy nuclei can create a compound nucleus in an excited state
above the activation energy barrier.As we have seen, compound nuclei have many final states into which they can decay:
nYXUUn A
Z
A
Z 2
2
1
1
236
92
235
92*
where Z1+Z2=92, A1+A2+=236
...in general:
Experimentally find the average A1/A2 peaks at 3/2
PROMPTNEUTRONS
nSrXeUUn 2* 95
38
139
54
236
92
235
92
The incident neutron itself need not be of high energy.
Thermal neutrons E< 1 eVSlow neutrons E ~ 1 keVFast neutrons E ~ 100 keV – 10 MeV
Typicalof decayProducts& nuclearreactions
“Thermal neutrons” (slowed by interactions with any material they pass through) have been demonstrated to be particularly effective.
This merely reflects the general ~1/v behavior we have noted for all cross sections!
Cro
ss s
ecti
on
incident particle velocity, v
At such low excitation there may be barely enough available energy to drive the two fragments of the nucleus apart.
Thus the individual nucleons settle into the lowest possible energy configurations
Division can only proceed if as much binding energy as possible
is transformed into the kinetic energy separating them out.
involving the most tightly bound final states.
(so MOST of the available Q goes into the kinetic energy of the fragments!)
There is a strong tendency to produce a heavy fragment of A ~ 140 (with double magic numbers N = 82 and Z = 50).
PdU 119
46
238
922
A possible (and observed) spontaneous fission reaction
238U119Pd
8.5 MeV/A
7.5 MeV/A
Gains ~1 MeV per nucleon!2119 MeV = 238 MeV
released by splitting
238 MeV represented an estimate of the maximum available energyfor symmetric fission.
For the observed distribution
of final statesthe typical average is
~200 MeV per fission.
Fragment kinetic energy 165 MeV
Prompt neutrons 5 MeV
Prompt gamma rays 7 MeV
Radioactive decay fragments 25 MeV
This 200 MeV is distributed approximately as:
235U
Isobars off the valley of stability (dark squares on preceding slide)-decay to a more stable state.
and decays can leave a daughter in an excited nuclear state
187W
1/2
5/2
187Re
0.13425
0.20625
0.61890
0.68610
198Au
2
0
198Hg
0.412 MeV
1.088 MeV
nKrBaUUn 3* 90
36
143
56
236
92
235
92
With the fission fragments radioactive, a decay sequence to stable nuclei must follow
143
57
143
56 eLaBa
neZrNdUUn 388* 90
40
143
60
236
92
235
92
eCe 143
58
143
59 ePr
143
59 edN
90
37
90
36 eRbKr
eSr 90
38
90
39 eY
90
40 eZr
nRbCsUUn 2* 93
37
141
55
236
92
235
92
With the fission fragments radioactive, a decay sequence to stable nuclei must follow
Pr 141
59
141
58
141
57
141
56
141
55 CeLaBaCs
CeLaBaCs 140
58
140
57
140
56
140
55 n
0.03%
25 sec
18 min
4 hr
33 d
65 sec
13 d
40 h
NbZrYSrRb 93
41
93
40
93
39
93
38
93
37
ZrYSrRb 92
40
92
39
92
38
92
37 n
1.40%
6 sec
7 min
10 hr
106 yr
5 sec
3 hr
4 h
nePrCeUUn 2888* 141
59
140
58
236
92
235
92
n3 n4sometimes or
nBrLaUUn 2* 95
35
139
57
236
92
235
92
nRbCsUUn 2* 93
37
141
55
236
92
235
92
nSrXeUUn 2* 95
38
139
54
236
92
235
92
nKrCsUUn 3* 90
36
143
56
236
92
235
92
For 235U fission, average number of prompt neutrons ~ 2.5
with a small number of additional delayed neutrons.
with every neutron freed comes the possibility of additional fission events
This avalanche is the chain reaction.
235U will fission (n,f) at all energies of the absorbed neutron.
It is a FISSILE material.
However such a reaction cannot occur in natural uranium (0.7% 235U, 99.3% 238U)
Total (t) and fission (f) cross sections of 235U.
1 b = 10-24 cm2
238U has a threshold for fission (n,f) at a neutron energy of 1MeV.
The difference between these two isotopes of uranium is explained by the presence of the pairing term
in the semi-empirical mass formula.
Notice:
0
4/3
4/3
Aa
Aa
pair
pair for Z even, N even
for Z odd, N odd
for A odd
Like nucleons couple pairwise into especially stable configurations.
Note the strong resonant capture of neutrons (n, ) in the energy range
10-100 eV(particularly
for 238U where the
cross-section reaches
high values)
The fission neutron energy spectrum peaks at around 1 MeV
At 1 MeVthe inelastic cross-section (n,n') in 238U exceeds the
fission cross-section.
