[en] A steam generator comprising a tubular plate with holes to which are connected the extremities of the U-tubes of a bundle, said tubes being dipped in a secondary fluid and containing a circulating primary fluid, is described. This generator is characterized in that the extremities of the U-tubes of said bundle are distributed in the tubular plate with a non-constant pitch, the tube portions in which flows the hot primary fluid being arranged with a pitch greater than that of those tube portions in which flows the cold primary fluid. Such an arrangement permits to improve the circulation of the secondary fluid in that portion of the bundle in which flows the hot primary fluid, in particular in that portion which is farthest from the secondary-fluid admission peripheral zone