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Radioecological research during 25 years after the Chernobyl accident Sweden Society Radioecology Conference , 22-23 of March. The Dnieper River Aquatic System Radioactive Contamination; 25 Years of Natural Attenuation and Remediation. Voitsekhovych Oleg.

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Radioecological research during 25 years after the Chernobyl accident

Sweden Society Radioecology Conference , 22-23 of March

The Dnieper River Aquatic System Radioactive Contamination; 25 Years of Natural Attenuation and Remediation

Voitsekhovych Oleg

Head of Environment Radiation Monitoring Department.

Ukrainian Hydrometeorological Institute. Kiev Ukraine

scope of presentation
Scope of presentation


1. Radionuclide release and deposition

2. Radioactive contamination of the catchments and aquatic environment

physical and chemical forms of radionuclides and its transformation

radionuclides in the aquatic systems

surface waters (rivers and reservoirs),

groundwaters in the Chernobyl exclusion zone

marine system (Black Sea and global context)

3. Assessment of the water protection countermeasure

Initial phase

Intermediate phase

Later phase and current situation

Chernobyl cooling pond decommissioning project

4. Radiation Dose and Risk assessment. Public perception and Assessment of the countermeasure effectiveness.

5. Lesson learned

Key natural attenuation processes

Developments and validation of radionuclide transport model

Environment Radiation Monitoring strategies and methods development

Risk Assessment and Risk management at the radioactive contaminated lands.

Emergency preparedness aspects

Chernobyl Isotopes applications as markers for Environment studies

  • Twenty five years ago, an unprecedented large amount of radionuclides were released into the environment due to the major accident at Unit 4 of Chernobyl NPP. As a result, the catchment areas and water bodies of the Dnieper River Basin (third largest aquatic system in a Europe) had been significantly contaminated, primarily due to 137Cs and 90Sr.
  • In some cases, the level of radionuclides contamination in the water bodies returned to pre-accident conditions within the first decade following the accident, some aquatic ecosystems remained highly contaminated.
  • This report presents the past twenty five year time-series data of water body contamination in Ukraine and the dynamic impacts caused by natural factors and human activities in the Chernobyl affected areas.
  • This overview is trying to find answer on a question. What Have We Learned ?, assessing some successes and failures to mitigate water contamination over post-accidental years

The information in

Media in the first weeks

since the Accident

did not described

adequately an actual


On April, 26





Chernobyl Accident is the Highest Single Release of Radionuclides into the Global Environment



  • Airborne radionuclide transport >200,000 km2 of Europe with 137Cs >37 kBq/m2 (1 Ci/km2).
  • Fuel particles—finely dispersed, low volatility, settled primarily within the ChEZ
  • Condensed components—from radioactive gases, settled primarily along the atmospheric flow pathways
  • Hot particles—fuel particles, uranium dioxide, with a specific activity >105 Bq/g, size 1 to 100 µm, surface density ~ 1,600 per m2, to ~0.5 m depth

Uncertainties in Assessment and needs for experimental verification of the accidental consequences.

Radionuclide transport studies due to Runoff, sampling at the contaminated lands and water bodies


Specific phenomenon of the Chernobyl radioactive release --- significant amount of nuclear fuel particles were dispersed to the environment and deposited on catchment’s soils and bottom sediment of the affected water bodies

UOx matrix fuel particle

Fuel particle X-ray microanalysis spectrum of Zr-U-O fuel particles(Ahamdach 2000)

U-Zr-O matrix fuel particle

Median size of fuel particles ~ 4-6 m

Till 2000 about 70% of radionuclide activity was associated with hot particles particles (2000)

In 2008 most of particles in the soils of river water catchments have been destroyed due to weathering impact and chemical leaching, while significant amount of the “hot particles” still remained in the bottom sediment of lakes around ChNPP.

from Kashparov at al, 2003

geochemical conceptual model of the fuel hot particles behavior in soils and bottom sediment
Geochemical Conceptual Model of the fuel hot particles behavior in soils and bottom sediment
  • Low DO and high pH cause a very slow dissolution of fuel particles in bottom sediments.
  • Weathering effects, vegetation and microbiological are causing significant effects onto the hot particles physical structure In soils and dried wetlands increasing its dissolution rate.
  • Fuel particle dissolution will take ~15–25 years in exposed sediments, and ~100 years in flooded areas
  • The physical and chemical form of radionuclide transformation in the catchments' soils and bottom sediment allow to achieve significant progress developing radionuclide water transport models from the contaminated watersheds and river systems

J Environ Radioact. 2009 Apr;100(4):p.329-32..