This effectively prevents fission from occurring
in 238U.
Natural uranium (0.7% 235U, 99.3% 238U)
undergoes thermal fission
onlythe
Fission produces mostly fast neutrons
Mev
but is most efficiently induced by slow neutrons
E (eV)
Consider fission neutrons created deep enough in a lump of natural uranium
that we’ll just (for now) ignore that some neutrons may simply escaping from the sample.
1000
100
10
1
1000
100
10
1
10000
0.1
Cro
ss-s
ecti
on
(b
arn
s)C
ross
-sec
tio
n (
bar
ns)
The processes competing withneutron-induced fusion
have approximate cross-sections(read from the graphs at right) of
238U (n,n) elastic scattering ~ 5 barn(n,n’) inelastic scattering ~ 2 barn(n,) ~0.2 barn(n,f) fission ~0.6 barn
235U (n,n) elastic scattering ~ 5 barn(n,n’) inelastic scattering ~ 3 barn(n,) ~0.2 barn(n,f) fission ~ 2 barn
238U (n,n) elastic scattering 8.3 (n,n’) inelastic scattering 3.3(n,) 0.3(n,f) fission 1
235U (n,n) elastic scattering 6.7(n,n’) inelastic scattering 1.7(n,) 0.3
(n,f) fission 3.3
Giving a relative probability to each of:
0.7/99.3
With 2-3 neutrons generated by each fission,only ~20 neutrons in the second generation
- this is insufficient to sustain a chain reaction.
Of the first 100 fission neutrons we start with
238U (n,n) elastic scattering 63 (n,n’) inelastic scattering 25(n,) 2(n,f) fission 8
235U (n,n) elastic scattering 1(n,n’) inelastic scattering 0(n,) 0
(n,f) fission 0
~98 are captured in the dominant 238U
only 8 of these
captures result
in fission
mixing natural uranium with a material to slow (but not absorb) neutrons to lower energies
where the fission cross-section for 235U is large.
Most fissions are then induced by neutrons with thermal energies (~0.025 eV).
FAST REACTOR
a 50-50 mix of the two isotopes will sustain a chain reaction (most fission events occurring now in 235U by neutron energies in the range 0.3 - 2.0 keV.
Enriching the 235U content
THERMAL REACTOR moderating the neutrons to thermal speeds
Granulated powders can be mixed for this purpose. Powdered uranium
Or blocks of uranium fuel can be alternately stacked with graphite to form a nuclear pile.
Moderator(Graphite)
Moderator(Graphite)
FUEL
FUEL
FUEL
1. Starting with neutrons/fission
2. Avg of neutrons after fast fission
238U
3. p survive thermalization
4. pf number captured in 235U
235U5. k = pf(f /total) number producing fission
One fission event producesk = pf(f /total)
secondary fission events.
k is the reproduction factor.
A chain reaction requires k1.
If k=1 the core is “critical” and self-sustaining.
Typical values for natural uranium/graphite piles are
47.2 02.1 89.0p
88.0f 54.0/ tf
k=1.07
Uranium is not dumped into the core like coal shoveled into a furnace. Instead it is processed and formed into fuel pellets (~pencil eraser size).
The fuel pellets are stacked inside hollow metal tubes to form fuel rods 11 to 25 feet in length.
Before it is used in the reactor, the uranium fuel is not very radioactive.
The fuel rods are arranged in a regular lattice inside the moderator. The rods are typically 2-3 cm in diameter and
spaced about 25 cm apart.
The rods metal sheath or cladding – most commonly stainless steel or alloys of zirconium.
This cladding supports the fuel mechanically, prevents release of radioactive fission products into the coolant stream and
provides extended surface contact with the coolant in order to promote effective heat transfer.
A single fuel rod cannot generate enough heat to make the amount of electricity needed from a power plant. Fuel rods are carefully bound together in assemblies, each of which can contain over 200 fuel rods. The assemblies hold the fuel rods apart so that when they are submerged in the reactor core, water can flow between them.
In nuclear power plants, the moderator is often In nuclear power plants, the moderator is often water water (though some types do still use graphite).(though some types do still use graphite).
Fuel cell channelsin face
of reactor core.
Control rods slide in or out between the fuel rods to regulate the chain reaction.
contain cadmium or boron (high cross section for neutron absorption, without fission).
e.g., natural Boron is 20% 10B
Control rods act like sponges to absorb excess neutrons.
with a cross section for thermal neutrons of3840 b for the process
*710 LinB
When the core temperature drops too low, the control rods are slowly pulled out of the core, and fewer neutrons are absorbed.
When the temperature in the core rises, the rods are slowly inserted.
