Benchmark for Uncertainty Analysis in Best-Estimate Modelling for Design, Operation and Safety Analysis of Light Water Reactors (LWR-UAM)

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.

Coupled Multiphysics and Multi-Scale Light Water Reactor Analysis

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 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 WPRS  .

Benchmark Phases

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.

  • Phase I (Multi-Scale Standalone Neutronics Static Phase) ​
    • Exercise I-1: “Cell Physics”​
    • Exercise I-2: “Lattice Physics”​
    • Exercise I-3: “Core Physics”

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.

  • Phase II (Core Phase – Introduce Other Physics in the Core and Time Dependence)​
    • Exercise II-1: “Fuel Physics”​
    • Exercise II-2: “Time Dependent Neutronics”​
    • Exercise II-3: “Bundle Thermal-Hydraulics”  

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.

  • Phase III (System Phase – Introduce Multi-Physics and Core-System Coupling) ​
    • Exercise III-1: “Core Multi-Physics”​
    • Exercise III-2: “System Thermal-Hydraulics”​
    • Exercise III-3: “Coupled Core-System”​
    • Exercise III-4: “Comparison of BEPU vs. Conservative Calculations”​

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). 


Benchmark Organisation

Benchmark coordinators

Kostadin Ivanov, North Carolina State University, USA

Eric Royer, CEA, France

Maria Avramova, North Carolina State University, USA


Access to the benchmark is open to all OECD/NEA member countries and it requires only acceptance of the benchmark conditions. Please sign the conditions form and send it to the WPRS Secretariat

Working Area

The participants' working area is now hosted on MyNEA SharePoint 

Please follow these steps to sign in to MyNEA SharePoint:

  1. On the SharePoint sign in page, enter your email address as your username. Then type in your NEA password (reminder).
  2. A two-factor authentication is needed to log in. Select ‘MyNEA Access Authentication Agent 3.1’ as next step, which is the service for delegates.
  3. You will receive an email with an additional access code. Enter the access code on the website.

The MyNEA End User Guide is available online here: User's guide: myNEA

NEA Secretariat Oliver Buss (WPRS Secretariat)
Publications and reports

Reference Systems and Scenarios

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:

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

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:

  1. derivation of the multi-group microscopic cross-section libraries (nuclear data, selection of multi-group structure)
  2. derivation of the few-group macroscopic cross-section libraries (energy collapsing, spatial homogenisation.)
  3. criticality (steady-state) stand-alone neutronics calculations (keff calculations, diffusion approximation)
  4. fuel thermal properties relevant for transients performance
  5. neutron kinetics stand-alone performance (kinetics data, space-time dependence treatment) 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

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.

Related Data

External Publication

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