Fuel particles in the Chernobyl cooling pond: current state and prediction for remediation options.

Bulgakov A, Konoplev A, Smith J, Laptev G, Voitsekhovich O.


Radioactive contamination of the catchments and aquatic environment as versus of fallout formation date, its physical and chemical forms and also the landscapes at the deposited river watersheds

Calculated plume formation according to meteorological conditions for instantaneous releases on the following dates and times (GMT): (1) 26 April, 00:00; (2) 27 April, 00:00; (3) 27 April, 12:00; (4) 29 April, 00:00; (5) 2 May, 00:00; and (6) 4 May, 12:00

(Borsilov and Klepikova 1993).

137Cs activity concentration in different rivers per unit of deposition, Smith, 2004

radionuclides in rivers
Radionuclides in Rivers

Ratio of 90Sr and 137Cs in soluble forms in Pripyat River near Chernobyl

Annual fluxes of 137Cs in the DnieperRiver

1012 Bq Radionuclide inlet to the Kiev reservoir. Pripyat River

Desna River

Rain flood

Winter ice jam

Spring flood

Spring flood

Data of Ukr. Hydromet. Institute


Wash-out phenomenon for 137Cs and 90Sr

Pripyat River Flood 1999

Return water running off

from floodplain and drainages


Wash-off Snow

melting effect

Radionuclides runoff budget in the Pripyat river show 10-20% of Cs and 40-70% of Sr have contributing by contaminated waters washed out from the ChNPP zone



Pripyat River Floodplain around Chernobyl NPP was most heavy contaminated and identified as most significant source of the Dnieper system 90Sr-90 secondary contamination


Flood protective dam has been constructed

Site characterization studies and modeling results show that most efficient water protection strategy will be to control water level and to mitigate inundation of the most contaminated floodplains by the flood protection sandy dykes constructed at left and right banks of the Pripyat river




Annually averaged 90Sr activities in water of the Pripyat River downstream of Chernobyl town and effects of water contamination reduction due to construction of the protective dams, preventing flooding of the most contaminated floodplain area near NPP riverside in 1993


Before protective dam constructed



After protective dam constructed in 1993


radionuclides in the closed lakes of the most contaminated areas around chernobyl
Radionuclides in the closed lakes of the most contaminated areas around Chernobyl

137Cs and 90Sr in Gluboky lake near Chernobyl NPP

Radionuclides in water of the Chernobyl cooling pond, 1986-2009

Data of Chernobyl Ecocenter



Temporary Radioactive Waste Disposal Sites

are significant sources of the shallow ground water contamination

Its characterization and step by step removal to the specially organized places for long-term safe storage at the Radioactive Waste Reprocessing Plant become significant element of Environment Remediation Strategy at the Chernobyl Exclusion zone reducing their influence on further long-term ground waters contamination


chernobyl pilot site worst case scenario of near surface radioactive waste disposal
Chernobyl Pilot Site – “worst case” scenario of near-surface radioactive waste disposal

Trench Studies TRWDS (PVLRO “ Red Forest)

Bugay at al, 2003.

Schematic trench cross-section


Monitoring and Simulation of 90Sr Distribution in Groundwater

In some places 90Sr activities concentrations in the ground waters adjacent to TRWDS are continuing to growth.

Those, its moving toward the Pripyat river are very slow (1-10 m per year).

90Sr will reach the Pripyat River in ~50-60 yr from now,

However, even in case contaminated groundwater front will reach the river its flux contribution will be insignificant to the Pripyat River radioactive contamination at this time.

In any case observations on the groundwater regime and its contamination trends will be continued for a long time

90Sr in the groundwater TRWDS “Sand Plato” near Pripyat River (Kiereev et al. 2006)

Bugai et al. 1996

effects of the groundwater level drawdown
Effects of the Groundwater level drawdown


Predicted 90Sr concentrations in the aqueous phase without NSC after 100 yr.

Distance toward the Pripyat River from NSC

The groundwater water table will be reduced at mane places around the ChNPP Cooling pond from 1 to 7 m of present since it will be decommissioning

The ground water flow directions will be also changed.

