The present paper deals with the participation of the University of Pisa in the last International Standard Problem (ISP)
focused on system thermal hydraulic, which was led by the Korean Atomic Energy Research Institution (KAERI). The selected
test was a Direct Vessel Injection (DVI) line break carried out at the ATLAS facility. University of Pisa participated, together
with other eighteen institutions, in both blind and open phase of the analytical exercise pursuing its methodology for developing
and qualifying a nodalization. Qualitative and quantitative analysis of the code results have been performed for both ISP-50
phases, the latter adopting the Fast Fourier Transfer Based Method (FFTBM). The experiment has been characterized by three dimensional behavior in downcomer and core region. Even though an attempt to reproduce these phenomena, by developing a
fictitious three-dimensional nodalization has been realized, the obtained results were generally acceptable but not fully satisfactory
in replicating 3D behavio
