Nuclear Executive Update   
An EPRI Progress Report, September 2008
Research Finds Steam Generator Divider Plate Weld Cracking Not Safety Significant

Global experience guides analysis of U.S. reactor susceptibility to divider plate cracking.

Steam generator inspections conducted by non-U.S. utilities since 1991 have discovered cracking in the alloy 600 weld material between the steam generator divider plate and the divider plate stub runner. Cracks have been reported in eleven steam generators, with primary water stress corrosion cracking as the assumed cracking mechanism. To assess the safety significance of divider plate cracking, EPRI’s Steam Generator Management Program (SGMP) conducted a two-phase research project examining crack propagation, crack growth rates, and inspection or acceptance criteria. Technical analyses stemming from the research indicates that divider plate weld cracking is not safety significant and that U.S. plants do not need to begin divider plate inspections.

The divider plate provides a separation between the cold and hot leg of the channel head as the primary water enters the steam generator. The typical manufacturing process includes welding the divider plate to the channel head and then attaching it to the tube sheet via a weld to a strip of metal on the primary side of the tube sheet called the stub runner. The weld that connects the stub runner and the divider plate in some steam generators consists of alloy 600 material, which is susceptible to stress corrosion cracking.

Phase I of the SGMP project in 2007 evaluated whether the cracks observed in the non-U.S. steam generators would be capable of causing divider plate failure during accident or normal operating conditions. SGMP identified the Westinghouse Model 51 Steam Generator as the limiting case with respect to divider plate cracking for U.S. plants because it has the thinnest as-designed divider plate section under minimum material conditions and the greatest vertical displacements of the tube sheet under normal, accident and faulted conditions. Using conservative crack and fatigue life analyses for the Model 51 geometry, SGMP showed that that the observed cracks in the foreign steam generators are not capable of causing the divider plate to fail in the worst case U.S. steam generator during accident or normal operating conditions.

In 2008, SGMP expanded the Phase I work to analyze multiple crack geometries and origin sites to better understand crack initiation and growth rates and determine a potential inspection or acceptance criteria. The Phase II analysis concluded that cracking in the stub runner to divider plate weld and heat affected zone is not a safety significant issue during plant operations for either normal/design conditions or accident conditions. A fully degraded divider plate (i.e., cracked 100% of plate length and 100% of plate thickness) does not adversely affect steam generator performance during loss-of-coolant accident events or non-loss-of-coolant accident transients. Further, the fatigue limits and usage factors documented in the Model 51 ASME stress report would continue to be met even if the limiting case cracks reported in the foreign fleet were to develop on the limiting case divider plate. Therefore, there is no need for U.S. utilities to begin inspecting divider plate welds.

A report documenting the accident analysis and ASME code reviews will be available by November 2008. For more information, contact Jim Benson, +01 704-595-2050,