[en] Full text: The installing of helicoidal spacers in both moderator heat exchangers 3311 HX1 /HX2 is described. The operating experience data of both heat exchangers indicated a high level of vibrations due to the induced impacts by refrigeration water flux at secondary side. This vibrations level was reduced in order to avoid or to minimize the possibility of tubes damage resulting in a heavy water (with high isotopic level) leak to the environment. Vibrations analysis was done with flux distribution determination with the vein analysis of the HTRI (Heat Transfer Research Institute). This method finds the fluid vein fraction corresponding to the fractional velocities of different zones of heat exchangers The analysis was focused to the surface zones close to the tube U-bend and the by-pass partition by-pass because they are under the conditions with highest velocities of cross-flow The vibration induced phenomena is normally due to vortex, turbulence, instability, elastic flux. From results it concluded that the tubes of the by-pass lane, of U-bend and in the surface of the tube mallet are the most sensitive to the flux induced vibration. The tubes were structurally modified, trough additional supports installing with helicoidal spacers inserted in the tubes mallet. Practically there are not changes in the flux and the natural frequency of tube is augmented that is the mallet resistance to the flux induced vibration. The heat exchangers structurally modified are more resistant to tube vibration by at full rate flux and to 125% of full rate flux. After modifications vibrations tests were done in the heat exchangers without evidence of pulse signals or frequency variations due to impacts flux in the equipment. Conclusion is that the modification gives a proper supporting and avoids a damage of tubes mallet. An analysis about proper surveillance program for the heat exchangers is opened. (author)