implications of rock structure on the performance in the near field of a nuclear waste repository

E L S E V I E R Engineering Geology 49 (1998) 195 200

Implications of rock structure on the performance in the near field of a nuclear waste repository

Johan Andersson a,,, Peter Robinson b, Michael Impey b a QuantiSei AB. ]41vsj6 G~rdsviig 3, 125 30 ]4lvsj6, Sweden

b QuantiSci Ltd, Chiltern House, 45 Station Road, Henley-on-Thames, O.,~fordshire RG9 1AT. UK

Abstract

The performance of a deep repository for nuclear waste relies heavily on its potential to retain radionuclides at source. This is achieved through a durable waste package and favourable chemical conditions under which the most dangerous nuclides are extremely difficult to dissolve in groundwater. The persistence of these favourable conditions is, however, partially controlled by the groundwater flow in the near field and its capability to transport radionuclides released from the waste form. Most source-term models used in performance assessment assume a simple conceptual model, with a deposition hole intersected by a single fracture with the release controlled by diffusion in the buffer and the chemical conditions at the source. This conceptual model is certainly a simplification of reality, where the deposition hole is intersected by an irregular network of discrete fractures with spatially varying properties and where the fractures close to the deposition hole and the tunnel are altered by excavation effects. In terms of flow modelling, spatially varying discrete fracture systems can be simulated using a number of methods including stochastic discrete fracture network models and stochastic continuum models. This choice of models leads to uncertainty, as does incomplete knowledge of the parameters that characterize the chosen model. For modelling migration through the geosphere, the largest uncertainties are those in groundwater flow distribution and in the flow-wetted surface area. Sensitivity analyses with existing source-term models suggest that near-field release is sensitive to flow and to fracture geometry in an intermediate range only, but this result may be an artefact due to the simplifications made. The adoption of a more realistic geometrical description of the near-field rock may lead to a more useful description of which near-field rock properties affect source-term release. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: Nuclear waste; Performance assessment; Rock structure

I. Introduction

The pe r fo rmance o f a deep repos i to ry for nuclear waste relies heavi ly on its po ten t ia l to retain rad ionuc l ides at source. This is achieved

* Corresponding author. Present address: Golder Associates AB, Korta Gatan 7, S-171 54 Solna, Sweden. E-maiL: [email protected]; Fax: +46 8 562 003 61

0013-7952//98/'$19.00 (O 1998 Elsevier Science B.V. All rights reserved. Pll: S0013-7952 (97)00049-5

t h rough a durab le waste package and favourab le chemical condi t ions under which the mos t danger - ous nuclides are ext remely difficult to dissolve in g roundwate r . The persis tence o f these favourab le condi t ions is, however , pa r t i a l ly con t ro l led by the g roundwa te r flow in the near field and its capabi l i ty to t r anspo r t rad ionucl ides released f rom the waste form. A p rope r unde r s t and ing o f which near field rock proper t ies are i m p o r t a n t for source- te rm

196 ,I. 4raters.son el aL En~ineerinv, GeMogl' 49 / 199b¢) 195 200

behaviour is necessary if an evaluation is to be undertaken of whether it would be beneficial, in terms of safety, to actively select favourable deposition holes and to reject non-favourable ones. For such emplacement criteria to be effective, it is also necessary to show that the relevant properties are measurable.

This paper describes the general concepts behind source-term models in performance assessment, with emphasis on the geometrical description of the near-field rock. It also discusses current views on how to model groundwater flow and radionuclide migration in crystalline rock, as this migration affects the potential for near-field release. With this in mind this paper examines: ( 1 ) which near-field rock parameters affect source-

term release, based on present modelling; (2) whether a more realistic geometrical descrip-

tion of the near-field rock in such models would alter these conclusions.

2. Near field release models used in performance assessment

The purpose of the engineered barrier system of a waste repository is to contain the waste for an initial period, which may be very long, and to assure acceptably low releases to the far field following the loss of complete containment. The site properties ("the geosphere") affect both these functions.

In terms of containment, the key elements are chemical and mechanical stability. Evaluation of suitable ambient chemical conditions (i.e. those that ensure low solubilities and high sorption) requires an understanding of the evolution of the site on a large scale, at which the detailed characteristics at the scale of the deposition tunnels are relatively insignificant. It is hard to conceive that ambient chemical characteristics would be discrim- inators between deposition holes. In contrast, both the mechanical stability and the effective source- term release may be coupled to the local scale properties.

