The goal of the benchmark is to determine the uncertainty in light water reactor (LWR) systems and processes in all stages of calculations. It is estimated through a simulation process of ten exercises in three phases provided by the benchmarking framework.
There had been an increasing demand from nuclear research, industry, safety and regulation for best estimate predictions to be provided with their confidence bounds. Consequently, an in-depth discussion on uncertainty analysis in modelling was organised at the June 2005 Nuclear Science Committee (NSC) meeting, with three presentations covering relevant topics. Furthermore, discussions were held at the 2005 International Conference on Mathematics and Computational Methods Applied to Nuclear Science and Engineering (M&C 2005) in Avignon and the Washington American Nuclear Society (ANS) meetings, which led to a proposal for launching an Expert Group on Uncertainty Analysis in Modelling. Following the endorsement by the Nuclear Science Committee (NSC), a workshop on Uncertainty Analysis in Modelling UAM-2006 was held from 28-29 April 2006 at the University of Pisa, Italy to define future actions and a programme of work. Following the presentation made by Professor J. Aragonés of the results from the workshop at the 1-2 June 2006 meeting of the NSC, the creation of the Expert Group on Uncertainty Analysis in Modelling (UAM) was endorsed. This expert group reported to the Working Party on Scientific issues in Reactor Systems (WPRS). Because it addresses multi-scale/multi-physics aspects of uncertainty analysis, it also worked in close co-ordination with the benchmark groups on coupled neutronics-thermal-hydraulics and coupled core-plant problems, and the Committee on the Safety of Nuclear Installations (CSNI) Working Group on the Analysis and Management of Accidents (WGAMA). After a restructuring of WPRS, the LWR-UAM benchmark is nowadays supervised by the EGPRS , EGMUP and EGTHM expert groups of the WPRS .
The first phase focuses on understanding uncertainties in prediction of key reactor core parameters associated with LWR stand-alone neutronics core simulation. Such uncertainties normally occur due to input data uncertainties, modelling errors and numerical approximations. Input data for core neutronics calculations primarily include the lattice averaged few group cross-sections.
The second phase focuses on the prediction of key reactor parameters associated with LWR stand-alone simulations. Fuel performance, thermal-hydraulics and neutron kinetics simulations are all included without considering any coupling effects between the three physics models.
The third phase focuses on the prediction of key reactor parameters associated with LWR multi-physics simulations. Coupled fuel rod, thermal-hydraulics and neutron kinetics simulations were included with taking into account coupling/feedback effects between the three phenomena.
The exercises in all phases are based on three main types of LWRs selected in UAM: a pressurised water reactor (PWR), a boiling water reactor (BWR) and a water-water energetic reactor (VVER).
Kostadin Ivanov, North Carolina State University, USA
Eric Royer, CEA, France
Maria Avramova, North Carolina State University, USA
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|Oliver Buss (WPRS Secretariat)
Reference systems and scenarios for coupled code analysis were defined to study the uncertainty effects in all stages of calculations. Measured data from plant operation were available for the chosen scenarios.
The coupled code transient benchmarks developed, such as the BWR Turbine Trip (TT), VVER-1000 Coolant Transients (V1000CT) and BWR Full Bundle Test (BFBT) were used as a framework for the uncertainty analysis in best-estimate modelling for design and operation of LWRs. Such an approach facilitated the proposed benchmark activities since many organisations had already developed input decks and tested their codes on the above mentioned coupled code benchmarks.
From the LWR transient benchmark problems, the Peach Bottom 2 BWR Turbine Trip was chosen as the first reference system-scenario, although provisions were made to address the other LWR systems and scenarios such as TMI-1 PWR MSLB, PWR-RIA-ATWS, BWR-CRDA-ATWS (with boron modelling), VVER-1000CT, etc. The Peach Bottom 2 BWR Turbine Trip is well documented, not only in the NEA/NRC BWR TT benchmark specifications, but also in a series of EPRI and Peco reports, which include the design, operation and measured steady-state and transient neutronics and thermal-hydraulics data. The presence of cycle depletion, steady-state and transient measured data on both the integral parameter level and the local distribution level is a very important feature of the Peach Bottom 2 BWR Turbine Trip.
The interaction was made with the OECD/NEA/NRC BWR Full Bundle Test (BFBT) benchmark and the uncertainty analysis exercises performed in its framework. The interaction was also be extended to the CSNI BEMUSE-3 benchmark through the NEA internal co-operation among the NSC and CSNI Committees.
The idea was to:
The investigation of uncertainty effects was initiated for each step of calculation and therefore it is proposed to have a sequence of exercises as described below:
The recommendation was to use experimental data as much as possible (two interactions with known experimental data are indicated above, but others could be added). The host institution identified input (I), output (O) or target of the analysis, as well as assumptions for each step and target uncertainty parameters (U). The uncertainty from one step would be propagated to the others (as much as it was feasible and realistic).
The above-described approach based on the introduction of the ten exercises allowed the development of a benchmark framework which mixed information from the available integral facility and nuclear power plant (NPP) experimental data with analytical and numerical benchmarking. Such an approach compared and assessed the uncertainty methods on representative applications and simultaneously benefitted from different approaches to be able to make recommendations and provide guidelines.
Global Sensitivity Analysis - The Primer, by A. Saltelli, M. Ratto, T. Andres, F. Campolongo, J. Cariboni, D. Gatelli, M. Saisana, S. Tarantola, Wiley, 2008, ISBN 978-0-470-05997-5
The Working Party on Scientific Issues and Uncertainty Analysis of Reactor Systems (WPRS) and its expert groups co-ordinate benchmark activities on Reactor Single- and Multi-Physics. The benchmark activities serve the WPRS objectives to:
The activities address the the following reactor types: