Chernobyl
Worst (still?) nuclear-power accident
in human history
April-May 1986
1
The Chernobyl Power
Complex, about 130 km north
of Kiev, Ukraine, and about 20
km south of the border with
Belarus
To the southeast of the plant, an artificial lake of some 22 square kilometers was constructed next to the
Pripyat River, a tributary of the Dniepr, with objective to provide cooling water for the reactors in Chernobyl.
2
The Chernobyl nuclear power plant
consisted of four nuclear reactors
of the RBMK-1000 design, with
Units 1 and 2 constructed between
1970 and 1977, while Units 3 and
4 of the same design were
completed in 1983.
Two more RBMK reactors were
under construction at the site at
the time of the accident in 1986.
The RBMK-1000 reactor is a Soviet designed
and built graphite moderated-pressure tubetype reactor, using slightly enriched (2% U235)
uranium dioxide fuel. It is a boiling light-water
reactor, with direct steam feed to the turbines,
without an intervening heat-exchanger. Water
pumped to the bottom of the fuel channels boils
as it p
progresses
g
up
p the p
pressure tubes in which
fission reactions take place, producing steam
which feeds two 500 MWe turbines. The water
acts as a coolant and also provides the steam
used to drive the turbines. The vertical pressure
tubes contain the zirconium-alloy clad uraniumdioxide fuel around which the cooling water
flows.
The moderator, whose function is to slow down neutrons to make them more efficient in
producing fission in the fuel, is constructed of graphite. A mixture of nitrogen and helium
is circulated between the graphite blocks largely to prevent oxidation of the graphite and
to improve the transmission of the heat produced by neutron interactions in the
graphite, from the moderator to the fuel channel.
The core is about 7 m high and about 12 m
in diameter. There are four main coolant
circulating pumps, one of which is always
on standby. The reactivity or power of the
reactor is controlled by raising or lowering
211 control rods, which, when lowered,
absorb neutrons and reduce the fission
rate. The power output of this reactor is
3200 MWt (megawatt thermal) or 1000
MWe, although there is a larger version
producing 1500 MWe
MWe.
Various safety systems, such as an
emergency core cooling system and the
requirement for an absolute minimal
insertion of 30 control rods, were
incorporated into the reactor design and
operation.
3
The most important characteristic of the RBMK reactor is that it possesses a "positive
void coefficient". This means that if the power increases or the flow of water decreases,
there is increased steam production in the fuel channels, so that the neutrons that
would have been absorbed by the denser water will now produce increased fission in
the fuel. However, as the power increases, so does the temperature of the fuel, and
this has the effect of reducing the neutron flux (“negative fuel coefficient”).
Th nett effect
The
ff t off these
th
two
t
opposing
i characteristics
h
t i ti varies
i with
ith the
th power level.
l
l At the
th
high power level of normal operation, the temperature effect predominates, so that
power excursions leading to excessive overheating of the fuel do not occur.
However, at a lower power output of less than 20% the maximum, the positive void
coefficient effect is dominant and the reactor becomes unstable and prone to sudden
power surges.
This was a major factor in the development of the accident.
Final note before telling the story of the accident:
A nuclear power plant employs different types of engineers: nuclear engineers whose
responsibilities include the management and control of the fission reactions, mechanical
engineers who deal with steam piping, pumps and turbines, and electrical engineers who
handle generators, transformers and various forms of electric controls.
In later April 1986, plans were to test live a scheme for extracting emergency electric
power from one of the station’s two turbo-generating sets. Such tests had been carried
out on two previous occasions, in 1982 and 1984.
Plans for the 25th of April 1986 were to use the shut-down of Reactor #4 for routine
maintenance as an opportunity for a third set of experiment. Shut-down reactors need
external power for circulating coolant that carries away heat from radioactive fission
products accumulated in the fuel rods.
The idea was to tap energy from a decaying (spinning down) turbine that was expected
to still deliver a few megawatts of electricity to pump cooling water through the reactor
core to delay the time when diesel generators had to be used to continue the cooling
process. Electrical engineers conducted the test, while nuclear-reactor specialists had
gone home at the end of the day.
Because the run was being conducted at low power,
power certain alarms were expecting to
go on, and the electrical engineers disabled those controls.
The electrical engineers reduced the power excessively late on April 25th, and noticing
their excess, they attempted to rectify the error by bringing power moderately back up.
When power began to rise, it did so violently because of the positive void coefficient
effect.
