[en] Schemes of particular conditions of VVER-1000 reactor core are presented and illustrated by the example of simulation of one of the BDBA passing scenarios. The rupture of the Primary Circulation Pipeline with non-actuation of safety features is considered in this scenario. To calculate K_eff in random conditions of the VVER reactor core, a complex of codes and libraries of estimated nuclear data SAPFIR-2006 has been developed in RRC Kurchatov Institute. Algorithms of codes in the complex are based on use of the Monte Carlo method. The complex has been verified at calculations of more than 3000 benchmark experiments of randoms of geometry, composition and spectrum of neutrons. Besides, calculations of start-up experiments and particular conditions of VVER reactor at power have been carried out during verification of the complex. To minimize the number of options being calculated during the analysis of BDBA neutron and physical aspects, a combination of realistic models and K_eff conservative values for the so-called 'base conditions' is used. Base conditions are characterized for each specific moment of fuel loading burnup by a typical fuel configuration in the core, a set of variable parameters (temperature, density of water, sizes of fuel element debris, sizes of areas with destroyed FA etc.) and a range of change of these parameters. For each base condition a set of values of variable parameters is determined at which K_eff maximum values is achieved. Dependence of K_eff maximum values for BDBA with rupture of the Primary Circulation Pipeline, NPP blackout and diesel-generator failure at the beginning of burnup of steady-state fuel loading at enrichment of makeup fuel 5% is shown. K_eff approaching K_eff=1,0 with consideration of accuracies is observed. In the case being considered this approaching is conditioned by great conservatism during formation of the initial model, as there is no reliable information on distribution of Gadolinium isotopes and solid fission debris in fuel after melting, and they were not taken into account during calculations. Such calculation research makes it possible to lay down the tasks for further experimental and calculation works in the field of BDBA analysis. Finally, these works allow to lay down recommendations for BDBA control