Research activities are continuing between the University of Pisa and ENEA Brasimone Research Center to understand better the phenomena and processes that occur during a postulated in-box LOCA in the Water-Cooled Lead Lithium Breeding Blanket (WCLL BB) and during the system's safety response. Various activities are also going on to strengthen the reliability of numerical tools and validate computer models, codes, and procedures for their implementations. The study presented here aims to assist in the validation process of the SIMMER III code model, to simulate the Lead-lithium and water interaction under conditions like those expected for the WCLL BB during accident conditions. In particular, the current work has been performed by numerically reproducing the experimental Test E5.2, executed in the separate effect test facility LIFUS5/Mod3 (installed at ENEA Brasimone Research Center). The code used for the simulation was SIMMER III computational code, in a version modified by the University of Pisa to take into account also the chemical interaction between the two fluids. A comparison of significant parameters computed by the SIMMER code with those obtained in the experimental test during the transient has been carried out, and it is presented and described in this paper.