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POSIVA Report 1998-2

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Name:

A Working Group's Conclusions on Site Specific Flow and Transport Modelling

Writer:

Johan Andersson; Henry Ahokas; Lasse Koskinen; Antti Poteri; Auli Niemi; Aimo Hautojarvi

Language:

English

Page count:

69

ISBN:

951-652-040-5; 1239-3096

Summary:

Working report: POSIVA-raportti POSIVA 98-02, 67 sivua
ISBN 951-652-040-5


A WORKING GROUP’S CONCLUSIONS ON SITE SPECIFIC FLOW AND
TRANSPORT MODELLING


ABSTRACT

This document suggests a strategy plan for groundwater flow and transport modelling to be
used in the site specific performance assessment analysis of spent nuclear fuel disposal to be
used for the site selection planned by the year 2000. Considering suggested general regulations
in Finland, as well as suggested regulations in Sweden and the approach taken in recent safety
assessment exercises conducted in these countries, it is clear that in such an analysis, in
addition to showing that the proposed repository is safe, there exist needs to strengthen the link
between field data, groundwater flow modelling and derivation of safety assessment
parameters, and needs to assess uncertainty and variability.

The groundwater flow used as an input to the source term modelling should be within the
ranges calculated from the groundwater flow modelling. Regarding the far-field migration there
is a need to strengthen the estimation of the ratio between the channel width and the
groundwater flow from actual field data, to evaluate different migration paths, and not only the
most pessimistic ones, and to study the effect of variability in the fracture plane. It is also
recommended that the groundwater flow modelling would discuss implications for the
biosphere in terms of dilution in the geosphere and distribution of release points.

The suggested strategy plan builds on an evaluation of different approaches to modelling the
groundwater flow in crystalline basement rock, the abundance of data collected in the site
investigation programme in Finland, and the modelling methodology developed in the
programme so far. It is suggested to model the whole system using nested models, where larger
scale models provide the boundary conditions for the smaller ones. Furthermore, it would seem
motivated to adopt different conceptual models for the different scales studied. Given the
geometrical detail at the canister scale it seems that the most appropriate model would be a
discrete fracture network description. It is suggested to limit the size of the discrete model to
small blocks contained by larger size fractures. Outside this domain a stochastic continuum
model would be applied, but with the hydraulic and the transport properties derived from the
discrete network model. In a regional and repository scale, adoption of the equivalent porous
medium concept would seem the most appropriate. The regional scale takes the regional
fracture zones and watersheds into account whereas the repository scale model would be more
focused into the repository area and would describe the local fracture zones and deterministic
structures found in the site. These models provide boundary conditions for more detailed scale
models, but do not resolve the variability of the hydraulic properties.


Keywords: groundwater flow, transport, performance assessment, nuclear waste,
disposal


POSIVA-raportti POSIVA 98-02, 67 sivua
ISBN 951-652-040-5


A WORKING GROUP’S CONCLUSIONS ON SITE SPECIFIC FLOW AND
TRANSPORT MODELLING


ABSTRACT

This document suggests a strategy plan for groundwater flow and transport modelling to be
used in the site specific performance assessment analysis of spent nuclear fuel disposal to be
used for the site selection planned by the year 2000. Considering suggested general regulations
in Finland, as well as suggested regulations in Sweden and the approach taken in recent safety
assessment exercises conducted in these countries, it is clear that in such an analysis, in
addition to showing that the proposed repository is safe, there exist needs to strengthen the link
between field data, groundwater flow modelling and derivation of safety assessment
parameters, and needs to assess uncertainty and variability.

The groundwater flow used as an input to the source term modelling should be within the
ranges calculated from the groundwater flow modelling. Regarding the far-field migration there
is a need to strengthen the estimation of the ratio between the channel width and the
groundwater flow from actual field data, to evaluate different migration paths, and not only the
most pessimistic ones, and to study the effect of variability in the fracture plane. It is also
recommended that the groundwater flow modelling would discuss implications for the
biosphere in terms of dilution in the geosphere and distribution of release points.

The suggested strategy plan builds on an evaluation of different approaches to modelling the
groundwater flow in crystalline basement rock, the abundance of data collected in the site
investigation programme in Finland, and the modelling methodology developed in the
programme so far. It is suggested to model the whole system using nested models, where larger
scale models provide the boundary conditions for the smaller ones. Furthermore, it would seem
motivated to adopt different conceptual models for the different scales studied. Given the
geometrical detail at the canister scale it seems that the most appropriate model would be a
discrete fracture network description. It is suggested to limit the size of the discrete model to
small blocks contained by larger size fractures. Outside this domain a stochastic continuum
model would be applied, but with the hydraulic and the transport properties derived from the
discrete network model. In a regional and repository scale, adoption of the equivalent porous
medium concept would seem the most appropriate. The regional scale takes the regional
fracture zones and watersheds into account whereas the repository scale model would be more
focused into the repository area and would describe the local fracture zones and deterministic
structures found in the site. These models provide boundary conditions for more detailed scale
models, but do not resolve the variability of the hydraulic properties.


Keywords: groundwater flow, transport, performance assessment, nuclear waste,
disposal


Keywords:

ground water flow; transport; performance assessment; nuclear waste; disposal

File(s):

A Working Group's Conclusions on Site Specific Flow and Transport Modelling (pdf) (3.1 MB)


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