[en] At present the best-estimate approach in the safety analysis of nuclear power plants is widely used around the world. The application of such approach requires to estimate the uncertainty of the calculated results. Various methodologies are applied in order to determine the uncertainty with the required accuracy. One of them is the statistical methodology developed at GRS mbH in Germany and integrated into the SUSA tool, which was applied for the sensitivity and uncertainty analysis of the thermal-hydraulic parameters inside the confinement (Accident Localisation System) of Ignalina NPP with RBMK-1500 reactor in case of Maximum Design Basis Accident (break of 900 mm diameter pipe). Several parameters that could potentially influence the calculated results were selected for the analysis. A set of input data with different initial values of the selected parameters was generated. In order to receive the results with 95 % probability and 95 % accuracy, 100 runs were performed with COCOSYS code developed at GRS mbH. The calculated results were processed with SUSA tool. The performed analysis showed a rather low dispersion of the results and only in the initial period of the accident. Besides, the analysis showed that there is no threat to the building structures of Ignalina NPP confinement in case of the considered accident scenario. (author)

[en] The analysis of an unintended main safety valve opening at Ignalina NPP was performed with COCOSYS code in order to assess its capability in simulation of the transient processes that occur inside Accident Localisation System of Ignalina NPP. COCOSYS has several user-selected options, e.g. zone model (EQUIL._MOD, NONEQUILIB), water flow model (BAL_DRAIN, DRAIN_BOT), etc for nodalisation development. The influence of a zone model selection, a water overflow model selection and efficiency of heat exchanger in Condenser Tray Cooling System was investigated and presented in the paper. The performed analysis supported introduction of new water overflow model in COCOSYS code and showed that COCOSYS code can be applied for the analysis of Accident Localisation System of Ignalina NPP

[en] In tins paper the analysis of the Main Safety Valve opening at Ignalina NPP unit 2 is presented. RALOC4 code applied for calculations. The calculated results reflect the real measured behaviour of parameters. The model should be further developed to investigate the influence of various parameters. (author)

[en] The question of the RBMK power plants response to Beyond Design Basis Accidents (BDBA) and especially to Severe Accidents is very important. Until now very little attention was paid to it and much speculation remains after the Chernobyl accident. This paper aims to present the current knowledge regarding the response of Ignalina NPP confinement (it is titled as Accident Localisation System (ALS) to severe accident). A large Loss-of-Coolant Accident with failure of pumped water injection to Reactor Cooling System was selected for the analysis. The analysis was performed using a detailed nodalisation developed for COCOSYS code. The analysis showed that many uncertainties remain in the BDBA phenomena analysis for the RBMK reactors and the work should be continued investigating other accident scenarios, e.g. station blackout, multiple rupture of fuel channels, etc. (author)

[en] In order to protect the public, environment and workers from the radiation hazards, nuclear power plants are equipped with the barriers that limit the spreading of the radioactive material. The last barrier, which prevents the radioactive material from release into the environment, is the containment or confinement, which encloses the piping of the reactor cooling system and the reactor itself. The Accident Localisation System and the Reactor Cavity which employs the pressure suppression principle perform a function of containment at the Ignalina NPP with an RBMK-1500 reactor. The paper presents a review of the containment / confinement types available around the world, discusses their main features, presents the processes that occur in the containments / confinements and compares the key parameters of the Ignalina NPP Accident Localisation System with other containments / confinements. (author)

[en] At present the lumped-parameter codes is the main tool to investigate the complex response of the containment of Nuclear Power Plant in case of an accident. Continuous development and validation of the codes is required to perform realistic investigation of the processes that determine the possible source term of radioactive products to the environment. Validation of the codes is based on the comparison of the calculated results with the measurements performed in experimental facilities. The most extensive experimental program to investigate fission product release from the molten fuel, transport through the cooling circuit and deposition in the containment is performed in PHEBUS test facility. Test FPT-2 performed in this facility is considered for analysis of processes taking place in containment. Earlier performed investigations using COCOSYS code showed that the code could be successfully used for analysis of thermal-hydraulic processes and deposition of aerosols, but there was also noticed that diffusive deposition on the vertical walls does not fit well with the measured results. In the CPA module of ASTEC code there is implemented different model for diffusive deposition, therefore the PHEBUS containment model was transferred from COCOSYS code to ASTEC-CPA to investigate the influence of the diffusive deposition modelling. Analysis was performed using PHEBUS containment model of 16 nodes. The calculated thermal-hydraulic parameters are in good agreement with measured results, which gives basis for realistic simulation of aerosol transport and deposition processes. Performed investigations showed that diffusive deposition model has influence on the aerosol deposition distribution on different surfaces in the test facility. (authors)

[en] During the course of severe accident in water-cooled nuclear power plant, large amount of hydrogen could be generated and released into containment. Therefore it is necessary to investigate the gas mixing phenomena's in the containment. The investigation of gas mixing in the containment was performed in the TOSQAN test facility. The helium was used instead the hydrogen. Simulation of the TOSQAN test facility was performed with COCOSYS code. The calculation results shows gut agreement with the experiment data. (author)

[en] Accident Localisation System (ALS) of Ignalina NPP is a 'pressure suppression' type confinement, which protects the population, employees and environment from the radiation hazards. According to the Safety Analysis Report for Ignalina NPP ∼110 m³ of hydrogen is released to ALS compartments during the Maximum Design Basis Accident. If the volume concentration of hydrogen in the compartment reaches 4 %, there is a possibility for a combustible mixture to appear. To prevent the possible hydrogen accumulation in the ALS of the Ignalina NPP during an accident the H2 control system is installed. The results of the performed analysis derived the places of the possible H2 accumulation in the ALS compartments during the transient processes and assessed the mixture combustibility in these places for a maximum design basis accident scenario. Such detailed analysis of H2 distribution in the ALS of Ignalina NPP was not performed before. (authors)

[en] The influence of different types of Ignalina NPP structure modeling on the results of thermo-hydraulic evaluation of ALS (accident localization system) compartments is analyzed. There are five types of structure models applied for calculations. The results showed that in the case of maximum design-based accident the difference between maximum overpressure in an accident compartment did not exceed 2.7%. In the future it is intended to apply the model with equivalent reinforced (concrete) material considering the paint layer covering the metal lining of ALS structures, because this model provides the most conservative results, and consumes less computer time. (author)

[en] The Accident Localisation System (ALS) of Ignalina NPP is a containment of pressure suppression type designed to protect the environment from the dangerous impact of the radioactivity. The failure of ALS could lead to contamination of the environment and prescribed public radiation doses could be exceeded. The purpose of the presented analysis is to perform long term thermal-hydraulic analysis of compartments response to Group Distribution Header rupture and verify if design pressure values are not exceeded. (authors)