The few-group cross section libraries, used by reactor dynamics codes, are affected by the spectral history effect – a dependence of fuel cross sections not only on burnup, but also on local spectral conditions during burnup. Neglecting this effect leads to an additional component of error in neutron-physical characteristics. Two solution approaches to this problem implemented in the reactor dynamic code DYN3D are described and compared in this paper: a cross section correction method based on239Pu concentration and separate cross sections treatment for each axial layer of reactor core. Steady-state and burnup characteristics of a PWR and a WWER-1000 cores, calculated by DYN3D with and without cross section corrections, are compared. An impact of the correction on transient calculations is studied for a control rod ejection example. Studies have shown a significant influence of spectral history on axial power and burnup distributions as well as on calculated cycle length. Two different correction methods have shown similar major effects.
The preparation of a few-group neutron cross-section library is an important step in implementation of the computer packages that are based on solution of the neutron transport equation in the few-group diffusion approximation into the safety analysis practices. The accuracy of modelling the physical neutron kinetic processes in the reactor core directly depends on the quality of few-group cross-section library. It is important to note that such cross-section library should be prepared in the format applied in the computer package and with use of a spectral code that models the fuel assembly quite adequately. The best option for preparing the few-group neutron crosssection library for the PARCS few-group diffusion code, which is being introduced into SSTC NRS safety analysis practices as a part of the TRACE/PARCS coupled neutron kinetic/thermal hydraulic package, is to adapt the previously developed and validated models of fuel assemblies for the HELIOS spectral program. The adaptation procedure for HELIOS models for WWER-440 including the fuel follower and transition part forming the input file structure required for correct work of the GenPMAXS program is presented. The approaches to the choice of reference states and branch parameters in the PARCS code format are presented. The results from correctness analysis of the adaptation of the HELIOS WWER-440 fuel assembly computer models are presented. The results are based on a comparative analysis of the fuel assembly multiplication properties obtained by the HELIOS model that was developed for preparation of the cross-section libraries for the DYN3D program (validated and widely used at SSTC NRS at present), and by the HELIOS model that was adapted for the GENPMAX program.
The State Scientific and Technical Center for Nuclear and Radiation Safety (SSTC NRS), a Ukrainian enterprise with a 29-year experience in the area of scientific and technical support to the national nuclear regulator (SNRIU), has been actively involved in international research activities. Participation in the IAEA coordinated research activities is among the SSTC NRS priorities. In the period of 2018–2020, the IAEA accepted four SSTC NRS proposals for participation in respective Coordinated Research Projects (CRPs). These CRPs address scientific and technical issues in different areas such as: 1) performance of probabilistic safety assessment for multi-unit/multi-reactor sites; 2) use of dose projection tools to ensure preparedness and response to nuclear and radiological emergencies; 3) phenomena related to in-vessel melt retention; 4) spent fuel characterization. This article presents a brief overview of the abovementioned projects with definition of scientific contributions by the SSTC NRS (participation in benchmarks, development of methodological documents on implementing research stages and of IAEA technical documents (TECDOC) for demonstration of best practices and results of research carried out by international teams).
Currently, new types of fuel are being considered to be introduced or already in the introduction process at Ukrainian NPPs with WWER. By means of a new version of the TRANSURANUS code, new functions of the gas gap thickness in dependence on the burnup have been created and implemented into the gas gap model of the reactor dynamics code DYN3D. These new functions cover all actual and perspective fuel types for the Ukrainian NPPs with WWER.
The use of best-estimate approach for WWER safety analysis in RIA is considered. The relevance of this problem is concerned with small margin to acceptance criteria under the conservative approach and becomes stronger under power uprate of nuclear power plants. Previous experience in this area for WWER-1000 reactor types is overviewed. The necessity to extend these activities for successful implementation of the best-estimate approach is noticed and areas of further work are discussed.
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