th
The 12 International Conference of
International Association for Computer Methods and Advances in Geomechanics (IACMAG)
1-6 October, 2008
Goa, India
Activity Limit for Earth Trench Disposal of Radioactive Solid Waste
based on Radionuclide Leaching and Well Groundwater Yield
P. Vijayan
Atomic Energy Regulatory Board, Mumbai, India
D. N. Singh
Department of Civil, IIT, Bombay, India
George Thomas
Atomic Energy Regulatory Board, Mumbai, India
Keywords: Earth trench, dose apportionment, ingestion toxicity index
ABSTRACT: Low and intermediate level radioactive solid waste generated from nuclear power plants, is
generally disposed in near surface solid waste disposal facilities(NSDF) such as earth trenches, reinforced
concrete trenches (R.C.T) and tile holes. The wastes with very low concentration of radionuclide content are
disposed off in earth trenches due to economic reason. The radionuclides disposed off in the earth trenches may
migrate from the disposal system to the groundwater while interacting with the rainwater and may also reach the
groundwater utilization point of the public during the transportation process. Intake of water containing
radionuclides concentration above certain levels results in radiation dose to the public. A public dose limit of
0.05mSv/y is apportioned to the near surface solid waste disposal facilities through the terrestrial route including
groundwater pathways. This paper is an effort to derive the acceptable/benchmark concentration of the important
radionuclides in groundwater based on the terrestrial dose limits. This benchmark concentration can be used to
monitor the regulatory compliance and assess the quality of groundwater with respect to radiation safety. The
paper also discusses the acceptable distance of the groundwater utilization points of the public from the waste
disposal facilities based on easily mobile radionuclides. Efforts are also made to derive the permissible activity
limit for the earth trench disposal of radioactive solid waste based on terrestrial dose apportionment, permissible
groundwater radionuclide concentration, well groundwater yield and ingestion toxicity index.
1 Introduction
Low-level radioactive solid wastes generated from the nuclear power plants are generally disposed in near
137
134
90
60
Cs, Cs, Sr, Co,
surface disposal facilities. The important radionuclides present in these wastes include
54
3
Mn and H. Earth trenches, reinforced concrete trenches and tile holes are generally used for the disposal of
these wastes depending on the activity content. The details of the design and operational criteria of reinforced
concrete trenches and tile holes are given in reference (AERB/SG/D-13, 2003). These engineered disposal
systems are designed to isolate the waste from the biosphere and contain the radionuclides within the system for
a predetermined period up to 300 years.
Earth trenches are used for the disposal of radioactive solid waste having very low concentration of radionuclides
3
content. Operational experience of these practices shows the presence of easily mobile radionuclide like H in
borewells at the proximity of the disposal facilities. Figure 1 indicates the maximum concentration of tritium
observed in the borewells at a distance about 10 to 15m away from the earthen disposal trenches of a typical
inland disposal site. Observance of prescribed radionuclide concentration in groundwater, total activity limit,
adequate distance of groundwater utilization point of the public from the waste disposal facilities and other basic
safety requirements stipulated by the regulatory body help to minimize radiation dose to the public due to the
migration of radionuclides. To assess the radiobiological impact of the public and the environment due to the near
surface disposal of radioactive solid waste, it is important to know the parameters like the permissible
concentration of the radionuclides in drinking water, the toxicity index and the quantity of activity to be disposed
off in the disposal facilities. This paper is an attempt to derive these parameters.
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Concentration(Bq/ml)
600
500
400
300
200
100
0
2000
2001
2002
2003
2004
2005
2006
YEAR
Figure 1: M axim um C oncentration of Tritium O bserved in B orew ells of a
Typical S olid W aste D isposal S ite
2 Permissible radionuclide concentration in water
In India, near surface solid waste disposal facilities are designed, constructed and operated with the dose limits of
0.05mSv/y through the terrestrial route (AERB/NF/SG/RW-4, 2006). The radiation dose to the public through
human intrusion pathways is controlled by the implementation of institutional controls (G.S.R-125, 1987).
Therefore, the radiation dose to the public due to the human intrusion is not anticipated during the operational
phase of the solid waste disposal facilities. During the operational phase of the solid waste disposal facilities, the
radionuclides may get released from the disposal facilities and migrate through the groundwater media. The
transportation of radionuclides through groundwater media and subsequent consumption of contaminated water
by the public may form the major pathways of radiation dose to the public during the operational phase of the
disposal facilities. Sampling of the borewell water located around the disposal facilities monitor the migration of
radionuclides through the groundwater media. Distribution pattern of tritium in borewells over a period of time of a
typical waste disposal site is given in Figure 2.
