The use of expert opinion in formulating conceptual models of underground disposal systems and the treatment of associated bias

Reliability Engineering and System Safety 42 (1993) 161-180

The use of expert opinion in formulating conceptual models of underground disposal systems and the treatment of associated bias M. C. T h o r n e Electrowatt Engineering Services (UK) Limited, Electrowatt House, North Street, Horsham, West Sussex, UK, RH12 IRF

The use of formal expert judgement procedures in developing a conceptual model of a hypothetical deep geological repository for the disposal of low and intermediate level radioactive wastes is described. This model is appropriate to the post-closure radiological assessment of such a waste disposal system and has been developed in the context of a recent trial assessment of such a system.

1. INTRODUCTION

For this reason, there has been interest in a scenario-based approach to this topic, v which aims to generate a comprehensive and well-documented set of factors, phenomena and processes, and their interrelationships, which have to be taken into account. This scenario-based approach is regarded as a useful first step, but it has not, generally, been developed to the stage at which the results are useful in defining conceptual models of the system as a whole which are useful for simulation purposes. Furthermore, such scenario development has not typically been undertaken in a formal expert judgement framework. In this context, it is important to recognise that a variety of studies have shown that problem decomposition is a key issue in decision analysis. Thus, for example, Fischoff et al. 8 demonstrated that differently structured fault trees for the same problem resulted in different probability assignments for the same classes of events. Taking this into account, Bonano et al. 9 recommended, in the context of high-level radioactive waste disposal, that formal expert judgement procedures be applied when a problem is complex, or when several experts are employed, either redundantly or as a team. In relation to assessments of the safety of nuclear reactors, Thorne and Williams 1° have emphasised that there are three levels at which formal expert judgement procedures are properly employed:

In the UK, the nuclear industry proposes to dispose of low and intermediate level solid radioactive wastes in an engineered facility located at a depth of several hundred metres in a suitable geological formation and accessed from land. 1' 2 On behalf of the industry, U K Nirex Ltd has undertaken an extensive programme of research and development in relation to this proposed disposal option, z In parallel, the U K Regulatory Authorities, notably H e r Majesty's Inspectorate of Pollution (HMIP) have developed a substantial capability to assess the post-closure radiological performance of such a facility. 3 Both these programmes make extensive use of overseas experience on this topic and collaboration in international comparative studies is commonplace. However, it has to be recognised that the assessment approaches adopted 4~ have developed in an ad h o c manner over several years in parallel with new insights arising from research and development activities. Thus, while the developers of these approaches typically consider them fit-for-purpose, or progressing towards fitness-for-purpose with some enhancements, the concern remains that these assessment approaches may omit some phenomena and factors of relevance to post-closure radiological safety, and as such may give rise to biased assessment results, irrespective of the adequacy of the models employed or the data used with those models.

(i)

definition of the p h e n o m e n a and factors to be taken into account in the assessment process (conceptualisation); (ii) selection of particular mathematical and

Reliability Engineering and System Safety 0951-8320/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. 161

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M. C. Thorne

computational tools to represent these phenomena and factors, and their interactions (representation); (iii) selection of data values, ranges or distributions (input specification). This paper describes a study in which formal expert judgement procedures were used to develop an overall conceptual model of the phenomena and factors to be taken into account in disposal of low and intermediate level solid radioactive wastes in a well-defined facility at a specified site. ~ The exercise was carried through to the stage at which the conceptual model could be used to assess the adequacy of the various tools used in a trial assessment of this facility (Dry Run 3) 4` ~ undertaken on behalf of H M I P by a team of contractors (henceforth referred to as the Assessment Team). An outline of the approach adopted in this study has been published previously] 3 Because of time and resource limitations, the further stage of representation was not attempted in this exercise. In contrast, within Dry Run 3 as a whole, significant attention was directed to the use of expert judgement in input specification, as is described elsewhere.~2

2. A P P R O A C H

In developing an appropriate procedure, it was recognised that a variety of experts from widely different fields would have to be consulted and would have to work closely together. For this reason, it was decided that only an expert judgement procedure utilising a highly interactive group approach would be appropriate, since groups tend to produce more hypotheses in problem-solving tasks than individuals TM and their solutions to problems tend to be better. ~5 While unstructured group processes have a number of well-documented shortcomings 16 including group think, lv'~8 inhibition of contributions and premature closure, experience in decision conferencing suggests that Lock ~6 is overly pessimistic when he remarks that there may be an ineradicable tension between the forces leading to effective cohesive groups and those leading to the expression of a variety of viewpoints. As Watson and Buede ~9 have very properly remarked 'even if a decision conference does not lead to consensus, it provides all the participants with a clearer understanding of the issues involved and why different members of the group prefer different actions'. In this context, it is relevant to note that even when a highly interactive group approach has been identified as appropriate, the methodology that should be used for eliciting the views of the members of that group is not fully defined. In particular, there is

considerable disagreement between those who insist that group elicitation should best be done by working with individuals and pooling their knowledge without getting them to meet and those who believe that the best advice is obtained through direct interactions. ~'~ Given that a group approach was to be adopted, the various stages of the exercise were defined as follows: (i) selection of the group; (ii) development of a comprehensive list of factors and phenomena that might be of relevance: (iii) elimination of factors and phenomena from further consideration; (iv) derivation of groups of factors and phenomena that should be included in a model of the system: (v) comparison of the conceptual model structure obtained with the approach adopted in the trial assessment. These topics are discussed in more detail below. 2.1 Selection o f the group

From an evaluation of the literature, it was recognised that the initial stage of defining an expert group and justifying the choice of experts is the one which tends to be least well recorded. For this reason, particular attention was given to documentation of this stage of the work (see also the review by Bonano et al.9). First, a list of factors and phenomena which might be of relevance to post-closure radiological assessment was drawn up, in the form of a structured list, by the analyst in consultation with members of the assessment team. This structured list contained 18 broad categories of factors and phenomena (see Section 2.3). It was decided that, as a minimum, the expert group should include individuals with a detailed knowledge of each of these areas. From knowledge within the assessment team, and by consultation with other individuals and organisations, the analyst drew up a short list of 59 potential expert group members, from a longer list of proposed candidates, according to the following criteria: (a) expertise in one or more of the listed areas: (b) preferably, some knowledge of the background to radioactive waste disposal in the UK; (c) resident in the UK and available for consultation: (d) not a member of the Dry Run 3 Team or UK Nirex Ltd Disposal Safety Assessment Team (this did not exclude individuals involved in research relating to radioactive waste disposal funded in whole, or in part, by either the U K Department of the E n v i r o n m e n t / H M I P or U K Nirex Ltd).

