nuclear physics - 4 · 2012-10-31 · 3 today’s plan 30 october nuclear physics 4 nuclear...
TRANSCRIPT
Nuclear Physics - 4
Quantum, Atomic and Nuclear Physics, Year 2
University of Portsmouth, 2012 - 2013
Prof. Glenn Patrick
2
Last Week - Recap
Ionising Radiation
Interactions with Matter – Charged & Uncharged
Neutron Radiation
Linear Energy Transfer
Activity, Absorbed Dose, Equivalent Dose
Relative Biological Effectiveness, Effective Dose
Origins of Radiation and Risks
Radon
Biological Effects – Direct and Indirect Action
Hiroshima & Nagasaki, Chernobyl
Beneficial Uses of Radiation
Nuclear Medicine – Proton Therapy, PET.
3
Today’s Plan 30 October Nuclear Physics 4
Nuclear Reactions, Conservation Laws
Reaction Energy, Q Value
Cross-section
Nuclear Fission: Induced and Spontaneous
Neutron Reactions, Fission Energy Release
Chain Reaction
Uranium Fuel Cycle
Fission Reactor Designs
Thorium
ADSR
Nuclear Fusion
Magnetic Confinement
Inertial Confinement
Copies of Lectures:
http://hepwww.rl.ac.uk/gpatrick/portsmouth/courses.htm
4
Nuclear Reactions
HONHe 1
1
17
8
14
7
4
2 1919, Rutherford
First nuclear reaction in the laboratory.
α particles from a radioactive source (214Bi).
Accelerators came later in the 1930s.
CONSERVATION LAWS
Conservation of total energy ΔE=0
Conservation of linear momentum Δp=0
Conservation of total charge ΔZ=0
Conservation of total angular momentum ΔI=0
Conservation of parity ΔP=0
Conservation of atomic mass number ΔA=0
Conservation of proton and neutron number
At low nuclear energies, no process related to nuclear forces is capable of
transforming protons into neutrons and vice versa.
The weak force is very slow, but we will see in particle physics that this law
does not hold at higher energies.
We will touch on conservation laws and symmetries again in particle physics.
Some of
these not
always exact
5
Reaction Energy The reaction energy, Q, is the net energy released in a reaction A+B C+D
The same as the difference between the masses of the initial and final states:
particles. 22 ),(),( cAZMcAZMQf fi i
Q > 0 Exoergic (or Exothermic) Reaction
Nuclear mass or binding energy is released as kinetic energy.
Q < 0 Endoergic (or Endothermic) Reaction
Initial kinetic energy is converted into nuclear mass or binding energy.
For elastic collisions, Q=0.
For Q>0 or Q=0, reaction is always possible.
When Q<0, there must be a minimum
threshold energy for reaction to proceed.
M
mMQK th
HONHe 1
1
17
8
14
7
4
2 MeVK th 531.114
144)192.1(
M= target
m=projectile
)( 2222
2222
cMcMcMcMTTT
TTcMcMTcMcM
DCBAADC
DCDCABA
Conservation of Energy
(T = Kinetic Energy).
6
Cross-Section
)()()()( dLdddFnddnvnddN bbaa
na = no. of beam particles
va = velocity of beam particles
nb = no. target particles/area
Incident flux F=nava
dN = no. scattered particles
in solid angle dΩ
d d
d
Total
cross-section
d
dN
Ld
d 1Differential
cross-section
.d.ddΩ sin
Measured in barns. 1 b = 10-24 cm-2
LN rateEvent
Luminosity L = flux x no. targets (cm-2s-1)
Cross-section quantifies
rate of reaction. Depends
on underlying physics.
7
Induced Nuclear Fission
INDUCED 1938: Otto Hahn, Fritz Strassman,
and Lise Meitner, Otto Frisch.
Followed earlier work by Enrico Fermi.
1944 Nobel Prize in
Chemistry to Otto Hahn.
Meitner nominated many
times…..
8
Spontaneous Nuclear Fission
SPONTANEOUS 1940: Konstantin Petrzhak and
Georgy Flerov
First to observe spontaneous fission of
uranium
Only a few nuclei are known
to fission spontaneously.
404 isotopes between 230-Th and
289-Fl have some SF activity
according to Nucleonica.
Only Actinides and trans-actinides
can undergo spontaneous fission…so
26 elements at the moment
9
ACTINIDES & TRANSACTINIDES
ACTINIDES
TRANSACTINIDES
Transuranic are those elements with Z > 92 (i.e. beyond uranium)
10
Neutron Reactions
UU
UUn
235*236
*236235
Elastic scattering.
No change in particles.
Neutron Capture.
Neutron combines with target
nucleus to form an excited nucleus
nUUn 235235
nKrBaUUn 389144*236235 Fission
nSrXeUUn 294140*236235
Using 235U as the example:
Inelastic scattering
important at higher
energies.
sUUUn ' 235*235235
11 1 MeV
Note that both 235U and 239Pu can
easily be split by slow neutrons.
