The feeder pipes are of carbon steel material conforming to ASME material specification SA106 Gr B in RAPS, MAPS and NAPS units and to SA333 Gr 6 in KAPS units onwards. This material as PHT pressure boundary material has been tested extensively and is found to be a good option for LBB application. During en-masse coolant channel replacement (EMCCR) of RAPS-2 in 1998, extensive in-service inspection (ISI) of the feeders was carried out. Thinning was observed on the feeder elbows immediately after Grayloc joints particularly on outlet feeders.

Mapping of the thickness indicated that the thinning was generally local. All inlet and outlet feeder elbows were inspected. The study of reduction of wall thinning and assessment of structural integrity of these feeder elbows was carried out. The phenomenon called “flow assisted corrosion” (FAC) was understood to be the cause of local thinning of the feeder elbows. As the thinning is predominantly on extrados of the elbow, the thinning is classified as localized thinning, which needs to have different treatment compared to the normal pipe as far as structural integrity is concerned.

To assess the structural integrity of these elbows of RAPS-2, ASME Code Case N-480 was used which defines the approach. A streamlined procedure and acceptance criteria for these elbows was evolved and which is being used to assess the wall thickness reduction in feeder elbows of Indian PHWRs. Some of the elbows in RAPS-2, which were having balance life less than 10 years, were weld buildup to increase the thickness.

In MAPS-1 also the thinning of feeder elbows was observed, though the thinning was to a lesser extent than that of RAPS-2. However from the consideration of long term corrective measure and enhancement of feeder life commensurate with the life of replaced coolant channels, part length, from Grayloc end including Grayloc hub and elbow(s) of all feeders were replaced successfully during 2005 EMCCR campaign. The new elbows are of higher thickness, sch.160. The material of new elbows & pipes is alloyed with 0.2 % minimum Chromium. It is expected that the refurbished feeders now will have the life of about 25-30 years.

On the similar lines as in MAPS-1, it is planned to replace part length of all feeders in NAPS — 1 also during EMCCR campaign in the year 2006.

The ISI programme for Indian PHWRs includes the thickness measurement of feeder elbows. However, in light of RAPS-2 experience, more refined procedure and newly evolved technique have been developed for UT thickness measurement and volumetric examination of elbows on sample basis for crack detection. The criteria of selecting feeders for periodic ISI is also revised based on operational experience.

In operating stations, strict chemistry control of primary fluid is required to have reduced effect of FAC. It is also required to take wider base data for inspected feeders based on previous assessment. Repeatability of thickness measurement for the feeders with balance life less than 5 years is required for periodic assessment. Such feeders are subjected to more frequent assessment. For some feeder elbows, weld builtup can be carried out to increase the balance life as done in RAPS-2. It is required to balance between economy and man-rem budget, while deciding to discard the thinned elbow and reinstall with the new one. Replacement of critically thinned elbows can be planned in long shutdown such as EMCCR.

In future reactors (TAPP-3 & 4/ RAPP — 5& 6 /Kaiga 3 & 4), the material SA333 Gr.6 with minimum Cr of 0.2% as an alloying element has been used. This coupled with higher size elbows can reduce FAC effect to great extent and expected to give adequate design life for the feeders.