Formulating conceptual models of underground disposal systems Item (d) arose because it was considered important to ensure that the deliberations of the group would not be constrained by the knowledge that they would have to undertake an assessment using the limited set of computational tools currently available. A matrix of potential group members and their areas of expertise was then drawn up by the analyst to provide a basis for the final stage of the selection procedure. On the basis of this matrix, and taking into account the need to keep the group to a reasonable size, the analyst selected individuals with competence in several of the major areas of interest, with the secondary criterion of selecting individuals from different organisations, as far as this was consistent with maintaining the high overall level of expertise of the group. By this method, the final group contained two or more individuals with expertise in each main area and provided access to a much wider range of expertise in the organisations from which the group members were drawn. The final selection was discussed and confirmed with the Dry Run 3 Assessment Team.

2.2 Development of a comprehensive list of factors and phenomena The first task of the expert group was to develop a comprehensive list of factors and phenomena that might be of relevance to post-closure radiological assessment. To this end, they were supplied with the initial structured list that had been drawn up previously and used to assist in the selection of the expert group plus background information on the hypothetical repository and its environs. This information was supplied separately by mail to each individual member of the expert group. Each expert, acting independently, was requested: (a) to review the general headings to see if any item had been omitted, then to repeat the procedure at each lower level of indexing; (b) where specific factors and phenomena were not listed at the lowest order, to give consideration to the specific factors and phenomena that should be listed; (c) to identify factors and phenomena that should be considered, but which did not fall naturally within the framework provided; and (d) to prepare a brief note listing the additional factors and phenomena to be included, their position in the table (if appropriate) and a comment as to why each factor or phenomenon requires consideration. It was stressed that at this stage factors and phenomena should not be excluded on the grounds of negligible impact or low probability. A variety of detailed responses were received and

163

used to expand the original structured list. This provided the basis for the second stage of the exercise, described below. It will be noted that no group interactions occurred in this stage of the exercise, since the emphasis was on generating a comprehensive list of factors and phenomena rather than on assessing their interactions. In practice, group interactions at later stages of the exercise produced some amendments to the list generated by this non-interactive approach.

2.3 Elimination of factors and phenomena from consideration The expanded structured list plus comments by the secretariat (the analyst and his support staff) was issued to the individual members of the expert group by mail. The members of the group were invited to: (a) identify those factors and phenomena that should be excluded from consideration on the grounds of negligible impact; (b) identify those factors and phenomena which will undoubtedly be of significance in postclosure radiological assessments, or which have to be included in order to evaluate their significance; (c) identify groups of factors and phenomena that could be considered together in the assessment process, because of similarities in their implications. Although the members of the expert group were provided with the views of the secretariat on the various items in the list of factors and phenomena, it was suggested that they might wish to defer consideration of this material until after they had formed their own initial views on the various factors and phenomena. On the basis of responses to this note, various factors and phenomena were eliminated from further consideration and relationships were established between those which remained. This formed a basis for the third stage of the work of the group, derivation of groups of factors and phenomena for modelling and assignment of degrees of belief (see Section 2.4). However, before proceeding to this stage, a meeting was held to ensure that factors and phenomena had not been excluded inequitably and to reintroduce any such phenomena. Following this meeting, an updated list of factors and phenomena was produced. This was very close to the final version adopted, but further discussions, as the work proceeded, resulted in minor modifications and clarifications. Because of its very general relevance to post-closure radiological safety assessments, the final list of factors and phenomena considered is given in full in Table 1.

M. C. Thorne

164

Table l.--(Cont.)

Table 1. Comprehensive list of all factors and phenomena considered Item Item

1. 1.1 1.1.1 1.1.1.1 1.1.1.2 1.1.1.3 1.1.1.4 1.1.2 1.1.2.1 1.1.2.2 1.1.2.3 1.1.2.4 1.1.2.5 1.1.3 1.1.3.1 1.1.3.2 1.1.3.3 1.1.3.4 1.1.4 1.1.4.1 1.1.4.2 1.1.4.3 1.1.4.4 1.1.4.5 1.1.4.6 1.1.4.7

Description

1.2

Gas production, transport and flammability

1.2.1 1.2.1.1 1.2.1.2 1.2.1.3 1.2.1.4 1.2.1.5 1.2.1.6 1.2.1.7 1.2.1.8

Hydrogen by metal corrosion Structural steel Container steel Waste steel Waste Magnox Waste aluminium Waste Zircaloy Waste other metals Effects of microbial growth on concrete Methane and carbon dioxide by microbial degradation Cellulosics Other susceptible organic materials Aerobic degradation Anaerobic degradation Effects of temperature Effects of lithostatic pressure Effects of microbial growth on properties of concrete Effects of biofilms Effects of hydrogen from metal corrosion Inhibition due to the presence of toxic materials Carbonate/bicarbonate exchange with concrete Energy and nutrient control of metabolism Effects of radiation on microbial populations Gas generation from concrete

1.2.2 1.2.2.1 1.2.2.2 1.2.2.3 1.2.2.4 1.2.2.5 1.2.2.6 1.2.2.7 1.2.2.8 1.2.2.9 1.2.2.10 1.2.2.11 1.2.2.12 1.2.2.13 1.2.3

1.2.7

Active gases Tritiated hydrogen Active methane and carbon dioxide Other active gases Toxic gases Transport In the waste container In the vault between containers Between vaults In the near-field, including vicinity of shafts and adits Flammability

1.3

Radiation phenomena

1.3.1 1.3.2

Radioactive decay and ingrowth Nuclear criticality

1.4

Mechanical effects

1.4.1 1.4.2 1.4.3 1.4.4 1.4.4.1 1.4.4.2 1.4.5 1.4.6

Canister or container movement Changes in in situ stress field Embrittlement Subsidence/collapse Repository induced Natural Rock creep Fracturing

J

1.5

Hydrological effects

1

1.5.1 1.5.1.1 1.5.1.2 1.5.2 1.5.2.1 1.5.2.2 1.5.3 1.5.4 1.5.4.1 1.5.4.2 1.5.4.3 1.5.4.4 1.5.4.5

Changes in moisture content Due to dewatering Due to stress relief Groundwater flow (unsaturated) Initial Due to gas production Groundwater flow (saturated) Transport of chemically active substances into the near-field Inorganic ions Humic and fulvic acids Microbes Organic complexes Colloids

1.6

Thermal effects

1.6.1 1.6.2 1.6.3 1.6.3.1 1.6.3.2 1.6.4 1.6.4. l 1.6.4.2 1.6.4.3 1.6.5 1.6.5.1 1.6.5.2 1.6.5.3 1.6.5.4 1.6.5.5 1.6.5.6 1.6.5.7 1.6.5.8 1.6.5.9 1.6.6 1.6.6.1 1.6.6.2 1.6.6.3

Differential elastic response Non-elastic response Fracture changes Aperture Length Hydrological changes Fluid pressure Density Viscosity Chemical changes Metal corrosion Concrete degradation Waste degradation Gas production Complex formation Colloid production Solubility Sorption Species equilibrium Microbial effects Cellulose degradation Microbial activity Microbial product reactions

1.7

Transport out of the repository

NEAR-FIELD Chemical]physical degradation Structural and container metal corrosion Metal corrosion: localised Metal corrosion: bulk Metal corrosion: crevice Stress corrosion Physical degradation of concrete Cracking Sealing of cracks Pore blockage Alkali-aggregate reaction Cement-sulphate reaction Chemical degradation of concrete Changes in pore water composition, pH, Eh Exchange capacity exceeded Alkali-aggregate reaction Cement-sulphate reaction Degradation of wastes Metal corrosion Leaching Complex formation Colloid formation Microbial degradation of organic waste Microbial corrosion Radiolysis

Description

Inc.