To assess the radiological impact due to transport of radionuclides from the waste disposal facility to
groundwater, the quality of groundwater around the waste disposal facilities with respect to the radiation safety
standard needs to be benchmarked by suitable method. One of the acceptable methods is to benchmark the
radionuclide concentration in borewell around the waste disposal facilities to the derived permissible radionuclides
concentration in drinking water (PRCW). PRCW of various radionuclides in drinking water at the public
groundwater utilization point can be derived from the following equation (WHO,2003) using the terrestrial dose
apportionment;
PRCW( Bq / l ) =
DL(Sv / y)
Wi (l / y) × DCF(Sv / Bq)
(1)
Where PRCW is the permissible concentration of radionuclide in water (Bq/l)
DL is the apportioned dose limit (Sv/y) for solid waste disposal site
Wi is the annual average water intake of an adult individual (l/y)
DCFis the dose conversion factor (Sv/Bq)
It is possible that drinking water may contain two or more radionuclides and that may affect the same organ or
tissue after ingestion. In case, the drinking water contains two or more radionuclides, the following relationship
should be satisfied;
Ci
∑ PRCW
≤1
(2)
i
where Ci and PRCWi are the observed and permissible concentrations, respectively, for each contributing
radionuclide.
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PRCWs of typical radionuclides derived based on the terrestrial dose apportionment of 0.05mSv/y corresponding
to the average drinking water intake of 3 liter per day of an adult individual (BARC/I/010, 1995) are given in Table
1.
Table 1 :PRCWs of typical radionuclides
Radionuclide
3
H
Dose Conversion Factor (Sv/Bq)
(IAEA, BSS115,1996)
PRCW (Bq/l)
-11
2540
1.80x10
134
Cs
1.90x10-8
2.40
137
Cs
1.30x10
-8
3.51
Cr
5.30x10
-11
862
51
57
Co
3.50x10
58
Co
60
Co
131
-9
13
6.80x10
-9
6.72
9.20x10-8
0.49
-8
2.08
I
2.20x10
55
Fe
4.00x10
-10
114
59
Fe
3.10x10-9
14.7
54
Mn
7.30x10
106
-10
62.6
Ru
1.10x10
-8
4.15
90
Sr
2.80x10-8
1.63
65
Zn
3.80x10
-9
12.0
1.30x10-9
35.1
95
Zr
600
Concentration(Bq/ml)
500
400
300
200
100
0
O- D- F- A- J- A- O- D- F- A- J- A- O- D- F- A- J- A- O- D- F- A- J- A- O- D- F- A- J- A01 01 02 02 02 02 02 02 03 03 03 03 03 03 04 04 04 04 04 04 05 05 05 05 05 05 06 06 06 06
Month
Figure 2: Distribution Pattern of Tritium in Borewells of a Typical Solid Waste Disposal Facility
2250
3 Ingestion toxicity index and derivation of activity limit
Radioactive waste disposed in earth trenches may start migrating from the disposal trenches to the groundwater
from the first monsoon. The radiological impact of such radionuclide migration depends on the concentration of
radionuclide in groundwater at the groundwater utilization / drinking point. To account the extent to which human
are exposed to such radionuclide contamination via groundwater intake can be expressed in ingestion toxicity
index given by the following equation (National Academy Press 1996);
∑ λi Ni (Bq)
ITm (l ) =
i
PRCWi ( Bq / l )
(3)
where ITm is the ingestion toxicity index, which is defined as the volume of ground water required to dilute a
radionuclide to the permissible concentration for pathway m, (l);
λiNi is the product of decay constant and number of radionuclide i which yield the activity (Bq)
PRCWi permissible concentration of radionuclide i in water for pathway m (Bq/l)
Rewriting equation (3), the total activity of particular radionuclide to be disposed of in earthen trenches can be
derived from the following equation:
∑ λi N i (Bq) = ITm (l ) × PRCW (Bq / l )
where
∑λ N
i
(4)
i
i
is the total activity in (Bq); and
i
Average groundwater yield of borewells and dugwells in various hydrological regions in India varies from 3.2x107
9
7
to 1.0x10 l/y (CGWB,2006). A moderate aquifer provides an average groundwater yield of 3.2x10 l/y in
borewells and dugwells at near surface depth. Considering the average volume of groundwater yield, the total
activity to be disposed off in earthen trenches in a year can be derived using equation (4). These values are
presented in Table 2.
The average and effective operational life of an inland near surface solid waste disposal facilities (twin unit
nuclear power plants) for a given site are estimated to be about 25 years excluding monsoon period. During
waste disposal practices, the emplacement of waste packages to the disposal facilities is generally carried out
after the rainy season. The interaction of radioanuclide and the rainwater may take place in subsequent rainy
season of each disposal practices. It is also possible that the leaching of radionuclide from the disposal trench to
the groundwater may take place at the end of the operational life or at a later stage after closer of the disposal
facilities. In such cases, the cumulative radionuclides inventories of the waste disposal facilities are available for
migration and transportation through the groundwater media. These Migration and transportation is considered
as the worst-case scenario of earth trench disposal facilities. In such worst-case scenarios, the concentration of
radionuclides in the monitoring wells may likely to exceed the PRCWs prescribed for the dose limit of 0.05 mSv/y.
To take care of such worst events a safety factor of 25 is applied arbiterly (considering 25 years of effective
operational period of NSDFs) while deriving the activity limit for earth trench disposal of radioactive waste.
