2013:36 Technical Note, Review and assessment of aspects of the Qeq concept – Main Review Phase

This technical review assignment considers the models and abstractions that Swedish Nuclear Fuel and Waste Management Company (SKB) developed to represent transport of dissolved constituents in the near field at the Forsmark site, in particular the Qeq abstraction for diffusive transport.  SKB uses the Qeq parameter to scale concentration gradients in order to estimate dissolved-species fluxes from the far-field environment to the canister surface and from a canister breach to the far field.  To accomplish the review assignment, we (i) reviewed relevant SKB reports that used the Qeq approach; (ii) summarized SKB’s approaches, checking for consistency in the descriptions and parameter choices; (iii) identified risk-significant aspects of the approach; (iv) compared SKB calculations and approaches to independent calculations, including independent numerical modelling; (v) and independently assessed a worst-case corrosion scenario linked to the Qeq approach.

We consider the Qeq approach implemented by SKB to be a reasonable and practical numerical method, widely applied across a variety of branches of mathematical physics, for approaching the transport of corrodants and radionuclides within the near field.  The methods for calculating resistances are based on analytical approaches.  Our independent calculations using detailed numerical models provided results consistent with the Qeq approach.  The Qeq approach is most accurate for nonsorbing or weakly sorbing dissolved species, which (i) describes the species of interest for corrosion and (ii) typically provide the largest contributions to dose.  Therefore, we conclude that the method performs best on the most risk-significant dissolved constituents.  SKB inconsistently describes transport assumptions in the release model; we estimate that an incorrect implementation would overestimate release rates by up to a factor of three.