The effects of the groundwater level declining in the CP will create positive effects in regarding of the number of temporary waste disposal sites situated around and also is beneficial for lowering inundation levels at Chernobyl NPP NSC (New Safe Confinement) site

Bugai D., Skalsky A. 2001


90Sr in the waters of the Dnieper’s reservoirs

90Sr in the reservoirs of the Dnieper cascade is still above of its pre-accidental levels observed in 2010 in range 40-100 Bq m-3 ( the same levels as were observed during 2002


137Cs in the waters of the Dnieper reservoirs

137Cs activity concentration in water at the lowest reservoir returned to its pre-accidental level still in 1996-1998.

In 2010 137Cs activities in Kiev (upper reservoir) in a cascade were observing in range 10-20 Bq m3, while in Kakhovka (lower reservoir) -- 0,5-1,0 Bq m3

137 cs in the bottom sediments of reservoirs
137Cs in the bottom sediments of Reservoirs


Upper part of Kiev Reservoir

Pripyat River


Low part of Kiev Reservoir




Kremetchug reservoir bottom, 1994

Dam near Kiev

137 cs in freshwater fish and other aquatic biota
137Cs in freshwater fish and other aquatic biota

137Cs in predatory and non predatory fish species in Kiev reservoir. (after I.Ryabov et al., 2001)

Gudkov, et al. 2008)

137Cs and 90Sr in predatory and non-predatory fish species. Gluboky Lake


Gluboky Lake

Absorbed dose rate caused by incorporated radionuclides in the algae and non-predatory fishes.

Published by D.Gudkov et al. 2008

Averaged chromosome aberration rates in the fresh water lake mussels in the lakes of the ChNNP area and in the reference clean lakes near Kiev

summary long term doses from aquatic pathways
Summary. Long-term doses from aquatic pathways.

Human exposure via the aquatic pathway took place as a result of consumption of drinking water, fish catch in reservoirs and agricultural products grown using irrigation water from Dnieper reservoirs.

In the middle and lower areas adjacent to the Dnieper reservoirs, which were not significantly subjected to direct radionuclide contamination in 1986, a significant proportion (10–20%) of the Chernobyl exposures were attributed to aquatic pathways.

Estimates were that individual doses via aquatic pathways would not have exceeded 1–5 μSv y-1.

Furthermore, in some closed lakes, the concentration of 137Cs remains high and high levels of contamination are found in fish species. People who illegally catch and eat contaminated fish may receive internal doses in excess of 0,5-1 mSv per year from this source.

The most significant individual dose was from 131I and was estimated to be up to 0.5–1.0 mSv for the citizens of Kyiv during the first few weeks after the Chernobyl accident.

aquatic pathway of radiation risk forming and its public perception

Dose realization (%) during a 70 years for children born in 1986

Aquatic pathway of Radiation Risk forming and its Public perception

For 1-st year about 47 %

  • In spite of doses were estimated to be very low, there was an inadequate understanding of the real risks of using water from contaminated aquatic systems.
  • This created an (unexpected) stress in the population concerning the safety of the water system. This factor made reasonable to provide assessment of collective commitment doses as a basis for justification of some water protection actions

For 10 years about 80%

From I.Los, O.Voitsekhovych, 2001


Actual dose

Public perception about

Food product, milk water external inhalation

Long-term probabilistic assessment of the Dnieper River contamination -- as a basis for collective dose simulation

Estimates were made of the collective dose to people from these three pathways for a period of 70 years after the accident, i.e. from 1986 to 2056

A long-term hydrological scenario was analysed using a computer model (Zheleznyak et al 1992).

Historical data were used to account for the natural variability in river flow.

Dose-assessment studies were carried out to estimate the collective dose from the three main pathways (Berkovski et al 1996),.

Concentration of 90Sr (1 pCi = 3,7 *10-2 Bq) in water of the upper and downstream reservoirs for the worst (top) and best probabilistic hydrological scenarios to be possible expected at the Dnieper reservoirs (Zheleznyak et al., 1997).


Collective effective dose for Kyiv region population due to water consumption from Kiev reservoir usage pathways as a function of years after 1986

Collective effective dose for Poltava region population due to water usage pathways from

Kremetchug reservoir as a function of years after 1986


Dose estimates for the Dnieper system show that if there had been no action to reduce radionuclide fluxes to the river, the collective dose commitment for the population of Ukraine (mainly due to Cs and Sr) could have reached3000 man Sv.

