1. Treatment of colloids and related issues in the safety case BELBaR 1 st Workshop, Helsinki 5-7 March Rebecca Beard, NDA RWMD

2. WP 1 Safety Assessment WP1: Produce Synthesis Report: Colloids and related issues in the long- term safety case Formulate the issues that are important for the long-term safety assessment Identify the main uncertainties related to colloids in the safety assessment Provide the focus for the research work in BELBaR 2

3. WP1 Current state-of-the-art report 3

4. WP 1 Role at project outset State of the Art report – Treatment of colloids and related issues in long-term safety assessments published in October 2012; Identifies merits and shortcomings Limitations of previous studies Identifies uncertainties Discusses needs for additional studies Identifies relevance of work in WPs 2 – 5 4

5. BELBaR Project WP Linkages: Information flow at start of project 5 WP1 Safety Assessment WP5 Conceptual and mathematical models WP3 Radionuclide and host rock interactions WP2 Erosion WP4 Colloid stability

6. Posiva – current treatment Transport of radionuclides facilitated by colloids has been omitted in the latest safety analysis Naturally occurring colloids can usually be neglected in performance assessments 6

7. Posiva - Limitations & uncertainties current treatment Groundwater has been assumed to be deionised All exchanger sites in clay components occupied with Na Formation of accessory mineral bed layers during erosion of bentonite is neglected Erosion will be significant only in a few locations Radiological consequences are small Mass loss rate assumed to scale with groundwater flow velocity 7

8. SKB – current treatment Bentonite colloids found in the groundwater near deposition holes and along the geosphere transport pathways Radionuclides will sorb onto bentonite colloids and may be transported through the geosphere Colloid facilitated transport involves a complicated combination of processes, many of which can mitigate the transport Sorption of radionuclides onto bentonite is a reversible process The effect of colloids in facilitating transport may be modelled through the introduction of effective transport parameters 8

9. SKB - Limitations & uncertainties current treatment The model is based on monovalent ions The model does not consider face to edge interactions between bentonite platelets Filtering effects by accessory minerals could limit release of colloids from the buffer Expansion has been taken to be horizontal, thus neglecting gravity The concentration limit for cation charge is based on experimental observations Most of the uncertainties are treated with pessimistic assumptions 9

10. NDA RWMD – current treatment Diffusion through the bentonite buffer was not explored as a variable as part of recent post closure safety analysis Previous scoping calculations performed for irreversible sorption of radionuclides to colloids –A cautious method for scoping the impact of colloids in the near field, i.e. assuming irreversible radionuclide sorption to mobile colloids. Only if such scoping calculations indicate that the impact of the near-field colloid population is potentially significant would consideration of colloids in more complex near-field models be justified 10

11. NDA RWMD - Limitations & uncertainties current treatment In some situations to assume reversibility of sorption is pessimistic –the radionuclide concentration dissolved in the groundwater and the amount of radionuclide that may be sorbed onto colloids may be overestimated. –Irreversibility of radionuclide sorption onto colloids could increase radionuclide mass transport. The understanding of the reversibility of sorption of radionuclides to colloid surfaces needs to be improved 11

12. Synthesis of issues: Erosion (WP2) IssueRelevance to the safety case Need for additional studies Mechanisms of erosion of clay particles from the Bentonite surface Erosion will cause a loss of bentonite buffer performance under some conditions. This may lead to corrosion failures of the canisters. Corrosion failure leads to the largest impact on risk, a less pessimistic approach may have significant impacts on the calculated risk. The mechanisms of clay colloid release are not fully understood. This will be studied in WP2. The quantitative models can be improved with new data. This will be done in WP5. Characteristics of Bentonite clay Divalent cations have not been studied that systematically. Should the existence and quantitative effect of divalent cations be argued, the importance of this outstanding uncertainty would reduce. The role of divalent cations can be pursued in WP2 and WP4. Insoluble accessory minerals are a topic for a potential parallel (smallish) investigation in WP2. The stability of different bentonites will be evaluated in WP4 12

13. Synthesis of issues: Erosion (WP2) 13 IssueRelevance to the safety case Need for additional studies Groundwater Chemistry The key factor for colloid stability is the ionic strength and the content of divalent cations. pH should have an effect, but the pH-range considered in the safety case is rather limited. The effect of mixed monovalent/divalent systems is still one of fundamental uncertainties. This issue will be studied in WP2, WP4 and WP5 Clay – Groundwater interactions Changes in bentonite porewater solute concentrations can be modelled. The related rates assumed to be limited by the availability of different porewater solutes. Mass loss rate assumed to have hydrodynamic contribution. The buffer and the groundwater never reach a true equilibrium. Observations from small scale experiments can therefore not be directly used in the safety case. A validated argumentation for (the conditions for) maximum mass loss rate to be used in the safety case can be prepared. This is a cross-WP effort. WP5 and WP1 will discuss how the studied processes should be integrated in the safety case.

14. Synthesis of issues: Erosion (WP2) 14 IssueRelevance to the safety case Need for additional studies Groundwater velocity Groundwater velocity has been considered as a variable. The loss of bentonite will be affected by the groundwater velocity and it is important to verify this dependence for erosion rates. It is important to verify the dependence between the groundwater velocity and the erosion rate. Clay extrusion paths Fractures have been assumed to be planar with a constant aperture. Extrusion of clay into a fracture is an integral part of the current model and will have a strong impact on the mass loss. Piping may occur before full saturation of the buffer under certain circumstances. Variable aperture tests should be discussed and possibly initiated within WP2. The effect of fracture geometry is studied in WP2.

