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Fast Multiple-Path Model to Calculate Radionuclide Release from the Near Field of a Repository

L. Romero, L. Moreno, I. Neretnieks

Nuclear Technology

Volume 112 / Number 1 / October 1995 / Pages 89-98


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Radionuclides leaking from a damaged canister spread into the backfill material surrounding the canister and then migrate through different pathways into water-bearing fractures in the rock surrounding the nuclear waste repository. If the backfill and other materials surrounding the canister have a low permeability, water flow is then excluded from these materials, and the solute transport is by diffusion only. Some nuclides are delayed by sorption on the materials through which they move, and those nuclides with short half-lives may decay to insignificant concentrations before they reach the flowing water in the fractures in the rock. This complex and variable transport geometry is modeled using a compartment model. The NUCTRAN compartment model is a useful tool to calculate the nonstationary transport of single nuclides or radionuclide chains. The model, which is a very coarsely discretized integrated finite difference model, is devised to be very fast and compact by embedding analytical solutions at sensitive points such as entrances and exits from small holes and fractures. The nuclide inventory in the source may be calculated using a solubility limit approach or a congruent dissolution approach. The model is flexible and can easily be adapted to various geometries. NUCTRAN agrees well with models using a very detailed discretization. Accuracy is gained if compartments with very large capacities are subdivided into a few compartments.

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