page 1nuclear familiarisation - radioactivity & fission pdw familiarisation with nuclear...
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Page 1Nuclear Familiarisation - Radioactivity & Fission
PDW
FAMILIARISATION WITH
NUCLEAR TECHNOLOGY
RADIOACTIVITY AND FISSION
Peter D. Wilson
DURATION ABOUT 40 MINUTES
Page 2Nuclear Familiarisation - Radioactivity & Fission
PDWCOMPONENTS OF AN ATOM
Atom of lithium-7 (conventional representation)
Not to scale - the electrons would be better regarded as a cloud of negative charge occupying a volume around 1/100,000,000 cm across, i.e. some 10,000 times the diameter of the nucleus
Proton Positive (+1) 1In nucleus
Around nucleus
Electric chargeParticle Relative mass
Neutron None 1
Electron Negative (-1) 0.00055
Page 3Nuclear Familiarisation - Radioactivity & Fission
PDWENERGY RELEASED BY FISSION
-20
-10
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100 120Atomic number
Packing fraction
Packing fraction = (Isotopic mass/Mass number - 1) x 10,000
Only the most abundant isotope is shown for each element
} Mass loss converted to energyE = m c2
Packing protons and neutrons into a nucleus involves some gain or loss in mass per nucleon, a quantity represented by the packing fraction. For fission products it is lower than in the parent nucleus. Fission of heavy elements thus releases some surplus mass as energy.
Page 4Nuclear Familiarisation - Radioactivity & Fission
PDWMORE ABOUT ATOMIC COMPONENTS
The central nucleus contains practically all the mass
Electrons occupy practically all the volume
The simplest nucleus (of hydrogen) consists of a single proton
Other nuclei have more protons held together by a similar or larger number of
neutrons
In a neutral atom the positive protons are matched by negative electrons, each
with unit charge
Chemistry depends on the behaviour of electrons
The number of protons (the atomic number) therefore determines the
chemical identity of the atom
Page 5Nuclear Familiarisation - Radioactivity & Fission
PDWPERIODIC TABLE
Elements in vertical columns have more or less similar chemistry
( VIIIA )
1 (IA) GROUP NUMBER 18 (VIII)
1H
2(IIA)
13(IIIB)
14(IVB)
15(VB)
16(VIB)
17(VIIB)
2He
3Li
4Be
5B
6C
7N
8O
9F
10Ne
11Na
12Mg
3(IIIA)
4(IVA)
5(VA)
6(VIA)
7(VIIA)
8 9 10 11(IB)
12(IIB)
13Al
14Si
15P
16S
17Cl
18Ar
19K
20Ca
21Sc
22Ti
23V
24Cr
25Mn
26Fe
27Co
28Ni
29Cu
30Zn
31Ga
32Ge
33As
34Se
35Br
36Kr
37Rb
38Sr
39Y
40Zr
41Nb
42Mo
43Tc
44Ru
45Rh
46Pd
47Ag
48Cd
49In
50Sn
51Sb
52Te
53I
54Xe
55Cs
56Ba
57La
72Hf
73Ta
74W
75Re
76Os
77Ir
78Pt
79Au
80Hg
81Tl
82Pb
83Bi
84Po
85At
86Rn
87Fr
88Ra
89Ac
104 105 106
Lanthanides(Rare Earths)
58Ce
59Pr
60Nd
61Pm
62Sm
63Eu
64Gd
65Tb
66Dy
67Ho
68Er
69Tm
70Yb
71Lu
Actinides 90Th
91Pa
92U
93Np
94Pu
95Am
96Cm
97Bk
98Cf
99Es
100Fm
101Md
102No
103Lr
(Alternative designation in parentheses)
SHOWING CHEMICAL ELEMENTS BY ATOMIC NUMBER AND CHEMICAL SYMBOL
Page 6Nuclear Familiarisation - Radioactivity & Fission
PDWISOTOPES
The number of protons determines chemical identity
Neutrons provide the remaining nuclear mass but may vary somewhat in number without other effect on chemistry
Forms of an element with different numbers of neutrons are called isotopes, distinguished by mass number (sum of protons + neutrons)
e.g uranium-235, uranium-238
Isotopes have identical chemistry (except for usually trivial effects of mass) but different nuclear properties
Not all nuclear combinations are stable - many decay spontaneously and are radioactive
Specific combinations of protons and neutrons are generically called nuclides, and if unstable radionuclides
Page 7Nuclear Familiarisation - Radioactivity & Fission
PDW
COMMON DECAY REACTIONS
Alpha () decay and fission are confined to the heaviest elements;beta () and gamma () decay may occur in any element.
