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

Proceedings of 2010 LWR Fuel Performance/TopFuel/WRFPMOrlando, Florida, USA, September 26-29, 2010Paper 097Fig. 7. Margin to PCI During Open Valve Test fromThreshold ModelAlthough every pin was evaluated for susceptibility toPCI, a simple way to summarize the event is shown in thefigure as PCIUTL (shown in blue). PCIUTL is defined asthe margin to PCI for the most limiting pin per time stepduring the event. The model accommodates the situationwhere the limiting pin location is changing from time stepto time step. Basically, the margin is calculated for everypin and only the most limiting margin value is presented inthe figure. The results show that there was sufficientmargin during the event to PCI failure.V. C. Thermo-Mechanical Fuel Performance AnalysisResultsThe last PCI analysis conducted for the open valve testwas to analyze every pin via the ONUS integrated fuelperformance analysis (ENIGMA and CMS using detailedoperational histories from CMSOps CMSOps) 10 .The most limiting parameter analyzed was the marginto maximum clad yield stress during the open valve test.The result is summarized here as Fig. 8. The ENGIMAanalysis assumed a failure yield stress of 93.5 kPSI. (Theactual limit is fuel design type- and exposure-dependent,but the variation is insignificant with the amount of marginthat exists for the fuel during this test.)Fig. 8. Margin to Yield Stress Limit of Most Limiting PinDuring Open Valve TestThe cases shown in the figure's x-axis correspond tothe time during which the open valve test was conducted,(approximately 8 hours). In the figure, the most limitingpin – in terms of smallest margin to yield stress (cladfailure) – was in one of the assemblies which the leadcontrol bank was partially inserted/withdrawn. Theminimum margin occurred axially near the core mid-plane.The figure also shows the lead control bank positionduring the test and the LHGR of the limiting pin. Theminimum margin (highest stress) occurs at the deepestinsertion of the lead control bank, most probably due to theLHGR being redistributed as the lead control banksuppresses power in the upper axial region of theassemblies into which they are inserted.Other findings from the ONUS results are that the pinswith the highest crack lengths (albeit with crack lengthsonly a fraction of 1% of the total clad thickness) are fromhigher depleted assemblies (pin-average exposures ofabout 45 GWd/T) where pellet clad gap closure will havealready occurred. However, because the higher depletedassemblies were located on the periphery of the core, thepin LHGRs before, during, and after the event wererelatively low and consequently SCC propagationnegligible.Detailed pin-by-pin results for minimum margin yieldstress during the event are presented in Fig. 9. The axiallocation where the minimum margin occurs is in thebottom half of the core. When the lead control bank ispartially inserted, the power in this lower region of thecore increases leading to a systematic increase in stress onthe clad (margin decrease). When this lead control bank isfully withdrawn, the power in this lower region decreasesagain and the stress on the clad relaxes (margin increase).