FAST MULTIPLE-PATH MODEL TO CALCULATE RADIONUCLIDE RELEASE FROM THE NEAR-FIELD OF A REPOSITORY

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 nucl ear waste repository. If the backfill and other materials surrounding the canister have a low permeability, waterflow is then excluded from these materials, and the solute transport is by diffusion only. Some n uclides are delayed by sorption on the materials through which they mo ve, and those nuclides with short half-lives may decay to insignifican t concentrations before they reach the flowing wafer in the fractures in the rock. This complex and variable transport geometry is modeled u sing a compartment model. The NUCTRAN compartment model is a useful fo ol to calculate the nonstationary transport of single nuclides or radi onuclide chains. The model, which is a very coarsely discretized integ rated finite difference model, is devised to be very fast and compact by embedding analytical solutions at sensitive points such as entrance s and exits from small holes and fractures. The nuclide inventory in t he source may be calculated using a solubility limit approach or a con gruent dissolution approach. The model is flexible and can easily be a dapted 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.