The SOLPS-ITER code has been applied to the analysis of the scrape-off layer (SOL) plasma associated with the fast flow Lithium (Li)) divertor design for Fusion Nuclear Science Facility (FNSF). Two flat divertor configurations are investigated to determine puff levels that provide robust operational windows that meets the FNSF design requirements on upstream density and divertor flux with a flat divertor for flowing LM. Deuterium (D) puff and neon (Ne) seeding levels are scanned to find operational windows that have acceptable core density and divertor heat flux. We found that simplified open geometry presents challenges for neutral control and impurity retention, but balanced geometry with baffling appears promising. Balanced geometry is found to have acceptable Ne seeding and D2 gas puff operational windows that meet reactor’s design requirements on upstream density and divertor flux with a flat divertor for flowing LM. Lithium (Li) is sourced into the balanced configuration and the effect of Li on the plasmas is investigated by sourcing Li0 uniformly along the divertor surfaces and scanning over a large range to understand what level of Li emission is needed to influence the divertor and upstream plasma conditions. At low sourcing level (ϕ_Li≤〖1×10〗^23/s), the Li shows a minor effect on the plasma and is well confined in the Private flux region (PFR) and divertor surfaces. At moderate sourcing level (〖〖1×10〗^23<ϕ〗_Li≤〖1×10〗^24/s), the Li starts affecting the upstream and divertor plasmas, and Li radiation and electron cooling are almost linearly increasing with the Li sourcing level. At high Li sourcing level (ϕ_Li>〖1×10〗^24/s), the Li strongly affects the upstream and divertor plasma conditions and leads to strong detachment by increasing momentum and power losses. At high sourcing level, Li reaches above the x-point that can result in a substantial Li radiation above the x-point and sufficient dissipation to strongly affect the divertor and upstream plasmas.