15. Synthesis of issues: Colloid, RN & host rock interaction (WP3) 15 IssueRelevance to the safety case Need for additional studies Colloid mobility controlling processes Clay colloids have not been considered radionuclide carriers due to the assumed low contribution. Rather than attempting to develop detailed process models for colloid-facilitated transport, potential mitigating processes are ignored so as to place an upper bound on the possible effect. WP3 could either validate or invalidate this assumption i.e. explicitly revise this assumption. Retention processes Retardation of colloid transport in the far field, will delay the arrival of radionuclides in the biosphere. The extent of this isn’t currently taken into account. WP3 will study possible retardation mechanisms

16. Synthesis of issues: Colloid, RN & host rock interaction (WP3) 16 IssueRelevance to the safety case Need for additional studies Radionuclide sorption To assess the possible role of rapid reversible sorption/desorption onto colloids in facilitating transport, the following assumptions are adopted; 1.equilibrium sorption of radionuclides onto mobile and immobile colloids, 2.equilibrium sorption of colloids onto fracture surfaces, and 3.colloid-free matrix pore space (conservative assumption, but also realistic for the small pore sizes of granitic rock). Reversible, linear sorption of radionuclides onto colloids has been assumed. Reversibility is assumed and this needs to be verified in WP3

17. Synthesis of issues: Colloid stability (WP4) 17 IssueRelevance to the safety case Need for additional studies Colloid stability controlling processes Stability of compacted bentonite in dilute porewater conditions has been evaluated by laboratory measurements. The controlling process is hydration of exchangeable cations limited by the availability of cation free water. Currently the uncertainties in geochemical conditions are greater than in uncertainties in the stability limit. Colloid stability studies have found that model colloids that possess a significant net negative charge at neutral pH, i.e. silica and illite clay, show the greatest stability under neutral pH conditions. All aspects of colloid stability should be studied

18. Synthesis of issues: Colloid stability (WP4) 18 IssueRelevance to the safety case Need for additional studies Influence of other factors to colloid stability Accessory minerals seem to enrich near the bentonite- groundwater interface. Filtration has been discussed as a possible mean to reduce erosion. Colloid size, solution ionic strength and water flow rate are factors which strongly influence colloid migration. Association of inorganic particles with natural organic compounds is an important mechanism for colloid stabilisation. This mechanism could potentially operate to stabilise and enhance colloid populations in the near- field porewater, this remains an area of uncertainty Unclear how much effort will be put into this issue within BELBaR

19. Synthesis of issues: Conceptual and mathematical models (WP5) 19 IssueRelevance to the safety case Need for additional studies Current model(s): Erosion of the Bentonite buffer The factors considered are; 1.groundwater velocity 2.fracture aperture 3.transport resistance of bentonite gel in terms of diffusivity 4.gel cohesivity in terms of viscosity Small-scale tests suggests groundwater ionic strength and the presence of divalent cations are the dominant factors. Pessimistic assumption neglecting safety promoting aspects have been used. Model improvement and testing will be done in WP5

20. Synthesis of issues: Conceptual and mathematical models (WP5) 20 IssueRelevance to the safety case Need for additional studies Current model(s): Radionuclide transport mediated by Bentonite colloids Clay colloids have not been considered radionuclide carriers in Posiva's safety case due to the assumed low contribution. SKB incorporated the effective transport parameters using the MARFA code. At the colloid concentrations likely in the far field, a significant increase in risk could arise if a proportion of the radionuclides associated with colloids are irreversibly sorbed. In that case the risk will depend on the mobility and particle lifetimes. The issue is if the assumption of reversible sorption is correct. If so, the impact of radionuclide transport with colloids in a repository will be insignificant. This assumption will be considered in terms of reversibility of sorption in WP5 and WP3.

21. WP 1 Role throughout project Integrate information produced by all WPs Examine the extent to which other WPs’ findings provide satisfactory answers to the needs identified at the project outset Define scenarios for colloid issues, discuss uncertainties in these scenarios and identify model and data needs for their treatment Make recommendations for updated treatment of colloids and related issues in relation to long-term safety assessments 21

22. BELBaR Project WP Linkages: Information flow at end of project 22 WP1 Safety Assessment WP5 Conceptual and mathematical models WP3 Radionuclide and host rock interactions WP2 Erosion WP4 Colloid stability

23. 23 Summary WP2 Bentonite erosion and production of colloids –to understand the main mechanisms of erosion of clay particles from the bentonite surface and quantify the (maximum) extent of the possible erosion under different physico-chemical conditions. WP3 Colloid, radionuclide and host rock interactions –to determine the sorption reversibility of radionuclides to the colloids as this is a current assumption in the safety case. WP4 Clay colloid stability –The stability and mobility of clay colloids under site conditions is important for assessments of long-term performance of radioactive waste repositories. WP5 Improve current models –The target is to obtain a validated advanced model for the purposes of geological disposal of high level nuclear waste.