To maintain a controlled nuclear chain reaction, the control rods are manipulated until each fission results in just one neutron on average, all other neutrons effectively absorbed by the control rods.
Temperature changes in the core are generally very gradual. However should monitors detect a sudden change in temperature, the reactor immediately shuts down automatically by dropping all the control rods into the core. A shutdown takes only seconds and halts the nuclear chain reaction.
This very common type makes use of the excellent properties of water as both coolant and moderator (ordinary water does absorb neutrons – converting hydrogen into deuterium).
The Boiling Water Reactor (BWR) allows the water to boil in the reactor core and uses the steam to drive the turbines.
The highest temperature possible for liquid water (critical temperature 374°C) is a limitation for devices that use water to convey heat.
In this ideal case the heat is received isothermally (the working fluid at T1)
but rejected isothermally (at T2) with all processes reversible.
No real power plant operates on an ideal Carnot cycle, but the expression shows the higher T1, the higher the efficiency (T2 cannot be lower than
the outside temperature).
12 /1 TTFurthermore recall: the Carnot engine efficiency is
The core must be contained within a pressure vessel of welded steel (typically withstanding pressures of about 1.55 x107 Pa or 153 bar.
1st land based pressurized water reactor: Shippingport USA (1957).
Pressure vessels are enormouswith 9 inch thick walls, often weighing more than 300 tons.
The pressure vessel surrounds and protects the reactor core,providing a safety barrier and holding the fuel assemblies, control rods, and coolant.
Pressure vessels are made of carbon steel and lined with a layer of stainless steel to prevent rust. The pressure vessel is located inside the containment building, a thick concrete structure reinforced with steel bars.
A Fast Reactor has no moderator
and consequently a much smaller core.
The very high power involved means that liquid metals have to be used as coolants!
Liquid sodium is the most common but has the disadvantage of becoming
radioactive itself through 23Na(n, )24Na.
As well as generating power fast reactors are used for breeding fissile material.
Breeder Reactors
If uranium fission reactors used as sole source of electrical power needs
all high-grade ores used up within a few decades!
Fermi, Zinn (1944)
Can fissile nuclei be grown? (the result of any nuclear reaction)Can we create fissile material as a by product of any reaction?
The parent nuclei that spawns the fissile material is described as being a fertile nuclide.
Example: build a reactor core that runs on 239Pu (the fuel)
packed within a bed of 238U (the fertile nuclide)• =2.91 fast neutrons/239PU fission
Only one of these on average producing an additional fissionis sufficient for sustainability.
If the rest are incident on 238U there’s a chance of inducing
UUn 239238
U239 Np239
Pu2391/2= 25 min
1/2= 2.3 days
A well designed breeder reactor can double the amount of fissile material
in 7 – 10 years.
While neither natural Uranium cannot maintain a chain reactioneven small lumps of pure 235U or 239Pu cannot explode
simply because of the number of neutrons that escape before inducing fission.
Recall we need k = pf(f /total) > 1
A large enough (critical) mass of 235U or 239Pu can chain reactand the reaction set off by any accidental initial neutron
(even from a rare spontaneous fission event).
If N neutrons (initially even 1 or 2) are present at time ttheir number will increase during the next moment dt by
NdtdN where is related to k and obviously depends on
the fissile material and its geometry.
For critical samples of 235U ~108 Hz.
As long as we can treat as a constant
/)0()( teNtN where =1/ is the
“generation time.”
Five grades of uranium are commonly recognized:
1. Depleted uranium: containing < 0.71% 235U.2. Natural uranium: containing 0.71% 235U. 3. Low-enriched uranium (LEU), between 0.71 – 20% 235U.
• commercial power reactors use 2-6 % 235U fuels. • cannot be used to make nuclear explosives
4. Highly enriched uranium (HEU): containing > 20% 235U. Research and naval reactors use either LEU or HEU fuel.5. Weapon-grade uranium: HEU containing > 90% 235U.
Uranium- 233: fissile, weapon-useable isotope, derived from irradiating 232Th with neutrons, ½ =160,000 years; 10-20 kg required for a
nuclear device; less common than U-235 for making nuclear explosives.
Uranium-235: best suited for fission bomb (or fast reactor) when enriched to > 90% purity; 6-25 kg required for a nuclear bomb; “significant quantities” standards (for UN inspections) is as little as 3 kg; a recent international study estimates 1,750 tons of highly enriched 235U have been produced worldwide.
Plutonium-239: Highly carcinogenic ray emitter.
Unlike uranium, all (but trace quantities) of Pu are manufactured. 239Pu is produced in nuclear reactors when 238U is irradiated with neutrons.
½ = 24,000 years, and it is a fissile material.
