ANALYSIS OF STATISTICAL STUDY OF ELECTRIC LOADS

The paper discusses the issues of determining the statistical characteristics and coefficients in the modes of power consumption. The basic calculations of the statistical coefficients of power consumption and characteristics of the electrical load are presented. The presented results can be used for calculations and improvement of power consumption modes; optimization calculations.В работе рассматриваются вопросы определения статистических характеристики и коэффициентов в режимах электропотребления. Приведены основные расчеты статистических коэффициентов электропотребления и характеристик электрической нагрузки. Представленные результаты могут быть использованы для расчетов и улучшения режимов электропотребления; проведения оптимизационных расчетов

The Multiple Pressure Tube Rupture (MPTR) issue in RBMK safety technology

The RBMK core is constituted by more than one-thousand pressurized channels housed into stacked graphite blocks and connected at the bottom and at the top by small diameter (D) and long length (L) pipes (less than 0.01 and more than 10 m, respectively) that end up into headers and drum separators. Control valves are installed in the bottom lines. Due to the large L/D value and to the presence of valves and other geometric discontinuities along the lines connecting with the pressure channels, the Fuel Channel Blockage (FCB) event is possible and already occurred in two documented NPP events. Pressure tube rupture occurred in a third NPP event not originated by FCB. Previous investigations, have shown the relevance of these events for the safety technology, and the availability of proper computational technique for the analysis, see the first and the third companion paper in this journal issue, respectively. The occurrence of the FCB event remains undetected for a few tens of seconds because of the lack of full monitoring for the individual channels, fourth companion paper in this journal issue. Therefore, fission power continues to be produced in the absence of cooling. This brings in subsequent times to fuel rod overheating, pressure tube failure, damage of the neighbouring graphite brick and ejection of damaged fuel. Following the pressure tube rupture, reactor cavity pressurization, radioactivity release into the same area and change of fluid properties occur that allow the detection of the event and cause the reactor scram at a time of a few tens of seconds depending upon the channel working conditions and the severity of the blockage. Notwithstanding the [delayed] scram and the full capability of the reactor designed safety features to keep cooled the core, the multiple pressure tube rupture (MPTR) issue is raised. The question to be answered is whether the ‘explosion’ of the blocked pressure tube damages not only the neighbour graphite bricks but propagates to other channels causing the potential for several channel failure. In order to address the MPTR issue fuel channel thermal-hydraulics and three-dimensional (3D) neutron kinetics analyses have been performed, as well structural mechanics calculations for the graphite bricks and rings (graphite rings surround the pressure tube to accommodate for thermal and radiation induced expansions). The bases for the analysis and the results of the study are presented. The conclusion, not reported within a licensing based format, is that the MPTR consequences are not expected to be relevant for the safety of the RBMK installations. This is supported by the analysis of experiments performed at the TKR facility available at the EREC research Centre near Moscow

Deterministic accident analysis for RBMK

Within the framework of an European Commission sponsored activity, an assessment of the deterministic safety technology of the ‘post-Chernobyl modernized’ Reactor Bolshoy Moshchnosty Kipyashiy (RBMK) has been completed. The accident analysis, limited to the area of Design Basis Accident, constituted the key subject for the study; events not including the primary circuit were not considered, as well as events originated from plant status different from the nominal operating conditions. Therefore, the notorious Chernobyl Unit 4 event was outside the scope of the investigation. Following the evaluation of the current state of the art in the area including the identification of critical issues, targets for the analysis were established together with suitable chains of computational tools. The outcomes from this part of the study are (a) the list of transient scenarios whose parameter values are assumed to constitute the boundaries for the evolution of any relevant safety transient and; (b) a set of computational tools with characteristics consistent with current technological achievements, suitable for performing safety analyses. The availability of computational tools, including codes, nodalisations and boundary and initial conditions for the Smolensk 3 NPP, brought to their application to the prediction of the selected transient evolutions that, however, are not classified as licensing studies. The results demonstrated proper safety margins and relatively long time constants associated with the huge values for the ratios between mass of moderator and mass of coolant and unit generated power. The results at the item above, suggested a qualitative, though non rigorous, comparison between accident analysis aspects in LWR and RBMK having the main purpose to show strengths in RBMK safety features heavily criticized not always in a consistent way following the Chernobyl event. The results of supporting analyses for the present paper are discussed in five companions papers in this Journal volume. The second (over six) and the third paper deal with the RBMK Main Coolant Circuit and Confinement thermal-hydraulic performance, respectively. Key specific issues in the RBMK safety technology, constituted by addressing of the “Multiple Pressure Tube Rupture (MPTR)” and by the application of coupled three-dimensional neutron-kinetics thermal-hydraulics, are discussed in the fourth and fifth papers. The proposal to instrument the core channels (ICM = Individual Channel Monitoring) has been formulated in this context and is discussed in the sixth companion paper