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Binding energy curve
Fission
Nuclear 0ission • Fission happens when a heavy atomic nucleus splits into two lighter nuclei
• The nucleus is deeply divided into two large fission fragments of roughly equal mass
• The mass decrease (and hence the energy released, Q) is appreciable
• Other neutrons are emi?ed in the process (called fission neutrons)
92235U + 0
1n→ 56144Ba+ 36
90Kr + 2 01n+Q
• Chain reac4on: fission neutrons produce further fission
Nuclear 0ission 92235U + 0
1n→ 56144Ba+ 36
90Kr + 2 01n+Q
92235U + 0
1n→ 56142Ba+ 36
91Kr +301n+Q
92235U + 0
1n→ 55140Cs+ 37
93Rb+301n+Q
92235U + 0
1n→ 62160Sm+ 30
72Zn+ 401n+Q
Example • EsCmate the energy released (in Joules) by the fission of 1g of U-‐235 according to the equaCon
atomic masses:
Avogadro’s number: 6.023 x 1023
1u = 931 MeV 1MeV = 1.6 ×10-‐13 J
92235U + 0
1n→ 56144Ba+ 36
90Kr + 2 01n+Q
92235U = 235.04438u 56
144Ba =143.92279u 3690Kr = 89.92089 u
01n =1.00866 u
Controlling chain reactions • Cri4cal mass: the minimum mass of fuel required to produce a self-‐sustaining chain reacCon
• Chain reacCons can be controlled by inserCng absorber (control) rods into the Uranium
• These rods contain elements such as Boron or Cadmium which can absorb many neutrons without fissioning themselves
• Absorber rods can be liXed to speed up chain reacCons • or can be lowered to decrease the reacCon rate or stop the reacCons
510B+ 0
1n→ 37Li+ 3
4He+ 2.8MeV
Nuclear reactors • First nuclear reactor built in 1942 • Fuel used was natural Uranium (contains less than 1% U-‐235) • Slow neutrons are be?er at producing fission • Many nuclear reactors use enriched Uranium • the presence of the U-‐235 isotope has been arCficially increased
• Control rods regulate the heat output of the reactor
• Water is used to prevent overheaCng
• OverheaCng would result in a Uranium meltdown
Nuclear reactors • The reactor works by turning water into steam • The steam drives a turbine which spins a generator to produce power
• Reactors housed in thick layer of concrete which acts as a radiaCon shield
• The concrete layer lies inside a steel container which provides extra shielding and prevents leakage of radioacCve waste
• A strong outer concrete building serves as a final layer
Charnobyl disaster (April 1986) • Despite warnings from the InternaConal Atomic Energy Agency that the protecCve measures were not strict enough, the Soviet Union did not take acCon remedy this
• There were no sufficiently secure secondary containment faciliCes
• There was also a problem with the design of the control rods
Fukushima 2011 • Earthquake and resulCng tsunami seriously damaged the nuclear plant
• Water was drained from the reactor core which made it impossible to control core temperatures
• Resulted in a parCal meltdown • Tepco admi?ed for the first Cme on October 12, 2012 that it had failed to take stronger measures to prevent disasters for fear of inviCng lawsuits or protests against its nuclear plants
• In 2013 the World Health OrganizaCon indicated that the residents of the area were exposed to so li?le radiaCon that it probably won't be detectable. They indicated that a Japanese baby's cancer lifeCme risk would increase by about 1%
Pros and Cons of nuclear power • Pros: • doesn’t depend on fossil fuel • Does not emit CO2 so does not contribute to climate change • fission produces a million Cmes more energy per unit weight than fossil fuel alternaCves
• Cons: • Costly to construct and maintain faciliCes • Once fuel is used it is sCll radioacCve (radioacCve waste) • Waste remains radioacCve for thousands of years
Nuclear fusion • Fusion takes place when two lighter nuclei combine to form a heavier nucleus
Fusion
• Energy is released in the process • Fusion can only happen at very high energies
• atomic masses
12H + 1
2H→ 23He+ 0
1n+Q
12H = 2.015u 0
1n =1.009u 23He = 3.017u
Fusion inside the Sun • Solar energy is produced by fusion at the core of the Sun
• Temperatures of ~108 K are reached • Atoms are stripped of electrons (hot plasma state) • Nuclei have very high energies and can overcome Coulomb forces
• Fusion takes place • Thermonuclear reacCons:
11p+ 1
1p→ 12H + +1
0β +ν + 0.4MeV
12H + 1
2H→ 23He+ 0
1n+3.7MeV
23He+ 2
3He→ 24He+ 21
1p+12.9MeV
Example What is the minimum speed required for protons to achieve fusion? Assume that the energy required is 0.4MeV. Use the kineCc theory of gases (Nmv2/3=RT) to get an esCmate of the temperature required to achieve such speeds. Use the following informaCon • mass of proton = 10-‐27kg • 1MeV = 1.6 × 10-‐13J • mass of 1 mole of protons is approximately 0.001kg • molar gas constant R = 8.3J mol-‐1 K-‐1
Fusion reactors • The main difficulty with gelng fusion reactors to work is achieving the enormous temperatures required
• D-‐T reacCon: • At such high temperatures Hydrogen is in hot plasma form • The plasma is confined using two types of reactors: • MagneCc confinement
• plasma confined inside reactor by a magneCc field • (e.g.JET, TFTR)
12H + 1
3H→ 24He+ 0
1n+17.6MeV
JET
Fusion reactors • The main difficulty with gelng fusion reactors to work is achieving the enormous temperatures required
• D-‐T reacCon: • At such high temperatures Hydrogen is in hot plasma form • The plasma is confined using two types of reactors: • MagneCc confinement
• plasma confined inside reactor by a magneCc field • (e.g.TFTR)