Protective measures, which were carried out during 1992–1993 on the left-bank flood plain of the Pripyat River and later on right bank (1999) decreased exposure by approximately1000 man Sv.(Voitsekhovich et al. 1996).

water protection and remediation
Water protection and Remediation
  • Many remediation measures during initial period after the accident (1986-1988) were put in place, but because actions were not taken on the basis of dose reduction, most of these measures were ineffective.
  • Because of the importance of short lived radionuclides, early intervention measures, particularly changing supplies, can significantly reduce doses to the population, mainly because 131I. However this opportunity has been missed during first month since the accident.
  • During first months after the accident restrictions on fishery and irrigation from the contaminated water bodies have been established. The number of water regulation actions at the small river in the Chernobyl exclusion zone were applied.
  • Numerous countermeasures put in place in the months and years after the accident to protect water systems from transfers of radioactivity from contaminated soils were, in general, ineffective and expensive and led to relatively high exposures to workers implementing the countermeasures.
  • The water regulation at the most contaminated floodplains and water runoff regulation from the wetlands in the close zone around ChNPP only can be considering as effective.

Decommissioning of the Chernobyl Cooling Pond

  • Decommissioning means:
  • Restore Monitoring network
  • Stop water pumping from the river
  • Separating the inflow and outflow channels (to use as fire reservoir)
  • Construct alternative source of cooling water--groundwater pumping wells
  • Declining water from the CP (filtration)
  • Remediation of the remained bottom area if needed. Institutional control.


Pripyat River

Water level above the sea

Days after start point


As the result of the water level decline the area covered about 60-70% of the bottom sediment territory may be dried and exposed for wind human access. The new artificially forming bottom sediment relief will be created by the 3 types - always dry- always covered by water - intermediate wetland (dried or wet) depend of water mode and climate conditions)

Bottom Sediment landscape transformation

The geochemistry of the wetland lakes will be transformed.

рН will be reduced and NH4 will be increased

The radionuclides in the water column will be increased

radionuclides in the bottom sediment uhmi 2005
Radionuclides in the bottom sediment (UHMI, 2005)




According to UHMI report in the CP is currently accumulated about

280 TБк 137Cs, 42 TБк 90Sr and 0,75 TБк Pu

The major activities of these radionuclides accumulated at the depth deeper of 7 meters and will remain flooded in a new transformed water ecosystem


Possible effects of soil particles atmospheric dispersion and fire

  • The effects of re-suspension to be local and may increase contamination of the surrounding areas no more then 5 % of existing contamination level.
  • NO significant effects for personnel, working at the Chernobyl NPP site due to effect of wind re-suspension or grass fire at the CP (less 1 mSv a year)
  • Radiological Risks associated with decommissioning of ChNPP for population living along the Dnieper River is negligible.
  • Transition period of the water infiltration may take 5-7 years, since the current CP will be transformed in to the new ecosystem

Preliminary assessment show that combination of OPTIONS

  • Do nothing and “Partial Remediation”, i.e. remediation of the most contaminated sediments ( relatively small areas 0,1-0,5 km2 by removing them and placing in the waste disposal site can be reasonable .
  • Natural attenuation process such as natural vegetation covers of the exposed sediment to be most reasonable selected remediation strategy.
  • New transformed ChNPP cooling pond ecosystem will pose a unique natural ecosystem laboratory.
  • It is still uncertain understanding how fast new transformed ecosystem will be restored as wetlands with a new sustainable conditions
from chernobyl forum 2005 to 2011
From Chernobyl Forum, 2005 to 2011

International Conference 25 Years after Chernobyl, Kiev

What has been changed ?

radionuclides in the black sea
Radionuclides in the Black Sea
  • After Chernobyl 137Cs inventory in the 0-50 m layer increased by a factor of 6-10 and the total 137Cs inventory in the whole basin increased by a factor of at least 2 ( pre-Chernobyl value of 1.40.3 PBq)
  • 137Cs input from the Danube and the Dnieper rivers (0.05 PBq in the period 1986-2000) was insignificant in comparison with the short-term atmospheric fallout.
  • The contribution of Chernobyl-origin 137Sr from atmospheric fallout was estimated at 0.1-0.3 PBq.
  • At the same time, a relatively important input of 90Sr from the Dnieper and Danube Rivers was observed.

Vertical profile of Radiotracers

137Cs activity and 238Pu/ 239,240Pu activity ratio profiles in deep-sea sediment core BS2K-11 (water depth 1880 m), 2002

137Cs in core BS98-03illustrate a history of sedimentation typical of riverine suspended particles deposited near the Danube River Delta

resolution of the core cutting method is 5 7 slices for 1 cm of the bottom sediment core

The result illustrates the low sedimentation rates, the upper peak of 137Cs corresponding to the Chernobyl input (1986) and the lower one to the time of maximum input from global fallout in the early 1960s.

