IEC 61502:1999 pdf download - Nuclear power plants - Pressurized water reactors - Vibration monitoring of internal structures

IEC 61502:1999 pdf download – Nuclear power plants – Pressurized water reactors – Vibration monitoring of internal structures

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IEC 61502:1999 pdf download – Nuclear power plants – Pressurized water reactors – Vibration monitoring of internal structures.
IEC 61502 applies to systems used for monitoring the vibratory behaviour of the internal structures of pressurized water reactors (core barrel, thermal shield, upper and lower core support, etc.) and fuel assemblies on the basis of neutron fluctuations observed outside the vessel and vessel vibrations. The main objective of monitoring described in this standard is to detect degradation of internal structures. Primary circuit measurements can be considered together with internal monitoring to provide further information and make it possible to detect degradation of primary circuit structural supports. This standard covers the system characteristics and gives recommendations for monitoring.
2 Definitions
For the purposes of this International Standard, the following definitions apply
core barrel
cylindrical structure situated in the vessel and supporting the core (see figure 1)
core barrel clamping system
hold-down spring (see figure 1) or any other structure (e.g. springs) compressed between upper and lower internals
thermal shield
metallic structure mounted around the core barrel, attached to it by clamps, intended to limit embrittlement of the pressure vessel steel under radiation (see figure 1). Not all reactors are fitted with this type of structure
ex-core neutron detector
ionization sensor situated outside the vessel (on a level with the core), measuring the neutron flux in order to monitor reactor power (see figures 1 and 2)
neutron noise
fluctuations in the neutron flux caused by variations in emission at the neutron source, or by variations in the transport of neutrons towards the outside of the vessel. Ex-core neutron noise corresponds to the fluctuations in the signal emitted by an ex-core neutron detector and is proportional to the signal of fluctuations in the flux reaching the detector
preferred vibratory frequency of a given structure. For a structure with a distributed mass. there are an infinite number of distinct eigenfrequencies, each of which is associated with a natural mode. The lower the eigenfrequency, the higher the vibration magnitude (amplitude in frequency domain) of a given mode (modal magnitude). This explains the fact that only the first few vibratory modes of the structures can be detected, which is to say those at the lowest frequencies (essentially modes 1 and 2 on internal structures)
beam mode vibratIon
natural vibratory mode of a structure behaving like a beam with respect to a given axis of observation (see figure 3). The first beam mode of the core barrel corresponds to pendular motion
shell mode vIbratIon
natural vibratory mode of a structure having an axis of symmetry. One example of this type of structure is a cylindrical thermal shield (see figure 4)
neutron energy spectrum
distribution of the population of neutrons emitted in a flux, dependent on their energy
fuel burnup
energy liberated per mass unit by the fuel since being loaded in the core
spectral signature
function depending on frequency (see annex A). As an example:
mono-channel signature (for a single signal)
autospectrum, expressed as power spectral density (PSO), normalized autospectrum, expressed as a relative value of neutron fluctuation or normalized power spectral density
cross-signatures (for signals associated two by two)
cross-spectrum, expressed as cross power spectral density (CPSD), phase and coherence functions
portion of a spectral signature culminating in a maximum between two minima. The frequency of the maximum is the peak frequency. Under certain conditions (in particular, the appearance of peaks at the same frequency on other signatures, and respecting certain laws of coherence and phase). it may correspond to a natural mode for the structure.

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