May-June 2019 NPJ

30 NuclearPlantJournal.com Nuclear Plant Journal, May-June 2019 CASL... ( Continued from page 29) displays the VERA simulation of crud buildup for Watts Bar 1 which is resolved in detail to the level of all individual fuel rods within the reactor core. Through an improved understanding of reactor operating behavior, especially as relates to the modeling of complex phenomena as embodied in the challenge problems, higher confidence in predicted reactor operational performance is achieved. b. What measures are being taken to increase the efficiency of nuclear power plants? The transfer of VERA technology to the commercial nuclear power industry is well underway and is based on close collaboration with nuclear utilities, fuel vendors, and nuclear service providers who are working with CASL through the VERA Users Group. VERA has been applied to a range of applications that includes core design, core follow, plant lifetime assessment, reactor safety assessments, advanced fuels, and refueling outage support. To date, VERA has been validated on over 150 fuel cycles representing a broad spectrum of plant types, fuel designs, and operating conditions for the current and future operating fleet. Industry applications of VERA are being driven by the CASL challenge problems as well as new, high-value applications, such as the introduction of advanced fuels, which hold the promise of increased reactor power output, increased operating flexibility, improved safety margins, and reduced fuel cycle costs. It is noted that many of the applications are based on first-of-a-kind analyses made possible by the fully integrated modeling and simulation capabilities of VERA. The use of VERA high fidelity simulations with quantified uncertainties will help identify and reduce the inherent conservatisms in existing analyses which will allow for additional margin within reactor core designs and flexibility during plant operations. c. How has the cost efficiency been achieved in current fuels in existing plants and also what new fuels are being developed to optimize plant efficiency? The challenge problems related to reactor operations, including crud, GTRF, and PCI have a direct impact on operational and fuel costs as well as power production. As an example, the need to minimize the operational risks associated with crud (CIPS and CILC) for many PWRs necessitates design of reactor cores with flatter power distributions as a means to limit the amount of subcooled boiling (a key driver of crud formation). The greater the uncertainty in crud formation the greater the accommodation that must be made within the core design to assure reactor operational performance. Such designs can be costly due to a need to increase the fuel loading which results in reduced fuel utilization and efficiency. A secondary effect is often increased core leakage that results in higher vessel fluence which, over the long term, can decrease plant lifetime. Accurate prediction of crud formation and its impact on core behavior as afforded by VERA allows designers and operators to be much better-informed regarding risk enabling improved core designs and thus decreased plant costs. CASL is working with industry on modeling of advanced fuels with VERA including coated clads, doped fuel pellets, and alternate fuel forms such as FeCrAl clad and U3Si2 fuel pellets. Also, considered are increases in fuel enrichment as well as burnup beyond current licensing limits. Each of these have or are currently being assessed with respect to improvements in reactor performance and energy production via industry partners. As an example, the extension of currently operating 4-loop Westinghouse PWRs using advanced fuels from an 18 month to 24 month refueling cycle for reload core fractions of less than 50% would have significant economic benefit to those class of reactors within the operating fleet. d. Describe any other areas of research to reduce plant downtime and reduce O&M costs for the plant. Load follow operation for reactors is becoming increasingly common and VERA has been used to model such Figure 1.