They are both fissile.
238U requires fast neutrons with
energy > 1 MeV.
238U cannot sustain a chain reaction
as scattering reduces neutron
energies to below 1 MeV. It is
fissionable, but not fissile. 1 eV
Resonance Region
Energy Levels
12
Remember – Binding Energy Binding energy is the amount of energy that would have to be added to the
nucleus to break it up. Nuclei with a higher binding energy/nucleon have a
lower atomic weight per nucleon. Elements in the middle of the plot have a
higher binding energy (lower atomic weight) per nucleon. In fission of a heavy
nucleus, the products have a slightly smaller combined atomic mass – this
difference is converted to energy.
13
Energy Release
Atomic Mass(u) BE/A(keV) BE(MeV) 235U 235.044 7590.9 1783.9
89Kry 88.918 8617.0 766.9 144Ba 143.923 8265.5 1190.2
Energy release/fission = 766.9 + 1190.2 – 1783.9 = 173.2 MeV
Alternatively,
Energy release = (235.044 - 88.918 - 143.923 - 2 x1.0087) x 931.494
= 172.9 MeV
nKrBaUUn 389144*236235
Compare this with a chemical reaction: U + O2 UO2
Heat of combustion is 4,500 J/g or only ~11 eV/atom!!
14
Fission Fragments
Mass Number A
Fu
sio
n Y
ield
(%
)
Vertical scale is logarithmic
Average fragment of U-235
has mass of ~118, but few
fragments found at that
mass.
More probable is to break up
into fragments with unequal
masses.
The most frequent have A
~95 and A~137.
15
Fission Timescales
16
Energy Distribution
Energy Form Amount (MeV)
INSTANTANEOUS ENERGY FROM FISSION
Kinetic energy of fission products 167
Prompt fission neutrons 5
Gamma rays 15
TOTAL PROMPT ENERGY 187
DECAY PRODUCTS OF FISSION FRAGMENTS
Kinetic energy of beta particles 7
Gamma rays following beta decay 6
Neutrinos (which escape) 10
TOTAL DELAYED ENERGY 23
17
Liquid Drop Model of Fission
Niels Bohr, John Wheeler, 1939
In the ground state, the nucleus is nearly spherical in shape.
After the absorption of a neutron, the nucleus will be in an excited
state and start to oscillate and become distorted.
If the oscillations cause the nucleus to become shaped like a
dumbbell, the repulsive electrostatic charges will overcome the
short-range nuclear forces, and the nucleus will split in two.
18
Chain Reaction
generation previousin neutrons ofnumber
generation onein neutrons ofnumber k
k > 1 leads to
supercritical reaction
used in a bomb.
k<1 leads to subcritical
reaction and the
reaction dies out.
k=1 is known as critical
and leads to a stable
reaction.
Prompt neutrons are emitted during fission process (after 10-14 s).
Delayed neutrons are emitted when a fission fragment decays – vital for control.
19
The First Reactor
Chicago Pile 1 (CP-1) built by Enrico Fermi et al underneath the
University of Chicago’s football stadium.
2 December 1942. The first self-sustaining chain reaction.
20
First UK Reactors GLEEP, Harwell
1947 - 1990
First reactor in Western Europe
Research, materials testing
3 kW
Calder Hall, Windscale
1956 - 2003
First nuclear power station
in the World!
4 Magnox reactors
180 MW heat, 40 MW electricity
21
Uranium Fuel Cycle ~99% of natural uranium is 238U.Only ~0.7% is 235U.
For use in LWRs, it needs to be enriched to contain 3-5% 235U.
Gas
Yellowcake
UO2 pressed into pellets
Centrifuges/diffusion
0.2 to 0.4% 235U
ρ=19.1 g/cm3
22
UK Nuclear Reactors I
Magnox built in UK 1956-1971.
Wylfa the last station operating.
Named after the magnesium alloy used
to encase the fuel.
Fuel rods loaded into graphite core.
Cooled by blowing CO2 gas past fuel.
Control rods vary fission rate.
23
UK Nuclear Reactors II
PWR most widely used type in world.
Sizewell B only in UK.
Uses enriched uranium dioxide (~3.2%
U235).
Water pumped at high pressure
through steel vessel acts as both
moderator and coolant.
AGR still uses graphite moderator.
7 of 14 UK stations from 1976-89 still
operating.
Natural uranium metal (~0.7% U235).
Higher efficiency with higher temps
and cooling gas pressure.
Steam generators & gas circulators
put in pressure vessel.
24
Fast Breeder Reactor All commercial reactors use
thermal neutrons to maintain the 235U chain reaction.
Even when enriched the fuel
contains a majority of 238U which
is not fissile, but instead gets
converted by neutron capture to
plutonium 239, which is fissile.
It is possible to design a fast
reactor which produces more
fissile material than it consumes.
