Uncertainty Analysis of the 35% Reactor Inlet Header Break in a CANDU 6 Reactor Using RELAP/SCDAPSIM/MOD4.0 with Integrated Uncertainty Analysis Option
NURETH-14 - 2011 September 25-30

Presented at:
2011 September 25-30
Toronto, Canada
Session Title:
O4-2 BEPU (Best Estimate code Plus Uncertainty) method, CSAU, Statistical Methods

Daniel Dupleac (Politehnica University of Bucarest)
Marina Pérez (Technical University of Catalonia)
Francesc Reventós (Universitat Politécnica de Catalunya)
Chris Allison (Innovative Systems Software)


The RELAP/SCDAPSIM/MOD4.0 code, designed to predict the behavior of reactor systems during normal and accident conditions, is being developed as part of an international nuclear technology Software Development and Training Program (SDTP). RELAP/SCDAPSIM/MOD4.0, which is the first version of RELAP5 completely rewritten to FORTRAN 90/95/2000 standards, uses the publicly available RELAP5 and SCDAP models in combination with (a) advanced programming and numerical techniques, (b) advanced SDTP-member-developed models for LWR, HWR, and research reactor analysis, and (c) a variety of other member-developed computational packages. One such computational package is an integrated uncertainty analysis (IUA) package being developed jointly by the Technical University of Catalonia (UPC) and Innovative Systems Software (ISS).

RELAP/SCDAPSIM/MOD4.0(IUA) follows the input-propagation approach using probability distribution functions to define the uncertainty of the input parameters. The main steps for this type of methodologies, often referred as to statistical approaches or Wilks’ methods, are the ones that follow:

  1. Selection of the plant;
  2. Selection of the scenario;
  3. Selection of the safety criteria;
  4. Identification and ranking of the relevant phenomena based on the safety criteria;
  5. Selection of the appropriate code parameters to represent those phenomena;
  6. Association of uncertainty by means of Probability Distribution Functions (PDFs) for each selected parameter;
  7. Random sampling of the selected parameters according to its PDF and performing multiple computer runs to obtain uncertainty bands with a certain percentile and confidence level;
  8. Processing the results of the multiple computer runs to estimate the uncertainty bands for the computed quantities associated with the selected safety criteria.

RELAP/SCDAPSIM/MOD4.0(IUA) calculates the number of required code runs given the desired percentile and confidence level, performs the sampling process for the user-selected parameters including code embedded correlations and models such as the wall-to-fluid heat transfer package, executes the code runs to propagate the uncertainty through, and processes the output data to derive the tolerance interval defining the uncertainty region.

This paper includes a:

  1. Brief description of RELAP/SCDAPSIM/MOD4.0(IUA) and associated CANDU 6 base input model for the 35% reactor inlet header (RIH) break scenario,
  2. Discussion of the safety criteria, relevant phenomena, and associated code parameters used in the uncertainty analysis,
  3. Discussion and conclusions for the RIH transient and associated uncertainties.

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