This internship will take place in several stages: Get to grips with the RHEOLEF tool (computer environment which serves as a practical laboratory for applied mathematics calculations involving finite element methods and which provides a set of commands and C++ algorithms and containers), Improve your skills on the THEMA spreading code. Compare the modeling and results between codes based on a test which will be defined by the LMAG and the research engineers in charge of the internship. This project will be carried out under Linux and requires knowledge of physics (fluid mechanics, thermal, and/or nuclear physics), computer science (source code in Fortran and tools in Python), and basics of applied mathematics.