Inc."

x x x

] ]

J J

J J J l x x x

1 J x

x x x

x 1 x x

1.2.4 1.2.4.1 1.2.4.2 1.2.4.3 1.2.5 1.2.6 1.2.6.1 1.2.6.2 1.2.6.3 1.2.6.4

J J J X X X

Formulating conceptual models of underground disposal systems Table 1.--(Cont.) Item

Description

1.7.1 1.7.2

Solubility Sorption

2. 2.1 2.1.1 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.6.1 2.2.6.2 2.2.6.3 2.2.7 2.2.7.1 2.2.7.2 2.2.7.3 2.2.8 2.2.9 2.2.10 2.2.11 2.3 2.3.1 2.3.2 2.3.3 2.3.3.1 2.3.3.2 2.3.3.3 2.3.3.4 2.3.4 2.3.4.1 2.3.4.2 2.3.4.3 2.3.4.4 2.3.4.5 2.3.4.6 2.3.5 2.3.6

FAR-FIELD Extra-terrestrial Meteorite impact Geological Regional tectonic Magmatic Metamorphism Diagenesis Diapirism Seismicity Repository induced Externally induced Natural Faulting/fracturing Activation Generation Change of properties Major incision Weathering Effects of natural gases Geothermal effects Hydrological Variation in groundwater recharge Groundwater losses Rock property changes Porosity Permeability Microbial pore blocking Channel formation, closure Groundwater flow Darcy flow Non-Darcy flow Intergranular (matrix) Fracture Effects of solution channels Unsaturated Salinity Variations in groundwater temperature Transport and geochemical Advection Diffusion Bulk Matrix Surface Hydrodynamic dispersion Solubility constraints Sorption Linear Non-linear Reversible Irreversible Effects of pH and Eh Effects of ionic strength Effects of naturally occurring organic complexing agents Effects of naturally occurring inorganic complexing agents Effects of complexing agents formed in the near-field Effects of naturally occurring colloids

2.4 2.4.1 2.4.2 2.4.2.1 2.4.2.2 2.4.2.3 2.4.3 2.4.4 2.4.5 2.4.5.1 2.4.5.2 2.4.5.3 2.4.5.4 2.4.5.5 2.4.5.6 2.4.5.7 2.4.5.8 2.4.5.9 2.4.5.10

165

Table 1.--(Cont.) Inc.

Item

Description

]

2.4.5.11

Effects of colloids formed in the nearfield Effects of major ions migrating from the near-field Effects of microbial activity Fracture surface changes Organic colloid transport Porous media Fractured media Effects of pH and Eh Effects of ionic strength Inorganic colloid transport Porous media Fractured media Effects of pH and Eh Effects of ionic strength Transport of radionuclides bound to microbes Isotopic exchange Gas transport Solution Gas phase Gas-induced groundwater transport Thermally induced groundwater transport Repository induced Naturally induced Biogeochemical changes

2.4.5.12

x x x x

] x

x x ] ]

2.4.5.13 2.4.6 2.4.7 2.4.7.1 2.4.7.2 2.4.7.3 2.4.7.4 2.4.8 2.4.8.1 2.4.8.2 2.4.8.3 2.4.8.4 2.4.9 2.4.10 2.4.11 2.4.11.1 2.4.11.2 2.4.12 2.4.13 2.4.13.1 2.4.13.2 2.4.14 3.

]

] ] ] x x

x x ] ]

]

3.1 3.1.1 3.1.1.1 3.1.1.2 3.1.1.3 3.1.1.4 3.1.1.5 3.1.1.6 3.1.2 3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.2.5 3.1.2.6 3.1.2.7 3.1.2.8 3.1.2.9 3.1.2.10 3.1.3 3.1.3.1 3.1.3.2 3.2 3.2.1 3.2.1.1 3.2.1.2 3.2.1.3 3.2.2 3.2.2.1 3.2.2.2 3.2.2.3 3.2.2.4 3.2.3

BIOSPHERE Climatology Transient greenhouse-gas induced warming Precipitation Temperature Sea level rise Storm surges Ecological effects Potential evaporation Glacial/interglacial cycling Precipitation Temperature Sea level fall Storm surges Ecological effects Seasonally frozen ground Permanently frozen ground Glaciation Deglaciation Potential evaporation Exit from glacial/interglacial cycling Greenhouse-gas induced Other causes Geomorphology Generalised denudation Fluvial Aeolian Glacial Localised denudation Fluvial (valley incision) Fluvial (weathering/mass movement) Glacial Coastal Sediment redistribution

Inc.

l

l J ] ] J

x x

] ] x x J ] ] ] J x ]

] l ] ] ] ] 1 x

166

M. C. Thorne Table 1.--(Cont.)

Table 1.--(Cont.)

Item

Description

Inc.

Item

Description

3.2.3.1 3.2.3.2 3.2.3.3 3.2.4 3.2.4.1 3.2.4.2 3.3 3.3.1 3.3.2 3.3.2.1 3.3.2.2 3.3.2.3 3.3.2.4 3.3.2.5 3.3.2.6 3.3.3 3.3.4

Fluvial Aeolian Glacial Effects of sea level change River incision/sedimentation Coastal erosion Hydrology Soil moisture and evaporation Surface hydrology Overland flow Interflow Return flow Macropore flow Variable source area response Stream/aquifer interactions Groundwater recharge Surface flow characteristics (freshwater) Stream/river flow Sediment transport Meander migration or other fluvial response Natural lake formation/sedimentation Effects of sea level change Surface flow characteristics (estuarine) Tidal cycling Sediment transport Successional development Effects of sea level change Coastal waters Tidal mixing Residual current mixing Effects of sea level change Ocean waters Ecological development Terrestrial Agricultural systems Semi-natural systems Natural systems Effects of succession Estuarine Coastal waters Oceans Radionuclide transport Erosive Fluvial Aeolian Glacial Coastal Groundwater discharge to soils Advective Diffusive Biotic Volatilisation Groundwater discharge to wells or springs Groundwater discharge to freshwaters Groundwater discharge to estuaries Groundwater discharge to coastal waters Surface water bodies Water flow Suspended sediments