4
Within 4 seconds, power peaked at about 100 times the nominal power rating of the
reactor. An internal explosion ensued, fuel rod ruptured, hot-fuel particles contacted
cooling water directly, turning it immediately into steam.
A massive explosion followed.
The reactor containment envelop
was breached, and graphite core
exposed to oxygen in the air caught
fire, vaporizing radionuclides of the
core and sending them into the air.
It was 1:23am.
It has been estimated that the
temperature of the fire reached
2500oC. (For comparison, the
surface temperature of the sun is
5506oC.)
Two major releases of radionuclides took place, the first on 26 April and the second
on 5 May following a second explosion.
5
Release during the first few days has been estimated at
45 x 1016 (±50%) Bq = 12 x 106 Ci,
while the second explosion on 5 May probably released an additional
30 x 1016 Bq = 8.1 x 106 Ci.
Aside from the explosions, there also was some continuous release over a period
of 10 days. The estimated total release is
10.6 x 1018 Bq = 286 million curie
of radionuclides with half lives of more than 1 day.
In comparison, release from the Three-Mile Island accident in Pennsylvania in
March 1979, which virtually stopped nuclear energy in the United States, was
9 million Curie.
Nuclear weapon tests above ground during the 1950s and 1960s amounted to
36 million curie of Cesium-137.
Definitions:
1 Bq = 1 becquerel = 1 disintegration per second;
1 Ci = 1 curie = 37 billion disintegration per second
Thirty different radionuclides were released in the atmosphere, all generating  and 
radiation:
Iodine-131
Cesium-134
Cesium-137
with 8-day half life (= 20% of core inventory)
with 2-year half life
with 30-year half life (=13% of core inventory)
Iodine-131 is volatile, breathable and highly toxic.
30 people died immediately, thousands were hospitalized, and hundred thousands
were relocated, most permanently.
How much is left by now in 2012?
We are now 26 years later, and all of the Iodine-131 and Cesium-134 is gone, but a
significant fraction of the Cesium-137 remains.
m(t )  mo e  Kt
with
K 
ln 0.5
 0.02310 / year
30 years
t  26 years  m  0.548 mo
So, 55% of it still remains
out there!
6
Cleaning up…
7
Chernobyl in relation to major cities and seas of Europe
Surface synoptic weather chart at 12:00 GMT on 26 April 1986 showing the location of
Chernobyl in relation to the pressure systems that were to govern the initial transport of
radionuclides from the accident. (Adapted from Wheeler, 1988)
8
There is clear evidence that the maximum release occurred close to 1700 m altitude.
At 850mb level (about 1500 m high), the
winds were decidedly south-south-easterly
with speeds of about 35 km/h
km/h.
At 700mb above Chernobyl, adding to the
complexity of the situation, winds were
northerly at 40 km/h. Material reaching
that level (altitude of about 3000 m) were
swept southwards before curving around
the southern flank of the middle-level low
pressure which at that time lay over
Crimea (northern bank of Black Sea).
There was an inversion at 700mb.
The meteorology of 26
April – 3 May 1986 over
Europe.
9
Leading edge arriving
in Finland in 28 hours.
Rainfall occurred when
the plume reached
Scandinavia,
depositing radioactivity
on the ground.
The Swedes were the
first to notice.
Trajectories at 850mb. Radioactive clouds reached Finland, Norway and Great Britain in the week
following the first explosion. (Adapted from Dennis A. Wheeler, 1988, p. 856)
10
Deposition over
Great Britain
(Wheeler, 1988, page 859)
Put this in a computer simulation model and obtain the following hindcasts…
11
Radioactive fallout in
the immediate
vicinity.
12
13
14
Contours of the 800mb surface (= weather patterns around 2000m above ground) on 1 May 1986.
Note the low pressure center over Russia, drawing radioactivity across Asia and to Japan.
(Source: Wheeler, 1988, page 862)
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16
17
Chernobyl today:
Reactor 4 buried
in a sarcophagus
Chernobyl today:
Abandoned houses
(http://elenafilatova.com/)
18
For comparison: Three-Mile Island in Pennsylvania, 28 March 1979
Normalized dose patterns (in units of mSv) released by Three-Mile-Island-2 on 29 March (first
24h), 30 March (48h total), 31 March (72h total) and 7 April (240h total).
(From Scope 50 – Radioecology after Chernobyl, edited by Sir Frederick Warner and Roy M. Harrison, John
Wiley & Sons, 1993)
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