The monitoring of vibration characteristics of complicated structures can detect mechanical defects or abnormal changes in the fastening conditions of structural components at an early stage. Vibration characteristics are related to such parameters as rigidity and inertia matrices, therefore there appears an opportunity of diagnostics followed by the identification of the cause for a particular anomaly.

The vibration noise diagnostics system is intended for reactor system vibration monitoring and diagnostics in order to detect the abnormal equipment vibration conditions caused by defects, changes in the equipment fastening conditions, increased hydrodynamic loads induced by the primary circuit coolant flow in the normal operation modes.

VNDS principal tasks are:

• Identification of coolant flow-induced hydrodynamic loading conditions of the reactor system equipment and pipelines to ensure the early detection of increased loads caused by the conditions of manufacturing, assembly and balancing of MCP rotary equipment;
• Monitoring of the fastening conditions of reactor internals in the course of operation to ensure the early detection of relaxation and wear of fastening components;
• Monitoring of thermal displacement of equipment and pipelines in the mode of reactor scheduled heating-up and cooling in order to confirm the reproducibility of displacement values and isolation of equipment components jamming;
• Evaluation of coolant flow rate per channel. This function is primarily required to define the fuel assemblies operating conditions;
• Evaluation of reactor internals actual life based on the vibration load conditions. This task is related to the possibility of extension of reactor internals service life, and it is relevance is due to the fact that vibration stress estimates made at the stage of scheduled maintenance can appear to be non-representative because they had not considered possible changes in reactor internals fastening conditions during continuous operation.

The object to be monitored with the help of VNDS is WWER-1000 (WWER-440) reactor system, including the reactor and internals, basic equipment and pipelines of main circulation circuit (MCC).

Changes of reactor system components vibration are the most quick-response indicators for the isolation of such changes in the mechanical equipment as:

• Redundant strain of elastic tubular members of reactor core barrel and loosening of the core barrel contact at the reactor head side;
• violation of design clearance gaps of the flow splitter;
• fuel assemblies abnormal vibration due to their camber as well as the displacement of the spacer grids;
• wear of vibration absorber contact surfaces in the upper and lower attachment points of the core barrel;
• fuel assembly levitation;
• abnormal changes in stiffening behavior of steam generator supports, main circulation pumps, core barrel support ledge resulting in inadmissible amplitudes and oscillation types of the above mentioned MCC components;
• beyond design basis displacements of SG, MCP and pipelines due to high temperature.

The created VNDS software-hardware suite enables the early detection of such malfunctions and prediction of their dynamics. Together with the monitoring of the change of the equipment mechanical state VNDS software-hardware suite solves the task of temperature–induced displacement monitoring of MCC basic equipment in the dynamic modes (heating up/cool down). This VNDS function is ensured by the relative displacement sensors that can measure vibration-induced micro-displacements of the equipment both in stationary modes and in case of coolant temperature changes (displacements of various MCC components within 10 sm).

VNDS consists of:

• Software-hardware suite – equipment for processing and analysis of sensor signals, main computer with the software and data transmission and reception facilities over LAN;
• Absolute displacement measuring device as part of primary transducers (absolute displacement sensors) and electronic unit;
• 24-channel programmable displacement measuring device as part of primary transducers (relative displacement sensors) and carrier frequency amplifier;
• Pressure fluctuation measuring channel as part of pressure fluctuation sensors and frequency carrier amplifier;
• Ionization chamber (IC) measuring channel as part of ionization chamber diagnostic suspensions and IC current measuring device;
• Unit of signal reception from in-vessel direct charge detectors that receives direct charge detector signals from standard in-core instrumentation system;
• Terminal boxes;
• Sensor attachment points.

VNDS is local automatized data measuring system consisting of measuring channels, software and hardware facilities. VNDS software-hardware suite is based on the newest measuring and computing equipment: various converters and programmable controllers in data measuring and processing channels and industrial PC for measurement control and data processing and analysis.

VNDS differs from other diagnostic systems in several aspects. The first and foremost aspect concerns the noise components of sensor signals of various physical nature. For example, the sweep of noise fluctuations of in-core neutron sensors is 10-12 - 10-9 А is at the level of the equipment own noise requiring high-quality analogue processing: use of high-precision measuring amplifiers and high-quality filters, galvanic decoupling of input and output signals, use of 16-bit analog-digital converters for simultaneous digitization of the whole frequency range. Besides, the volume of calculations and the complexity for the retrieval of diagnostic attributes from the recorded signals are very high, requiring the implementation of complicated algorithms.

VNDS Detector Layout of Kola NPP WWER-440
Taken as an Example

The layout of VNDS detectors of Kola NPP WWER-440 taken as an example is given in Figure above.

Pressure fluctuation sensors installed on the «cold leg» of the main circulation pipeline between MCP and reactor, and absolute displacement sensors installed on MCP casings are not included in the picture.

VNDS software is based on the modular concept and contains components intended to control the system hardware, record digitized sensor signals, analyze measurements and build trends of equipment vibration parameters. The above functions are started by the operator using the user shell. VNDS software is controlled by ОС Redhat LINUXTM. The user interface is based on X-WINDOW. VNDS system and application software performs:

• measurement control including calibration and balancing of absolute and relative displacement sensor measuring channels;
• preliminary preparation of sensor signals;
• data recording;
• data storage and archiving;
• automatic measurement analysis.

Vibration diagnostics algorithm is implemented as scenarios i.e. calculation procedures performing diagnostics of a certain component of reactor system equipment and performing monitoring functions. The launch library consists of the following scenarios:

• vibration of basic equipment of main circulation circuit (steam generator and MCP);
• mutual vibration of reactor system vessel and core barrel;
• vibration-inducing forces (acoustic standing waves, turbulent coolant flow);
• fuel assembly vibration.