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Paper presented at the Third International Symposium and Exhibition on Environmental Contamination in
Central and Eastern Europe, September 10-13, 1996, Warsaw, Poland
THE CHARACTERIZATION AND RISK ASSESSMENT OF THE “RED FOREST”
RADIOACTIVE WASTE BURIAL SITE AT CHERNOBYL NUCLEAR POWER PLANT
D.A.Bugai; A.S.Skalskij; S.P.Dzhepo:
Institute of Geological Sciences, Gonchara Str. 55-b, Kiev, Ukraine., Tel.: 380 44 216-6353
R.D.Waters:
Sandia National Laboratories, Albuquerque, NM, 87185., Tel.: 1 505 844-4672
Abstract
The “Red Forest” radioactive waste burials created during emergency clean-up activities at
Chernobyl Nuclear Power Plant represent a serious source of radioactive contamination of the
local ground water system, with
90
Sr concentration in ground water exceeding drinking water
standard by 3-4 orders of magnitude. In this paper we present results of our hydrogeological and
radiological “Red Forest” site characterization studies, which allow us to estimate
90
Sr
subsurface migration parameters. We use then these parameters to assess long term radionuclide
transport to ground water and surface water, and to analyze associated health risks. Our analyses
indicate that
90
Sr transport via ground water pathway from “Red Forest” burials to the adjacent
Pripyat River is relatively insignificant due to slow release of
90
Sr from the waste burials (less
than 1% of inventory per year) and due to long enough ground water residence time in the
subsurface, which allows substantial decay of radioactive contaminant. This result and our
previous analyses indicate that though conditions of radioactive waste storage in burials do not
satisfy Ukrainian regulation on radiation protection, health risks caused by radionuclide
migration to ground water from “Red Forest” burials do not justify application of expensive
countermeasures.
Introduction
The “Red Forest” radioactive waste burials were created during emergency clean-up
activities at Chernobyl Nuclear Power Plant (Ch.NPP) in 1986-87. The radioactivity
contaminated materials (i.e., dead pine “red forest” killed by radiation, soil, litter and debris)
were buried in situ in shallow (2-3 m deep) unlined trenches. The goals of this hastily planned
mitigative measure were to reduce the external exposure rate and to remove the threat of
resuspension of radioactivity associated with a fire in the dead “red forest”. The inventory of
long lived radionuclides disposed at about 1 km
2
in size “Red Forest” site is estimated at 2160 Ci
of
90
Sr, 3120 Ci of
137
Cs, and 28 Ci of
239
Pu and
240
Pu. A 1 to 2 m rise of the ground water table
occurred in the zone of the burial site from 1987 to 1989, which induaded some burials. An
intense radionuclide migration occurred to the local unconfined Quaternary sand aquifer. The
primary radioactive contaminant of concern for the groundwater migration is
90
Sr. The
90
Sr
activity in ground water below some burials reached 1.3 10
-6
Ci/L in 1989-91, which exceeds the
Ukrainian drinking water standard for
90
Sr of 100 pCi/L by four orders of magnitude [1,2].
Contaminated groundwater migrates towards Pripyat River, which is tributary to the major
Ukrainian water course Dnieper. Also, contamination of the unconfined aquifer threatens water
quality in the confined Eocene aquifer underlying Quaternary sediments, which is used as a
potable water source for Ch.NPP. Therefore, ground water contamination at “Red Forest” site
was considered a high priority environmental problem by several authors, and remediation
measures have been proposed (e.g., [3]).
Paper presented at the Third International Symposium and Exhibition on Environmental Contamination in
Central and Eastern Europe, September 10-13, 1996, Warsaw, Poland
In addition, radionuclide migration by biological pathway (i.e., radionuclide transfer to
vegetation) occurs from the buried waste. In 1989-91 herb and trees (mainly pines and birches)
have been planted at the study area in order to stabilize the soil cover of the waste burials.
According to few preliminary sampling data specific activity of the vegetation is about 10
-6
Ci/kg
with respect to
90
Sr and
137
Cs, and about 10
-12
-10
-11
Ci/kg with respect to
239
Pu,
240
Pu [ 4].
Though some preliminary site characterization studies has been conducted at “Red Forest”
burials in 1989-91, a number of important hydrogeological and geochemical parameters has not
been yet adequately characterized [1]. The major uncertainty is connected with specific chemical
form of Chernobyl radioactive contaminants. Radionuclides in the fall-out were associated with
fine dispersed reactor fuel (UO
2
matrix) “hot particles”. The geochemical mobility of this type of
contamination has been a subject of considerable controversy [1,5]. In this paper we present
results of our hydrogeological and radiological “Red Forest” site characterization studies, which
allow us to estimate in situ
90
Sr leaching rate from radioactive waste to ground water system as
well as other radionuclide subsurface transport parameters. We use then these parameters to
assess long term radionuclide transport to ground water and surface water, and to analyze
associated health risks.