] ] ] j ] x

3.5.7.3 3.5.7.4 3.5.7.5 3.5.8 3.5.8.1 3.5.8.2 3.5.8.3 3.5.8.4 3.5.8.5 3.5.8.6 3.5.8.7 3.5.9 3.5.9.1 3.5.9.2 3.5.9.3 3.5.9.4 3.5.9.5 3.5.9.6 3.5.10 3.5.10.1 3.5.10.2 3.5.10.3 3.5.10.4 3.5.10.5 3.5.10.6 3.5.10.7 3.5.11 3.5.11.1 3.5.11.2 3.5.11.3 3.5.ll.4 3.5.11.5 3.6 3.6.1 3.6.2 3.6.2.1 3.6.3 3.6.4 3.7 3.7.1 3.7.1.1 3.7.1.2 3.7.1.3 3.7.2 3.7.2.1 3.7.2.2 3.7.2.3 3.7.2.4 3.7.2.5 3.7.2.6 3.7.3 3.7.3.1 3.7.3.2 3.7.3.3 3.7.3.4 3.7.3.5

Bottom sediments Biogeochemical cycling Effects of fluvial system development Estuaries Water flow Suspended sediments Bottom sediments Effects of salinity and pH variation Biogeochemical cycling Effects of estuarine development Effects of sea level change Coastal waters Water transport Suspended sediment transport Bottom sediment transport Effects of sea-level change Effects of estuarine development Effects of coastal erosion Plants Root uptake Deposition on surfaces Vapour uptake Internal translocation and retention Washoff and leaching by rainfall Leaf-fall and senescence Cycling processes Animals Uptake by ingestion Uptake by inhalation Internal translocation and retention Cycling processes Effects of relocation and migration Planning considerations Urbanisation Management of water resources Lake formation Agricultural policy Recreation policy Human exposure External Land Sediments Water bodies Ingestion Drinking water Agricultural crops Domestic animal products Wild plants Wild animals Soils and sediments Inhalation Soils and sediments Gases and vapours (indoors) Gases and vapours (outdoors) Biotic material Salt particles

4.

SHORT-CIRCUIT PATHWAYS RELATED TO HUMAN ACTIVITIES Related to repository construction Loss of integrity of borehole seal Failure

3.3.4.1 3.3.4.2 3.3.4.3 3.3.4.4 3.3.4.5 3.3.5 3.3.5.1 3.3.5.2 3.3.5.3 3.3.5.4 3.3.6 3.3.6.1 3.3.6.2 3.3.6.3 3.3.7 3.4 3.4.1 3.4.1.1 3.4.1.2 3.4.1.3 3.4.1.4 3.4.2 3.4.3 3.4.4 3.5 3.5.1 3.5.1.1 3.5.1.2 3.5.1.3 3.5.1.4 3.5.2 3.5.2.1 3.5.2.2 3.5.2.3 3.5.2.4 3.5.3 3.5.4 3.5.5 3.5.6 3.5.7 3.5.7.1 3.5.7.2

j ] ] ] ] ] ] j ] ]

1

] ]

x

x ] j ] ] ] ] ] ]

] ] ] ]

4.1 4.1.1 4.1.1.1

lnc

] ] t 1 t ] ] l ] ] ] ] ] ] l t ] ] ] 1

j ] ] ] 1 ] i ] ] ] ] j ] ] ]

i x x

Formulating conceptual models of underground disposal systems Table 1.--(Cont.) Item 4.1.1.2 4.1.2 4.1.2.1 4.1.2.2 4.1.3

4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8 4.2.9 4.2.10 4.2.11 4.2.12 4.2.13

Description

Inc.

Degradation Loss of integrity of shaft or access tunnel seal Failure Degradation Damage to the host medium around shafts or access tunnels Post-closure Deliberate recovery of wastes or associated materials Malicious intrusion Exploratory drilling Exploitation drilling Geothermal energy production Resource mining Tunnelling Construction of underground storage/disposal facilities Construction of underground dwellings/shelters Archaeological investigations Injection of liquid wastes Groundwater abstraction Underground weapons' testing

l l

167

It is emphasised that this list pertains specifically to a cementitious repository for low and intermediate level radioactive wastes located in an argillaceous formation in the general vicinity of the Harwell Laboratories in southern England. 11 It would not be appropriate to use this list for a different type of repository at another location, but it could be useful as an initial input to an expert group drawing up a list appropriate to such a repository and location.

x x x J

x

Inc. = requires consideration; ] = yes, x = no.

2.4 Derivation of groups o f factors and p h e n o m e n a for modelling On the basis of the provisional reduced list of factors and p h e n o m e n a to be considered, the secretariat drew up an initial proposal for a minimal assessment. This minimal basis was discussed at the first meeting of the expert group. The discussion allowed the secretariat to produce a revised minimal assessment, which was documented, and then reviewed and modified at the second meeting of the expert group. The final version of the minimal assessment is summarised in Table 2. In this context, it should be emphasised that 'minimal assessment' was defined in discussions with

Table 2. Characteristics of the minimal assessment (a)

Neglect metal corrosion, but include physical and chemical degradation of concrete and degradation of wastes. Item 1.1.1 1.1.2 1.1.3 1.1.4

(b)

Description Generation of hydrogen by metal corrosion Generation of methane and carbon dioxide by microbial degradation Gas generation from concrete Generation of active gases Generation of toxic gases Transport in the near-field, especially in the vicinity of shafts and adits Flammability

Status H H x H x M/H x

Radiation phenomena must be included, but not criticality. Item 1.3.1 1.3.2

(d)

Status x L/M H H

Gas generation in the repository must be included, but only selected aspects of transport in the near-field need be included explicitly. Item 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.2.7

(c)

Description Structural and container metal corrosion Physical degradation of concrete Chemical degradation of concrete Degradation of wastes

Description Radioactive decay and ingrowth Nuclear criticality

Status H x

Effects of fracturing in the near-field should be included. Mechanical effects of gas production on the stress field should be studied outside the assessment. All other mechanical effects can be neglected. Item 1.4.1 1.4.2 1.4.3 1.4.4 1.4.5 1.4.6

Description Canister or container movement Changes in in situ stress field Embrittlement Subsidence/collapse Rock creep Fracturing

Status x s x x x L/M

M. C. Thorne

168

Table 2.--(Cont.) (e)

Only groundwater flows in saturated conditions in the near-field should be included. Item 1.5.1 1.5.2 1.5.3

(f)

Description Transport of chemically active substances into the near-field

Status x

Repository induced thermal effects should be included. Thermo-mechanical effects should be the subject of a supplementary geotechnical study outside the assessment. Thermal modifications of microbial effects can be neglected. Item 1.6.1 1.6.2 1.6.3 1.6.4 1.6.5 1.6.6

(h)

Status x x H

Neglect transport of chemically active substances into the near-field. Item 1.5.4

(g)

Description Changes in moisture content Groundwater flow (unsaturated) Groundwater flow (saturated)

Description Differential elastic response Non-elastic response Fracture changes Hydrological changes Chemical changes Microbial effects

Status s s s L L x

Transport out of the repository should be included, taking account of solubility constraints and sorption in the near-field. Item 1.7.1 1.7.2

Description Solubility controls on transport Sorption controls on transport

Status H H

(i)

Extra-terrestrial processes can be neglected.

