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Chernobyl

Worst (still?) nuclear-power accidentin human history

April-May 1986

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The Chernobyl PowerThe 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.

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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.

The RBMK-1000 reactor is a Soviet designed and built graphite moderated-pressure tube-type 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 progresses up the pressure tubes in which

Two more RBMK reactors were under construction at the site at the time of the accident in 1986.

p g p pfission 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 uranium-dioxide 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 MWeproducing 1500 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.

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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 t ff t f th t i h t i ti i ith th l l At thThe net effect of these two opposing characteristics varies with the power level. At the 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 certain alarms were expecting toBecause the run was being conducted at low 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.

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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 envelopThe 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 issurface 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.

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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 curieof 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 toNuclear 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 with 8-day half life (= 20% of core inventory)Cesium-134 with 2-year half lifeCesium-137 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?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.

o

Kto

mmt

Kemtm

548.0years26

year/02310.0years30

5.0lnwith)(

So, 55% of it still remains out there!

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Cleaning up…

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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)

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At 850mb level (about 1500 m high), the winds were decidedly south-south-easterly with speeds of about 35 km/h

There is clear evidence that the maximum release occurred close to 1700 m altitude.

with speeds of about 35 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.

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Leading edge arriving in Finland in 28 hours.

Rainfall occurred whenRainfall 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)

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Deposition over Great Britain

(Wheeler, 1988, page 859)

Put this in a computer simulation model and obtain the following hindcasts…

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Radioactive fallout in the immediate vicinity.

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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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Chernobyl today:

Reactor 4 buried in a sarcophagus

Chernobyl today:

Abandoned houses

(http://elenafilatova.com/)

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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)