Resolution of the core cutting method is 5-7 slices for 1 cm of the bottom sediment core

Black Sea Cruise 1998

1986 (89)


NW Tests

1963 (66)

environment radiation monitoring findings
Environment Radiation Monitoring Findings
  • ERM should be a tool for decision making
  • ERM should be Task and Site specific
  • Sampling programs to be based on screening assessment and also have adequately designed according to the tasks, expected way of the data utilization and regulatory requirements
  • Parameters for analytical measurements have to be corresponded with source term analyses and strategies for Safety or Environment Assessment .
  • QA/QC principles to be obligatory for all partners of the ERM programs
  • Data reporting and Data management should be well coordinated, agreed and based on DPSIR principles

DPSIR = Drivers, Pressures, States, Impacts and Responses

network of monitoring stations and wells in the chnpp close in zone
Network of Monitoring Stations and Wells in the ChNPP close-in zone.

Schematic of Monitoring Wells







  • 40 cross sections and aerosol pump stations;
  • 138 wells, 2 water supply stations;
  • 4 stations of surface water and bottom sediments


Chernobyl Ecocenter, S. Kireev.

groundwater modeling
Groundwater Modeling
  • VisualModflow and MT3D96 codes
  • Regional model of the Chernobyl exclusion zone and a 2D cross-section model


Infiltration model

After Bugai et al.

surface water modeling
Surface Water Modeling

Radionuclide transport modeling codes RIVTOX, COASTOX and THREETOX developed in IMMSP of the National Academy of Sciences of Ukraine

Water Quality Analysis Simulation Program (WASP)--EPA framework for modeling contaminant fate and transport in surface water.

Kd depends on the N ammonia concentration (M. Zheleznyak et al., (2005)—INTAS-2001-0556 Project Report “Radionuclide and Sediment Transport Modelling Within the Cooling Pond Ecosystem“)

Zheleznyak et al., 2002; Maderich et al, 2005


Processes Affecting Radionuclide Transport in the lake-reservoir systems

Microbial communities

Suspended sediments



Radionuclides in

suspended sediments



Dissolved radionuclides






Radionuclides in bottom sediments

Modified after M.Zheleznyak

Do We Have Reliable Monitoring and Modeling Tools?



Chernobyl NPP



Modeling of Cooling pond Dam Break and Sr-90 release

M. Zheleznyak et al. 2005

some conclusive comments
Some conclusive comments
  • Basic knowledge of geological, hydrological and ecosystem peculiarities at the area of potential radiation impact allows to select right strategy on imitative and environment protection
  • Any countermeasure and remediation planning must be based on detailed monitoring data and exhausting site characterization results.
  • Scientifically defensive assessment tools and required data must be developed and applied
  • Countermeasure and remediation selection must be based on a cost-risk analyses that directly connects the main physical and chemical processes to environment (ecosystem) or human heath risks and costs
  • Decision makers must be knowledgeable on phenomena being evaluating, they should efficient use expert’s experience and expert’s analytical and modeling systems, which can help to accept right and reasonable decisions aiming to mitigate or prevent expose of people and also allow to safe as always limited resources available , when measures can not be justified or may be postponed.
  • Decision makers must communicate facts quickly and honestly to the affected public

This this comprehensive overview is based on the results taken from number of previous national and international projects, which have been implementing with contribution of many people during recent 25 years.

Special thanks to:.

G. Laptev, V.Kanivets, A. Kostezh, L.Pirnach, S.Todosienko (UHMI)

  • and also appreciate to our colleagues:
  • D.Bugay, A.Skalsky (Institute Geological Sciences)
  • S. Kireev ( Chernobyl Centre),
  • V. Kashparov (Institute agriculture radioecology)
  • Konoplev, A. Bulgakov, (SPA, “Typoon”)
  • M.Zheleznyak, (IPMMS)
  • V.Berkovsky ( IRP)
  • O.Nasvit and D.Gudkov (IGB)

Many thanks to all analyst, engineers and technicians, which contribution to field and analytical studies make possible this syntheses and analyses

Many thanks to Chernobyl NPP authority and Administration of the Chernobyl Exclusion zone for supporting remediation projects and monitoring programs at the Chernobyl exclusion zone

thank you very much for your attention
Thank you very much for your attention

UHMI, Nauki prospect, 37. Kiev 03028. Ukraine