1. Dmitri Bugai, Consultant Waste Technology Section NEFW
Experiences in planning and implementing decommissioning and remediation strategy for the cooling pond of Chernobyl NPP
Dmitri Bugai, Consultant
Waste Technology Section NEFW
Plenary Meeting of the Network on Environmental Management and Remediation (ENVIRONET)
5—7 December 2017, IAEA Headquarters, Vienna
The subject of my presentation is ….

2. Contributors of information and data for this presentation
This presentation is based on the IAEA TECDOC on a similar subjected which was finalized by a team of Ukrainian scientists and IAEA experts during June - September 2017
The responsible IAEA officer for this report is Horst Monken-Fernandes
The important contributions to the report were provided by V.Kashparov, D.Gudkov, V.Protsak, A.Skalskyy, B.Faybishenko, G.Laptev, V.Kanivets, O.Voitsekhovitch, and others.
WATEC 2017

3. Background
D&ER activity for the Chernobyl cooling pond is an element of the general D&ER program for the Chernobyl NPP
The challenges are related to serious contamination of the pond in the course of the Chernobyl accident in 1986, and it location inside the Exclusion Zone
The feasibility study for decommissioning cooling pond was prepared with the assistance from the IAEA in 2013, and approved by Ukrainian Regulatory authorities in 2014
Decommissioning (drainage) of the cooling pond started in 2014, and continues until present time
2013
2017

4. Contents Engineering design and site description
Radioactive contamination
Remedial planning (strategy; end-state criteria)
Radiological impact assessment analyses (providing bases for remedial design)
Remediation experience ( ) and Lessons learned
Conclusions

5. 1.1 Site description Basic characteristics of the pond Important dates
Parameter
Value
Length, km
11.5
Mean width, km
2.2
Operational water level”, m a.s.l.
111.0
Surface area, km2
22.9
Volume, million m3
151
Average depth, m
6.6
Max depth, m
18.5
Important dates
1976 – First section of the pond (12.5 km2) was commissioned
1981 – Pond area extended to 22.9 km2
– Chernobyl accident
– Last Unit 3 of ChNPP was stopped
May 2014 – Start of decommissioning (drainage) of the cooling pond

6. 1.2 Engineering design and water balance (operational period)
Design of the dam of the pond
Water level in the pond was about ~7 m above the level of Pripyat River
The sandy bottom and dam did not have any lining which resulted in large seepage losses
Water losses from the pond were compensated by pumping water from Pripyat River
Components of water balance of the pond
Water balance of the pond
Component
Value, x106 m3/year
Seepage
~ 100
Evaporation (with heat load from 3 units)
30-35
Evaporation (without heat load)
~ 5
( m3) m3
Maintenance of the pond entailed large exploitation costs (for pumping water)

7. 1.3 Bathymetry (depth distribution) of the cooling pond
Bottom topography of the pond based on results of 2001 survey [Buckley et al., 2002]
Areas with depths of 7.5 m and less occupied about 70% (~ 15.8 km2) of the pond.
These areas can be potentially exposed in case of the drawdown of level in the pond
Bottom sediment types at different depths
Depth range, m
Prevailing type of bottom sediments
Bottom area, km2
Fraction of the total cooling pond bottom area, %
0 – 3.5
Sand
2.1
9.6
3.5 – 7.5
Silty sand
13.7
62.6
Silt with admixture of sand (sandy silt)
10-12
Silt
1.7
7.8
> 12
2.3
10.5

8. 2.1 Radioactive contamination of the cooling pond
Contamination mechanisms
Atmospheric fallout of radioactive aerosols on water surface (nuclear fuel particles, condensation particles)
Releases to the pond of the ~5000 m3 of highly contaminated water from the damaged Unit 4 circuits/water used for firefighting
137Cs distribution in bottom sediments
(2012) [Voitsekhovitch et al., 2013]
Mid-May 1986 : Activity of water in the pond was ~ 104 Bq/L; the total inventory of radionuclides was estimated at ~ 2000 TBq
Time dynamics of 137Cs activity in the pond water
Media of concern – bottom sediment
By the end of 1986:
About 95% of 137Cs and 99% of 90Sr in the cooling pond were accumulated in the bottom sediments (sorption on suspended particles, sedimentation…)

