KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
! 1934 - Fermi irradiates uranium
with a neutron source and observes
a variety of new radionuclides. This
is erroneously interpreted as first
evidence of transuranium elements
! 1938 - Hahn and Stra$mann find
that the “transuraniumelements” in
reality are isotopes of Ba, La etc.
! 1939 - Meitner and Frisch explain
this as follows: The nucleus devides
into two equally large parts,
releasing almost 200 MeV energy,
when uranium is irradiated with
thermal neutrons
! 1939 - Frisch reports strongly
ionising fragments
Per Hoff
Autumn 2004
Fission, short history
Fission, short history
Fission, short history
! 1939 - Niels Bohr and John A. Wheeler
explain the phenomenon theoretically in
simple terms
! 1939 - Bohr explains that thermal
fission only occurs with 235U and not
with 238U
! 1939 - von Halban and Joliot, as well as
Szilard and Fermi, realise that the net
gain of neutrons may give a chain
reaction, a basis for a reactor
! 1940 - Seaborg, McMillan and Wahl
discover Np and Pu, and find that Pu
undergoes thernmal neutron fission
! 1942 - Enrico Fermi leads the project
building the first nuclear reactor at the
Chicago University Stadion.
Per Hoff
Autumn 2004
! 1940 - Flerov and Petrzhak discover
spontaneous fission
! 1941-45. The Manhattan-district
project develops the atomic bomb.
Two bombs are used against Japan,
one based of isotope enriched 235U,
one on reactor produced 239Pu.
! 1956 - First ordinary nuclear power
plant, Calder Hall, England, followed
by build-up of nuclear power i many
contries
! 1960-1962 Extensive fallout from
atmospheric bomb tests
! 1963 - Agreement for stopping
atmorpheric bomb testing (Moscow
treaty)
! 1979 - Harrisburg accident
! 1986 - Tsjernobyl accident
Autumn 2004
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Chain reaction
Neutron induced fission
Why does fission occur ?
Neutron
235
The reason for fission (induced and
spontaneous) is that the coulomb forces
become stronger than the nuclear forces
in large nuclei. A parameter assessing
that is the
fissionability:
Z2/A
U
142
91
Ba
Kr
Neutrons
Barium and krypton are examples on
fission products which may be formed
when the intermediate fissions
Autumn 2004
Per Hoff
Per Hoff
! Because the number of neutrons
produced are larger than the
number consumed, one can obtain
a chain reaction.
! This was probably first recognised
by Leo Szilard. He also tried to
patent it.
Autumn 2004
Per Hoff
Purely energetically, fission is possible
wherever energy can be gained by
splitting a heavy nucleus into two
fragments, but the coulomb barrier
prevents that from happening for
elements lighter than U.
Autumn 2004
Per Hoff
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Energy release in fission
Energy release in fission
The fission process
Consequence; The residual-heat effect
117
The reactor must be cooled also after it
is shut down, otherwise the release of
heat from the decay of fission products
could melt the fuel, eventually making
a hole in the reactor tank
234
Energy difference: ~0.9 MeV pr. Nucleon
Total energy release pr.fission process
is then 4 x ~0.9 = 200 -210 MeV
About 180 MeV at fission, the rest is
delayed and released as $- and (
Per Hoff
Autumn 2004
That happened in the Harrisburg
accident 1979. About 50 % of the fuel
turned out to be molten, but it did not
manage to make a hole in the tank.
This accident was the second most
serious at a nuclear power plant ever,
but did not lead to significant release of
radioactivity. But the situation was
totally out of control.
Per Hoff
Autumn 2004
The yield curve is asymmetric
with a minimum at 117
Autumn 2004
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Energy dependence
Fission yield
Per Hoff
Charge dependence
14 MeV
Fission neutrons
Thermal
The lower mass peak moves
upwards, when the fissioning mass
increases, the upper peak remains.
Autumn 2004
Per Hoff
When the neutron energy
increases, the “valley”is filled for
the fission of 235U
Autumn 2004
Per Hoff
Along a single isobar the yields
distribute as whown on this curve,
with a maximum at Zp, corresponding
to the proton/neutron ratio in the
fissioning nucleus
Autumn 2004
Per Hoff
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Proton reactions -bismuth
Fission products
Transuranium elements
Fission products are nuclei with a Z/Aratio corresponding roughly to 234U:
For A = 139 it gives Zp = 54 (139Xe)
For A = 95 it gives Zp = 38 (95Sr)
Giving the following sequences
139
Xe(39s) Y 139Cs(2.3m) Y 139Ba(63m)
Y139La (stab.)
95
95
95
95
Sr(24s) Y Y(10m) Y Zr(64d) Y
Nb(35d) Y 95Mo(stab.)
Hence, one gets a waste problem at
A= 95, but not atA=139
Autumn 2004
Per Hoff
At energies up to 100 MeV there is
almost only spallation. Above 100 MeV
spallation and high energy fission
compete
Autumn 2004
Per Hoff
Neutron capture in 238U with two
consecutive disintegrations give 239Pu
as indicated. More neutrons will be
captures, and form transuraniums up
to 252Cf.
Per Hoff
Autumn 2004
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Principle for a nuclear
reactor
There are many different types of nuclear
power plants, from ordinary civilian plants
to submarine reactors with very special
function.
In general, the construction must make it
possible to bring the fission process to
criticality in a controlled way. This is done
by means of control rods made in a
material with high ability to capture
neutrons with thermal energies, i.e. with
high Fthermal, usually Cd or B (as boron
carbide)
Alsmost all reactors are based on
thermal fission.