Subsequent neutron captures lead to accumulations of 240Pu, 241Pu and 242Pu.241Pu is fissile, but 240Pu and 242Pu are not. However, all are fissionable by fast neutrons, and can be used either in combination or alone in nuclear explosives;
best fission explosive nuclear material. 3-8 kg required for nuclear explosive; "significant quantities standard" 1 kg.
Plutonium is ~10 times more toxic than nerve gas. When inhaled, the smallest particles cause cancer: inhaling 12,000 micrograms (millionths of a gram) causes death within 60 days. The dispersal of 3.5 ounces of plutonium could kill every-one in a large office building.
For weapon production, plutonium has to be at least 93% enriched. Plutonium technology for bomb construction is judged to be more difficult than 235U techniques.
The bomb dropped on Nagasaki contained 6.1 kg. There are about 1,200 metric tons of Plutonium on our planet of which some 230 tons have been produced for military purposes.
Weapons can be made out of plutonium with low concentrations of 239Pu and high concentrations of 240Pu, 241Pu, or 242Pu.The plutonium used in nuclear weapons typically contains mostly 239Pu and relatively small fractions of other plutonium isotopes. Plutonium discharged in power reactor fuel typically contains significantly less 239Pu and more of other plutonium isotopes.
The following grades of plutonium are widely used:1. Weapon-grade plutonium: containing < 7% 240Pu.2. Fuel-grade plutonium: 7 - 18 % 240Pu.3. Reactor-grade plutonium, containing over 18 percent 240Pu.The term "super-grade plutonium" is sometimes used to describe plutonium containing less than 3 percent plutonium 240. The term "weapon-usable plutonium" is often used to describe plutonium in separated form and, thus able to be quickly turned into weapons components
Before triggering, fissile material is kept in subcritical quantities to prevent accidental explosions. An electrical trigger sets off chemical explosives that drive the subcritical parts together.
Gun Trigger Assembly
Propellant
TamperTamper
Active Materialeach 2/3 critical
Hiroshima
Enola Gay
Little Boy
Size: length - 3 meters, diameter - 0.7 meters.
Weight: 4 tons.
Nuclear material: Uranium 235.
Energy released: equivalent to 12.5 kilotons of TNT.
Code name:"Little Boy"
/)0()( teNtN Once triggered the
chain reactionbuilds exponentially.
Note logarithmic scale!
After ~50 generations (0.50 sec) the energyreleased is increasing sorapidly it heats thematerial to the pointit expands explosively.
This scatters the remaining fissile material in subcriticalquantities, and the chainreaction ends.
Dropping the first atomic bombAt 2:45am local time (August 6, 1945), the Enola Gay, a B-29 bomber took off from the US air base on Tinian Island in the western Pacific. 6½ hours later, at 8:15 A.M. Japan time, its atomic bomb was dropped and exploded a minute later at an estimated altitude of 58020 meters over central Hiroshima.
Initial explosive conditionsMaximum temperature at burst point: several million degrees C. A 15m radius fireball formed in 0.1 millisecond, with a temperature of 300,000o C, and expanded to its huge maximum size in one second. The top of the atomic cloud reached an altitude of 17,000 meters.
Black rainRadioactive debris fell in a “black rain” for > hour over a wide area.
Damaging effects of the atomic bombThermal heatIntense thermal heat emitted by the fireball caused severe burns and loss of eyesight. Thermal burns of bare skin occurred as far as 3.5 kilometers from ground zero (directly below the burst point). Most people exposed to thermal rays within 1-kilometer radius of ground zero died. Tile and glass melted; all combustible materials were consumed.
BlastAn atomic explosion causes an enormous shock wave followed instantaneously by a rapid expansion of air (the blast); these carry ~half the explosion's released energy. Maximum wind pressure of the blast: 35 tons per square meter. Maximum wind velocity: 440 meters per second. Wooden houses within 2.3 km of ground zero collapsed. Concrete buildings nearground zero (blast from above) had ceilings crushed, windows and doors blown off.
RadiationExposure within 500 meters of ground zero was fatal. People exposed at distances of 3-5 kilometers later showed symptoms of aftereffects, including radiation-induced cancers.
DeathsWith an uncertain population figure, the death toll could only be estimated. According to data submitted to the United Nations by Hiroshima City in 1976, the death count reached 140,000 (plus or minus 10,000) by the end of December, 1945.
Active Material(235U or 239Pu)each 1/3 critical
electricaltrigger
chemicalexplosive
Active Material(235U or 239Pu)
subcritical density
chemicalexplosive
Implosion Assembly Designs
Nagasaki
Fat Man
The atomic bomb dropped on Nagasaki exploded at 11:02 A.M. on August 9. Using 6.1 kg of 239Pu it delivered the explosive power of 20 kilotons of TNT-equivalent,
And left an estimated 70,000 dead by the end of 1945.
