The CALIBRE source-term code: Technical documentation for version 2

The CALIBRE source term model has been updated, to make it a computationally more robust and faster code, and to incorporate some new features. These include the addition of a canister pinhole release model and a 'tunnel' option. The tunnel is represented by a zero concentration boundary condition at the interface of the backfilled tunnel with the bentonite and rock. The redox front model has been incorporated into the radionuclide transport code, so that a redox calculations may be run alone or prior to a radionuclide migration calculation. Redox front results can also be saved and re-used for radionuclide transport. More generally, the code simulates the diffusion of radionuclides from a high-level waste canister, through a backfill region and into a fractured rock matrix. The model includes chain decay and ingrowth, linear equilibrium sorption, solubility limiting and response to a redox front as it emerges from the canister and migrates through the near-field. Radial advection (which approximates the advection downstream from the canister and buffer) is applied in the fracture, in addi- tion to diffusion. There is also an option to allow for fixed total concentrations (i e concentrations sorbed on the solid and dissolved in the pore water) of naturally occurring isotopes in the fractured rock. This document describes the mathematical model and numerical methods used in developing CALIBRE, together with a number of verification tests which compare the results with those computed using analytic solutions. 11 refs, tabs