Transcript nuclear fusion

NUCLEAR
FUSION
3 simple questions:
• why does the sun shine ?
• why is gold more expensive than iron ?
much rarer
• what is radioactivity ?
3 chapters
• basic physics of nuclear fusion
• nuclear fusion in stars
• nuclear fusion as energy supply on earth ?
basic physics
of
nuclear fusion
of elements
atomism
explains chemistry: 92 elements (from H to U)
increasing weight,
regularities in properties (alkalines, halogenes,
noble gases, ...)
all of them found in nature !
WHY ?
 job for the physicists
heat up pure elements --> emission of light in characteristic colours
„spectra“
all explained by quantum mechanics :
atoms are not fundamental
but composed of
a small nucleus
and a cloud of electrons
10-15 m
10-10 m
the electrons are negatively charged, the nuclei positively,
cloud and nucleus are held together by the electromagnetic force
analogy: electron cloud the size of a football stadium 100 m
--> size of football (nucleus) at the centre point: 1 mm
nuclear physics
H (lightest nucleus) has 1 electron and
1 positively charged particle as nucleus
= proton
2 problems for heavier nuclei:
- how can many protons stay together at such small distances ?
(repulsion of equal electric charges !)
- nuclei with charge Z have a mass mZ ≥ 2Z mp
possible explanation:
need another neutral particle with mn ≈ mp
and a special force between n and p
neutron postulated 1920, experimentally proven 1932 (Chadwick)
new force: strong interaction
-->
with p and n and strong int. nuclei can be built up
„rules of the game“ for building up nuclei out of p and n :
• quantum mechanics
• special relativity
• weak interactions
Epot
quantum mechanics :
p and n have spin 1/2 (fermions)
no 2 fermions can be in the same state
additional p and n are less tightly bound
--> „shell structure“
n
p
special relativity :
E = m.c2
combination of a and b to system S
-->
mS < ma + mb
„mass defect“ ∆m = mS - (ma + mb)
energy liberated = ∆ m.c2
weak interaction :
= „binding energy“
p + e- <--> n + e
n.b. also need to explain why some nuclei send out
various types of radiation
„radioactivity“
, , 
„construction on paper“ of nuclei
assume unlimited availability of p and n
in either spin state (up or down )
notation: AZ , e.g. hydrogene = 1H
Epot
2 nuclei: pp, nn, or pn
S.I. is stronger for parallel spins
not allowed for identical particles
--> only pn bind !
= deuteron (p n + p n ) = 2H
3 nuclei: ppp and nnn don‘t bind,
pnn OK = triton
ppn OK = 3He
= 3H
n
p
4 nuclei: pppn and pnnn don‘t bind
ppnn
Epot
jackpot !
= 4He
all 4 nuclei in ground state
total spin = 0
very high binding energy
=  particle
5 nuclei: all unstable
6 nuclei: 3p3n OK
= 6Li (=  )
7 nuclei: 4p3n unstable
3p4n OK = 7Li
8 nuclei: 4p4n unstable

n
(= 
p
 - radiation
Epot
Epot
transition with radiation of
high energetic photon
n
p
+ 
n
p
 - radiation
Epot
Epot
transition with transmutation of
p into n
+ e++ e
with emission of
positron and neutrino
n
p
n
p
(also n
n + e++ e
p + e- + e )
p
 - radiation
Epot
Epot
Epot
nucleus splits
+
n
p
n
mostly in heavy nuclei
p
n
p
answer to
„what
•
 - radiation:
•
 - radiation:
•
 - radiation:
is radioactivity“ ?
split up of nucleus
resulting in a lighter nucleus + 4He
transmutation of p (or n) into n (or p) and
emission of e+ (or e-) and a (anti-)neutrino
transition of a p or n from a higher to a lower
energy level with
emission of a 
 high energy photon (keV)
nuclear fusion
in the
universe
basic astronomical facts
classification of stars:
Hertzsprung-Russel diagram
luminosity
vs.
colour index
spectra of starlight
same spectra as for known elements on earth !
all the same stuff .....
expansion of the universe
nebulae are other galaxies
their light is the more shifted to longer wavelenghts the
further they are away
redshift
soon interpreted as expansion of the universe
(Einstein‘s GTR)
sun
BAS11
Z = 0.07
Big Bang nucleosynthesis
big bang
at t = 0
???
expanding universe is cooling
1ms (300 MeV)
free particles
p, n, e-, e+, , 
s ( 1 MeV)
e- p
n
mn > mp
neutrons decay
d = (pn) forms, but destroyed by 
s (100 keV)
300 s ( 50 keV)
lifetime of d increases
neutrons captured into 3T, 3He
nucleosynthesis finished !
92% p, 7% 4He, < 1% d, 7Li
no free n left
4He
not much happens to universe of p, 4He, and e- between 5 min and 350000 y
universe needs to cool below 3000° to form H and He atoms
now GRAVITATION takes over
enormous potential gravitational energy in H and He atoms!
expansion counteracted by the formation of gas clouds of various sizes
contracting gas clouds
rise of T at centre
„proto-star“
if mass of cloud big enough ( > 0.1 m ) after several million years (My)
T will rise to reionisation and eventually to neutron production !
p+p
p + n + e+ + 
free neutrons are back !
p+n
d+
after 500 My:
back to fusion of H to 4He
the heavier the star
the hotter in the centre
the faster it will burn
enormous stable energy release for billions of years
we understand
why the sun shines
but we haven‘t found gold yet
keep going ...
stellar evolution
study core: highest T
production of 4He increases density
eventually all H in core burnt up
fusion moves to shell, less dense
small star (m < 0.4 m
big star:
and T
fusion speeds up
positive feed-back !
„shell burning“
) : H fusion will stop
brown dwarf
m ≈ 20 m
H fusion will continue
mass and density of 4He and T will further increase
„ 3  process“
helium burning
4He
+ 4He
8Be
OK, but 8Be unstable with very short lifetime
need a third 4He to hit 8Be
4He
+ 8Be
12C
„helium flash“
enormous rise of E output and T
H burning in outer shell also increases
aside:
nuclei with A = 4n (multiples of 4He) dominate fusion processes
other nuclei produced as well but play little role
nuclei with odd number of p or n much less stable
only 5 stable odd - odd nuclei
helium burning will produce 12C (and 16O ) in core
increase of T
ignition of consecutive further burning stages
helium burning
carbon burning
neon burning
oxygen burning
silicium burning
3 4He
2 12C
2 20Ne
2 16O
2 32S
12C
, 16O
24Mg
20Ne , 16O
24Mg
32S
28Si , 24Mg
56Ni
56Co , 56Fe
all processes at the same time in consecutive shells
every new process shorter and with less E output
no more fusion processes beyond
56Fe
/ 56Ni !
beyond 56Fe binding energy of nucleons decreases
formation of heavier nuclei requires energy instead of releasing it
great job done ! all elements up to Fe , Ni produced
but still no gold
!!?
detail:
once C, N and O are present
a second H fusion process contributes: „CNO cycle“
C, N and O
just act as
catalysts
heavy star unstable:
Fe accumulates in the core,
when mcore > 1.3 m
no energy output
gravitational collaps
supernova explosion
e- p
n
supernova
Fe core
only n
collaps to incompressible nucleus
gigantic release of


nuclei falling in from outer shells stopped, partially disrupted
made to bounce back as fragments
release of p, n, 
high energy collisions
production of heavy n -rich nuclei
beyond U
supernova
collapsed core will form a neutron star or a black hole
nuclei produced will settle down into full range of elements
including gold !!!
ejected matter:
material for new star formation
sun & solar system probably 3rd generation star
heavy elements all from previous stars
chart of nuclei
total nb. of known nuclei ≈ 3300
of which ≈ 250 (quasi-) stable
relative abundance of elements in earth crust
nAu
nFe
now we understand
why gold is much rarer than iron
≈ 10-7
nuclear fusion
as
energy supply on earth ?
intriguing prospect:
practically unlimited and cheap supply of fuel: 1H, 2D, 3T
main problem:
how to attain necessary high T
to overcome electrostatic repulsion ?
brute force solution:
hydrogene bomb
fusion triggered by fission bomb !
first „successful“ explosions by the US (1952)
and the Soviet Union (1953)
controlled fusion
60 years of technical developments:
fuel (almost) always
2D
and 3T
two techniques:
- fusion of plasma enclosed in magnetic fields
( plasma = fully ionised atoms = bare nuclei )
- inertial fusion
ignition of fuel by focussed lasers or particle beams
3 critical parameters to initiate fusion:
- density
- temperature
of fuel
- confinement time
(Lawson criterium )
goal: keep fusion going for a time long enough
so that energy output > energy input
plasma fusion
enclose plasma in toroidal magnetic field
„Tokamak“
„stellarator“
chamber circumference ≈ 20 m
diameter
≈ 3m
magnetic coils: no problem, field ≈ 2-5 Tesla
nuclei will spiral around magnetic field lines
and around the torus
plasma current will induce poloidal field
--> instabilities
injection of 2D and 3T and acceleration to fusion temperature
fusion should keep up T
n produced will escape and should heat up He gas in surrounding
large number of experimental sites all over the world
most advanced: TFTR (US) first ignition 1986
JET (EU)
first ignition 1991, energy O/I ≈ 0.65, 2 sec
new international project (EU, US, Russia, Japan, China, South Korea, India)
ITER
International Thermonuclear Experimental Reactor
in planning/construction in Cadarache (south of France)
plans: test with hydrogene plasma in 2020
ignition with 2D and 3T
goals: energy output/input ≈ 10
burning time > 400 sec
in 2027
inertial fusion
main project: National Ignition Facility (Livermore, Cal., USA)
driven by 192 high power lasers focussing on small target (mm)
about to reach ignition (more difficult than expected )
has also military aims
advantages of fusion reactors w.r.t. fission reactors
- fusion reactors cannot explode
- unlimited supply of fuel
- major radioactive material: only 3T with half-life 12.3 y
disadvantages of fusion reactors
- very expensive to construct and operate
economically viable ?
summary
we wouldn‘t exist
• without the hydrogen and its gravitational potential
produced during the cosmic evolution
• without all the elements
produced by nucleosynthesis in the stars
• without the energy
steadily supplied by the sun for billions of years
we would not exist
without
nuclear fusion