• InerCal confinement reactors • heat and compress confined fuel using laser-‐light • extreme heaCng up and compression can ignite fusion
12H + 1
3H→ 24He+ 0
1n+17.6MeV
Example Show that the energy released in the D-‐T reacCon is 17.6MeV [Assume 1u=931MeV and use the following atomic masses:]
12H = 2.01410u 0
1n =1.00867u24He = 4.00260u1
3H = 3.01605u
Advantages of Fusion power • No carbon emissions. The only by-‐products of fusion reacCons are small amounts of helium, which is an inert gas that will not add to atmospheric polluCon
• Abundant fuels. Deuterium can be extracted from water and triCum is produced from lithium, which is found in the earth's crust. Fuel supplies will therefore last for millions of years
• Energy efficiency. One kilogram of fusion fuel can provide the same amount of energy as 10 million kilograms of fossil fuel
• No long-‐lived radioac4ve waste. Only plant components become radioacCve and these will be safe to recycle or dispose of convenConally within 100 years
• Safety. The small amounts of fuel used in fusion devices (about the weight of a postage stamp at any one Cme) means that a large-‐scale nuclear accident is not possible
• Reliable power. Fusion power plants should provide a baseload supply of large amounts of electricity, at costs that are esCmated to be broadly similar to other energy sources
Progress in fusion research • The experimental faciliCes have achieved sustainable results • JET has produced 16 megawa?s of fusion power and proved the technical feasibility of fusion using deuterium and triCum
• The next step is proving it can work on a power plant scale but may take several more decades
• The main problems are the extreme condiCons required for sustainable reacCons and large scale plasma confinement
The A-‐Bomb • Atomic bombings of Hiroshima (6 Aug 1945) and Nagasaki (9 Aug 1945)
• 90,000–166,000 people died in Hiroshima • 60,000–80,000 in Nagasaki • Most were civilians • Hiroshima – was it necessary? h?p://www.doug-‐long.com/
Hiroshima Nagasaki
The A-‐Bomb • “Li?le Boy” (Hiroshima bomb) was a gun-‐type fission bomb made using U-‐235
• “Fat Man” (Nagasaki bomb) was an implosion-‐triggered fission type bomb using Pu-‐239
• Gun-‐type: • Fuel masses subcriCcal and kept separate • Brought together forcefully à supercriCcal mass • If brought together slowly, the iniCal explosion will push them apart and the explosion will fizzle
li?le boy fat man
The A-‐Bomb • “Li?le Boy” (Hiroshima bomb) was a gun-‐type fission bomb made using U-‐235
• “Fat Man” (Nagasaki bomb) was an implosion-‐triggered fission type bomb using Pu-‐239
• Implosion: • Fuel mass subcriCcal and kept at the core • Surrounded with chemical explosives • Chemical explosion compresses core à core supercriCcal
li?le boy fat man
Fusion bombs (H-‐bomb) • fission used to trigger fusion of heavy Hydrogen • fusion releases large numbers of neutrons • these can be used to make a more violent explosion by surrounding the bomb with a layer of natural Uranium
• fission drives fusion which drives further fission • about 1000 Cmes more destrucCve than A-‐bombs
• Energy released: • blast and shock (50%) • thermal radiaCon (35%) • immediate nuclear radiaCon (10%) • residual nuclear radiaCon (15%)
EMP generation • High-‐alCtude nuclear explosions will generate EMP • gamma-‐rays ionize atoms in the atmosphere • resulCng high-‐energy free electrons spiral down the magneCc field lines of the earth
• give rise to a rapidly rising radiated electromagneCc field called an electromagne4c pulse
• induce very high currents in electronic components • loss of power
• Oppenheimer: • aXer WW2 strongly lobbied for internaConal control of nuclear power
• used influence to try to avert nuclear arms race with Russia
• had his security clearance revoked
“We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad Gita; Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his mulC-‐armed form and says, 'Now I am become Death, the destroyer of worlds.' I suppose we all thought that, one way or another.”
• Bohr: • was not directly involved in the making of the bomb but was invited as an advisor
• oXen expressed social concern about such weapons and an eventual nuclear arms race
• believed atomic secrets should be shared by the internaConal scienCfic community
• conCnued to work for internaConal control of nuclear weapons unCl his death in 1962
• Einstein: • was a pacifist • supported but did not take part in the Mahna?an project
• hoped that the bomb would not be used unless absolutely necessary
• gave all the energies not spent on his scienCfic work to campaigns for peace
'I loathe all armies and any kind of violence; yet I'm firmly convinced that at present these hateful weapons offer the only effecCve protecCon.' Should Nazi militarism prevail, 'you can be sure that the last remnants of personal freedom in Europe will be destroyed'.
‘We have learned, and paid an awful price to learn, that living and working together can be done in one way only -‐ under law. Unless it prevails, and unless by common struggle we are capable of new ways of thinking, mankind is doomed.’
To Do • Read chapter 30 [p.614-‐634 Nuclear Energy] • Assignment wk10: Prac4cal DC3 • Homework: 27.14, 29.5, 30.3, 30.7, 30.11, 31.7 • Hand it in no later than 4:00pm next Wednesday -‐ LATE WORK WILL NOT BE ACCEPTED