(j)

Item Description Status 2.1.1 Meteorite impact x Regional tectonic effects, seismicity and the effects of faulting and fracturing should be included in supplementary studies outside the assessment. Weathering should be included, but as a component of geomorphology rather than under geology. Item 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 2.2.8 2.2.9 2.2.10 2.2.11

Description Regional tectonics Magmatic effects Metamorphism Diagenesis Diapirism Seismicity Faulting/fracturing Major incision Weathering Effects of natural gases Geothermal effects

Status s x x x x s s x moved x x

(k) Far-field hydrological characteristics generally need to be included. Item 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6

(l)

Description Variation in groundwater recharge Groundwater losses Rock property changes Groundwater flow Salinity effects on flow Effects of variations in groundwater temperatures on flow

Status H H M H x x

Transport should be assumed to be in aqueous form, taking speciation and complexation into account. Item 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.4.7

Description Advective transport Diffusive transport Hydrodynamic dispersion Solubility constraints Sorption Fracture surface changes, notably demineralisation Organic colloid transport

Status H H H x H M M

Formulating conceptual models of underground disposal systems

169

Table 2.re(Cont.) 2.4.8 2.4.9 2.4.10 2.4.11 2.4.12 2.4.13 2.4.14 (m)

X

Description Transient greenhouse-gas induced warming Glacial/interglacial cycling Exit from glacial/interglacial cycling

Status M H X

Description Generalised denudation Localised denudation Sediment redistribution Effects of sea-level change Weathering

Status H L M/H x H

Description Soil moisture and evaporation Terrestrial near-surface hydrology Groundwater recharge Surface flow characteristics (freshwater) Surface flow characteristics (estuarine) Flow characteristics (coastal waters) Flow characteristics (ocean waters)

Status H, d H, d H, d H, d s s x

Description Effects of terrestrial ecological development on hydrology Effects of estuarine ecological development on hydrology Effects of coastal water ecological development on hydrology Effects of oceanic ecological development on hydrology

Status M, d s s x

Radionuclide transport processes in the environment should generally be included. However, groundwater discharges to estuaries and coastal waters can be ignored, while transport in estuaries and coastal waters can be treated in a supplementary study outside the assessment. Item 3.5.1 3.5.2 3.5.3/3.5.4/3.5.7 3.5.5 3.5.6 3.5.7 3.5.8 3.5.9 3.5.10 3.5.11

(r)

X

Effects of ecological development on the terrestrial surface hydrological system should be included. Effects of estuarine and coastal water ecology can be investigated in a supplementary study and not included directly in the assessment. Effects of oceanic ecology need not be considered. Item 3.4.1 3.4.2 3.4.3 3.4.4

(q)

H M/H

Surface hydrology should be included in a comprehensive and coherent way. Development and application of a detailed model outside the assessment would be appropriate. Estuarine and coastal water hydrology could be investigated in supplementary studies and not included directly in the assessment. Item 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.3.7

(p)

X

Geomorphological change should generally be included. Item 3.2.1 3.2.2 3.2.3 3.2.4 2.2.9

(o)

H M

Glacial/interglacial cycling should be assumed. Greenhouse-gas warming giving rise to an end of such cycling is considered to be a low-probability event with huge non-radiological consequences. Item 3.1.1 3.1.2 3.1.3

(n)

Inorganic colloid transport Transport of radionuclides bound to microbes Isotopic exchange Gas transport Gas-induced groundwater transport Thermally-induced groundwater transport Biogeochemical changes

Description Erosive transport (N.B. glacial and coastal erosion to be treated outside the assessment) Groundwater discharge to soils Groundwater discharge to wells, springs, streams, rivers and surface water bodies Groundwater discharge to estuaries Groundwater discharge to coastal waters (see above) Radionuclide transport in estuaries Radionuclide transport in coastal waters Radionuclide uptake, retention and cycling in plants Radionuclide uptake, retention and cycling in animals

Status H H H x x H s s H H

Planning considerations, including land and water management policies, should be taken into account in determining the range of potential human influences on the disposal system. Item 3.6.1

Description Urbanisation

Status M

170

M. C. Thorne

Table 2.--(Cont.) 3.6.2 3.6.3 3.6.4

(s)

Status H H H

Description External exposure Ingestion Inhalation

Short-circuit pathways related to repository construction, including those associated with damage to the host media in the vicinity of boreholes, should be included. Item 4.1.1 4.1.2 4.1.3

(u)

H M L

The assessment should include a comprehensive treatment of human exposure pathways. Item 3.7. ! 3.7.2 3.7.3

(t)

Management of water resources Agricultural policy Recreation policy

Description Loss of integrity of borehole seals Loss of integrity of shaft or access tunnel seals Damage to the host medium around shaft or access tunnels

Status M M H

A limited number of post-closure short-circuit pathways should be included, others could, if necessary, be covered by scoping or scaling calculations. Item 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8 4.2.9 4.2.10 4.2.11 4.2.12 4.2.13

Description Deliberate recovery of wastes or associated materials Malicious intrusion Exploratory drilling Exploitation drilling Geothermal energy production Resource mining Tunnelling Construction of underground storage/disposal facilities Construction of underground dwellings]shelters Archaeological investigations Injection of liquid wastes Groundwater abstraction Underground weapons' testing

