ATHLET Framework for Embedding/Extending: An Efficient, Flexible & Easy-to-use Python Framework to a Large FORTRAN Code

— Over the years, large scientific code packages have been successfully developed using FORTRAN. Today, maintaining and extending these codes is a challenging task and often forbids several possibilities. Rewriting legacy code packages is almost prohibitive due to lacking resources and inertia against changing functional codes. One way of tackling these challenges is to integrate scripting languages into existing codes thereby retaining performance-critical kernels in FORTRAN and add an interface to a flexible, dynamically typed scripting language. Amongst the scripting languages, Python stands as a forerunner, especially due to the availability of scientific packages such as NUMPY and SciPy. In this project, an interface to Python is developed for a thermal-hydraulic simulation tool ATHLET, which is mainly used for nuclear reactor safety problems. The FORTRAN to Python interface generator tool (f2py) is used in an innovative way to have access to FORTRAN subroutines and module variables from Python and vice versa. Standard interfaces for three purposes have been developed. The interfaces allow for processing the simulation results during the simulation, calling Python subroutines within a simulation time step and coupling external simulations with ATHLET. The result of the project is an easy-to-use Python Framework to a large FORTRAN code which is very low on overhead, provides standard interfaces and requires very little changes to the original code. This framework allows the user to extend the functionality of ATHLET, by writing new modules in Python and coupling with external simulation codes, without having to deal with the rigours of FORTRAN programming.