Page 1Nuclear Familiarisation - Radioactivity & Fission PDW FAMILIARISATION WITH NUCLEAR TECHNOLOGY RADIOACTIVITY AND FISSION Peter D. Wilson DURATION

Page 1Nuclear Familiarisation - Radioactivity & Fission

PDW

FAMILIARISATION WITH

NUCLEAR TECHNOLOGY

RADIOACTIVITY AND FISSION

Peter D. Wilson

DURATION ABOUT 40 MINUTES


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


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.


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


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


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


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


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


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


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


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


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


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):-


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.


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


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)


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


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.


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.


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


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


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.


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?

Documents

Page 1Nuclear Familiarisation - Radioactivity & Fission PDW FAMILIARISATION WITH NUCLEAR TECHNOLOGY RADIOACTIVITY AND FISSION Peter D. Wilson DURATION