Status x x H x, sc x x~sc x, sc x, sc x, sc x x H x

Notes: x--excluded

L--low priority M--medium priority H--high priority s--should be the subject of a supplementary study outside the assessment d----component of a detailed model of the surface hydrological system to be applied outside the assessment sc--appropriate topic for scoping or scaling calculations the group to mean the assessment with the lowest degree of complexity which would not be grossly in error because of excluded factors and p h e n o m e n a , given that the available mathematical models and data were adequate for representing those factors and p h e n o m e n a that were included. It had been intended to define both augmented and depleted assessments relative to the minimal assessment, and to investigate the changes in expert perceptions of bias in both directions relative to the minimal assessment. In practice, time constraints were such that only the minimal assessment and depleted variants could be investigated. The depleted variants were defined by assigning priorities to the c o m p o n e n t s of the minimal assessment (Table 2). On the basis of these priorities, the secretariat constructed structural diagrams of the minimal and depleted assessments. These were transmitted to individual m e m b e r s of the expert group

by mail and all replies received were taken into account in synthesising revised versions (see Figs 1-3, and Table 3 which comments on the interactions shown in these figures). Having defined the minimal assessment and its depleted variants, individual m e m b e r s of the expert group received, by mail, a questionnaire asking them to evaluate the adequacy of these assessments (see Table 4). It was of interest that responses to this questionnaire, which potentially could have been very diverse, were extremely coherent, indicating that disagreements on technical issues had largely been resolved in previous rounds of correspondence and discussion. On this point, it should be emphasised that, whenever the views of individual experts were elicited by mail, their replies were made available, in full, to all other m e m b e r s of the expert group in the next briefing note issued by the secretariat. On some occasions, this

Formulating conceptual models of underground disposal systems

171

Thermal effects

1.6.4/1.6.5

tl,

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I

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ingrowlh

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repository construclion

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in the near-l~eld 1.2.1/1.2.2/ 1.2.4/1.2.6

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1.31

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Land malqagement prac~ces 3.6.1/3 6 Zj 3.6.3/3.6 4

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material was supplemented by explanatory material from the secretariat, i.e. the analyst was responsible for providing guidance on how issues might be resolved and technical information relevant to resolving those issues. In respect of the evaluations of the two reduced assessments provided by the expert group, an interpretive analysis by the secretariat yielded the following results.

In respect of the first reduced assessment, (Fig. 2), with some caveats as to modelling requirements, there was broad agreement on the judgements shown in Table 5. These judgements form a coherent set and indicate a general view of the group that this reduced assessment would yield results within an order of magnitude of those obtained from the minimal assessment. This was confirmed by distributing a

M.C. Thorne

172

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1.7.1/1.7.2

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4.1.1/4.1.2/ 4.1.3

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management pracbces 3 6 1 / 3 6 T'.,' 363

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.

.

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.

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.

.

! I

Fig. 2. The minimal assessment - - first level of reduction. (Note: The individual items are summarised in Table 2 and the relationships between items, indicated by arrows, are summarised in Table 3.) briefing note including this interpretation of their judgements to the members of the expert group for comment. In respect of the second reduced assessment (Fig. 3), the general response of the group was that the implications could not be quantified without further modelling studies. Overall, the response to the questionnaire essentially confirmed previous views relating to the minimal assessment. There was general agreement as to its structure, small deletions could be tolerated, but more extensive deletions resulted in a general feeling that

too much of substance had been deleted and that the degree of bias would be unquantifiable without modelling studies, which effectively corresponds to reintroducing the factors a n d / o r phenomena into the assessment.

2.5 Comparison of the derived conceptual model with Dry Run 3 Finally, the secretariat compared the characteristics of the minimal assessment with those factors and phenomena included in the Dry Run 3. This

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t 3.63 3.6.1/3.6PJ Fig. 3. The minimal assessment -- second level of reduction. (Note: The individual items are summarised in Table 2 and the relationships between items, indicated by arrows, are summarised in Table 3.) comparison is shown in Table 6, which distinguishes between: (a) those factors and phenomena included in the overall systems model VANDAL/TIME4; ~2 (b) those factors and phenomena included in detailed models such as the detailed chemical speciation and transport code CHEMTARD used to provide guidance on input parameter values or distributions to be used with VANDAL/TIME4; (c) those factors and phenomena not included in items (a) and (b) above, but which were

investigated, in respect of their potential implications for post-closure radiological safety, through a variety of ad hoc scoping calculations. Table 6 demonstrates that, in the judgement of the expert group, Dry Run 3 was significantly sub-minimal when evaluated against the requirements for a comprehensive post-closure radiological assessment. This was as expected, since the main objectives of the Dry Run 3 related to the performance of a demonstration time-dependent probabilistic risk assessment, emphasising those aspects not covered by earlier trials. ~2

M. C. Thorne

174

Table 3. Structural relationships incorporated in the minimal assessment

Targets

Origins 1.1.3

1.7.1/1.7.2

1.1.4 1.1.4

1.2.1/1.2.2/1.2.4 1.7

1.2.1/1.2.2/1.2.6

1.1.2

1.2.1/1.2.2/1.2.6

1.5.3

1.2.1/1.2.2/1.2.6

1.4.6

1.2.1/1.2.2/1.2.4/1.2.6 1.3.1

1.7 1.1/2.4/3.5

1.4.6

t .5.3

1.4.6

1.2.6

1.5.3

1.7

1.6.4

1.5.3

1.6.5

1.1.2/1.1.3/1.1.4

1.7

2.4

1.7

4.1

2.3

1.5.3

2.3 2.3

2.4 4.1

2.3

3.3.2/3.3.4

2.4

3.5

2.4

4.2.12

2.4.11 3.1

3.7.1/3.7.3 3.2.1/3.2.2/3.2.3/2.2.9

3.1 3.1

3.3 3.4.1

3.1

3.6.2/3.6.3

3.2

2.3

2.2.9

2.4.5/2.4.6

3.2

3.3

3.2 3.2.1/3.2.2/3.2.3

3.4 3.5.1

Comment Degradation of wastes produces substances which infuence solubility and sorption, and hence transport out of the near-field Degradation of wastes is the source of gas production Degradation of wastes is the source of radionuclides for transport out of the near-field, as well as leading to the formation of organic complexing agents and sorption sites Over-pressurisation may lead to cracking of concrete. CO2 production and sorption may also be significant Gas production can induce or impede groundwater flow in the near-field Gas production can lead to over-pressurisation and fracturing Gas can migrate out of the near-field Radioactive decay and ingrowth occurs primarily during residence in the near-field, transport through the far-field and transport in the biosphere Fracturing influences groundwater flow characteristics of the near-field Fracturing influences gas transport properties of the near-field Groundwater flow in the near-field is a primary determinant of radionuclide transport out of the near-field Thermal effects modify near-field hydrological transport Thermal effects modify near-field chemical degradation Transport out of the near-field is the source term for far-field transport Transport out of the near-field is the source term for transport via preferential pathways associated with repository construction Far-field hydrology controls groundwater flow through the repository Far-field hydrology controls far-field transport Far-field hydrology controls transport via preferential pathways associated with repository construction Groundwater discharge as a determinant of surface hydrology (e.g. springs, baseflow) Far-field transport as a source of radionuclides to the biosphere Far-field transport as a source of radionuclides via groundwater abstraction Direct exposure to active gases Climate as a primary determinant of denudation and weathering Climate as a primary determinant of surface hydrology Climate as a control on terrestrial ecological development Climate as a determinant of the management of water resources and of agricultural policy Land form change modifies the boundary conditions of the far-field hydrological system, weathering alters the hydraulic properties of far-field materials Weathering alters the transport properties of far-field materials Land form change modifies the characteristics of the surface hydrological system Land form change influences ecological development Denudation and sediment redistribution control the erosive transport of radionuclides

