VUJE, a.s.

Nuclear safety

VUJE performs analyses of thermal-hydraulic transients and accidents, probabilistic safety assessments, and core neutronic calculations, analyses of beyond design-basis accidents and accidents with severe core damage, by means of advanced computer codes. It carries out thermal-hydraulic measurements not only at nuclear units, but also at other nuclear and non-nuclear facilities. The results of these analyses are used for designing and reviewing safety enhancement, for safety evaluation, and for the development of operating procedures and emergency operating procedures. VUJE develops concept issues of nuclear safety, carries out analyses of safety related events in plant operation. It develops drafts of safety rules and guidelines for the Nuclear Regulatory Authority (UJD) of the Slovak Republic (SR).

The activities in the field of nuclear safety relate mainly to the following areas:

Thermal-hydraulic computer analyses

VUJE has available a comprehensive set of tools and know-how for meeting the requirements of nuclear industry in the field of thermal-hydraulic analyses and evaluation of nuclear safety. It uses extensive experience with modeling thermal-hydraulic processes by means of the most modern computer codes such as TRANSURANUS and CALOPEA for detailed modeling of processes in fuel assemblies, KING and SPACPOV for unit energy balance analysis, RELAP5, CATHARE and TRAC-P for the analysis of system behavior in the whole spectrum of transients idents, MELCOR and MAAP for the analysis of beyond-design basis accidents, severe accidents with core melting, and transport of radionuclides in reactor coolant system and plant areas, CONTAIN and TRACO for the simulation of processes in hermetically closed plant areas (containment).

The models of nuclear power plant components, as established in these computer codes, are verified by sets of experimental data. The results from the resolution of various problems of nuclear power plant safety are used for the demonstration of the acceptable safety level of operating units (comprehensive development of appropriate parts of the Final Safety Analysis Report for the V1 Bohunice following the implementation of safety upgrading, of the Final Safety Analysis Report for the V2 Bohunice, review of parts from Mochovce safety documentation), as well as for continual safety upgrading of plant operation and also for operative resolution of current needs of operators. The abilities of VUJE in this field are thoroughly verified and used within the framework of broad international cooperation. VUJE participates in International Atomic Energy Agency (IAEA) projects (for validation of computer codes, development of internationally accepted guidelines and recommendations), provides the solution of thermal-hydraulic part for PSA level 1 and 2 studies, and participates in developing PHARE, PECO etc. studies.

Neutronics

VUJE performs calculations of core neutronic characteristics as needed for unit commissioning and operation. It proposes fuel charge schemes and patterns and analyzes the effects of fuel structural modifications on power distribution in the core and on authorized operating modes. For this, VUJE uses diffusion computer codes BIPR-7, PERMAK, KASSETA and HELIOS. VUJE has developed and introduced into practice the advanced in-reactor system TOPRE that monitors power distribution in VVER 440 reactor core in on-line mode. VUJE specialists innovated the method for fast neutron fluency monitoring by extending the original set of neutron detectors using fission detectors of 238U and 237Np. Such monitoring is used in the specimen program for monitoring the conditions of reactor pressure vessels. Codes ANSIN, TORT-DORT for coupled transport calculations of neutron and photon shielding, libraries of transport cross sections BUGLE 96, VITAMIN B6, and also I SCAMPI code for the preparation of libraries ith transport effects (those are codes or libraries from the collection of ORNL computer codes used in the US) were adapted in VUJE. These codes are used for safety analyses for interim spent fuel pools using noise diagnostics by means of signals from ionization chambers and self-powered detectors, for spent fuel in containers, etc., and a system for neutron noise diagnostics using signals from ionization chambers and SPD detectors was developed and instaled as well.

Thermal-physical measurements

VUJE performs measurements of thermal-hydraulic plant parameters and designs automatic evaluation of technical and economical performance indicators. It carries out metrology activities in a calibration center for pressure and temperature meters. After refueling, it calibrates standard temperature measurements in reactor. For plant instrumentation qualified for accident conditions, VUJE proposes, designs, and installs devices for the measurement of temperature and level in reactor.

Instrumentation and control system

VUJE proposes, designs, delivers and performs installation of measurement and control equipment both in nuclear and conventional power plants. With the objective to enhance both nuclear and operational safety during manipulations with nuclear fuel, VUJE has developed and installed a modernized control system for refueling machines. The system was implemented at four units at V1 and V2 Bohunice plants where it made possible to reduce refueling time by about 30%. Innovated system SIPPING for the control of fuel cladding integrity was developed. Within Siemens/VUJE consortium, instrumentation and control system for the V1 Bohunice plant was reconstructed (VUJE provided concept, design, assembly, test programs and implementation, evaluation and operating procedures). Within the modernization of the V2 Bohunice plant, VUJE develops the concepts of solution. In the field of instrumentation and control, VUJE addressed and implemented safety measures at the Mochovce plant, including tests during the commissioning of units 1 and 2. VUJE carries out measurements of dynamic properties of instrumentation and control systems.

Probabilistic safety assessment

VUJE performs reliability analyses of systems, equipment and process parts of nuclear power plants and carries out comprehensive Probabilistic Safety Assessment level 1 and 2 studies, including development of reliability characteristics. The results are used for the review of benefits from measures implemented at V1 Bohunice and also at Mochovce units 1 and 2 for safety upgrading and evaluation of core damage risk. The whole work is done in the environment of internationally used and commercially available software RISK SPECTRUM (product of RELCON, Sweden) that makes it possible to implement not only PSA model at all levels, but also reliability analyses of selected systems. VUJE has elaborated a methodology for analyses by means of PSA studies based on internationally accepted methodology procedures presented in NRC documents NUREG, in IAEA documents TECDOC and Safety Series. VUJE also has available a database of operational events, which is based on comprehensive operational experience from V1 Bohunice. The database enables to calculate reliability characteristics according to the requirements of PSA and system reliability analyses. The database is used by the US DOE within its activities in the Russian Federation and in Ukraine.

Analyses of events in plant operation

VUJE carries out analyses of events in the operation of both domestic and foreign nuclear power plants in relation to their effect on safety to enhance the efficiency of the operational safety experience feedback. It prepares reports on safety significant events into the IAEA Incident Reporting (IRS) and for the World Association of Nuclear Operators (WANO).

Concepts and nuclear safety upgrading

VUJE processes the knowledge from the concepts of nuclear safety issues abroad with regards to their application in Slovakia. Based on this knowledge, it develops safety concepts of VVER 440 plants. VUJE has developed the safety concept for the V1 Bohunice and now the safety concept for the V2 Bohunice is being developed. For public information about the nuclear safety concepts, VUJE develops basic materials for presenting the level of nuclear safety according to the internationally accepted criteria. It distributes news from the international nuclear press agency NucNet within Slovakia and contributes by reporting domestic information to the NucNet.

VUJE in cooperation with SIEMENS within the REKON consortium ensures coordination and implementation of projects for the safety upgrading of V1 Bohunice. The measures are focused on the area of core cooling during accidents, mitigation of accident consequences, improvement of instrumentation and control systems and of electrical systems, and on updating safety documentation. It also coordinates preparation and implementation of projects for strengthening seismic resistance, capacity extension of the Interim Spent Fuel Storage Facility in Bohunice and completion of the Republic Repository of Radioactive Waste in Mochovce. It provided support for the completion of construction of the first two units in Mochovce and their safety enhancement for the operators, for the general vendor Skoda Prague and also for the international consortium EUCOM consisting of Siemens and Framatome.

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