A plant for synthesis of urea with a CO2-stripping process, comprising a natural-circulation synthesis loop, said loop including at least a urea reactor (1), a carbon dioxide stripper (2) and a condenser (3), said reactor, stripper and condenser operating substantially at the same elevated pressure, said loop comprising also a reactor effluent flow line (5), connecting said urea reactor to said stripper, which comprises means (15) for directly or indirectly detecting the flow rate and/or the direction of the flow through said reactor effluent flow line (5).