# Pathways-based uncertainty

Propagation of uncertainty in inventories and responses is challenging for cases where the nuclide inventory depends strongly upon the production paths, particularly where there may be a great many. Using the uncertainties in reaction rates determined using full variance-covariance data in the nuclear data files, a sampling method can be used to determine uncertainties in various quantities, but this can be computationally impractical. Particularly in cases where many reactions and decays are responsible for the production of some nuclides, this requires full sensitivity analyses followed by potentially a great many nuclear data samples.

To both make these calculations more practical and allow full uncertainty quantification on observables, rigorous pathway analysis and pruning algorithms were developed. These identify and weight the possible combinations of reactions and decays which result in the production of dominant nuclides by using the **UNCERTAINTY** keyword. The nuclide set can be extended to others of interest using other keywords such as **ROUTES**, **LOOKAHEAD** and **PATHRESET**. Once the pathways are calculated, the uncertainty along each is calculated using the uncertainties along each as an uncorrelated sum of squares of individual links within the path. The weighted sum of these gives the uncertainty in the production of the nuclide.