2014:38 Technical Note, Detailed assessment of radionuclide Kd-values for the geosphere-main review phase

The Swedish Radiation Safety Authority (SSM) is reviewing an application submitted by the Swedish Nuclear Fuel and Waste Management Company (SKB) in 2011 to construct and operate a deep geologic repository for spent nuclear fuel at the Forsmark site in the municipality of Östhammar, Sweden.  SKB has presented details of its long-term safety assessment, SR-Site, in a main report (SKB, 2011, TR-11-01) and in multiple supporting technical documents that are cited by the main report.  In developing the safety case for SR-Site, SKB identified the geosphere’s capability to provide favourable radionuclide transport conditions as an important safety function that contributes to the optimum performance of the disposal system.  One such favourable condition is the substantial retention of radionuclides owing to high sorption coefficient (Kd) values for the host rock.  SKB has chosen to model radionuclide retention using a linear Kd approach and has supported the development of Kd values with site-specific laboratory experiments and data from the open literature.

As part of its initial review phase activities related to SKB’s Forsmark application, SSM reviewed SKB’s selection of Kd values used in SR-Site performance assessment modelling (e.g., Randall, 2012, SSM Technical Note 2012:63).  Those reviews identified some potential concerns regarding the development of Kd values and recommended additional work such as (i) conducting a detailed examination of how sorption experimental data have been transferred through the Kd derivation process, (ii) conducting a detailed review of the sorption experimental methodology and development of original Kd-data, and (iii) evaluating the span of probability distribution functions used in performance assessment models.  The present review is part of SSM’s main review phase and was undertaken to address comments of the previous reviews and to provide a comprehensive assessment of SKB’s approach to Kd value development, including the methods and results of sorption experiments and the relevance of data derived from those experiments.  Also examined in this review were the transfer factors used to correct sorption data for surface area, mechanical damage, cation exchange capacity, and groundwater chemistry variations.  In the present review, data were considered from a select number of radioelements (Cs, Ra, Np, Pu, and U) that span a range of characteristics including sources of data, primary mechanisms of sorption, and importance to performance.