The neutrons are unmoderated. Fast reactor needs a high fissile core (~20%
plutonium) surrounded by a blanket of 238U.
Heat removal requires high conductivity coolant like liquid sodium.
More expensive, but increased uranium prices would make economic.
UK prototype at Dounreay closed in 1994.
25
Nuclear Waste
NDA, Radioactive Wastes in the
UK- 2010 Inventory
http://www.nda.gov.uk/ukinventory/
Total waste today plus forecast over next century.
Cubic metres
Low Level Waste (LLW) 4,400,000
Intermediate Level Waste (ILW) 290,000
High Level Waste (HLW) 1,000
TOTAL 4,700,000
Fill this ~4 times over
Temp may rise significantly
HLW contains 95%
of all the radioactivity!
Easily fit inside one
Olympic swimming pool.
Not exceeding 4 GBq/tonne
alpha &12 GBq/tonne
beta/gamma.
Waste does not include:
• Spent Nuclear Fuel
(may be reused)
• Unirradiated Fuel
26
UK Repository?
CORWM – Committee on Radioactive Waste Management
http://corwm.decc.gov.uk/
Geological disposal for management of higher activity waste in long term.
27
Global Energy Consumption
World Economic and Social Survey 2011, DESA, United Nations
Primary
Energy
(exaJoules)
28
Energy and Electricity Supply
2012 Key World Energy
Statistics, International Energy
Agency
World Total Primary Energy Supply
(Mtoe)
World Electricity Generation by Fuel
(TWh)
29
Thorium as a Fuel • Natural Thorium (232Th) is not naturally fissile.
• It is fertile and can be converted to 233U by neutron
capture within the ADSR core.
• Thorium 3 times more abundant than uranium (similar
to lead abundance).
• Proliferation resistant fuel cycle.
UPaThThn 233
92
233
91
233
90
232
90
DRAGON, Winfrith
30
Accelerator Driven Sub Critical Reactor ADSR – Accelerator Driven Sub Critical Reactor
Towards an Alternative Nuclear Future, Thorium Energy Amplifier Association, 2009-10
Core of the reactor is
sub-critical.
Neutrons provided by
protons from an
accelerator hitting a
target.
If accelerator is
switched off, reactor
processes shut down
safely.
Numbers
10 MW proton accelerator,
requires 20 MW to operate.
1,550 MW in reactor
Extract 600 MW.
580 MW to Grid…. Can use Thorium as the fuel
31
Nuclear Fusion
Nuclear fusion – 2 or more lighter nuclei come together
to form a larger nucleus.
Binding energy/nucleon after fusion is greater than before.
Total mass of products is less than before – exothermic reaction.
32
Reminder – Hydrogen Isotopes
33
Deuterium-Tritium (D-T)
34
Why D-T? Ignition Temperature • A number of reactions could provides fusion energy on Earth.
• Tritium is not naturally occurring, but can be produced relatively easily.
• Deuterium can be obtained from seawater.
• Problem is not so much the raw materials, but the basic physics of
overcoming the Coulomb barrier.
35
These days we use the triple product:
Lawson Criterion
Lawson Criterion:
Named after John D. Lawson (Harwell).
For a fusion reactor, need:
1) High Temperature, T. Must be high enough that
fusing particles overcome Coulomb barrier. Need
100-200 million K.
2) High Density, n. Must be high enough probability
of particles being close enough to fuse. Densities
of ~2-3 x 1020 particles/cm3 needed.
3) Sufficient Confinement Time, τ, at high
temperature and density. Must be held together
for 1-2s.
320 / 10 x 5.1 msn ee
re temperatuT
t timeconfinemen τ
density plasman where
e
e
321 s/ keV 01 x 6 mTn ee
36
Magnetic Confinement
Contain plasma in a vacuum vessel
Use magnetic fields to keep away from walls
Ohmic heating from plasma current
Add extra heating
37
Magnetic Confinement: Tokamak
38
All done with Magnetic Fields
39
Joint European Torus (JET)
40
JET
41
Joint European Torus (JET)
42
Fusion Triple Product
43
ITER
Cadarache, France. 30 m tall, 23,000 tonnes. Operation from 2020?
44
Inertial Confinement A capsule of D-T is irradiated by lasers, X-rays or particle beams.
45
National Ignition Facility (NIF)
192 laser beams. Expected to achieve ignition within next ~2 years.
Lawrence Livermore National Laboratory, California, 2009 -
46
High Power Laser for Energy Research (HiPER)
European Collaboration of 10 countries. UK possible location.
Transition from proof of principle to a demonstration power plant.
48
Energy Generation in the Sun
MeV252e2Hep4 e
4 Overall process:
Actual process: e
2 eHpp
or
e
2Hpep
32 HepH
e
77 LieBe
or 2pLi7
87 BpBe
e
8*8 eBeB
2Be8*
p2HeHe 33 or
743 BeHeHe
e
43 eHepHe
or