1. The need for process understanding in the derivation of a credible dose model for geological repositories Shulan Xu 2010-01-26

2. Contents Introduction Critical issues Modelling exercises Considerations

3. Introduction  Historical development of biosphere models For thirty years or more, dose models have employed a compartmental representation of the surface hydrogeology to model the dynamic concentration of contaminants in various ecosystems –BIOMOVS I & II, BIOMASS-6 Landscape models in SR-Can (SKB, 2006) –A number of ecosystems connected –Evolution of site Dose assessment in the overall assessment –Dose factor, DF, (Sv/yr per Bq/yr) –Defensible, transparent –ACM, –possible impacts

4. Developments in safety assessment SSM has initiated research and development projects in the area of performance assessment (PA) modelling a few years ago. –Processes understanding of transport and distribution of radionuclides –Independent modeling capacity to perform replication calculations and development of alternative models/tools to give critical analysis for the safety assessment. Performance of reviews –SR-97, SR-Can –SAFE, SAR-08

5. Characteristics of discharge areas Lake, stream or wetlands Thick soil layer (QD) Coincidence with fracture zone

6. How a size of biosphere object can be identified? Path 1 Path 2 Contaminated area

7. Critical issues Representation of discharge areas Process descriptions –Accumulation/GBI –subsurface chemical zonation Model discretisation

8. Research and modelling work in 2009 Project on radionuclide transport through the ”GBI” Modelling exercises –Effect of accumulation –Effect of different combinations of discharge points

9. Modelling exercises in 2009 Assume discharge point moving with time due to uplift

10. Modelling exercises in 2009 0 – 3000 years 3000 – 20000 years 1 Bq/y river lake wetland Agr. land a) b) QD

11. Modelling exercises in 2009 Simulated inventory of top soil vs time

12. Estimates of dose rates for agricultural land compared with doses for different ecosystems. NuclidesAgricultural Land/River Agricultural Land/Lake Agricultural Land/Mire 226 Ra0.7333 36 Cl165810 135 Cs0.42.517 129 I159021 59 Ni2014017 Modelling exercises in 2009

13. Findings from modelling exercises It is not always that the discharge point/object has highest dose effects A clear link between FEP matrix and PA models is necessary

14. Considerations Do we know enough about dilution to make credible dose calculations? Consensus