Evaluating the effect of decay and fission yield data uncertainty on spent nuclear fuel source term using Serpent 2

Knowledge of spent nuclear fuel (SNF) source term (decay heat, reactivity, nuclide inventory and other relevant properties of SNF) is essential in the safe handling and final disposal of SNF. For example, decay heat power determines how densely the fuel canisters can be packed in the final disposal tunnels. Computational characterization of SNF involves numerous uncertainty sources, one of which is the uncertainty in nuclear data. Different nuclear data libraries are known to yield significant differences in nuclide inventory calculations. How these uncertainties affect each component of the source term is not generally known. This work aims to identify the major uncertainty components in decay and fission yield data in relation to the SNF source term.

The uncertainty propagation from decay and fission yield data uncertainties into the uncertainties of the source term will be conducted using a sampling based technique and an extended version of the Monte Carlo particle transport code Serpent 2. Normally, Serpent 2 only uses the tabulated values for the fission yield and radioactive decay data from ENDF-6 format data files. In the extended version, the nominal values are perturbed based on random sampling using the uncertainty data present in the ENDF-6 format files. Producing the SNF source term several times using different randomly perturbed nuclear data allows the propagation of the input uncertainties to all output quantities included in the SNF source term. Sub-components of the resulting output uncertainty can be identified by limiting the perturbation to a specific class of data, e.g. by only perturbing the fission yield data of U-235 and Pu-239.

The analysis will investigate the decay and fission yield data related uncertainties in the SNF source term of a TVEL second-generation type VVER-440 fixed assembly with an average enrichment of 4.37 % U-235 and six gadolinium-doped fuel rods with 3.35 % Gd2O3. Other nuclear data related uncertainties are ignored in this study, and only fixed, nominal depletion conditions are considered.