ENHANCING THE PERFORMANCE OF FISSION SOURCE CONVERGENCE WITH FUNCTIONAL EXPANSION TALLIES IN SERPENT 2 MONTE CARLO CODEAna Jambrina, Jaakko Leppänen and Heikki SuikkanenEPJ Web Conf., 247 (2021) 04001DOI: https://doi.org/10.1051/epjconf/202124704001