Formulating conceptual models of underground disposal systems

175

Table 3---(Cont.) Origins

Targets

3.3

2.3

3.3.1/3.3.2/3.3.4

3.4.1

3.3.2/3.3.4

3.2

3.3

3.5.2/3.5.3/3.5.4/3.5.7

3.4.1

3.2.1/3.2.2/3.2.3

3.4.1 3.4.1

3.3.1/3.3.2/3.3.3 3.5.10/3.5.11

3.5

3.7

3.6.2

2.3.4

3.6.1/3.6.3/3.6.4

3.2.1/3.2.2/3.2.3

3.6

3.3

3.6

3.4.1

3.6

3.5

3.6

3.7

4.1

3.5

4.2

3.5

4.2.3

3.7

Comment Surface hydrology defines the boundary conditions for far-field hydrology Surface hydrology constrains the types of ecosystems that can develop Surface hydrology is a major determinant of denudation, sediment redistribution and weathering Surface hydrology is a major determinant of radionuclide transport in the environment, both in solution and bound to sediments Vegetation cover as a control on denudation and sediment transport Vegetation cover as a control on surface hydrology Ecological development partly defines the foodchain pathways Biosphere transport is the primary determinant of human expsoure pathways, except possibly for some active gases Management of water resources modifies groundwater flow Land management controls denudation and sediment redistribution All aspects of land management can influence the surface hydrological system Land management and management of water resources are primary determinants of ecological development Land management determines the type of biosphere pathways likely to occur, while management of water resources can influence the utilisation and distribution of contaminated water Management practices partly determine behavioural characteristics and hence exposure pathways Short-circuit pathways related to repository construction primarily provide source terms for biosphere transport Post-closure short-circuit pathways provide source terms for biosphere transport Exploratory drilling results directly in exposures of those involved in the operation or examination of the extracted material.

Notes: Where secondary headings only are listed, all the tertiary headings included in Fig. 1 are implied.

3. DISCUSSION A N D CONCLUSIONS In reviewing the study in retrospect, the following conclusions were drawn, tl In the context of the expert judgement studies undertaken, the methodology generally operated well, though there is r o o m for the use of m o r e meetings relative to communications by correspondence. As a general rule, dates for such meetings must be set at least two months in advance, because of the limited availability of the senior staff involved, which is a major constraint on the flexibility of the process and is an important determinant of its timescale. In respect of communication by correspondence, the method of briefing notes, questionnaires and

reproduction of all material received for consideration by the group worked well. It is emphasised that the secretariat cannot merely reproduce contributed items in the briefing notes. There must be a substantial input of interpretation and comparison, in order to help the group progress its work. For example, in this study, the secretariat was responsible for drawing up an initial version of the minimal assessment (see Section 2.4). Considerable care has to be exercised, when providing supplementary material, not to direct or constrain responses from group members. It might be argued that interpretation of material by the secretariat should be avoided. However, this would place a considerable requirement for synthesis of material on individual expert group members, who are

176

M. C. Thorne Table 4. Questionnaire on assessment adequacy

1.

Are there any factors or phenomena excluded from the minimal assessment (Fig. 1) which could render estimates of peak individual risk substantially in error? 2. What are these factors and/or phenomena? 3. What are your estimates of their probability of occurrence on the following timescales. 0-102 years post-closure 102-103 years post-closure 103-104 years post-closure 104-105 years post-closure 105-106 years post-closure Note that these timescales increase as a geometric progression. 4. What are your estimates of their likely separate effects on the values of peak individual risk calculated from the minimal assessment? Limited (less than a factor of two) Moderate (less than a factor of ten) Severe (greater than a factor of ten) Unquantifiable without modelling studies Unquantifiable even with modelling studies 5. If several factors and/or phenomena are listed under Item (2), are there any interactions between their various probabilities of occurrence and/or their likely effects on the results of the assessment? 6. The following components were excluded from the minimal assessment to produce the reduced assessment shown in Fig. 2: Item Description

7. 8.

1.6.4 Thermally induced hydrological changes in the near-field 1.6.5 Thermally induced chemical changes in the near-field 3.2.2 Localised denudation 3.6.4 Recreation policy developments All of these are likely to occur. What is your estimate of the effects on calculated values of peak individual risk of their combined exclusion from the assessment? Limited (less than a factor of two) Moderate (less than a factor of ten) Severe (greater than a factor of ten) Unquantifiable without modelling studies What were the main considerations you took into account in responding to Item (6)? The following components were excluded from the minimal assessment to produce the reduced assessment shown in Fig. 3. Item Description

1.1.2 Physical degradation of concrete 1.4.6 Fracturing in the near-field 1.6.4 Thermally induced hydrological changes in the near-field 1.6.5 Thermally induced chemical changes in the near-field 2.3.3 Modification to far-field hydrology due to rock property changes 2.4.6 Fracture surface changes in the far-field, notably demineralisation 2.4.7 Organic colloid transport 2.4.9 Transport of radionuclides bound to microbes 3.1.1 Transient greenhouse-gas induced warming 3.2.2 Localised denudation 3.6.4 Recreation policy developments 4.1.1 Short-circuit pathways relating to loss of integrity of borehole seals 4.1.2 Short-circuit pathways relating to loss of integrity of shaft or access tunnel seals 8a. What are your estimates of their probabilities of occurrence on the following timescales. 0-102 years post-closure 102-103 years post-closure 103-104 years post-closure 104-105 years post-closure 105-106 years post-closure Note that these timescales increase as a geometric progression. 8b. What are your estimates of the effects on calculated values of peak individual risk of their combined exclusion from the assessment? Limited (less than a factor of two) Moderate (less than a factor of ten) Severe (greater than a factor of ten) Unquantifiable without modelling studies 9. What were the main considerations you took into account in responding to Item (8).

