Integral Performance Assessment: Coupling NEAR3D and CHAN3D

An integrated code that may follow the transport of nuclides all the way from the damaged canister to the biosphere has been developed. This new package is based on the codes NEAR3D for near-field calculations and CHAN3D for far-field calculations. It couples the near- and far-field models in a transparent, effective and simple way. In addition the package comprises a Graphic User Interface, which controls the function of the other parts of the package, in which the Coupling code connects the codes NEAR3D and CHAN3D. The near-field code takes all the important processes within the near field into account, including dissolution within the canister, solubility of radionuiclides, radioactive chain decay, and transport in the buffer material surrounded the canister. The code uses analytical or semi-analytical expressions in locations where other codes have to use very fine discretization. In the far-field code, CHAN3D, fluid flow takes place in a network of interconnected flow channels. The channel density, transmissivity distribution, and flow wetted surface of the rock are chosen so that they agree with field observations. The main characteristics of the CHAN3D code are that it may handle chain decay and account for several embedded fields (rock domains) with different properties. Safety assessment calculations for a potential repository for spent fuel located at Laxemar area were performed. This is one of the possible locations for the future repository in Sweden. The model includes, in deterministic way, the main fracture zones, tunnels and other features of the repository. The near-field simulations with SKB’s code, using the same data gave an excellent agreement. Stochastic simulations for the far field were also made, and results were summarized in terms of the F-ratio and the water residence time. The statistics for these entities compare well with those obtained by SKB, where the code CONNECTFLOW was used.