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Proceedings of 2010 LWR Fuel Performance/TopFuel/WRFPMOrlando, Florida, USA, September 26-29, 2010Paper 097III. OPEN VALVE TEST EVENT DESCRIPTIONTo demonstrate the application of the various PCIevaluation methods to actual operating data, a specificevent from an operating PWR in Southern Nuclear's fleetwas selected. The event is called an open valve test. Thetest itself should result in fairly mild changes in LHGRvalues. However, upon completion of the test in the actualplant, coolant off-gas activity increased indicating fuelfailure. The analysis was motivated by the need toeliminate PCI as a possible cause.In an open valve test the reactor core inlet coolanttemperature is reduced to facilitate testing on thesecondary side. The temperature reduction is mild, on theorder of a few degrees.This particular event was conducted near BOC at fullreactor power (HFP). The MTC for the core at thiscondition was slightly negative in value (meaning astemperature decreases positive reactivity is inserted). Inorder for the reactor to remain critical this positivereactivity insertion must be balanced with the insertion ofnegative reactivity by partially inserting the lead controlbank. During this event, the lead bank was partiallyinserted approximately 10% into the core.Once the testing on the secondary side is complete, thelead control bank is withdrawn to a full out position as theinlet coolant temperature is slowly raised back to nominal.During this movement of the lead control bank at HFPLHGR is redistributed axially and radially within the core.Since PCI is dependent on changes in LHGR vs. time, thisevent was chosen to apply the various PCI methodspreviously discussed.IV. ANALYSIS MODELS OF SPECIFIC OPERATINGEVENTS DESCRIPTIONIn order to assess PCI during operating events, severalcalculational components are necessary: an accurate threedimensional,cycle-specific core neutronic model, detailedcore follow data for the actual transients, an accuratethermo-mechanical fuel performance code, a systemcoupling these models together, and a data managementsystem to digest and resolve all the data points into asuccinct summary for the events. This section describesthese analysis models.IV. A. CMS Model DescriptionA cycle-specific CMS neutronics model, comprised ofStudsvik’s CASMO 1 and SIMULATE 2 codes, has beenused for several cycles at Southern Nuclear for fuel vendoroversight, operational support, and operator simulatortraining. These models employ CASMO as the crosssectionlattice code and SIMULATE as the steady-statenodal simulator.IV. B. CMSOps Model DescriptionStudsvik’s CMSOps 3 was used to integrate detailedplant operational data during the events analyzed withinthis paper 4 . CMSOps manages the high-fidelity CMSneutronics model for reactivity management, including athreshold based PCI assessment.CMSOps ensures the CMS core neutronics modelaccurately reflects the plant’s actual detailed operatinghistory by automating core follow. CMSOps reads plantmeasured signals on a minute-by-minute frequency anddetects any changes in the values of these signals. Basedon certain change criteria being met (e.g., changes inpower level, lead control bank position, inlet temperaturechange, etc.), CMSOps automatically activates executionand updating of the cycle-specific SIMULATE model.CMSOps is built on a very efficient database thatarchives all of the measured signals and all calculatedneutronic parameters, making it easy to assess PCI usingother techniques, such as the delta-kW/ft and the thresholdmodels.Studsvik, with assistance from Southern's engineers,implemented CMSOps for the plant in which the operatingevent (open valve test) was conducted. This event resultedin several SIMULATE calculations over a small amount oftime (~8 hours), each of which produced detailed threedimensionalLHGR pin-maps.IV. C. ENIGMA Thermo-Mechanical FuelPerformance Model DescriptionThe UK National Nuclear Laboratory (NNL)participated in the analysis by evaluating the CMS datafrom the operational event using their fuel performancecode, ENIGMA 5 .
Proceedings of 2010 LWR Fuel Performance/TopFuel/WRFPMOrlando, Florida, USA, September 26-29, 2010Paper 097ENIGMA is a fully validated fuel performance codethat has been used for several definitive design andlicensing assessments for both UO2 and MOX fuel,including licensing UO2 and gadolinia-doped UO2 inSizewell B (the UK’s only PWR) and the Finnish LoviisaVVER-440 reactor, and mixed oxide (MOX) fuel in theSwiss Beznau-1 PWR. ENIGMA is also used in support ofMOX production in the Sellafield MOX Plant (SMP) andto perform feasibility studies for advanced fuel concepts.Recently NNL and Studsvik have coupled the CMScore neutronics model and ENIGMA’s fuel performanceanalysis capabilities, producing ONUS 6,7 to performthermo-mechanical analysis of every fuel pin in the core.Since CMSOps automatically generates CMS casesfor the entire operational event, detailed LHGR's generatedare available for every fuel pin and axial pin node in thecore. This data was coupled directly with ENIGMA toperform PCI evaluation for every pin in the core, using itsdetailed power history. ONUS is also then used to displaythe 3-D pin-by-pin data available following the analysis.V. PCI RESULTS FROM OPEN VALVE TESTThis event provided an opportunity to explore thevarious ways of monitoring PCI limits in order to mitigatePCI violations. The three various methods explainedpreviously are applied.Fig.3. presents a graphical result from ONUS(ENIGMA/CMS), of the normalized, axially integrated(radial) pin-by-pin LHGR at the point of deepest insertionof the lead control bank. The core loading pattern is easilyrecognized as a contemporary low-leakage one.Fig.3. Relative Power Density Distribution and LeadControl Bank Locations during Event (Circles)Note also the locations of the lead control bank, whichis partially inserted during the open valve test. Since thelocations are in relatively high-powered regions, theconcern of PCI during the event is warranted.V. A. Delta-kW/ft Results for Open Valve TestUsing data from the open valve test event, the deltakW/ftanalysis was applied to assess margin to PCI limit.This analysis calculated delta-kW/ft/hr, as compared to theimmediately previous case, calculated with data for everynode in the core, as shown in Fig. 4. and Fig. 5.Fig. 4. Nodal delta-kW/ft/hr during Open Valve Test vs.Exposure..
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