A method for determining the ratio (f) between the height of the foams (H f ) and the height of the reactor (H R ) through an algorithm defining the gas hold-up in three zones of a slurry phase reactor, a first lower zone in which a bubble regime is established, a second intermediate zone where there can be the presence of foams, a third zone situated in the upper hemispherical part in which the multiphase mixture is accelerated until it reaches outlet conditions, the average gas hold-up being given by the weighted average of each of the three gas hold-ups of the three zones, characterized in that it uses three nuclear densimeters positioned inside the reactor positioned at a height between H R /4 and H R /5, between H R /9 and H R /11 and between H R /6 and H R /7 of the reactor, H R being the height of the cylindrical part of the reactor, and comprises: • measuring, for each nuclear densimeter used, gas density values, relating to different gas and/or slurry velocities, which correspond through said algorithm to calculated gas hold-up values, • revealing, with a calculated gas hold-up of less than 40%, the absence of foams at least up to the height at which the densimeter is positioned, whose density measured corresponds to said gas hold-up, with a calculated gas hold-up higher than 70%, the presence of foams starting at least from the height of the reactor in which the densimeter is positioned, whose density measured corresponds to said gas hold-up, • finally, determining through said algorithm, the ratio f and the extension in height of the possible presence of foams, calculating the consequent height H f wherein the formula for calculating the hold-up of the lower bubble zone is the model of Krishna with two bubble groups, whose parameters are the rise velocity of the bubbles and the transition hold-up, the rise velocity V ∞ , om of the small bubbles being V ∞ , om = a σ ρ l − ρ V g ρ l 2 1 4 exp bH f ε f wherein σ and ρ l are respectively the surface tension (expressed in N/m) and the density (expressed in kg/m 3 ) of the liquid phase inside the reactor, p v is the density (expressed in kg/m 3 ) of the vapour phase inside the reactor, H f and ε f are the height and hold-up of the possible foam phase present in the upper zone, and a and b are two parameters to be determined, the transition gas hold-up ε tr being estimated with the expression: ε tr = C ρ V 0.96 σ 0.12 ρ l 1 2 1 − C 1 f V exp − C 2 U L wherein f V is the volumetric fraction of solid in the reactor, U L is the slurry velocity, C, C 1 and C 2 are three parameters to be determined, and the gas hold-up in the foam phase ε f being obtained from the following expression: ε f = 1 1 + Ω U L U G wherein Ω is a slip parameter to be determined and U G is the gas velocity. • measuring, for each nuclear densimeter used, gas density values, relating to different gas and/or slurry velocities, which correspond through said algorithm to calculated gas hold-up values, • revealing, with a calculated gas hold-up of less than 40%, the absence of foams at least up to the height at which the densimeter is positioned, whose density measured corresponds to said gas hold-up, with a calculated gas hold-up higher than 70%, the presence of foams starting at least from the height of the reactor in which the densimeter is positioned, whose density measured corresponds to said gas hold-up, • finally, determining through said algorithm, the ratio f and the extension in height of the possible presence of foams, calculating the consequent height H f <EOS>