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Proceedings of 2010 LWR Fuel Performance/TopFuel/WRFPMOrlando, Florida, USA, September 26-29, 2010Paper 097II. PCI ANALYSIS METHODS OVERVIEWFuel performance has long been a part of reload coredesign and analysis. Typically, only bounding analysis ofthe most limiting fuel pins is included in the core designprocess.The difference in the analysis presented here is twofold.First, an actual transient from operating reactors thatresulted in changes to LHGR is analyzed for margin toPCI. Second, since PCI is a pin-based phenomenon, allfuel pins in the core are equally analyzed (e.g., nobounding analysis of only limiting pins was used).Fig. 1. Limitations to delta kW/ft modelThe assessment demonstrates that the various methodsfor examining PCI can be applied to actual operatingevents and screening criteria for bounding analysis is notnecessary.II. A. Delta-kW/ft ModelIn a delta-kW/ft model, changes in linear heatgeneration rate (LHGR) between successive calculationpoints (statepoints) are examined to ensure the values arewithin acceptable limits. The LHGR values can be nodeaverageddata, where LHGR's for each pin in an assemblyare normalized within an assembly and divided axially intonodes.The delta-kW/ft model uses the maximum change inLHGR as a potential indicator of a PCI violation. Thedisadvantage to this approach is that it may ignore arelatively small change in LHGR that could indicate a PCIviolation if the LHGR was already near the limit.Additionally, this approach could incorrectly recognize arelatively large change in LHGR as a potential PCI issue,even though the large change occurred in a pin that wasoperating well below violation limits. This is illustrated inFig. 1.To compensate for these shortcomings, conservativevalues for the limiting delta-kW/ft are typically applied.Although this does lend itself to mitigating PCI concernsduring a power maneuver, for example, it can beeconomically inefficient, requiring a slower return topower than is actually needed to preclude PCI.The advantage of the delta-kW/ft model is that data isreadily available from the core simulation neutronicmodels, with relatively fast computational times.II. B. Threshold LHGR PCI ModelA more sophisticated method of monitoring PCI usingLHGR data is to account for the physical phenomenon ofpin conditioning, the time-dependent annealing that occurswhen a pin LHGR remains at the same power for anextended period. By modeling conditioning, greaterchanges in LHGR are achievable without violating PCIlimits, since the pellet and cladding conditioning arefactored into the calculation.Fig. 2 presents a graphical depiction of how athreshold model works. The lower dashed line representsthe conditioning threshold. The upper dashed line is thePCI violation limit, which is a delta above the PCIthreshold. If, during plant operation, a pin LHGR valueexceeds the threshold, "conditioning" begins for that pin.During conditioning the threshold increases and, since thePCI limit is a fixed delta above the threshold, the PCIviolation limit also increases. Provided the LHGRincreases at a rate slow enough to remain below the PCIlimit, no violation occurs. However, if the LHGR changesmore rapidly than the threshold changes, a PCI violationmay occur.
Proceedings of 2010 LWR Fuel Performance/TopFuel/WRFPMOrlando, Florida, USA, September 26-29, 2010Paper 097fuel performance perspective are maximum localconcentrated clad hoop stress and maximum local cladcrack length.Clad Hoop Stress - is the stress that forces a crack inthe clad wall to “open up” and propagate through thecladding. Formation of a crack is a prerequisite toPCI and is therefore an ideal indication of thesusceptibility of a fuel pin to PCI failure. However,PCI failure will not necessarily occur only byformation of the crack by itself.Fig. 2. Threshold PCI Model using LHGR dataConversely, once the LHGR is above the thresholdand then begins to decrease, the threshold and PCIviolation limits begin to decrease or de-condition.The drawback with this particular model for PWRs isthat the threshold level, the PCI violation limit delta, andthe conditioning & de-conditioning rates are not typicallysupplied from the fuel vendor. They can be determinedfrom detailed thermo-mechanical fuel performancecalculations or via experimental data. Additionally,detailed neutronic data of the actual LHGR's duringoperation has previously been difficult to obtain.II. C. Detailed Thermo-Mechanical FuelPerformance AnalysisWhen the LHGR of a fuel pin increases, the fuel pinbecomes prone to a failure mechanism known as PCI. Thespecific mechanism is referred to as stress corrosioncracking (SCC) and occurs at the inner wall of the fuel pincladding. Although the power increase causes a differentialthermal expansion in the fuel clad, which in turn generatesa tensile stress in the cladding (hoop stress), a certain finalpower level is also necessary for failure to occur.This behavior implies that a high temperature in thefuel, leading to the release of chemically aggressive fissionproducts (such as iodine), is also required for PCI. Thefailure is not immediate, suggesting that the crack has todevelop and propagate until the crack length is greater thanthe cladding thickness, requiring that the fuel pin be atpower for a certain period of time.Clad Yield Stress - the point at which the materialmoves from elastic to plastic deformation, i.e., doesnot "recover" when the stress is removed. In thisanalysis the difference in the calculated clad hoopstress to clad yield stress, or margin to clad failure, isused. The clad yield stress limit is dependent on thefuel design and conditions during plant operation.Clad Crack Length – Once a crack has formed in theclad wall, the length and growth of the crack is adirect indication of the potential for a PCI failure.However, the failure threshold occurs when thecalculated crack length exceeds the clad wallthickness.It is important to note that the clad hoop stress andclad crack length are very much dependent on theirradiation history (LHGR vs. exposure) as well as thelocal LHGR of every fuel pin. Therefore, a full powerhistory (LHGR vs. time) of every fuel pin for the entiretime it has been in the reactor core must be modeled as partof the PCI assessment. (Note this is for Zircaloy clad fueland is not necessarily applicable for other claddingmaterial.)The deficiency with this approach has been in itsapplication; typically only the most limiting pins from acore neutronic simulation are selected for this morethorough fuel performance analysis. This brings intoquestion the criteria used to select the candidate pins.During an actual operational transient it is very difficult toset inclusive criteria to ensure the most limiting pins arecorrectly chosen. As with the other methods, until now,there has not been an efficient way to perform this morerobust assessment during plant operations.Therefore, in order to properly assess vulnerability toPCI due to local power changes, the key indicators from a
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