A Burnup-Dependent Isomeric Production Branching Ratio Treatment

To take the production of isomeric states into account during the irradiation of a material, a depletion code needs the proper isomeric branching ratio s for every isomeric state s produced in a reaction. The composition of some nuclides such as, for example, ²³⁸Pu and some Cm isotopes is quite sensitive to the value of the isomeric branching ratio for the ²⁴¹Am neutron capture reaction. Existing depletion codes use constant burnup-independent values for the isomeric branching ratio data, which were calculated in advance for a particular type of spectrum (e.g., pressurized water reactor, boiling water reactor, or fast reactor systems). In this paper, we propose a burnup-dependent treatment using evaluated nuclear data from ENDF files as a function of the irradiation history. This treatment has been implemented into the VESTA Monte Carlo depletion code using both the multigroup binning approach and Monte Carlo estimators. The validity and usefulness of this new treatment has been demonstrated using experimental data from the MALIBU program and has shown that it improves the prediction of ^242mAm when using JEFF 3.1 data. It is also shown that more work is required on the measurement and evaluation of the cross-section data for the Am isotopes in general and the energy-dependent ²⁴¹Am branching ratio in particular to improve the results of depletion calculations.