Uncertainty and sensitivity analysis of a VVER-1000 start-up experiment using the coupled code DYN3D/ATHLET and the statistical code package SUSA


Uncertainty and sensitivity analysis of a VVER-1000 start-up experiment using the coupled code DYN3D/ATHLET and the statistical code package SUSA

Kliem, S.; Mittag, S.; Langenbuch, S.

The transition from the application of conservative models to the use of best-estimate models raises the question about the uncertainty of the obtained results. This question becomes especially important, if the best-estimate models should be used for safety analyses in the field of nuclear engineering. Different methodologies were developed to assess the uncertainty of the calculation results of computer simulation codes. One of them is the methodology developed by Gesellschaft fuer Anlagen- und Reaktorsicherheit (GRS) which uses the statistical code package SUSA. In the past, this methodology was applied to the calculation results of the advanced thermal hydraulic system code ATHLET. In the frame of the recently finished EU FP5 funded research project VALCO, that methodology was extended and successfully applied to different coupled code systems, including the uncertainty analysis for neutronics. These code systems consist of a thermal hydraulic system code and a 3D neutron kinetic core model. One of the code systems applied was ATHLET coupled with the Rossendorf kinetics code DYN3D. Two real transients at NPPs with VVER-type reactors documented within the VALCO project were selected for analyses. One of them was a test with the switching-off of one of two main feed water pumps at the VVER-1000 Balakovo-4 NPP. The current paper is dedicated to the different steps of the use and implementation of the GRS methodology to coupled code systems and to the assessment of the results obtained in the calculations of the VVER-1000 transient by the DYN3D/ATHLET code.

Based on the relevant physical processes in the transient, a list of possible sources of uncertainties was compiled. Besides control parameters like control rod movement and thermal hydraulic parameters like secondary side pressure, mass flow rates, and pressurizer heater performance, different neutron kinetic parameters were included into the list of possible sources of uncertainties. These are the burn-up state of the core, the control rod efficiency for different control rod groups and the coefficients for Doppler and moderator density feed back. By use of the SUSA package, sets of input data with statistical variation of the relevant parameter values were generated for a large number of runs of the coupled code. New tools were developed for the automated implementation of the varied input parameters into the different input decks of the DYN3D/ATHLET code. The same concerns the extraction of result parameters from the output data of the calculations and statistical analysis of these results. Time-dependent rank correlation coefficients were calculated showing the influence of the varied parameters on the output parameter under investigation. The most interesting output parameters are the physical parameters for which experimental data are available. First of all, these are the core power, upper plenum pressure, loop temperatures and steam generator levels. The calculation results allowed also the determination of time-dependent tolerance intervals for given coverage and confidence. The comparison of the experimental data, the (best-estimate) reference solution and the tolerance intervals showed how the agreement between experiment and calculation could be quantified. In most of the cases the tolerance intervals include the experimental curves. A compiled list of the most important input parameters based on the rank correlation coefficients shows, which input parameters and models are responsible for the deviations. This list gives indications for further model improvements and code developments.

  • Lecture (Conference)
    14. AER Symposium, 13.-17.09.2004, Helsinki, Finnland
  • Contribution to proceedings
    14. AER Symposium, 13.-17.09.2004, Helsinki, Finnland, Proceedings, 503-516

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Publ.-Id: 6623