Site characterization studies
General description of hydrogeological conditions and radioactive contamination at
Ch.NPP site is given in [1]. The research program reported in this paper was conducted in 1994-
96. It included regional and local scale studies. The regional monitoring well network was
installed at the site. From this network the regional hydraulic head gradient in the unconfined
aquifer is estimated at 0.003. Combined with hydraulic conductivity estimates of 1-10 m/day and
porosity estimate of 0.2 [1], this allows to estimate ground water advection velocity in the
unconfined aquifer at 5-55 m/y. Using chlorine ion balance method we derived estimate of
ground water recharge of 150-270 mm/y. This range of values is noticeably higher compared to
recharge estimates for areas undisturbed by clean-up activities (e.g., 50-60 mm/y [5]). Increase in
groundwater recharge may be explained by decrease in evapotranspiration caused by demolition
of the forest and removal of top soil layer. This increased recharge probably provoked the rise in
groundwater levels at the site in 1987-89.
Major field studies were conducted at the experimental plot located at the “typical” trench burial
No.22-T. To determine
90
Sr distribution in the subsurface, series of multilevel ground water wells
have been installed to the about 20 m thick unconfined aquifer, and soil sampling have been
carried out from the trench and underlying vadoze zone (Fig.1). Also, sediment cores were
extracted from the aquifer, and
90
Sr sorption distribution coefficient (K
d
) values in the range from
0.5 to 2 ml/g were estimated based on measurements of radioactivity of pore fluid and sediment
matrix. From the radiological sampling data,
90
Sr balance in the hydrogeological system was
calculated (Table 1). To interpret the data of Table 1, we use a simple first order kinetics source
term model, which has been shown in [6] to describe adequately radionuclide leaching from fuel
“hot particles”:
dA
G
= K
L
A(t) dt,
where dA
G
is amount of
90
Sr leached from a waste burial to vadoze zone and groundwater (Ci),
dt is time interval (y), A(t) is amount of
90
Sr in burial (Ci) at time t (y), and K
L
is leach rate
constant (y
-1
). From the data of Table 1 we estimate K
L
at 0.006 to 0.01 y
-1
(this corresponds to
leach rate of 0.6% to 1% of the total trench
90
Sr inventory per year). The above source term
model is used later in the
90
Sr ground water transport assessment.
Paper presented at the Third International Symposium and Exhibition on Environmental Contamination in
Central and Eastern Europe, September 10-13, 1996, Warsaw, Poland
Chemical forms of radionuclides in waste samples have been studied using sequential extraction
techniques, described in [6] . Results are presented at Fig.2. More than 50% of
90
Sr in the trench
is still associated with fuel particles, and therefore is geochemically immobile. This finding is
consistent with observed rather low radionuclide leaching rate to the groundwater system.
Radionuclide transport and risk assessment
The “Red Forest” site is situated within the 30-km Chernobyl exclusion zone, which serves an
institutional control to prevent public from accessing the highly contaminated land. Therefore it
is most appropriate to focus radiological risk assessment at populations outside the Chernobyl
zone in the downstream Pripyat - Dnieper River basin [3,5].
361800
14445
9180
361800
14445
9180
136890
171450
1026
278100
54000
1593
55890
32130
31050
39150
5886
675
27
0
5
10
15
20
25
107
108
109
110
111
112
113
114
115
X, m
LEGEND
Trench No.22-T
594000
3,000,000 -
30,000,000
90Sr activity in ground water, pCi/L 90Sr activity in soil, pCi/kg
Cross-section through "Red Forest" burial No.22-T
Sand screen
layer
Ground water flow
direction
Fig.1.
Z,
m
55%
12%
32%
1%
Residue (hot
particles)
Acid soluble (fixed)
Exchangable
Water soluble
Fig.2.
Chemical forms of
90
Sr in waste
Table 1.
Distribution of
90
Sr in the system „waste
burial - vadoze zone - aquifer‟ at burial
No.22-T
Compartment
90
Sr inventory,
%
Burial 92-95
Vadoze zone 2
Aquifer 3-6
Paper presented at the Third International Symposium and Exhibition on Environmental Contamination in
Central and Eastern Europe, September 10-13, 1996, Warsaw, Poland
We have estimated
90
Sr transport via ground water pathway from the “Red Forest” burials to the
Pripyat River using PAGAN computer code developed at Sandia National Laboratories [7]. We
assume that
90
Sr leaching rate from all “Red Forest” burials is same as for the trench No.22-T.
Hydrogeological and
90
Sr subsurface transport parameters have been assigned conservative
values based on our field data, e.g. seepage velocity = 55 m/y, K
d
=0.5 ml/g, K
L
=0.01 y
-1
.
