Probabilistic Analysis of Radionuclide Transport for Near-Surface Disposal Facilities in Spatially Varying Soils
Publication: Journal of Hazardous, Toxic, and Radioactive Waste
Volume 25, Issue 1
Abstract
The long-term safety of radioactive waste disposal facilities is ensured by developing performance assessment models. The general requirement of these models is to assess the risk caused by failure of disposal systems that leads to radionuclide release into the geosphere and their migration toward the near-field biosphere. The radiation dose and risk due to disposal practice are the endpoints of assessment of the model. A two-dimensional radionuclide transport model with a decaying source is modeled numerically to compute the radiological impact caused by radionuclide, iodine-129 (I-129) in the biosphere. The intrinsic part of the performance assessment model requires consideration of uncertainties and variabilities in the system that are indicated by the inherent variability (heterogeneity in the geological medium), measurement, and modeling uncertainties (the geohydrological, geochemical properties of the radionuclides being released and transported). In this study, the spatial variability in the geological medium is addressed by treating the hydraulic conductivity of the medium as a random field. The probability of the radiation dose exceeding the design threshold (i.e., the probability of failure) is computed using subset simulation method. The influence of autocorrelation length and coefficient of variation of hydraulic conductivity on the probability of failure and the transport behavior of radionuclide are also studied. Thus, the results showed that a probabilistic framework that accounts for spatial variability in the geological medium is necessary for performance assessment of radioactive waste disposal facilities.
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Acknowledgments
The work presented in this paper is part of research work carried out in the project “Development of probabilistic design and analysis procedures in radioactive waste disposal in NSDF and design of NSDF closure” sponsored by Board of Research in Nuclear Science (BRNS). The discussion with scientists of Bhabha Atomic Research Center (BARC) is gratefully acknowledged.
Notation
The following symbols are used in this paper:
- B
- width of the disposal facility;
- C0
- concentration of the nuclide at the source area;
- C(t)
- concentration of the radionuclide at time t;
- COV(Pf)
- coefficient of variation of failure probability;
- cr
- concentration of radionuclide in ground water;
- Dp
- permissible radiation dose;
- din
- drinking water intake (L/day);
- doin
- ingestion dose coefficient (Sv/Bq);
- g(ξ)
- limit state/performance function;
- H
- height of the disposal facility;
- H(x, θ)
- random field;
- Kl
- leach rate or fractional release rate of the nuclide;
- L
- length of the disposal facility;
- l
- auto-correlation length;
- m
- number of terms for truncation;
- max(D(ξi, t))
- maximum radiation dose computed from the model;
- Nss
- number of samples per subset ;
- Pf
- probability of failure;
- P(fj)
- probability of intermediate failure events;
- qd
- Darcy velocity field;
- RD
- radiation dose;
- Rf
- retardation factor;
- S
- source/sink term;
- Sr
- surface area of the disposal facility;
- V
- volume of the disposal facility;
- ɛ
- porosity of soil;
- λi, ϕi
- eigenvalues and eigenvectors of the autocorrelation function;
- λp
- radioactive decay constant;
- μln
- mean of underlying normal field;
- σln
- standard deviation of underlying normal field;
- υ
- infiltration rate of water from the disposal facility; and
- ψ
- transport quantity.
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Journal of Hazardous, Toxic, and Radioactive Waste
Volume 25 • Issue 1 • January 2021
Copyright
© 2020 American Society of Civil Engineers.
History
Received: Mar 10, 2020
Accepted: Jun 10, 2020
Published online: Aug 28, 2020
Published in print: Jan 1, 2021
Discussion open until: Jan 28, 2021
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