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Towards a Consolidated Approach for the Assessment of Evaluation Models of Nuclear Power Reactors

A. Epiney, S. Canepa, O. Zerkak, H. Ferroukhi

Nuclear Technology / Volume 196 / Number 2 / November 2016 / Pages 223-237

Technical Paper /

First Online Publication:September 26, 2016
Updated:November 2, 2016

The STARS project at the Paul Scherrer Institut (PSI) has adopted the TRACE thermal-hydraulic code. For analyses involving interactions between system and core, a coupling of TRACE with the SIMULATE-3K (S3K) light water reactor (LWR) core simulator has been developed. In this configuration, the codes and associated simulation models play a central role to achieve a comprehensive safety analysis capability. Therefore, efforts have now been undertaken to consolidate the validation strategy by implementing a more rigorous and structured assessment approach for TRACE applications. The principle is to systematically track the evolution of a given set of predicted physical quantities of interest (QoIs) over a multidimensional parametric space. If properly set up, such environment should provide code developers and code users with persistent (less affected by user effect) and quantified information (sensitivity of QoIs) on the applicability of a simulation scheme (codes, methodology, and input models) for steady-state and transient analysis of full LWR systems. Through this, for each given transient/accident, critical paths of the validation process can be identified that could then translate into defining reference schemes to be applied for downstream predictive simulations. To illustrate this approach, this validation strategy is applied to an inadvertent blowdown event that occurred in a Swiss BWR/6. The transient was initiated by the spurious actuation of the automatic depressurization system. Here, the validation approach progresses through a number of dimensions: (a) different versions of the TRACE code; (b) the methodology dimension—in this case imposed power and updated TRACE core models are investigated; and (c) the nodalization dimension, where changes to the input model are assessed. For each step in each validation dimension, a common set of QoIs is investigated. For the steady-state results, these include fuel temperature distributions. For the transient part of the present study, the evaluated QoIs include the system pressure evolution and water carryover into the steam line. It has been seen that the improvements to the model predictions resulted in a small impact on the system pressure gradient, thus confirming a persistency of the downstream mechanical stress estimate, whereas the water carryover could vary by up to 150% as a function of the adopted simulation methodology.