Advanced Reactor Physics Methods for Transient Analysis of Boiling Water Reactors

The safety assessment of nuclear power plants requires the consideration of the several physical phenomena taking place in the reactor core. Since the last decade, the simulations are performed using the so-called Best-Estimate (BE) thermal-hydraulic system codes coupled with 3D nodal diffusion reactor dynamics solvers. These numerical tools are well developed and validated for PWR but for BWR there are still improvements and developments to be done. In addition, homogenized cross sections libraries, depending on the individual thermal-hydraulic state parameters, e.g. fuel temperature, moderator density, etc., must be supplied to take into account the interdependencies of the involved physics.
In this PhD a new coupled code system named TRADYN was developed that consists of the novel and innovative integration of the core simulator DYN3D into the code system TRACE/PARCS using compiler preprocessor directives. This has the advantage to preserve the original TRACE/PARCS system “untouch” and to facilitate the maintenance, modification and debugging. In order to manage the transfer information between TRACE and DYN3D, a General Interface and Specific Data Map routines in FORTRAN were developed. ... mehrTo properly describe the tightly-coupled neutronics and thermal-hydraulic phenomena within the core of a BWR, different physical models were extended and new ones implemented in DYN3D and PARCS.
Another contribution of this doctoral thesis is the development of a new in-house methodology called GENSIM-XS for the generation of nodal cross sections for BWRs considering history effects of control rods and void. GENSIM-XS is able to greatly simplify the number of the neutronics regions present in the reactor core. This new methodology uses the AUDIT option of SIMULATE-3 (S3) to report the cross sections on the output file. Then, they are extracted and written in multi-group tabulated cross sections in NEMTAB format in an automatized manner using Python scripts.
The validation of TRADYN is based on Boiling Water Reactor Turbine Trip (TT) benchmark data, where TRADYN has demonstrated its capability to predict the stationary plant conditions as well as the temporal evolution of the main plant parameters, showing a good agreement with the measurements e.g. core power and dome pressure.
The static core of the cycle 4 of BWR Laguna Verde nuclear power plant was selected as second case for the validation not only of the GENSIM-XS methodology but also of TRADYN. The TRADYN results such as k-eff, axial power profile, axial void fraction exhibit an excellent agreement with the reference values. This underpins the capabilities of the new methodology to generate cross-sections for coupled thermal-hydraulic/neutronics calculations accounting also for history effects for BWR core analysis.