9. 2.2 Radionuclide inventory and speciation in bottom sediments
Example activity profile in bottom sediments at 12 m depth (2002) [Pirnach et al., 2011]
While 137Cs was initially largely in soluble form (condensation aerosols, liquid releases)
90Sr and TRU isotopes were mostly associated with fuel particles (FP) of ~ n 1 … n  10 m size
Radioactivity inventory in bottom sediments (2012) [Buckley et al., 2002]
Radionuclide
Inventory, TBq
Max. spec. activity, Bq/g
Cesium-137
164±32 ~
Strontium-90
24±9
~ 100
Plutonuim-239/240
0.5±0.2
~ 2
Americium-241
1.1±0.4
~ 4
Autoradiography analyses of FP in bottom sediments [Protsak et al., 2017]
Tran-sedimentation; Possibility of RN mobilization in oxidized conditions
Due to redistribution (trans-sedintation) highest activities have accumulated in the deep pond areas (>10 m)
Geochemical conditions in bottom sediments (anoxic regime, alkaline pH) favoured low dissolution rates of fuel particles
In most studies > 90% of activity were in non-exchangeable forms (presumably) associated with undissolved particles

10. 3.1 Remedial planning: Strategy – water level drawdown in the pond
Drawdown of water level in the pond has been identified in a sequence of pre-design research projects as an ultimate option for the Chernobyl pond decommissioning
Advantages
Savings of ~ USD/year (expenses for continued refilling of the pond and dam maintenance)
Reduction of radioactivity ex-filtration to Pripyat River
Decreasing of groundwater levels in the adjacent areas (including Sarcophagus and radwaste disposal sites)
Elimination of the risk of the accident of the dam breach and release of radioactivity

11. 3.2 Remedial planning: Risks of water level drawdown in the pond
Atmospheric resuspension of highly contaminated bottom sediments (e.g., accident in Chelyabinsk-65 with dried up Karachay Lake in 1967)
Possibilities for exposure to “hot spots” of highly contaminated bottom sediments
Increased mobility of radionuclides from bottom sediments (oxidized environment)
Ecological damage : “massive dying out of aquatic species”, deterioration of water quality and general ecological situation
Predicted configuration of drained bottom areas for different climatic conditions
Remedial design foresaw possibility for suspending drainage process (by resuming pumping of water) in case of occurrence of unforeseen negative impacts

12. 3.3 Remedial planning: Phases and Radiological End-state criteria
“Brown field” end-state criteria accounting for location of pond inside the Chernobyl Zone
End-state coordinated with the decommissioning of the ChNPP (and reflect mean background levels)
Zone 1 (NW): 14 µSv/hour
Zone 2 (SE): 7 µSv/hour (2012)
(criteria set as mean values for 100m x 100 m areas)
“Control levels” set for radionuclide airborne concentrations (similar to 10-km zone of ChNPP)
Phases of cooling pond decommissioning
Preparatory stage ( ) - Creating preconditions (e.g., characterization) and developing design project for the decommissioning
Water level drawdown (controlled) – 5-6 years (starting from 2014)
Transition period to “stabilization of new aquatic ecosystem” – 7-10 years
Period of the long-term institutional control (> 10 years)

13. 4.1 Hydrogeological modeling to asses cooling pond drawdown rate and end-state
A set of groundwater flow and radionuclide transport models (MODFLOW, MT3D, NORMALYSA) was developed to assess pond draw-down rate, resulting configuration of residual lakes, and off-site radionuclide fluxes for a set of different climatic scenarios
Configurations of residual lakes
Normal
scenario
(B) Dry
scenario