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Table 2. Permissible total activity of radionuclides in solid waste for earthen trench disposal
Radionuclide
3
H
Half-life (y)
Total Activity (Bq/y)
12.3
3.24x10
9
134
Cs
2.06
3.07x10
137
Cs
30
4.48x10
51
6
6
Cr
0.07
1.10x109
57
Co
0.74
1.66x10
58
Co
0.19
8.57x106
60
Co
5.27
6.33x105
131
7
6
I
0.02
2.65x10
55
Fe
2.70
1.46x108
59
Fe
0.12
1.88x10
Mn
0.85
7.98x10
106
Ru
1.01
5.30x106
90
Sr
29.1
2.08x10
Zn
0.66
1.53x10
0.17
4.48x107
54
65
95
Zr
7
7
6
7
4 Results and discussion
PRCWs and total activity of various radionuclides in solid wastes that can be disposed off in earth trenches are
presented in Tables 1 and 2, respectively. Activity limit of each radionuclide is derived from the PRCWs
7
corresponding to a terrestrial dose limit of 0.05 mSv/y and average groundwater yield (3.2x10 l/y). It is
presumed that in a worst-case scenario, the radionuclide present in the disposal trenches reaches to the
groundwater without any decay and groundwater will be the only available mechanism to dilute the radionuclide to
an acceptable concentration before reaching to the groundwater utilisation point of the public.
The volume of groundwater required to dilute the radionuclide to an acceptable concentration varies with
radionuclide and depends on the half-life and other physiochemical characteristics of the radionuclides. Various
factors retard the movement of radionuclides from the disposal system to the groundwater are reported (Godse
3
et.al.). However, radionuclides like H exhibit very low distribution coefficient in the soil and move along with the
velocity of ground water.
The groundwater velocity of the solid waste disposal site across the country varies from 1cm/d to 1m/d at near
3
surface depth (Balu et. al.). Considering the highest groundwater velocity of 1 m/d, the radionuclides like H will
take only 2.73 years to reach a longitudinal distance of 1 km from the disposal trench through the groundwater
movement calculated from the following equation:
T=
D
V
(5)
where T is the time(y), D is the longitudinal distance (m) and V is the groundwater velocity (m/y)
From Equation (5) it is evident that far distance groundwater utilization point and the low groundwater velocity
provide acceptable concentration of radionuclide in groundwater. These parameters also provide decay of shortlived radionuclide during the migration process. Therefore, the PRCWs need to be benchmarked at the public
groundwater utilization point rather than the vicinity and adjacent to the disposal trenches with respect to the
radiation safety of the public. From these data it is also concluded that near surface disposal facilities using
earthen trench for the disposal of radioactive solid should have adequate distance from the public groundwater
utilization point considering the radionuclide having highest mobility through the groundwater media.
5 Conclusion
Earth trenches are used for the disposal of radioactive solid waste having very low concentration of radionuclide
content. The radionuclide like 3H present in the waste migrates from the disposal trench to groundwater due to
the percolation of rainwater. In case of any release of activity, 3H is observed in borewells at very close distance
of the disposal facilities. Therefore, groundwater utilization point of the public should have adequate distance from
3
the disposal trench considering the fast and easily mobile radionuclides like H. PRCW derived for a dose limit of
2252
0.05mSv/y can be used as benchmarks for radionuclide concentration in groundwater at the public utilization
point. Activity limits derived based on the average groundwater yield can be used as the annual activity limit for
disposal of radioactive solid waste in earth trench disposal facilities.
6 Acknowledgment
Authors are indebted to Shri S.K. Chande, Vice-Chairman AERB for his interest and encouragement in the
study. The authors are also grateful to Shri R. Venkatraman, Director, OPSD, AERB, Shri S.A Sukeshwalla and
Shri S.N. Rao of OPSD, AERB for useful discussion.
7 References
AERB/SG/D-13, 2003. Safety Guide on “Liquid and Solid Radwaste Management in Pressurised Heavy Water Reactor Based
Nuclear Power Plants”
AERB/SG/O-5, (1998). Safety Guide, on Radiation Protection During Operation of NPPs.
AERB/NF/SG/RW-4, 2006. Safety Guide on “Near Surface Disposal of Radioactive Solid Waste”
G.S.R-125, 1987, Atomic Energy (Safe Disposal of Radioactive Waste) Rules.
WHO,2003. Guidelines for Drinking Water Quality-3, World Health Organisation, Geneva, Switzerland.
BARC/I/010, 1995. Environmental Radiation Measurement Around Narora Atomic Power Station (NAPS) Site.
IAEA BSS-115, 1996. International Basic Safety Standards for Protection Against Ionising Radiation and for the Safety of
Radiation Sources.
National Academy Press, Washington, D.C. 1996. Nuclear Wastes Technologies for Separations and Transmutation.
CGWB,2006, Hydrological studies – Hydrological Condition “Central Groundwater Board”, Ministry of Water Resources, India
(Web information)
V.B.Godse, A.L.Mohan, Mahesh Singh, R.V. Amalraj and K.T.Thomas “Characterisation of Trombay Soil for Disposal of
Radioactive Waste” SM-93/21.
K.Balu, A.L Mohan, P.K. Narayan, V.B. Godse and N.S. Sunderrajan “ Twenty years experince with shallow ground repositories
in India” IAEA-CN-43/135.
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