The release of radionuclides to the far-field rock, here referred to as the "source-term". may be divided into release from the waste form and

migration through the near field to the near- field-far-field interface. The release fi-om the waste form is controlled by the radionuclide inventory and its distribution in the waste and chemical constraints of release such as waste form reaction rate and radionuclide solubilities for the groundwater chemistry at the waste (Savage, 1995). For solubility controlled elements, this, in turn, implies that the effective release will depend upon the migration capacity from the waste form into the rock. The migration is controlled by the transport mechanisms, radioactive decay, groundwater chemistry and the geometry of the system. Special attention is required for the interfaces of the different barrier regions (such as the waste canister, canister buffer and buffer rock interfaces). Generally, the continuity of concentration and mass transfer exists at an interlace, but the migration properties may change drastically.

Most source-term models, used in performance assessment, such as CALIBRE (Worgan and Robinson, 1995), T U L L G A R N ( Kjellbert, 1995 ), REPCOM (Nordman and Vieno, 1988) or PAGODA (QuantiSci, 1996) assume a simple conceptual model for the rock mass geometry. Typically, it is assumed that the deposition hole is intersected by a single parallel plate fracture, or a parallel set of such fractures, that the groundwater flow is perpendicular to the deposition hole, and that a boundary layer develops due to the existence of the buffer (assumed to be impervious). Radionuclides may diffuse in the buffer and are transferred to the flowing groundwater in the fracture through the boundary layer. The geometrical and hydrological factors controlling the release are then the fracture aperture and the groundwater flow in the fracture. Some models (CALIBRE) also considers diffusion through the rock mass and acknowledge that the effective contact area between the rock matrix and flowing fracture may be less than the geometrical area owing to channelling in the fracture. In this case, additional parameters are the diffusivity of the rock and the local flow-wetted surface. In contrast, other models further simplify the rock description by assuming a zero concentration boundary condition at the roc~buffe r interface (e.g. PAGODA). In this case, the release is fully decoupled from the

J. Andersson et al. / Engineering Geology 49 (1998) 195-200 197

groundwater flow and the near-field structure geometry.

For a KBS-3 type emplacement, with vertical deposition holes drilled down from the emplacement tunnel, the models also usually consider the alternative pathway with release directly to the tunnel. This is usually modelled as diffusion through the buffer and a zero concentration boundary condition at the tunnel. No attempt is made to calculate the actual concentration at the tunnel.

The geometrical description of the rock mass and the groundwater flow in source-term models is certainly a simplification of reality. The buffer-rock interface plays an important role in controlling release, and the question may be asked as to whether a more detailed geometrical description of the interface would be significant. Furthermore, source-term models usually only consider a small portion of the rock mass, if any. It may also be beneficial to study the interface between the near- field and the far-field rock, in particular to evaluate if assumptions of zero concentration boundary conditions are far too conservative. Indeed, source- term models directly coupled to the far-field migration do now exist (e.g. AZURE, Williams et al., 1996), although not, as yet, applied in published performance assessment exercises. Before discuss- ing such models, however, we will discuss how groundwater flow and transport is modelled in the far field.

3. Migration in the far field

In reality, migration in a crystalline rock occurs in an interconnected network of discrete features. In addition, the plane of the fracture has an internal structure and the simplistic notion of a parallel plate fracture is evidently in conflict with field evidence. However, it is no way feasible to characterize every feature, at least in a deterministic fashion and simplifications are needed.

Groundwater flow and solute migration models build, to varying degrees, on models of the structure of the crystalline rock. The rock mass contain structures of different length scales. The possibility of actually finding such structures could be eval-

uated through the theory of geometrical prob- ability (Santal6, 1976) and is related to the size of the structures and the available observation window (surface outcrops, boreholes and tunnels). Structures that are large in relation to the available observation window may be identified deterministi- cally; but, below some scale related to the observation window, the uncertainty is too large to make deterministic predictions meaningful. Below this scale, the rock mass structure can only be described stochastically or in an average sense.

It should be emphasized that the smallest scale of a feature that can be included in a deterministic fashion depends on the scale of the observation window. Based on data from a pre-investigation (surface maps, surface outcrops and deep boreholes), deterministic determination of structures is meaningful for length scales larger than the order of 500-1000 m. In the vicinity of the near field, however, data from tunnels may possibly be used to infer structures with length scales of about 100 m, as was demonstrated in the SCV project in Stripa (Olsson, 1992).

In order to account for spatial variability between structures, there are various alternative conceptual models for the details of flow in heterogeneous rock, namely stochastic continuum, discrete fracture network and channel networks models. Different correlation structure models exist for the stochastic continuum models; for example, Gaussian, non-Gaussian and fractal, and information about structure could, to a limited extent, be incorporated through non-Gaussian fields. Discrete fracture network models use statis- tical structure information to produce connectivity estimates. Channel network models do not depend on structure information, but instead try to capture the flow variability directly.