14. 4.2 Assessment of impacts from drawdown: atmospheric transport of radioactive aerosols
Scenarios:
Normal conditions
“Dust storm”
Wildfire of the contaminated vegetation
Reference persons
Workers at ChNPP
Residents of Chernobyl Town
Impacts:
For all considered scenarios radiological impact impacts are low (µSv range)
Secondary contamination due to deposition is well below existing levels
Estimated inhalation 50-yr EED due to atmospheric transport of radioactive aerosols from pond bottom, µSv [KASHPAROV et al., 2011]
Scenario
Location
ChNPP
Chernobyl Town
“Normal” scenario
0.52
0.009
Dust storm
3.0
0.34

15. 4.3 Assessment of impacts from drawdown: Dose rates from bottom sediments
Predicted dose rates from bottom sediments [Kashparov et al., 2011]
Several sediment “hot spots” expected in in the Northern and Western parts of the pond
Maximum gamma dose rates are comparable to some other locations in Chernobyl zone
Managing contaminated bottom sediments:
As predicted atmospheric resuspension is low and there is no direct exposure of staff of ChNPP, there is no need to implement large-scale remedial measures for bottom sediments
Decisions for local hot spots should be taken on case-by-case basis
Conclusion: pond drainage appears to be an acceptable strategy, however the radiation and ecological situation needs to be monitored, and corrective actions shall be carried out in case of need (“controlled drainage”)

16. 5.1 Remediation experience: Water level drawdown dynamics
Satellite photo of the pond (August 2016)
Reasonable agreement of the actual pond drainage dynamics with a priori modelling predictions
By mid-summer 2017 drained bottom area reached ~40%
Some discrepancies in elements of bottom relief with the numerical DEM
Comparison of the predicted level drawdown rate and monitoring data

17. 5.2 Water level drawdown consequences: Re-vegetation of bottom areas
Silty drained pond bottom areas with a densely vegetation (as in 2017)
By summer 2017 about 60% of the drained pond bottom was overgrown by newly formed vegetation
Highly contaminated silty areas are overgrown by most dense vegetation cover (due to larger organic matter content)
Sandy poorly vegetated areas pose low risks of RN resuspension due to low contamination levels
Effect of retreating shoreline on “self-seeding” of sandy beaches” (2017)
Sandy areas covered by the dead Dreissena shells
Photos courtesy of V.Kashparov and V.Protsak, UIAR inst.

18. 5.3 Water level drawdown consequences: Radiological impacts
Dose rates from drained bottom areas [data of ChNPP]
Radiological impacts (airborne RN concentrations, external dose rates) are generally within the predicted acceptable ranges
The fuel particle dissolution in exposed bottom sediments is proceeding slower than expected (part of activity which can be mobilized over next 10 year is estimated at <30%)
Atmospheric monitoring data from monitoring stations in the vicinity of the cooling pond
Monitoring station
Annual averaged 137Cs airborne concentration, Bq/m3
2016
2017
“VRP-750”
2.5E-3
3.6E-3
1.2E-3
“Neftebaza”
6.1E-4
8.7E-4
3.0E-4
“BNS-3”
1.7E-4
2.9E-4
1.5E-4

19. 5.3 Water level drawdown consequences: “Ecological” parameters
No feared catastrophic consequences to the pond ecosystem have been observed so far (e.g., massive dying out of aquatic species leading to deterioration of water quality and general ecological situation), which allowed for continuous water level drawdown regime
Dissolved oxygen
Nitrogen (as ammonia)
Data of D.Gudkov, IHB inst.

20. 6 Conclusions
Behavior of the cooling pond during the initial phase (4 years) of water level drawdown mostly followed a priori modeling predictions
Radiological and ecological parameters generally conform to prescribed reference levels
A number of natural process/conditions in drained areas favor attenuation of radiological impacts
Experience of first 4 years also suggests that better understanding of some radioecological process of the cooling pond would be of broad scientific interest (e.g., fuel particle dissolution behavior, speciation radionuclides in residual reservoirs etc.)
Further in-depth radioecological studies of the cooling pond are planned in the frame of the Ukraine-Japan SATREPS project ( ); it is planned that activities of SATREPS will be carried out in close exchange with the IAEA (e.g., joint workshops, information exch. etc.)

21. Thank you!
Thank you!