Longtitudal and transverse dispersivities for
90
Sr transport in the saturated zone were assigned
values 5 m and 0.5 m respectively based on literature data. Estimated
90
Sr subsurface flux to the
Pripyat River, which is located at the about 2000 m distance down gradient from the “Red
Forest” site, is shown at Fig.3. The maximum projected
90
Sr transport via ground water to the
Pripyat River is about 0.02 Ci/y. This value is insignificant compared to the current off-site
90
Sr
hydrological transport from the Chernobyl zone, which is governed by direct
90
Sr wash-out from
the Pripyat River floodplain soils during spring snow melt and rains, e.g. 100 - 355 Ci/y in 1994-
95. The health risk caused by current radioactive contamination of water in the Dnieper system
are estimated at 10
-6
- 10
-7
y
-1
[3,5]. Thus,
90
Sr ground water transport from “Red Forest” burials
can only marginally contribute to the hydrological off-site radionuclide transport from Chernobyl
site, and associated health risks are far
below the level requiring counter-measures.
On-site health risks from radionuclide
migration to the unconfined aquifer,
assuming that this aquifer is used as a
potable water source by a hypothetical self-
sufficient resident of the exclusion zone,
are analyzed in [4]. It is shown that on-site
ground water risk is about two orders of
magnitude lesser than health risks due to
external and internal exposure (due to
consumption of radioactivity contaminated
foodstuff) from the surface contamination,
and therefore ground water remediation is
not “a number one” priority.
Probability of radioactive contamination of the confined Eocene aquifer and subsequent failure
of Ch.NPP water wells is also rather low [8].
Conclusions
Despite generally unfavorable hydrogeology (e.g., high recharge to ground water, low sorption
ability of sediments) estimated
90
Sr transport from “Red Forest” burials to the Pripyat River is
relatively insignificant due to slow release of
90
Sr from the waste burials (less than 1% of
inventory per year) and due to long enough ground water residence time in the subsurface, which
allows substantial decay of radioactive contaminant. More than 50% of
90
Sr in the trench is still
associated with fuel particles, and therefore is geochemically immobile. Results of this study
and our previous analyses [5,8] indicate that, though conditions of radioactive waste storage in
burials do not satisfy Ukrainian regulation on radiation protection, health risks caused by
radionuclide migration to the ground water from “Red Forest” burials do not justify application
of expensive countermeasures. However, besides the ground water pathway, other exposure
pathways (e.g., biological radionuclide transport from waste burials) may pose radiological
hazard, and research on characterization and risk assessment of “Red Forest” burials should
continue.
Acknowledgment
0.00
0.01
0.02
0
100
200
300
400
500
Time, y
Fig.3
Estimated
90
Sr flux from “Red Forest”
burials to Pripyat River, Ci/y
Paper presented at the Third International Symposium and Exhibition on Environmental Contamination in
Central and Eastern Europe, September 10-13, 1996, Warsaw, Poland
This work was supported by the Ministry on Chernobyl Affairs (Minchernobyl) of the Ukraine.
Additional support was provided by the United States National Academy of sciences/National
Research Council through the Collaborative Research in Sector Policy Program grant awarded to
Dr.R.Waters.
References:
[1] Dzhepo S.P., Skalskij A.S., Bugai D.A., Marchuk V.V., Waters R.D. “Impact on the
Hydrogeological Environment of the Major Radioactive Waste Burial Grounds at the
Chernobyl NPP Site,” Geologicheski Zhurnal, 4/6, 1994, 100-108 (in Russian)
[2] Waters R.D., Gibson D.J., D.A.Bugai et al. “A Review of PostAccident Measures Affecting
Transport and Isolation of Radionuclides Released from the Chernobyl Accident,” Proc. of
the Second International Symposium and Exhibition on Environmental Contamination in the
Central and Eastern Europe, Budapest, Hungary, September 20-23 1994.
[3] Voitsekhovitch O.V., Nasvit O., Los I. et al. “Assessment of Water Protection Operations to
Minimize the Risk from Using Water from the Dnieper Water System at Various Stages after
the Accident at the Chernobyl Nuclear Power Plant,” IAEA-SM-339/140, Proc. of
International Symposium on Environmental Impact of Radioactive Releases, Vienna, 8-12
May 1995.
[4] Kopeikin V.A. “Plutonium in the Wood of Natural Trees in the Near Zone of the Chernobyl
NPP,” Problems of Chernobyl Exclusion Zone, 2, 1994, 157-162.
[5] Bugai D.A., Waters R.D., Dzhepo S.P. et al. “Risks from Radionuclide Migration to
Groundwater in the Chernobyl 30-km Zone,” Health Physics, 71, 1996, 9-18.
[6] Konoplev A.V., Bobovnikova T.I. “Comparative Analysis of Chemical Forms of Long-lived
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Environmental Impact of Radionuclides Released During Three Major Nuclear Accidents:
Kyshtim, Windscale, Chernobyl. Luxembourg, 1-5 October 1990.
[7] Chu M.S.Y., Kozak M.W., Campbell J.E. et al. “A Self-Teaching Curriculum for the
NRC/SNL Low-Level Waste Performance Assessment Methodology,” SAND90-0585, Sandia
National Laboratories, Albuquerque, New Mexico, 1991.
[8] Bugai D., Smith L., Beckie R. “Risk-Cost Analysis of Strontium-90 Migration to Water
Wells at the Chernobyl Nuclear Power Plant,” Environmental & Engineering Geoscience, 2,
1996, 151-164.
