Multi Dimensional Boron Transport Modelling in a Subchannel Approach

Open Access
Author:
Ozdemir, Ozkan Emre
Graduate Program:
Nuclear Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 24, 2012
Committee Members:
  • Maria Nikolova Avramova, Dissertation Advisor
  • Maria Nikolova Avramova, Committee Chair
  • Kostadin Nikolov Ivanov, Committee Member
  • Turgay Ertekin, Committee Member
  • Fan Bill B Cheung, Committee Member
  • Dr Tanju Sofu, Special Member
  • Mr Kenya Sato, Special Member
Keywords:
  • boron tracking
  • thermal hydraulics
  • Godunov
  • COBRA-TF
  • STAR-CD
Abstract:
The risk of Small Break Loss Of Coolant Accident (SBLOCA) and other reactivity initiated transients caused by boron dilution in the Light Water Reactors (LWRs) and the complications of tracking the soluble boron concentration experimentally inside the primary coolant raised the interest in computational studies for accurate boron tracking simulations in nuclear reactors. This PhD work presents the development and implementation of a multidimensional boron transport model with modified Godunov scheme within a thermal-hydraulic code based on a subchannel approach. The cross flow mechanism in a multiple-subchannel rod bundle geometry, heat transfer and lateral pressure drop effects are considered to simulate deboration and boration case studies accurately. The Reactor Dynamics and Fuel Management Group (RDFMG) version of the COBRA-TF (CTF) code was chosen for the implementation of three different boron tracking models: First Order Accurate Upwind Difference Scheme, Second Order Accurate Godunov Scheme, and Modified Godunov Scheme. Based on the applied analytical and nodal sensitivity studies, the Modified Godunov Scheme approach with a physical diffusion term was determined to provide the most accurate and best estimate solution. As a part of the verification and validation activities, a code-to-code comparison was carried out with the STAR-CD Computational Fluid Dynamics (CFD) code and presented here. The objective of this study was two-fold: 1) to verify the accuracy of the newly implemented improvements of the CTF boron tracking model against CFD calculations; and 2) to investigate its numerical advantages as compared to other thermal-hydraulics codes. In addition, the model validation studies were performed against experimental data from the Rossendorf Coolant Mixing Model (ROCOM) test facility. Finally, the importance of the two-phase flow characteristics in modeling boron transient were emphasized, especially during long term cooling period after the LOCA condition in Pressurized Water Reactors (PWRs). The CTF capabilities of boron transport modeling were further improved based on the three field representation of two-phase flow utilized in the code. The boron transport within entrained droplets was modeled. In addition, a model for predicting the boron precipitation under transient conditions was developed and tested. It was aimed to extend the applicability of CTF for reactor transient simulations, and particularly Large Break Loss Of Coolant Accident (LB-LOCA) analysis.