Abstract
Current industry practice in fuel licensing often relies on thermo-mechanical modeling of a fuel rod with an artificially constructed bounding power history. The benefit of this approach is that it is computationally efficient; however, the drawbacks are that 1) such an approach is not always conservative, for instance when modelling phenomena related to late onset pellet-clad gap closure; and 2) it can poorly estimate available safety margins for fuel operating at high local power densities and/or to high burnup. For these reasons NNL developed an in-house whole-core fuel performance framework – NEXUS – to enable modelling of all fuel rods in the core using the ENIGMA fuel performance code and computed power histories from core simulation packages (currently limited to PARCS or SIMULATE). One of the main objectives was to create a tool that was both computationally efficient and user friendly. The former was achieved by making use of parallelisable architecture, while the latter was achieved by minimising necessary user input and providing tools for easy interrogation of the fuel performance output. NEXUS has been applied to several LWR operational scenarios, which we summarise in this paper, including steady-state operation of an ABWR, and a rod ejection accident in a small modular soluble boron free PWR and a GWe-class PWR. We also summarise current development activities related to integrating NNL’s in-house fuel performance Monte Carlo uncertainty analysis software CASINO into the NEXUS framework.
Highlights
There has been a reliance in using artificially constructed bounding power histories for fuel performance calculations related to fuel licensing
As part of a detailed licensing assessment it would be necessary to consider a range of cases for the small soluble boron free (SBF) PWR, including the REA initiating from hot full power (HFP) and throughout the cycle to ensure the most limiting case was captured
Developments within NEXUS are planned to allow NEXUS to automatically output complete ENIGMA input files for fuel pins that are most limiting with respect to certain design criteria to allow easy assessment for rods most pertinent for uncertainty analysis
Summary
There has been a reliance in using artificially constructed bounding power histories for fuel performance calculations related to fuel licensing This approach is not always conservative, for fuel phenomena related to late onset of pellet-clad gap closure. To overcome this limitation alternative artificial power histories are often employed (for example cycleaverage power histories, as was the case in licensing of the initial fuel cycles of the Sizewell B reactor) but such an approach can result in excessive conservatism, for instance when there is a large sensitivity to clad surface temperature and local power density. Future areas of development of NEXUS to enable greater automation of fuel design and licensing and uncertainty analysis via Monte Carlo methods are detailed
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