Managing Feedwater Heater Shell Thinning

Thinning of feedwater heater (FWH) shells has become an important issue throughout the U.S. nuclear industry. The thinning occurs primarily in the vicinity of the extraction steam and drain inlet nozzles where two-phase steam/water mixtures enter the heater. The thinning can be general wall thinning over a large area or highly localized thinning in a limited area. Eventually, shell repairs become necessary to ensure integrity of the pressure boundary and personnel safety, and to restore compliance with the ASME pressure vessel code. Ideally, long-term monitoring of the thickness of FWH shells would allow for timely repairs before the wall thickness decreases below the ASME Code minimum wall thickness. However, since most plants did not start inspecting FWH shells until recently, the initial inspection may find shells that are thinned to near or below the ASME Code minimum wall thickness. Because the cost and manpower requirements for shell repairs can be significant, it is imperative that shell repairs provide a long-term solution and that they be implemented as a planned outage evolution. Accordingly, the approach to managing FWH shell thinning must provide sufficient advance warning of the need for repairs while ensuring safe operation until the permanent repair can be implemented. The approach outlined in this paper does both. • Establish acceptance criteria to ensure safe, interim operation with thinned shells until a permanent repair can be implemented. • Develop contingency plans for interim repairs should the inspections identify thinning below the interim acceptance criteria. • Inspect FWH shells during a refueling outage or possibly during planned system outages or power reductions. If necessary, perform interim repairs to ensure safe operation until the next refueling outage. • Develop and implement permanent repairs that will prevent shell thinning.