DEVELOPMENT OF UNCERTAINTY METHODOLOGY FOR COBRA-TF VOID DISTRIBUTION AND CRITICAL POWER PREDICTIONS

Open Access
Author:
Aydogan, Fatih
Graduate Program:
Nuclear Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 07, 2008
Committee Members:
  • Lawrence Hochreiter, Committee Chair
  • Kostadin Nikolov Ivanov, Committee Chair
  • John Harlan Mahaffy, Committee Member
  • Turgay Ertekin, Committee Member
  • Kurshad Muftuoglu, Committee Member
Keywords:
  • dry-out elevation
  • uncertainty analysis
  • bwr
  • cobra-tf
  • fatih aydogan
  • pennstate uncertainty methodology
  • void distribution
  • critical power
  • bwr bundle
Abstract:
Thermal hydraulic codes are commonly used tools in licensing processes for the evaluation of various thermal hydraulic scenarios. The uncertainty of a thermal hydraulic code prediction is calculated with uncertainty analyses. The objective of all the uncertainty analysis is to determine how well a code predicts with corresponding uncertainties. If a code has a big output uncertainty, this code needs further development and/or model improvements. If a code has a small uncertainty, this code needs maintenance program in order to keep this small output uncertainty. Uncertainty analysis also indicates the more validation data is needed. Uncertainty analyses for the BWR nominal steady state and transient scenarios are necessary in order to develop and improve the two phase flow models in the thermal hydraulic codes. Because void distribution is the key factor in order to determine the flow regime and heat transfer regime of the flow and critical power is an important factor for the safety margin, both steady state void distribution and critical power predictions are important features of a code. An uncertainty analysis for these two phenomena/cases provides valuable results. These results can be used for the development of the thermal hydraulic codes that are used for designing a BWR bundle or for licensing procedures. This dissertation includes the development of a particular uncertainty methodology for the steady state void distribution and critical power predictions. In this methodology, the PIRT element of CSAU was used to eliminate the low ranked uncertainty parameters. The SPDF element of GRS was utilized to make the uncertainty methodology flexible for the assignment of PDFs to the uncertainty parameters. The developed methodology includes the uncertainty comparison methods to assess the code precision with the sample-averaged bias, to assess the code spreading with the sample-averaged standard deviation and to assess the code reliability with the proportion of specimens among the sample with a bias lower than the experimental uncertainty. Besides, the rankings of dominant phenomena are observed with the second comparison method (sensitivity analysis). Simple Random Sampling, Order Statistics, Richardson Extrapolation are some of the methods that are in the developed methodology. This uncertainty methodology was implemented for the COBRA-TF predictions. The uncertainty and sensitivity results are presented in the dissertation.