The application of these models demonstrate that they all could capture local flow variability. Table 1 displays the local-scale Darcy velocities predicted in performance assessments carried out in Sweden and Finland. The porous medium approach adopted in KBS-3 (SKBF, 1983) pro- duced overly optimistic flow predictions. The more recent studies SKB-91 (SKB, 1992), TVO-92 (Vieno et al., 1992) and SITE-94, (SKI, 1996) all attempt to capture local scale variability, but used

198 J. Andersson et u/. Engineering Geology 49 (1998) 195 200

Table I

R a n g e o f D a r c y velocities a s s u m e d in p e r f o r m a n c e assessments

in Sweden a n d F in l and

S tudy qmtn (m/yea r ) q . . . . (m/yea r )

KBS-3 2 x 10 ~' 6 x 10 '~ SKB-91 2 x 1 0 5 10 1

T V O - 9 2 3 x 1 0 5 2 × 1 0 ~

SITE-94 l0 ~ 10

[ a b l e 2

Ra t io ( F - a L / q ) ) between flow wet ted surface , a, d a r c y veloci ty,

q, a n d m i g r a t i o n d is tance , L, a s s u m e d in KBS-3, SKB-91 ,

T V O - 9 2 a n d SKI S ITE-94

S t u d y L o w F-va lue t y e a r / m ) H i g h k -va lue ( y e a r / m )

KBS-3 5 x 10 ~' 5 x 10 6

SKB-91 10 4 10 ~'

T V O - 9 2 4 x 103 6 × 105

S ITE-94 6 x 102 I() ~'

different models. These studies predict a large (and similar) variability in the Darcy velocity, despite the different model assumptions. However, depending on the assumptions on structure, connectivity and correlation, the models scale up differently. This has implications for their use in predicting migration properties over larger length scales.

The radionuclide migration properties of crystalline rock primarily depend upon: ( 1 ) the distribution of groundwater flow; (2) the distribution of flow-wetted surface along streamlines; (3) sorption capacity: and (4) rock matrix diffusivity and poros- ity. Long-term retention is essentially controlled by the ratio between the flow-wetted surface and the Darcy velocity along a streamline (see, for example, Vieno et al., 1992). The distribution of groundwater flow along different streamlines depends upon the connectivity of the permeable structures and is thus directly linked to the structure model of the rock. The flow-wetted surface depends both on the details of the flow distribution on individual fracture planes and the distribution of flow paths in the rock. This makes the ratio between the flow-wetted surface and flow both quite variable and uncertain, which can be seen from Table 2 which displays this ratio for the same assessments as in Table 1.

The tables display quite significant differences between the studies, both in terms of ranges and absolute values. However, the differences are prob- ably not due to differences between sites. They rather reflect changes in attitude in how to represent groundwater flow and flow-wetted surface. KBS-3 modelled the rock mass as a porous medium intersected by a few fracture zones. TVO-92 applied a similar model, but with far more detail. In addition, the parameters actually selected

for migration calculations were selected quite con- servatively from the flow modelling information. SKB-91 applied a stochastic continuum model, which results in significant spatial variation in the local scale, but tend to average behaviour on larger scales. SITE-94 applied different conceptual models, with different scaling properties.

Models of migration in crystalline rock need to account tbr the strong variability in flow paths, but there is a significant uncertainty in the distribution of these flow paths that originate from the conceptual uncertainty of how to represent the structure of the rock mass. There is also significant uncertainty in estimates of the flow-wetted surface, which is determined by a combination of both large-scale and detailed-scale geometry of flow paths. At more local scales (of the order of 50 m migration distance), however, it appears feasible to reduce this uncertainty, with proper access to detailed structure information and local scale site migration tests.

4. Geosphere properties influencing release

Analysis using the source-term models presently used in performance assessment suggest that the impact of the rock geometry and groundwater flow in the near-field rock mass on radionculide release is quite weak. Indeed, sensitivity analyses carried out with performance assessment codes in Finland (Vieno et al., 1992) or Sweden (SKB, 1992; SKI, 1996) show that near-field release is sensitive to the groundwater flow only for certain ranges of groundwater flow. For example, Vieno et al. ( 1992 ) assumed that the groundwater flow in the excavation-damaged zone was high enough to result in

J. Andersson et al. / Engineering Geology 49 (1998) 195-200 199

zero concentration at the top of the deposition hole. For this case, the release through the tunnel would dominate release through rock fractures for most realistic groundwater velocities. Sensitivity analyses with source-term models carried out by SKB (1992) and SKI (1996) showed that near- field release is relatively insensitive to groundwater flow for Darcy velocities above 10-3m/year, although the precise range over which this insensi- tivity persists depends on geometrical factors as well. Consequently, present-day source-term models rather suggest that active selection of deposition holes provide little scope for limiting release. In particular, a Darcy velocity of 10-3 m/s is far too low to be used as an emplacement criterion. However, such a conclusion may be premature considering the conservative over- simplification of the rock in the present source- term models. Analysing the problem with a more realistic and less conservative description would possibly show scope for useful emplacement crit- era. Prime candidates for an increased geometrical realism include the description of the flow paths close to the deposition hole and handling the interface between the near field and the far field.