Fission
may be spontaneous but more likely if neutron-induced
Alpha decay (emission of helium nucleus)
Beta decay (electron emission, leaving a neutron
converted to a proton)
Gamma emission (electromagnetic radiation)
UNSTABLE NUCLEI
Page 8Nuclear Familiarisation - Radioactivity & Fission
PDW
Atomic number reduced by 2, mass number by 4 (e.g. Pu-239 U-235)
-emission
-emission Atomic number raised by 1, mass number unchanged (e.g. Ru-106 Rh-106)
-emission Atomic and mass numbers unchanged (generally accompanies or follows other nuclear reactions)
Fission Nucleus splits into two main fission products and two or three free neutrons
Further neutron emission sometimes follows after a short delay
INTERNAL EFFECTS
Thus all but simple -emission result in a change of chemical identity
COMMON NUCLEAR REACTIONS
Page 9Nuclear Familiarisation - Radioactivity & Fission
PDWNUCLEAR RADIATION TYPESEXTERNAL EFFECTS
Alpha Short range, stopped by a surface film of water, but causes concentrated damage to material within range
Rather more penetrating, range about a centimetre in water - more diffuse damage
Beta
Range several metres in water with still more diffuse damageGamma
Also very penetrating, can induce radioactivityNeutron
A high dose of radiation to the body over a short time is likely to cause illness or death
A low dose (comparable with natural levels) may or may not have adverse effects; they cannot be identified against the natural background and the likelihood is subject to dispute
Page 10Nuclear Familiarisation - Radioactivity & Fission
PDWLOW-LEVEL RADIATIONRISKS OF HARMFUL EFFECTS (illustrative, not to scale)
Risk
Dose
Risk
Dose
Risk
Dose
RELIABLE EVIDENCE ONLY AT HIGH DOSES;ESTIMATES AT LOWER LEVELS DEPEND ON FORM OF EXTRAPOLATION ASSUMED
LINEAR HYPOTHESIS ADOPTED FOR REGULATION OF OCCUPATIONAL EXPOSURES AT INTERMEDIATE LEVELS ON GROUNDS OF CAUTION, DESPITE NEGLECTING MITIGATING FACTORS
OTHER RELATIONSHIPS AT LEAST EQUALLY PLAUSIBLE
Page 11Nuclear Familiarisation - Radioactivity & Fission
PDWSOURCES OF RADIATION EXPOSURE (UK PROPORTIONS, 2005)
Average proportions; levels vary widely from place to place
Radon, internal (e.g. K-40), terrestrial and cosmic contributions are natural
Page 12Nuclear Familiarisation - Radioactivity & Fission
PDW
Time
Amount
NUCLEAR DECAY CHARACTERISTICS
Half-life
Decay of radioactive nuclei is a matter of chance and can be predicted only statistically
A characteristic proportion of those present decays in any unit of time
The time taken for half to decay is the half-life (unalterable for any given radionuclide)
The longer the half-life, the feebler the radioactivity
The pattern of decay is identical for all pure radionuclides but with different time scales, so for a mixture can be complex
Page 13Nuclear Familiarisation - Radioactivity & Fission
PDWNEUTRON ABSORPTION EFFECTS
•Excitation
•Emission
•Conversion
•Fission
then loss of energy as gamma-rays but with no further change;
of one or more neutrons;
of a neutron to a proton with emission of a beta-particle - transmutation to next higher element in the Periodic Table (may be repeated);
splitting into two major parts plus some free neutrons.
Any nucleus can absorb a free neutron - likelihood varies enormously
Likelihood and consequence varies widely according to neutron energy and nuclear composition; exchanging a neutron in the nucleus for a proton can make an enormous difference
Absorption can have one of 4 possible consequences (not necessarily immediate):-
Page 14Nuclear Familiarisation - Radioactivity & Fission
PDWNEUTRON ABSORPTION TERMS
Neutron flux
The probability that a nucleus will interact with unit flux of neutrons (1 neutron per sq. cm. per second) in one second has the dimensions of area.
It may be imagined as the area presented by the nucleus to the neutron flow and is known as the cross-section.
The unit of cross-section is the barn (from the expression, “as easy as hitting a barn door”);1 barn = 1/1,000,000,000,000,000,000,000,000 sq. cm (10-24 cm2).
Neutron density in a given space is inversely proportional to speed.
Absorption is in general therefore likelier with slow than fast neutrons.