Formulating conceptual models of underground disposal systems

177

Table 5. Views of the expert group on the importance of the eliminations in the first reduced assessment

Item

Description

Effect (c.f. Table 4) Limited/Moderate

1.6.4/1.6.5 3.2.2 3.6.4 1.6.4/1.6.5/3.2.2/3.6.4

Thermally induced hydrological changes in the near-field Thermally induced chemical changes in the near-field Combination of above Localised denudation Recreation policy developments All effects combined

1.6.4 1.6.5

Limited/Moderate Limited/Moderate Limited Limited Limited/Moderate

Table 6. Comparison of the minimal assessment with the trial assessment

Component of the minimal assessment

(a) Near-field and waste degradation 1.1.2 Physical degradation of concrete 1.1.3 Chemical degradation of concrete 1.1.4 Degradation of wastes (b) Gas generation and transport in the near-field 1.2.1 Generation of hydrogen by metal corrosion 1.2.2 Generation of methane and carbon dioxide by microbial degradation 1.2.4 Generation of active gases 1.2.6 Transport in the near field, especially in the vicinity of shafts and adits (c) Radioactive decay and ingrowth 1.3.1 Radioactive decay and ingrowth (d) Near-field fracturing 1.4.6 Fracturing (e) Groundwater flow in the near-field 1.5.3 Groundwater flow (saturated) (f) Thermal effects in the near-field 1.6.4 Hydrological changes 1.6.5 Chemical changes (g) Transport out of the near-field 1.7.1 Solubility controls on transport 1.7.2 Sorption controls on transport (h) Far-field hydrology 2.3.1 Variation in ground water recharge 2.3.2 Groundwater losses 2.3.3 Rock property changes 2.3.4 Groundwater flow (i) Far-field transport 2.4.1 Advective transport 2.4.2 Diffusive transport 2.4.3 Hydrodynamic dispersion 2.4.5 Sorption 2.4.6 Fracture surface changes, notably demineralisation 2.4.7 Organic colloid transport 2.4.8 Inorganic colloid transport 2.4.9 Transport of radionuclides bound to microbes 2.4.11 Gas transport 2.4.12 Gas-induced groundwater transport (j) Climate change 3.1.1 Transient greehouse-gas induced warming 3.1.2 Glacial/interglacial cycling (k) Geomorphological change 3.2.1 Generalised denudation 3.2.2 Localised denudation 3.2.3 Sediment redistribution 2.2.9 Weathering

Status in trial assessment Vandal/ TIME4

Detailed model

Scoping calculation

x" x' ~

x x ~b

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x x

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178

M. C. Thorne Table 6---(Cont.) Component of the minimal assessment

Status in trial assessment VANDEL/ TIME4

Detailed model

Scoping calculation

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(o)

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3.3.1 Soil moisture and evaporation 3.3.2 Terrestrial near-surface hydrology 3.3.3 Groundwater recharge 3.3.4 Surface flow characteristics (freshwater) Ecological development 3.4.1 Effects of terrestrial ecological development on hydrology Biosphere transport 3.5.1 Erosive trasnport 3.5.2 Groundwater discharge to soils 3.5.3/3.5.4/3.5.7 Groundwater discharge to wells, springs, streams, rivers and surface water bodies 3.5.10 Radionuclide uptake, retention and cycling in plants 3.5.11 Radionuclide uptake, retention and cycling in animals Land management practices 3.6.1 Urbanisation 3.6.2 Management of water resources 3.6.3 Agricultural policy 3.6.4 Recreation policy Human exposure pathways 3.7.1 External exposure 3.7.2 Ingestion 3.7.3 Inhalation Short circuits; repository construction 4.1.1 Loss of integrity of borehole seals 4.1.2 Loss of integrity of shaft or access tunnel seals 4.1.3 Damage to host medium around shaft or access tunnels Post-closure short circuits 4.2.3 Exploratory drilling 4.2.12 Groundwater abstraction

Notes:

x - - n o t included --included " - - n o t modelled, but taken into account in choice of material properties ~'--see Vol. 5, Ch. 5 (here and in all subsequent notes, volume references relate to the Dry run 3 documentation)~,~2 ' - - s e e Vol. 8, Appendix B, Calculation Note 3 "--included throughout, wherever appropriate "--see Vol. 5, Ch. 3 of Sumerling (1992) ~ ' - - n o t modelled explicitly in Dry Run 3, but can be represented in the V A N D A L geosphere module ~--detailed models are available and have been used in previous Dry Run exercises. However, for this exercise, network representations, compatible with V A N D A L / T I M E 4 , were used throughout ~'--represented in the input data to the environmental simulation code TIME4 ~2 i--represented in TIME4 J--see Vol. 8, Appendix B, Calculation Note 2 k - - d e e p weathering is implied in this entry ~--represented implicitly in model boundary conditions "'--represented in the biosphere module of V A N D A L " - - m o d e l currently under development "--represented in terms of observed equilibrium factors " - - s e e Vol. 8, Appendix B, for non-groundwater-mediated pathways "--only so far as choices on these matters are reflected in the data used ' - - s e e Vol. 5, Ch. 2 ' - - s e e Vol. 8, Appendix B, Calculation Note 4 '--generalised abstraction from a chalk horizon was represented, but not discrete wells "--effects could, in principle, be represented in T I M E 4 / V A N D A L "--limited hydrological studies of 'super-interglacial' conditions (see Vol. 5, Ch. 5) " - - s e e Vol. 3, Ch. 6 ' - - s e e Vol. 5, Ch. 5

Formulating conceptual models of underground disposal systems not likely to be as skilled as the analyst in such synthesis for safety assessment purposes. In the context of the meetings, it is relevant to note the conclusions of an observer who was present, as a normative expert (in the sense of Bonano et al. 9) at the second meeting of the group. His view was that the meeting was well controlled and that, from a practical point of view, the potential for introduction of bias was limited. He commented as follows on areas which require additional consideration in the conduct of such meetings. (a) Physical facilities are important and provision should be made for rapid updating of the complex reference material which forms the basis of discussion at such meetings. Visual presentation of this material to facilitate the work is also important. (b) The potential motivational bias of the analyst (Chairman), who is typically a member of the secretariat, should be noted and a suitably qualified observer should be present, if practical, to detect such bias. (c) Positive confirmation of the secretariat's judgement should always be sought (this applies also to correspondence). (d) Scope of the work of the expert group and underlying assumptions should be defined clearly prior to discussions, as should definitions of the terms used. (e) Training of expert group members and peer review of their deliberations is important. The latter requires that a comprehensive audit trail be maintained of all discussions. Finally, it is relevant to note that, while the group was able to come to semi-quantitative judgements on the adequacy of different assessment structures, exploration of this topic was necessarily limited. Additional work in this area would be desirable, to further test and extend the methodology. In this context, it is relevant to note that the use of formal expert judgement techniques is necessarily resource intensive (see also a recent report from the US Nuclear Regulatory Commission, on reactor safety studies2°). Thus, the study described in this paper required approximately 100 man-days of professional effort, to which should be added approximately 15 man-days of secretarial support. Furthermore, this is probably close to a minimum estimate, since it assumes that the secretariat staff are familiar with all the areas to be addressed by the expert group.

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