Implementation and Preliminary Verification of the RELAP5/PARCS Code for Pb-Bi Cooled Subcritical Systems

In the framework of the Italian research program on Accelerator Driven Systems funded by MURST (Italian Ministry of the University, Scientific and Technological Research), the feasibility and operability of a Pb-Bi cooled ADS prototype are currently under investigation. Development and validation of a suitable tool for studying the dynamical behavior of such a system during operational transients and accidental sequences are clearly crucial issues among the R&D needs. This paper deals with the activity performed by ENEA and University of Pisa with the aim to implement and qualify the coupled thermal-hydraulic and neutronic code RELAP5/PARCS, initially developed for LWRs, and adapt it to Pb-Bi cooled subcritical systems. According to this goal, Pb-Bi eutectic thermal and physical properties have been added to RELAP5 libraries while PARCS module has been modified in order to have the possibility of taking into account fast spectrum and time-dependent neutron source driven subcritical systems. A preliminary verification of the modified coupled code RELAP5/PARCS has been carried out analyzing some basic transients. In all these cases, even if the transient is originated by a purely thermal-hydraulic or neutronic event, the interaction between thermal-hydraulic and neutronic phenomena has to be properly considered. Although an exhaustive validation of the modified coupled code has still to be performed by comparing the simulated transients with the results provided by subcritical system dedicated neutronic codes and, restricted to the thermal-hydraulic behavior, with the experimental results, it seems possible to state that the modified coupled code RELAP5/PARCS can acceptably describe the dynamical evolution of a Pb-Bi cooled ADS system.