Expert Group on Uncertainty Analysis in Modelling (UAM-LWR)

Coupled Multi-physics and Multi-scale LWR analysis

OECD Nuclear Energy Agency (NEA), North Carolina State University (NCSU)

In recent years there has 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 NSC meeting, with three presentations covering relevant topics. In addition, discussions were held at the Avignon M&C and the Washington ANS meetings, which led to a proposal for launching an Expert Group on "Uncertainty Analysis in Modelling". Following the endorsement by the OECD/NEA Nuclear Science Committee (NSC) a workshop on Uncertainty Analysis in Modelling UAM-2006 was held on 28-29 April 2006 at the University of Pisa, Italy with the aim of defining future actions and a programme of work. Following the presentation made by Prof. J. Aragon├ęs at the 1-2 June 2006 meeting of the OECD/NEA Nuclear Science Committee of the results from the workshop, the setting up of an Expert Group on Uncertainty Analysis in Modelling (UAM) was endorsed. This Expert Group will report to the Working Party on Scientific issues in Reactor Systems (WPRS) and because it addresses multi-scale / multi-physics aspects of uncertainty analysis, it works in close co-ordination with the benchmark groups on coupled neutronics-thermal-hydraulics and on coupled core-plant problems, and the CSNI Working Group on the Analysis and Management of Accidents (GAMA).


The expert group has the following mandate:

  1. To elaborate a state-of-the-art report on current status and needs of sensitivity and uncertainty analysis (SA/UA) in modelling, with emphasis on multi-physics (coupled) and multi-scale simulations.
  2. To identify the opportunities for international co-operation in this area that would benefit from coordination by the NEA/NSC.
  3. To draw a roadmap for the development and validation of the methods and codes required for uncertainty analysis including the benchmarks adequate to meet those ends, the schedule and organization of its realisation


To determine the uncertainty in LWR systems and processes in all stages of calculations.

Reference systems and scenarios

Reference systems and scenarios for coupled code analysis will be defined to study the uncertainty effects in all stages of calculations. Measured data from plant operation should be available for the chosen scenarios.

The already developed OECD/NEA/NSC coupled code transient benchmarks - such as the BWR Turbine Trip (TT), VVER-1000 Coolant Transients (V1000CT) and BWR Full Bundle Test (BFBT) - will be used as a framework for uncertainty analysis in best estimate modelling for design and operation of LWRs. Such an approach will facilitate the proposed benchmark activities since many organisations have already developed input decks and tested their codes on the above mentioned coupled code benchmarks.

From these OECD LWR transient benchmark problems, the Peach Bottom 2 BWR Turbine Trip was chosen as the first reference system-scenario, although provisions will be 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 OECD/NEA/NRC BWR TT benchmark specifications but also in a series of EPRI and PECo reports, which include 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.

Interaction will be made with the OECD/NEA/NRC BWR Full Bundle Test (BFBT) benchmark and the uncertainty analysis exercises performed in its framework.  The interaction will also be extended to the ongoing NEA/CSNI BEMUSE-3 benchmark through the NEA internal co-operation among the NSC and CSNI Committees.

The idea is to:

  1. subdivide the complex system/scenario into several steps (exercises);
  2. identify input, output and assumptions for each step;
  3. calculate the uncertainty in each step;
  4. propagate the uncertainty for the evaluation of the overall system/scenario.

The investigation of uncertainty effects is undertaken for each step of calculation and therefore it is proposed to have a sequence of exercises as described below:

  1. Derivation of the multi-group microscopic cross-section libraries (nuclear data, selection of multi-group structure, etc.).
  2. Derivation of the few-group macroscopic cross-section libraries (energy collapsing, spatial homogenisation, etc.).
  3. Criticality (steady state) stand-alone neutronics calculations (keff calculations, diffusion approximation, etc.).
  4. Fuel thermal properties relevant for transients performance.
  5. Neutron kinetics stand-alone performance (kinetics data, space-time dependence treatment, etc.) in PWR rod ejection and BWR control rod drop accidents.
  6. Thermal-hydraulic fuel bundle performance - interaction with the OECD/NRC BFBT benchmark and the available experimental data as well as the Uncertainty Analysis Exercises being performed in the framework of the BFBT benchmark.
  7. Coupled neutronics/thermal-hydraulics core performance(coupled steady state, coupled depletion, and coupled core transient with boundary conditions) - interaction with the Peach Bottom Cycles 1, 2 and 3 operating and measured data.
  8. Thermal-hydraulics BWR system performance - interaction with the Peach Bottom Turbine Trip and BEMUSE-3 experimental data.
  9. Coupled neutronics kinetics thermal-hydraulic core/thermal-hydraulic system BWR performance - interaction with the Peach Bottom Turbine Trip experimental data and Peach Bottom stability performance - interaction with EOC2 and EOC3 experimental data.

It is recommended to use experimental data as much as possible (two "interactions" with 'known' experimental data are indicated above but others can be added). The Host Institution shall identify 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 should be propagated to the others (as much as feasible and realistic).

The above described approach based on the introduction of nine steps (exercises) will allow to develop a benchmark framework which mixes information from the available integral facility and NPP experimental data with analytical and numerical benchmarking. Such an approach will compare and assess the current uncertainty methods on representative applications and simultaneously will benefit from different approaches to arrive at recommendations and guidelines.

These nine steps (exercises) will be carried out in three phases each covering a two-year period. The first phase will include the first three exercises (neutronics), with the final specifications discussed and adopted at a first workshop, held in May 2007 in Paris, France. A second workshop is planned for 2-4 April 2008 to discusss the preliminary results of phase I, the output parameters and formats for phase II and the prioriities for phases II and III. The 2008 workshop will be held in Garching, Germany.

The work of the group will address mainly the scientific aspects of the methodologies being developed and demonstrate their validity. This work will interface with the activities of the CSNI who would later address any licensing issues.

To summarise, uncertainty analysis in modelling (UAM) is to be further developed and validated on scientific grounds in support of its performance, in addition to LWR best-estimate calculations for design and safety analysis. There is a need for efficient and powerful analysis methods suitable for such complex coupled multi-physics and multi-scale simulations. The proposed sequence of benchmarks will address this need by combining the expertise in reactor physics, thermal-hydraulics etc. and uncertainty and sensitivity analysis, and will contribute to the introduction of advanced/optimised uncertainty methods in best-estimate reactor simulations. Such a task can only be undertaken within the framework of a programme of international co-operation that would benefit from the coordination of the NEA/NSC and from interfacing with the CSNI activities.

Exercises programmed for UAM Expert Group on Uncertainty Analysis in Modelling

Phase I (Neutronics Phase)

Phase II (Core Phase)

Phase III (System Phase)

For the core and systems applications three main LWRs types are selected, based on previous benchmark experiences and available data: BWR (Peach Bottom-2),PWR TMI and VVER-1000 (Kozloduy-6, Kalinin-3).

History / Workshops:

In-depth discussion on uncertainty analysis in modelling, NSC meeting, June 2005

Related data

Please note all files related to this benchmark are password protected

Conditions for Release of the LWR-UAM data

Related expert groups

Related workshops

Verification and Validation for Nuclear Systems Analysis Workshop, July 21st - July 25, 2008, Center for Higher Education, Idaho Falls, ID, USA

Related links

Recently published books of relevance for the project:

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


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Last reviewed: 29 November 2019