Autumn 2004
Per Hoff
! Consequences:
! One needs a moderator, which
slows neutrons down but does not
absorb them
! One needs a critical mass of fissile
material (235U,233U, 239Pu)
! One needs control rods with high F
to control the process
! Produced energy must be removed
! The contruction must give one new
neutron available for a new fission
after every fission (criticality)
! The process must be controlled,
done by means of delayed neutrons
! The neutrons must be slowed down
to thermal energies
Autumn 2004
Per Hoff
Energy release
One kg 235U = (1000/235) C 6.02C1023 atoms
=2.6C1024 atoms 235U
Released energy: 2.6C1024 C 200 MeV
= 5.1C1026 MeV = ( 5.1C1026 C1.6C10-13) J
= 8.2C1013 J
If this amount is developed pr.day, it is released
9.5C108 J pr.second from fission,which implies
an effect of 950 MW thermal, a typical average
power plant.
The electricity production is of course lower,
depending upon the transfer efficiency (pure
thermodynamics)
Autumn 2004
Per Hoff
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Reactor types -boiling
water (BWR)
Reactor types - Magnox
Sketch of swedish pressurised water
reactors (Source: SKI)
Sketch of swedish boiling water
reactors (Source: SKI)
Autumn 2004
Reactor types
Pressurised water (PWR)
Per Hoff
Autumn 2004
Per Hoff
Per Hoff
Autumn 2004
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Reactor types -Heavy
water (Canada)
The RBMK-reactor
(Tsjernobyl-type)
Security barriers
T.Henriksen, Biofysikk
In this type of reactor, each fuel element
stands in a separate channel, surrounded
by graphite as moderator material
Autumn 2004
Per Hoff
Autumn 2004
Per Hoff
!
!
!
!
!
!
1.
2.
3.
4.
5.
6.
The fuel itself
Fuel encapsulation
Reactor vessel
Concrete shielding
Primary enclosure
Bunker
Autumn 2004
Per Hoff
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Used fuel
Thyroid cancer with children
Tsjernobyl accident
T.Henriksen. Biofysikk
The remains of reactor 4 in Tsjernobyl
after the accident
The radiologic risk in used fuel
declines as shown here.
Autumn 2004
Per Hoff
Autumn 2004
Per Hoff
Kilde: SSI, Sverige
It is a dramatic increase in thyroid cancer
among persons between 0 and 15 years
at the time of the accident, probably due
to inferior proptection against exposure
to131I
Per Hoff
Autumn 2004
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Transport pattern from
Tsjernobyl April/May 1986
The weather in this period went
from east to west and gave
maximum fallout in the district
from Gävle to Snåsa
Autumn 2004
Per Hoff
Radioaktive fallout
Waste nuclides
! There are three sources to
radioactive waste in a reactor.
! Transuranium nuclides from (n,()processes in the fuel
! Fission products
! Other capture processes (in
construction materialser etc.)
! In addition comes waste from the
enrichment process, particularly
depleted uranium
A fallout after an accident will follow
different patterns in nature depending on
weather, radionuclide chemistry, initial
conditions etc.
Autumn 2004
Per Hoff
Autumn 2004
Per Hoff
KJM-5900
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Waste nuclides Most important
transuranium isotopes
(> 1 Ci pr. Ton used fuel)
Halflife > 25 y:
238
Pu
88 y
239
Pu
24 000 y
240
Pu
6560 y
242
( Pu 375 000 y)
241
Am 432 y
243
Am 7370 y
243
Cm 29 y
Halflife 1-25y
Pu
14.4 y (soft$, gives 241Am)
Cm 18 y
241
244
Per Hoff
Autumn 2004
Waste nuclides
Most important fission
products
Very longlived
(Halflife > 1000 y)
79
Se 65 000 y
93
Zr 1.5 mill y
99
Tc 213 000 y
107
Pd 6.5 mill y
126
Sn 100 000 y
129
I
16 mill y
135
Cs 2 mill y
Methods for final deposit shall prevent
also these from release into the
environment
Autumn 2004
KJM-5900
KJM-5900
Nuclear Chemistry, Dept.of Chem., University of Oslo
Nuclear Chemistry, Dept.of Chem., University of Oslo
Waste nuclidesSome important shortlived
89
Sr
Zr/Nb
Ru
131
I
133
I
132
Te
140
Ba
141
Ce
143
Pr
147
Nd
148g,m
Pm
149
Pm
151
Pm
153
Sm
95
103
Autumn 2004
Per Hoff
Forsmark 1, installation
50 d.
64/35 d
39 d
8d
21 h
3d
13 d (140La daughter)
33 d
14 d
11 d
5/41 d (activation prod.)
2.2 d
1.1 d
2.1 d
Per Hoff
Autumn 2004
Per Hoff
Waste nuclides
Most important fission
products
Average halflife (0.5-100 y)
85
Kr 11 y
90
Sr 29 y (90Y daughter)
106
Ru 1 y (106Rh daughter)
113m
Cd 15 y
125
Sb 2.8 y
134
Cs 2.0 y (activation product)
137
Cs 30 y
144
Ce 284d (144Pr daughter)
147
Pm 2.6 y
151
Sm 93 y
154
Eu 8.5 y (activation product)
155
Eu 4.9 y
Autumn 2004
Per Hoff