The SGs and other HXs (such as Moderator HXs, S/D HXs, Bleed coolers etc.) undergo periodic Eddy current Testing of tubes as a part of ISI. Steam generators of RAPS and MAPS are hair pin HXs type and are not amenable to ISI. These HXs use monel tubes. SG tube

failure took place in five HXs of MAPS. The cause after dismantling was found to be under deposit pitting near tube sheet. However similar HXs of RAPS when inspected neither did nor show any build up of deposit and tube degradation. By improving the chemical treatment /condensate polishing and blow down practice in MAPS (where condenser is sea water cooled) the failures were arrested. However, all hairpin HXS of these SGs are being replaced in the time slot for pressure tube replacement.

SGs from NAPS onwards are mushroom type and thus more amenable for In-service Inspection and use alloy 800 tubes. The indigenously developed automated robotic system is used for this purpose. These remotely controlled fully automatic devices are capable of traversing the probe head parallel to the tube sheet to the next selected tube, pushing the eddy current probe into the full length of the tube and recording information as the probe is subsequently retracted. This would reduce the time required for In-service Inspection of SGs as well as men-rem consumption. No generic degradation in SG tubes has been observed so far in any SG. However few SG tubes were found leaky due to damage caused by foreign material.