SAFER AND FASTER OUTAGES WITH MODERN PREDICTION TOOLS

摘要

Fuel assembly (FA) bow in pressurized waterreactors (PWR) is the result of a complex interactionbetween mechanical, hydraulic, and irradiation effects. Itis a general phenomenon in PWR. Increased FA bow cannegatively influence the handling of the fuel. During coreloading and unloading, increased fuel assembly bowcould lead to higher handling loads and time delays of theoutage. For extreme cases damage of the FA is possible.In the framework of nuclear safety and plant operability,these consequences must be prevented.Framatome has developed state-of-the-art simulationtools to predict the core-wide bow deformation. Thesetools rely on the coupling of a hydraulic model, whichcalculates the flow field and fluid-induced loads with adetailed mechanical model. The equilibrium of the fluidstructure-interaction is solved along the operation cycletime. All relevant effects such as creep and nonlinearitiesare considered. Framatome’s FA bow prediction toolshave been validated on full-scale laboratory flow testsand numerous predictions for actual cycles in variousplants. Based on the initial core bow conditions, the endof-cycle bow deformation of all FAs can be calculated. Anexcellent agreement of calculated and measured bowdeformation has been demonstrated.Framatome has developed a complementary tool(COLOSS), which can be used to calculate the frictionforces when a fuel assembly is loaded into or unloadedfrom the core. This tool considers the bow deformation,the burnup dependent FA stiffness, and the loading stateof the core. It can be applied for different FA designsincluding dummy FA, different plant types and specificfeatures of the FA loading machine. It calculates thefriction forces for single loading steps and for partial andcomplete loading sequences.The bow prediction and core loading tools have beenapplied over several years in the German PWR Brokdorfto facilitate the core loading and unloading. Based onprediction data or measurement data for FA bow, anoptimized strategy to unload and to reload the core hasbeen devised for the annual outage. With this optimizedcore unloading/loading sequence, critical situationswhich can lead to handling delays or even damage wereavoided. With the application of the tools a significantamount of core loading and unloading time could besaved. The good quality of the bow prediction also led toa reduction of the number of FA straightnessmeasurements.The development and application of the tools is anexcellent example of using state-of-the-art methods and ofthe close and successful cooperation between fuel vendorand utility.