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Modelling of Thermal Hydraulic Transient Processes in Nuclear Power Plants: Ignalina Compartments

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Modelling of Thermal Hydraulic Transient Processes in Nuclear Power Plants: Ignalina Compartments


The monograph presents the fundamentals and results of mathematical simulation of transient thermal hydraulic processes occurring at the Ignalina NPP compartments during the loss of cool-ant accidents. The building structures of the compartments constitute the last barrier for the fis-sion products to enter the environment. The maintenance of this barrier's structural integrity in case of any initial event is one of the nuclear power plant's safety ensurance tasks. The integrity of the structures during accidents is defined by the behavior of the thermal hydraulic parameters in the compartments.
The main Ignalina NPP compartments and systems, including heat exchange and flow trans-fer processes, occurring while the steam-gas mixture flows from the ruptured location into the special condensing pools or into the environment (if special pressure relief panels are opened in the compartments), are described in the monograph. A lot of attention is given to bubbling and condensation processes of the steam-gas mixture in the condensing pools of accident localization system, which functions as containment sites. Other processes, due to which energy is directed inwards or outwards (example, heat exchange with structures, cooling of gas and condensing pools in compartments with special sprinkler systems, heat and mass transfer during the drainage systems operation, etc.), and which have impact on the behavior of the thermal hydraulic pa-rameters in compartments, are also taken into account in the analysis.
Models for the analysis of thermal hydraulic processes at Ignalina NPP compartments are developed and described. The state-of-the-art computer code CONTAIN, developed at Sandia National Laboratory (USA), as well as computer codes DRASYS, RALOC and COCOSYS, de-veloped at GRS mbH (Germany), were employed for the modelling. These computer codes are recognized and employed for NPP safety ensurance worldwide. The analysis is performed and thermal hydraulic parameters in the compartments are evaluated in the case of guillotine ruptures of various size pipes. Since accidents analyzed in the work are postulated in different compart-ments with different release paths of gas-steam mixtures and different blowdowns through the rupture, several models were developed for the simulation of these accidents. Specific character-istics of the analyzed systems and of the compartments and their models are described.
The modelling of accident processes at Ignalina NPP compartments was used not only for the safety ensurance, but were also directly employed to propose and justify modifications. Some examples of practical analysis are presented in this work. The analysis results were used in im-plementing knock out safety panels, designed for the pressure suppression in the central hall, and evaluating the modification, which increases the drainage water capacity from the drum separa-tors compartments. The calculated results of the parameters' behavior at Ignalina NPP compart-ments are employed for the implementation of ‘leak before break’ conception, by choosing the most sensitive parameters, the change of which enables the operative determination of leaks through the pipe, and preventing the pipe crack from outgrowing into a full rupture.

212 pages, © 2007


Nomenclature and abbreviations
1 Introduction. Main features of NPP containments
1.1 Containment design types
1.2 Brief characterisation of Ignalina NPP ALS
1.3 Ignalina NPP ALS comparison with containments of other NPP’s
1.4 Ignalina NPP ALS similarities and differences with VVER-440/213 containment
1.5 Previous analysis of Ignalina NPP ALS performance
2 Basis for mathematical modelling of transient processes in the compartments
2.1 Main equations for modelling of thermal hydraulic processes in compartments
2.1.1 Momentum conservation equation
2.1.2 Mass conservation equation
2.1.3 Energy conservation equation
2.1.4 Thermodynamic state closure equations
2.2 Codes for modelling of thermal hydraulic processes in compartments
2.2.1 CONTAIN code
2.2.2 COCOSYS, RALOC4 and DRASYS codes
2.3 Acceptance criteria for Ignalina NPP accident localisation system analysis
3 Ignalina NPP compartments with piping of reactor coolant system
3.1 Accident localisation system
3.1.1 General description of accident localisation system
3.1.2 Reinforced leak-tight compartments of accident localisation system
3.1.3 Compartments of lower water piping and under-reactor compartment
3.2 Reactor Cavity Venting System
3.3 Compartments of reactor cooling system, which are located outside accident localisation system
3.4 Compartments of steam lines
4 Models of Ignalina NPP compartments
4.1 Description of accident localisation system models
4.1.1 ALS model for CONTAIN code
4.1.2 Accident localisation system model for RALOC and COCOSYS codes
4.2 Model of reactor cavity venting system
4.3 Model of affected compartments for loss of coolant accidents analysis in drum separators compartments
4.4 Model for steam lines compartments
4.4.1 CONTAIN model of affected compartments in the case of steam line break in the shaft between blocks D and G
4.4.2 CONTAIN model of affected compartments in the case of steam line break in the turbine hall
4.5 Codes validation for simulation of Ignalina NPP compartments
4.5.1 RALOC4 and COCOSYS codes validation against measured data at Ignalina NPP during single main safety valve opening
4.5.2 Comparative study of Ignalina NPP compartments response using different codes
5 Response of accident localisation system to loss of coolant accidents
5.1 Maximum design basis accident – pressure header rupture
5.1.1 Mass/energy release influence to accident localisation system response
5.1.2 Analysis of long term response of accident localisation system
5.1.3 Evaluation of accident localisation system functions
5.1.4 Sensitivity and uncertainty analysis
5.1.5 Influence of condensation pools modelling on thermal hydraulic parameters of ALS
5.1.6 Influence of heat transfer to the ALS building structures on the calculation results
5.2 Break of Group distribution header
5.2.1 Mass/energy source term
5.2.2 Main results
5.2.3 Comparison of RALOC4 and CONTAIN codes results
6 Accident localisation system and reactor cavity venting system response to fuel channels ruptures
6.1 Single fuel channel rupture with simultaneous loss of off-site power
6.2 Multiple rupture of fuel channels
6.3 Uncertainty and sensitivity analysis for multiple fuel channels rupture
7 The thermal-hydraulic analysis of compartments response in the case loss-of-coolant accidents outside accident localisation system
7.1 Rupture of downcomer pipes in the drum separators compartments
7.2 Steam line break in shaft between blocks D and G
7.3 Steam line break in turbine hall
8 Application of calculation results for modifications
8.1 Implementation of knock-out panels from the reactor hall
8.2 Evaluation of drainage system capacity increasing from drum separators compartments
8.3 Identification of most sensitive parameters in the compartments for Leak Before Break concept application