Abstract The Refueling Neutron Monitoring System and the Reactivity Monitoring System for startup measurements are both aged, the development of a new combined system fulfilling both functionalities has therefore been started. The new system is based on 6 autonomous measurement chains covering the whole neutron flux range and uses fixed fission chambers. The signals of the measurement chains are received by two redundant processing systems continuously providing measured and calculated data for external systems like the VERONA core monitoring system. The system generates emergency signals and events for the operators of the Main Control Room and the Refueling Machine. The pilot measurement chain has been tested at Units 2 and 3 of Paks NPP. The first implementation of the new system is planned to be installed at Unit 3 by the middle of the following year.

The core monitoring and surveillance system of Paks nuclear power plant has been replaced recently with a new version of the system VERONA utilizing virtualization technology and GPU accelerated numerical computations. In the new system, the process variables are monitored by the visual engine of a new simulation platform SIMTONIA (SIMulation TOols for Nuclear Industrial Application). In this paper the hardware and software architecture of the new system is presented in details enlighten the advantages of the application of stateof- art computational technologies.

The Reactivity Monitoring System and the Refuelling Neutron Monitoring System of Paks NPP are aged and need to be reconstructed. Since both systems are based on neutron flux measurements, the new system is to be served by the same detectors and measurement instrumentation. In order to provide data during refuelling, start-up and at full power, a full range system is required, i.e. the detectors and the associated instrumentation should cover the full range of neutron flux measurements from 0% to 100% of reactor power. Additionally, the new system is required to operate continuously, to maintain a measurement archive, and to provide data for the Process Computer and the VERONA core monitoring system. In order to cover the full neutron flux range, Photonis CFUL08 type fission chamber was chosen. The interface module will serve all three operation modes of the detector: pulse, Campbell (AC) and current (DC) modes. In order to obtain high reliability and dependability, the system will be built from independent and redundant components.

Between 2003 and 2007 the Hungarian Paks NPP performed a large modernization project to upgrade its VERONA core monitoring system. The modernization work resulted in a state-of-the-art system that was able to support the reactor thermal power increase to 108% by more accurate and more frequent core analysis. Details of the new system are given in Végh et al. (2008), the most important improvements were as follows: complete replacement of the hardware and the local area network; application of a new operating system and porting a large fraction of the original application software to the new environment; implementation of a new human-system interface; and last but not least, introduction of new reactor physics calculations. Basic novelty of the modernized core analysis was the introduction of an on-line core-follow module based on the standard Paks NPP core design code HELIOS/C-PORCA. New calculations also provided much finer spatial resolution, both in terms of axial node numbers and within the fuel assemblies. The new system was able to calculate the fuel applied during the first phase of power increase accurately, but it was not tailored to determine the effects of burnable absorbers as gadolinium. However, in the second phase of the power increase process the application of fuel assemblies containing three fuel rods with gadolinium content was intended (in order to optimize fuel economy), therefore off-line and on-line VERONA reactor physics models had to be further modified, to be able to handle the new fuel according to the accuracy requirements. In the present paper first a brief overview of the system version (V6.0) commissioned after the first modernization step is outlined; then details of the modified off-line and on-line reactor physics calculations are described. Validation results for new modules are treated extensively, in order to illustrate the extent and complexity of the V&V procedure associated with the development and licensing of the new calculations running in version V6.22 of VERONA. Some details on the experience collected during the operation of the new reactor physics calculations are also discussed. Finally conceptual plans for the next system modification phase are outlined briefly; these changes are induced by the forthcoming introduction of 15 month long fuel cycles (instead of the present 12 month long cycles).

In 2002 the Hungarian Paks NPP started a large-scale reconstruction project to upgrade its core monitoring system called VVER On-Line Analysis (VERONA). The main reason for the reconstruction decision was the planned reactor thermal power increase from 1375 MW to 1485 MW (108%), in order to achieve 500 MW electric power output. Considering safety margins and operation limits the basic approach was to keep all previously valid limits at 108%, as well. However, increased core power and a new type of fuel required a more accurate and more frequent core analysis; therefore modernization of the VERONA system was necessary and unavoidable. At this moment Unit 3 and Unit 4 are served by the new system. The reconstruction project is planned to be fully completed in 2008, when all Paks units will have a modernized core analysis system, providing state-of-the art services and operator support.The paper first presents a brief overview of the reconstruction process, and then main design principles and functions of the new VERONA system are outlined. Details of the new system architecture, hardware devices and software tools are also given, and then elements of the new human-machine interface are described and illustrated. A special emphasis is devoted to the new core analysis software with a detailed description of the validation methods and results. The process of reaching 108% power at Unit 4 is discussed; core behavior is illustrated by real data taken during the power increase procedure. Finally experience related to the development and operation of the new system is discussed briefly. (C) 2007 Elsevier B.V. All rights reserved.