Robust PCI Monitoring During PWR Operation at ... - Studsvik

Proceedings of 2010 LWR Fuel Performance/TopFuel/WRFPMOrlando, Florida, USA, September 26-29, 2010Paper 097Pins with the lowest margin during the event are fromthe same assembly locations in which the lead control bankwas partially inserted during the event. These assemblieshave pin exposures high enough that the fuel/clad gap isexpected to have closed prior to the event.Furthermore, using these thorough and rigorousmethods, it has provided confidence that the fuel failureexperienced during the open valve test in the SouthernNuclears PWR was not caused by PCI. This analysishelped eliminated this potential cause from any furtherinvestigation and has allowed Southern Nuclear to focusresources onto other potential sources of fuel failure. (Rootcause analysis results based on poolside inspectionsconducted by Southern at the end of cycle turned out to beconsistent with results from these studies.)The techniques presented in this study were applied toanalyze a past operating event, but they can also easily beapplied to the simulation of a future power maneuvers toassess margin to fuel performance parameters (such as cladstress, strain) before the event actually occurs. The resultscould then be used to alter the power maneuver, ifnecessary, to ensure sufficient margin and help reducepotential fuel failures.Fig. 9. Minimum margin to clad yield stress (lead controlbank partially inserted)The results indicate there is still a large degree ofmargin before fuel failure would occur, even for the mostlimiting pin locations during this event. (Fuel failure mightstill be possible due to prior mechanical damage that couldhave been exacerbated during this event; missing pelletsurface, introduction of foreign material hitting the fuelpin, hydriding, etc.)VI. CONCLUSIONThree different methodologies to evaluate fuelperformance in terms of PCI were applied to an actualtransient from operating PWR. The methods range fromexamining changes in coarse nodal LHGR's (delta-kW/ftmodel) during the transient as an indicator of PCIsusceptibility, to a threshold-based pin-by-pin model, tothermo-mechanical fuel performance analysis for every pinin the entire core.The work here demonstrates that all three approachesare computationally feasible and that limitations ofperforming coarse analysis using nodal LHGR data (inwhich the margin to PCI is underestimated) can bereplaced with explicit pin-by-pin thermo-mechanical fuelperformance evaluation during plant operation.The coupled CMS-ENIGMA analysis hasdemonstrated how an explicit pin-by-pin PCI analysis canbe applied during actual operating events for fuelperformance assessment.NOMENCLATUREPCI – Pellet Clad InteractionLHGR – Linear Heat Generation RatePWR – Pressurized Water ReactorDelta-kW/ft – delta kilowatt per foot (energy)SCC - Stress Corrosion CrackingBOC – Beginning of CycleHFP – Hot Full PowerMTC – Moderator Temperature CoefficientCMS – Studsvik Core Management SystemkPSI – kilo-pounds per square inch (pressure)MPa – mega Pascals (pressure)GWd/T – gigawatt days per metric ton (exposure)REFERENCES1. J. D. RHODES, K. S. SMITH, J. D. LEE,“CASMO-5 Development and Applications,”PHYSOR 2006, Vancouver, Canada, (Sept 2006).2. K.S. SMITH et al., “SIMULATE-3 Methodology,”Studsvik Report, SOA-95/18 (1995).3. A. DIGIOVINE and A. NOËL, “GARDEL-PWR:Studsvik’s Online Monitoring and ReactivityManagement System,” Proceedings of Advances