RD&D-Programme 2004 - SKB

18.2.23 Radionuclide transport – sorption
Sorption of radionuclides in the backfill is of very limited importance for repository performance.
It was assumed in SR 97 that the distribution coefficients were proportional to the content
of bentonite and crushed rock.
Conclusions in RD&D 2001 and its review
SKI says there is no justification for SKB’s conclusion that determination of the K d values does
not require additional work.
Newfound knowledge since RD&D 2001
No new knowledge has been forthcoming.
Programme
No further research is considered necessary.
18.2.24 Radionuclide transport – speciation of radionuclides
The speciation of radionuclides is of importance for sorption and diffusion in the backfill. It
is influenced by what speciation the nuclide had at the boundary to the backfill, i.e. inside the
buffer, but also by the chemical conditions in the backfill.
The speciation process is discussed in section 15.2.14. In SR-Can, the same types of speciation
calculations are carried out for e.g. the buffer environment as in SR 97.
18.3 Integrated modelling – radionuclide transport in the
near field
The probabilistic risk calculations in SR 97 (and previous safety assessments) were carried
out with the software package Proper. Proper is a package of calculation codes which, besides
routines for generating probabilistic input data and routines for handling communication
between different parts of the code, consists of a number of submodules for calculating
radionuclide transport in different parts of the repository. The version of Proper that has been
used is written in Fortran 77 and has been run on a Unix platform. In SR 97, radionuclide
transport in the near-field was calculated with the Proper module Comp23. The conceptual
model that has been implemented in Comp23 includes such processes as fuel dissolution,
precipitation and dissolution of solubility-limited radionuclides, diffusion through a specified
breach in the copper shell, diffusion and sorption in the buffer and the backfill, transport out to
the surrounding geosphere, and chain decay.
Radionuclide transport in the interior of the canister was simplified in the modelling in SR 97
in the following manner: After a certain “delay time” has passed since a breach occurred in
the canister’s copper shell, the entire void in the canister, approximately 1 m 3 , is assumed to
be filled with water. The length of the delay time is determined on the basis of the size of the
breach in the copper shell and the subsequent water flux and corrosion. After the delay time,
all water in the canister was assumed to be available for the fuel dissolution process, i.e. to be
in direct contact with all fuel without being impaired by Zircaloy cladding or other structures.
The water was assumed to be constantly stirred so that there are no concentration differences
between different parts of the canister interior. The fuel dissolution process then determines
the rate of release of matrix-bound radionuclides. Segregated nuclides and radionuclides in the
structural parts of the fuel are assumed to be accessible for dissolution in water immediately
240 RD&D-Programme 2004