Extension of the reactor dynamics code DYN3D to SFR applications – Part I: Thermal expansion models

The reactor dynamics code DYN3D, initially developed for LWR applications, is being extended for steady-state and transient analyses of Sodium cooled Fast Reactor (SFR) cores. In contrast to LWRs, thermal expansions of SFR core and reactor components such as fuel, cladding, diagrid, control rod (CR) drivelines, vessel, etc. provide essential reactivity feedbacks under normal and transient conditions. Since DYN3D was originally oriented to the LWRs analyses, the modeling of thermal expansion mechanisms was not considered in the code. Therefore, the development of a new thermal-mechanical module accounting for thermal expansions has been initiated as a part of the SFR related activities. At first step, the DYN3D code was extended with the capability of treating two important thermal expansion effects occurring within the core, namely axial expansion of fuel rod and radial expansion of diagrid. Part I of the paper provides a detailed description of the newly implemented models and presents the results of the initial verification study performed on a full core level using a large oxide SFR core from the OECD/NEA benchmark and a Phenix end-of-life core from the static neutronic IAEA benchmark. Two IAEA benchmarks on the Phenix end-of-life experiments were used for a more detailed validation of the extended version of DYN3D. While Part II presents the validation study performed against the static neutronic benchmark of the control rod withdrawal tests, the results of the initial stage of the natural circulation test are covered in Part III of the paper.