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

More documents

Recommendations

Info

Proceedings of 2010 LWR Fuel Performance/TopFuel/WRFPMOrlando, Florida, USA, September 26-29, 2010Paper 097Fig. 5. Nodal delta-kW/ft/hr during Open Valve Test vs.TimeThe nodal delta-kW/ft/hr values presented in thefigure coincide with the expected behavior. During theevent nodes where power is being suppressed as the leadcontrol bank is inserted experience a negative deltakW/ft/hr.Conversely, nodes where power is beingincreased as the lead control bank is withdrawn experiencea positive delta-kW/ft/hr. The magnitude of the changes (indelta-kW/ft/hr) are consistent with the expected increase inclad strain – a key component of PCI – given that the leadcontrol bank movement occurs over a relatively short time.The event was also evaluated with the delta-kW/ftapproach by using data for each fuel pin in the core (datawhich is typically not available during operationaltransients).Exploring the same plot of delta-kW/ft/hr with datafor every pin in every node in the core (Fig. 6.) highlightsone major inadequacy of this monitoring method – namelythat it ignores local (pin-level) changes, which can besignificantly larger in magnitude than more coarse nodewisedata.This pin data is compared with the nodal results andpresented as Fig.6. Note that in the following plot, the datain red is pin-by-pin delta-kw/ft/hr, while the data in blackis the nodal delta-kw/ft/hr.Fig. 6. Pin-by-Pin Margin-to-PCI during Open Valve TestThe results demonstrate that using nodal data vs.detailed pin information does not show the significantlyhigher changes in LHGR that individual pins couldexperience during a power transient. (Even though for thisparticular case there is not sufficient evidence, even withthermo-mechanical fuel performance analysis to supportPCI failure.)Finally, neither the nodal data based delta-kW/ft/hrmodel nor the pin-by-pin delta-kW/ft/hr models aresufficient for diagnosing PCI. PCI is predicated not only inchanges in kW/ft but also a time component (e.g., howquickly the changes have occurred, has the fuel claddingbeen conditioned to the higher LHGR, etc.).V. B. Threshold LHGR Results for Results for OpenValve TestUsing the same detailed pin-by-pin LHGR datagenerated from CMSOps during the open valve test, thealternative threshold model 8 was applied. As previouslymentioned, this model requires certain constants for thecomponents for monitoring (e.g., conditioning/deconditioningrates, etc.).The results presented in Fig. 7. indicates similarmargin for the most limiting pin during the transient usingthe threshold model and empirical data from StudsvikNuclear 9 .In the figure the inlet temperature vs. time during theevent is plotted against the right-side y-axis and the leadcontrol bank position on the left-side y-axis.
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).
Page 1 and 2: Proceedings of 2010 LWR Fuel Perfor
Page 3 and 4: Proceedings of 2010 LWR Fuel Perfor
Page 5: Proceedings of 2010 LWR Fuel Perfor
Page 9: Proceedings of 2010 LWR Fuel Perfor

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

Create successful ePaper yourself

Delete template?

Save as template?