Improvements to the Transmutation Trajectory Analysis of depletion evaluation

To simulate the nuclide evolution process in a nuclear reactor core, the Transmutation Trajectory Analysis (TTA) method solves the depletion equations by decomposing the depletion system into a number of linear chains and then solving each one analytically. In this paper, two improvements are proposed for TTA to obtain better efficiency. Firstly, the pseudo node evaluation for linear chain cutoff check has been removed. Instead, a time-averaged nuclide number density is employed as the chain termination criteria, which in theory can improve the computational efficiency by a factor of two. Secondly, a new recursive formula has been derived to replace the legacy direct solution formula for solving the linear chains. Numerical tests have been carried out based on a typical PWR fuel cycle to demonstrate that these improvements enable the TTA method to solve decay problems efficiently and accurately.