68 Research focus and highlights – Nuclear energy and safety
PSI Scientific Highlights 2012
Improved temporal coupling schemes for thermalhydraulic/neutronic analysis of LWR transients Omar Zerkak, Damar Wicaksono, Konstantin Nikitin, Hakim Ferroukhi and Andreas Pautz, Laboratory for Reactor Physics and Systems Behaviour (LRS), PSI
The high-fidelity computational simulation of Light Water Reactor accident-scenario transients allows ultimately for a better evaluation of the performance and safety margins of these reactors. This requires the use of complex calculation methodologies based on the intricate space-time coupling of different computational models that were originally designed for standalone usage. In this context, the temporal coupling scheme of the thermal-hydraulic analysis code TRACE with the core simulator SIMULATE-3K has been revised for higher numerical accuracy and reduced CPU time. The improvements obtained with the coupling schemes have been verified on the basis of the analysis of a control rod ejection accident and a turbine trip event.
The STARS Project [Link 1] – Steady-state and Transient Analysis Research for the Swiss reactors – is an international
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effort, whose main partner is the Swiss Nuclear Inspectorate, ENSI. One central goal of the project is to improve space-time resolution in the simulation of accidental transients of the
CA
Swiss Light Water Reactors (LWRs), in order to more accu-
CB
rately evaluate how safe the reactors are. This requires the development of complex multi-physics computational
Convergence test
methodologies that operate an intricate space-time coupling
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of different simulation codes. The particular intention is to establish coupling schemes that provide more accurate and more efficient simulations when large detailed core models
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or very long transients are to be analysed. In 2012, emphasis was given to assessing enhanced temporal
Figure 1: Temporal coupling of two codes, CA and CB , based on a
coupling schemes for the TS3K coupled code, originally de-
Fixed Point Iteration method.
veloped by Studsvik Scandpower and maintained by PSI. TS3K implements space-time coupling of the thermal-hydraulics
enhancements were devised to improve the temporal coupling
(T-H) plant system code TRACE with the core transient simula-
in TS3K, with the objective of achieving a given target accu-
tion code SIMULATE-3K (S3K) [1]. The temporal coupling em-
racy using larger time-step sizes than with the conventional
ploys a conventional operator-splitting (OS) method, where
OS method. Semi-implicit methods are one option, but these
the T-H solver (TRACE) uses power densities evaluated in the
can lead to a deeper footprint in the source codes and result
core at the end of the previous time-step. The neutronics
in a heavy maintenance overhead, as the TRACE and S3K codes
solver (S3K) is provided afterwards with the new T-H feedback
are often modified by the developers. This prevents the use
quantities needed to compute the nuclear reactions cross-
of the most accurate schemes, such as Approximate Block
sections, thus completing the time-step integration of the
Newton methods. Thus, for practical reasons, the focus on
coupled problem. This results in a nearly fully explicit scheme
semi-implicit schemes had to be limited to the more workable
that exhibits poor numerical accuracy and a stability condi-
Fixed Point Iteration (FPI) methods, such as the one illus-
tioned by the time-step size.
trated in Figure 1, which shows an FPI coupling scheme at the
To overcome these limitations and following the experience
level of the solver outer-iterations of two codes, CA and CB.
gathered from participation in the EU Nuclear Reactor Inte-
Two temporal coupling schemes have been studied, namely:
grated Simulation Project (NURISP) [2], different possible
a) The application to the conventional OS of a weighted time-