After absorption, fission is likelier the higher the energy.
Absorption is especially likely at resonance energies.
Page 15Nuclear Familiarisation - Radioactivity & Fission
PDWRESONANCE SPECTRUM
Fission cross-section of uranium-233 (typical - no significance in choice)
1/V trend
Fission probability rises above 1/V trend
with increasing energy
Page 16Nuclear Familiarisation - Radioactivity & Fission
PDWACTIVATION AND TRANSMUTATIONActivation
n -
e.g. Co-59 Co-60 Ni-605.3 years
Transmutation n - -
e.g. U-238 U-239 Np-239 Pu-239 24 min 2.4 days
Activation and transmutation involve the same processes; the difference lies in the time-scale and point of interest.
In activation a stable nucleus is made radioactive, usually with a fairly long half-life (days to years)
In transmutation a new element is formed more or less quickly (seconds to days)
Page 17Nuclear Familiarisation - Radioactivity & Fission
PDWEXAMPLES OF TRANSMUTATION
U-239
Pu-239
U-238
Pu-240 Pu-241 Pu-24214.4 yr
nnn
n
Am-241
2.355 day
23.5 min
Am-242 Am-243 Am-244nnn
16.02 hr
Cm-242
10.1 hr
Cm-244Cm-243 nn
Np-239Thus fissile Pu-239 is generated from non-fissile U-238
Page 18Nuclear Familiarisation - Radioactivity & Fission
PDWPROPERTIES RELATED TO FISSION
Once a neutron is absorbed, the likelihood of fission rather than other effects increases with neutron energy.
Neutrons with energy matching their surroundings are thermal.
Neutrons as released by fission are fast.
Neutrons rather faster than thermal are epithermal.
Nuclei that can undergo fission with thermal neutrons (e.g. uranium-235) are fissile.
Nuclei that undergo fission only with fast neutrons (e.g. uranium-238) are fissionable.
U-238, not itself fissile, is converted by neutron absorption to fissile Pu-239 and so is fertile.
Page 19Nuclear Familiarisation - Radioactivity & Fission
PDWCRITICALITY
Fission in a heavy nucleus may occur spontaneously but is more readily caused by absorbing a neutron.
Each fission releases several initially fast free neutrons that in principle could cause a further fission, and so on in a chain reaction.
If on average exactly one neutron from each fission goes on to cause another, the chain reaction continues indefinitely at a constant rate - criticality - the condition required in a power reactor.
If less than one causes further fission, the chain dies away more or less rapidly.
If more than one causes further fission, the reaction accelerates until controlled naturally or artificially.
Page 20Nuclear Familiarisation - Radioactivity & Fission
PDWCONDITIONS FAVOURING CRITICALITY
Large mass of fissile material
Low surface/volume ratio to minimise
escape of neutrons
Reflector to return some
escaping neutrons
Few non-fissioning neutron absorbers
“Moderating” medium to slow down neutrons
A nearby fissile mass
Page 21Nuclear Familiarisation - Radioactivity & Fission
PDWFISSION PRODUCT DISTRIBUTION
Fission usually yields products differing considerably in mass. Symmetric fission is much less common, as shown here for U-235 in a thermal neutron flux.
Very fast neutrons lead to a distribution with a shallower minimum
Fission Yield (%)
0.00001
0.0001
0.001
0.01
0.1
1
10
70 80 90 100 110 120 130 140 150 160 170
Mass number
In terms of elements, fission peaks are roughly from krypton to palladium and iodine to europium
Page 22Nuclear Familiarisation - Radioactivity & Fission
PDWINSTABILITY OF FISSION PRODUCTS
0
20
40
60
80
100
120
140
160
0 20 40 60 80 100 120
2 or 3 free neutrons
Neutrons
Protons
Typical fission
-decay chain
~ 5 steps from fission to stability
line
The proportion of neutrons to protons needed for stability rises with atomic number.
Thus the primary fission products have too many.
They therefore convert some of the excess to protons by emitting energetic electrons (-particles), and usually -radiation
Accordingly they rise by one atomic number unit at each step but keep unchanged mass number.
Page 23Nuclear Familiarisation - Radioactivity & Fission
PDWA FEW THOUGHTS
Heavy elements are remnants from stars that exploded before the Earth was formed.
Only the heaviest are subject to fission.
All beyond lead are more or less unstable.
The heaviest to have survived is uranium.
The only natural fissile nuclide on Earth is U-235.
So the very possibility of nuclear energy depended upon the last element available to us ... or did it?