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Analysis of the instability event occurred on February 25, 1999 at the Oskarshamn-2 NPP using the advanced 3D Neutron Kinetic and Thermal Hydraulic coupled code RELAP5-3D©/PHISICS

March 15 @ 4:00 pm - 5:00 pm

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Paolo Balestra
Post Doc
North Carolina State University 


Several OECD/NEA benchmarks carried out in the past have established the validity of the 3D neutron kinetic thermal hydraulic codes for the simulation of most of the anticipated operational occurrences for light water reactors. The OECD/NEA proposed an international benchmark based on the data collected during an instability transient occurred in the Oskarshamn-2 NPP. This benchmark is aimed at testing the coupled codes on more challenging situations like diverging and unstable BWR power oscillations, with and without scram occurrence. The research activities have been carried out in this framework with the scope of developing and validating a 3D neutron kinetic and thermal hydraulic model capable of simulating such a complex scenario. The reference system code used for the analysis was RELAP5-3D©/PHISICS. Since PHISICS is a tool still under development and validation, the RELAP5-3D©/NESTLE code was also used for supporting the research activities. Models validation has been performed through the plant data obtained during the February 25, 1999 instability event occurred at the Oskarshamn-2 nuclear power plant, Sweden. The first part of the research effort has been devoted to the development and validation of system codes input decks and auxiliary tools. Dedicated software tools have been developed for automatically generate large input decks and cross section libraries and to easily post-process the coupled codes output. Estimation of the stability parameters (oscillation frequency and decay ratio) was achieved using two different methodologies (Graphical and ARMA model estimation) which were implemented in an in-house developed tool (FEDR) and validated using the data of the OECD/NEA “Forshmark 1&2” stability benchmark. The second part of the research activities was devoted to the steady-state and on-transient validations, resulting in the identification of the most important parameters for the reactor stability prediction. The most critical issue for the simulation models is the numerical diffusion, which has been investigated by a systematic study on the effects of the core active zone axial meshing. Then an ad-hoc scheme for minimizing numerical diffusion was implemented in the RELAP5-3D© input deck. Finally, the RELAP5-3D©/PHISICS model has been used to perform stability tests and to calculate the evolution of the stability parameters; to study the instability transient without scram; and to study the limit cycle oscillations. The results contributed to the PHISICS code assessment and allowed to identify code improvements.


Mr. Paolo Balestra got his  Master degree in energy engineering (2012) and PhD in energy and environment with focus on nuclear engineering (2017) at the “Sapienza” university of Rome. Currently he is working as a postdoctoral research fellow at the nuclear engineering department of the North Carolina state university conducting research on modeling development and validation as well as developing new multi-physics coupling schemes for the analysis of the existing light water reactor fleet and the assessment of new reactor designs. During his PhD he participated in collaboration with the Italian Energy Agency ENEA to many international projects devoted to the validation of 3D Neutron Kinetic and thermal hydraulic system codes for light water reactors and sodium cooled fast reactors such as and not limited to, the EU FP7 funded project “JASMIN” (2013-2015),  the IAEA Benchmark analysis for the EBR-II Shutdown Heat Removal Tests (2014-2016) or the OECD/NEA Oskarshmn-2 instability benchmark (2013-2016). Since the 2013 he is part of team of developers of the Idaho National Laboratory advanced thermal hydraulic and 3D neutron kinetic coupled code RELAP5-3D©/PHISICS. More recently he worked as a subcontractor at INL for the High-Temperature Gas-Cooled Reactor Methods and Simulation group to implement new modules in the RELAP5-3D©/PHISICS code such as: a new adaptive time step scheme, a perturbation theory module and a new quasi static module (2016-2017). Author of more than 8 papers in peer-reviewed International conferences and 4 publications in Journals.


March 15
4:00 pm - 5:00 pm
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1202 Burlington Labs
2500 Stinson Drive
Raleigh, NC 27695-7909 United States
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