The groundwater flow in the vicinity of the deposition hole deviates significantly from flow in a parallel plate fracture, which means that a boundary layer of the type assumed in most source- term models would not develop. Scoping calculations carried out by us, using the capability to model a spatially heterogeneous rock mass in the AZURE code (Williams et al., 1996), indicate that channelling in the rock mass close to the buffer may have a significant effect on release.

The present source-term models are also too pessimistic about the near-field-far-field interface. For example, scoping calculations made within the SKI SITE-94 project (SKI, 1996) suggest that a considerable groundwater flow is needed to sup- port a zero concentration boundary condition at the interface between deposition hole and tunnel. Introducing a more realistic description of the near-field-far-field interface would be a means to evaluate under which circumstances such conditions could actually occur. Such an analysis should also build on the fact that, at a scale of the order of 10-50 m, the uncertainty in fracture zones and

their hydraulic properties is less than further along the migration paths.

5. Conclusions

Present-day source-term models tend to conser- vatively simplify the migration in the rock mass and the near-field-far-field interface, which means that model results are quite insensitive to the geometry or properties of the rock mass in the vicinity of the deposition holes. This makes present-day models less useful when establishing criteria for suitable or non-suitable emplacement holes. There is, however, significant scope for more realistic models of the rock mass in source-term models, which would make them useful for design and planning purposes.

Combining the possibilities of a refined characterization of the rock mass on the scale of deposition tunnels and advances in modelling techniques may show that present-day source-term models are too pessimistic for most potential deposition holes and, in addition, would provide direct information on what near-field rock mass properties really are important for radionculide release.

References

Kjellbert, N., 1995. Tullgarn A Near Field Radionuclide Migration Code. Swedish Nuclear Fuel and Waste Manage- ment Co., Stockholm, AR 91-25.

Nordman H., Vieno, T., 1988. Validation of a Numerical Release Model (REPCOM) for the Finnish Reactor Waste Disposal System Part III: Comparison of Calculated Results and Intermediate Laboratory Measurements after the First Year. Nuclear Waste Commission of Finnish Power Companies, Helsinki, Report YJT-8812.

Olsson, O (Ed.), 1992. Site Characterization and Validation - Final Report. Stripa Project Report 92-22, Swedish Nuclear Fuel and Waste Management Co., Stockholm.

QuantiSci, 1996, PAGODA Version 3.2: Technical Description. QuantiSci Ltd, Alvsj6, Sweden.

Santal6, L.A., 1976. Integral geometry and geometrical prob- ability. Encyclopedia of Mathematics and its Applications. Addison-Wesley, Reading, MA.

Savage, D. (Ed.), 1995. The Scientific and Regulatory Basis for the Geological Disposal of Radioactive Waste, Wiley, New York.

SKB, 1992. SKB-91. Final Disposal of Spent Nuclear Fuel:

200 J. Anders,~on el a[. L)lgineermg Geology 49 (1998) 195 200

Importance of the Bedrock for Safety. SKB Technical Report 92-20, The Swedish Nuclear Fuel and Waste Management Co., Stockholm.

SKBF. 1983. Final Storage of Spent Nuclear Fuel. Swedish Nuclear Fuel Supply Co., Stockholm.

SKI, 1996. The SKI Deep Repository Performance Assessment Research Project SITE-94. Swedish Nuclear Power Inspecto- rate (in preparation).

Vieno, T., Hautoj~irvi, A., Koskinen, L., Nordman, H., 1992.

]-VO-92 Safety Analysis of Spent Fuel Disposal. Nuclear Waste Commission of Finnish Power Companies. Helsinki, Report YJT-92-33E.

Williams, M.J., lmpey, M.D., Sellar, C.C., 1996. AZURE User Guide. QuantiSci Report 1M4138, Version 1.0.

Worgan, K.. Robinson, P., 1995. The CALIBRE Source-term Code: Technical Documentation for Version 2 (SITE-94). SKI Report 95:13, Swedish Nuclear Power Inspectorate.

implications of rock structure on the performance in the near field of a nuclear waste repository

Documents