Basic PrinciplesChapter 10B

Nuclear Energy

1938: Fission discovered in Germany by Otto Hahn Fritz StrassmamJan 16, 1939: Lise Meitener & Otto Robert Frisch published a theoretical interpretation of

Hahnd Strassmam experiments in Natare (10 days after Hahn & Strassman publication)

Apr. 17, 1939: Frederich Isliet, Hans von H, Plutonium bomb, alban, & Lew Kiowarsk publish paper dealing with possiblility of nuclear chain reaction

Aug 2, 1939: Albert Einstein wrote letter to pres. F. D. Roosevelt drawing attention to the possiblilty of an atomic bomb

1940: Edwin McMillan & Glenn Seaborg discover PlutoniumDec. 1942: First Nuclear reactor went critical

-beneath the stands of the Universtiy of Chicago stadium (Chicagoo Pile 1)-Fermi Chain Reaction-core was 9m wide. 9.5m long, 6 m high

52 tons of natural uranium & 13,50 tons of graphiteCadmium rods used for control0.5W power for a few minutes

FISSION TIMELINE

1943: Town of Los Angeles constructed for atomic research & weapons construction

July 16, 1945: 1st Nuclear Explosion, Trinity site in New Mexico1952: Fusion Weapon,. Hydrogen Bomb (much less radioactive debris)June 1, 1954: 1st Nuclear Power Plant

* Obninsk Nuclear Power Station, near Moscow (ADS-1 Obninsk)* rated power 5 MW, 30μωeh

* graphite moderated, water cooled* shut down April 24, 2002

Jan 21, 1954: USS Nautilus (SSN.571) launched* Pressurized water reactors (Westinghouse)* 10 μωm

* decommissioned March 3, 1980

FISSION TIMELINE

COMPONENTS 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 1Electron Negative (-1)0.00055

ENERGY 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.

MORE 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

PERIODIC 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

ISOTOPES

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

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-inducedAlpha decay

(emission of helium nucleus)

Beta decay (electron emission, leaving a neutron

converted to a proton)

Gamma emission (electromagnetic radiation)

UNSTABLE NUCLEI

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

COMMON NUCLEAR REACTIONS

NUCLEAR 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 damage

Gamma

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

LOW-LEVEL RADIATION

RISKS 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

SOURCES 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

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

NEUTRON 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):-

NEUTRON 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.

RESONANCE 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

ACTIVATION AND TRANSMUTATION

Activation 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)

EXAMPLES OF TRANSMUTATION

U-239

Pu-239

U-238

Pu-240 Pu-241 Pu-242

14.4 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-239 Thus fissile Pu-239 is generated from non-fissile U-238

PROPERTIES 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.

CRITICALITY

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.

CONDITIONS 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

FISSION 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

INSTABILITY 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.

A 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?

Thank You!Sebilo, Eugene Paul Brian D.

BS ECE IV

References: http://www.peterwilson-seascale.me.uk/Basic.htm http://www.classroom-energy.org/energy_09/new-

window/4_nuclear_steam_turbine_plant.html http://www.youtube.com/watch?v=hZqA2hZW53o http://www.youtube.com/watch?v=H1ckdTlgvlU