RU2427937C1 - System for diagnosis of occurring inter-channel instability in reactor with water under pressure - Google Patents

Abstract

FIELD: power engineering. ^ SUBSTANCE: there are recorded signals from off-zones sensors of neutron flow (ionisation chambers). The recorded signal of neutron flow is converted into a signal neutron power and is spread into frequency spectre. There is revealed harmonic series with known frequency of fundamental harmonic characterising occurrence of inter-channel instability by means of amplitude-frequency discrimination. In case of revealing not-zero amplitudes in spectre of power signal there is generated diagnosis signal "Occurrence of inter-channel instability" by beforehand known dependence from level of reactor power with automatic change of level of amplitude discrimination in proportion to level of spectral noise in a measuring path. The signal is transferred to input of the reactor control system. ^ EFFECT: upgraded response of procedure and reduced probability of false operations. ^ 2 dwg

Description

The invention relates to nuclear energy, in particular to the control of the coolant in the reactor core, and is intended to control the occurrence of inter-channel instability (regular flow pulsations) in the reactor core in real time when controlling reactors with water under pressure.

A known method of controlling flow pulsations in fuel assemblies (and consequently the occurrence of inter-channel instability) by measuring the flow rate of the coolant in the channels of the active zone (Sadulin V.P. Complex measurements of energy release and flow rate in the fuel assemblies of a boiling reactor // Thermal-hydraulic aspects of the safety of active zones cooled by water and liquid metals / Abstract collection "Thermophysics-2008. Obninsk, October 15-17, 2008. P.62-64).

Among the disadvantages of the known method:

- the complexity of the intra-reactor placement of flow meters to control all fuel assemblies;

- the appearance of additional radioactive waste due to contamination of all internal reactor elements.

The closest in technical essence is the method that uses the noise diagnostic method [RU 2331120 C1, G21C 17/00 of 11/24/2006], designed to control abnormal pulsations of the flow of coolant reactors with water under pressure of the WWER type. In this method, the formation of a diagnostic feature is carried out according to the amplitudes of the noise pulsations, and during the processing, the variable component of the average signal of the extra- and intra-zone sensors is isolated, a generalized indicator of the average pulsation amplitudes and / or attenuation decrements for each sensor is determined at the program level, and the signals are recorded and processed sensors are simultaneously and continuously. According to the results of the processing of the sensor signals, a diagnostic signal is generated.

The disadvantages of the prototype are:

- signal averaging, which reduces the sensitivity of diagnostics;

- analysis of a wide band of the signal spectrum, which may contain other (spurious) noises having a variable component in addition to the useful signal, which does not exclude false triggering of diagnostics, so flow pulsations during inter-channel instability are masked by a large number of not only stochastic, but also harmonic noise, a systematic and variable type, for example, during movements of regulatory bodies, vibrations of fuel assemblies, stiffness gratings, and other reactor internals;

- the complexity of the placement, and then the disposal of intrazone sensors.

The challenge facing the inventor was to develop a method for diagnosing the occurrence of inter-channel instability in a pressure water reactor, devoid of the above disadvantages, and which provides an increase in the sensitivity of the control method and a decrease in the likelihood of false alarms using only extra-band neutron flux detectors.

The specified technical result is achieved by the fact that in the method for diagnosing the occurrence of inter-channel instability in a pressure water reactor, including recording signals from off-band neutron flux sensors (ionization chambers), the recorded neutron flux signal is converted into a neutron power signal, then it is laid out in the frequency spectrum, reveal a harmonic series with a known fundamental harmonic frequency, characterizing the occurrence of inter-channel instability, by amplitude-frequency dis rimination with automatic shift of the discrimination frequency bands in proportion to the change in the time the coolant passes through unheated sections of the fuel assemblies, according to a previously known dependence on the reactor power level and the automatic change in the amplitude discrimination level is proportional to the level of spectral noise in the measuring path, if a non-zero signal is detected (i.e. non-zero amplitudes in the spectrum of the power signal) form the diagnostic signal "The occurrence of inter-channel instability", which th passed to the input of the reactor control system.

The known method for noise diagnostics of reactors with water under pressure allows one to diagnose global boiling - the stage when boiling covers most of the active zone, since it is based on the dominance of one of the frequencies (the variable component of the average signal) against the background of noise, while the occurrence of inter-channel instability ( podkipaniya occurrence of local centers) characterized by the appearance in the spectrum of neutron power signal is not a single frequency, and the harmonic number f ^{n _N,} due to the addition of regular ripple coolant flow rate with the parameters in non-linear dependence of the density of coolant enthalpy (near the boiling point). In this case, previously unknown conditions for the unevenness of energy release in the core provide interchannel instability with equally probable possibilities for the appearance in the spectrum of the signal power of harmonics with amplitudes exceeding the noise level both at the frequency of the fundamental and subharmonics, harmonic series f ⁿ _N.

f ⁿ _N = n · f _G , where

f ⁿ _N is the harmonic series in the spectrum of the neutron power signal (N), containing n is the number of subharmonics that are multiples of the fundamental frequency of the inter-channel flow oscillations f _G ,

n is the serial number of the harmonic in a row (the region of the considered values of n is determined by the nature of the diagnostic tasks and the number of fuel assemblies in the reactor core);

f _G is the fundamental frequency (the frequency of inter-channel flow fluctuations).

Thus, instead of isolating the variable component of the power signal over a wide frequency band, where, in addition to the controlled features, harmonic noises indistinguishable from them may be contained, which will be falsely diagnosed as controlled features, the proposed method initially calculates the harmonic series of frequencies of inter-channel instability (taking into account the current power values) and only then on extremely narrow frequency bands, limited only by errors in the calculation of the signal spectrum, and often From the harmonic series in the spectrum of the signal, a number of harmonics characterizing the inter-channel instability are distinguished.

The essence of the patented method is as follows.

The signal from the output of the off-band sensors (ionization chamber current) is amplified in a known manner, the current is converted into voltage, then they are digitized by an analog-to-digital converter and thereby a digital reactor neutron power signal is obtained (proportional to the neutron flux density in the region of the off-band sensors). Next, the neutron power signal is decomposed into the frequency spectrum using the well-known method of spectral analysis (for example, fast Fourier transform), the spectral characteristic of the signal (signal spectrum) is obtained. The resulting neutron power spectrum is filtered, highlighting the frequency region where harmonics characterizing inter-channel instability can be located (a harmonic series f ⁿ _N with a fundamental frequency equal to the known inter-channel flow rate frequency f _{G is} isolated).

The fundamental frequency f _G depends on the power level of the reactor and is estimated by the formula

f _G = 1 / τ _(N) , where

τ _(N) - time of the coolant passage through unheated sections of fuel assemblies.

For each type of reactor, the functional dependence of f _G on the power level is determined through the known dependence of τ _(N) on the reactor power level

τ _(N) = γ ^TVS (N) · V _TBC / G (N), where

G (N) is the dependence of the flow rate of the coolant through the fuel assembly on the reactor power;

V _TBC - heat carrier volume in unheated sections of fuel assemblies;

γ ^{fuel assembly} (N) is the dependence of the density of the coolant in unheated sections of the fuel assembly on the power level of the reactor.

In accordance with the dependence f _{G, a} shift is calculated relative to the values corresponding to the nominal power value (N ₀ ), filtering bands (Δf _G ), proportional to the change in the passage time of the coolant through unheated sections of the fuel assemblies for the current power value (N ₁ )

где

Where

- время прохода теплоносителя по необогреваемым участкам ТВС при номинальной мощности N₀;

- transit time of the coolant along the unheated sections of the fuel assembly at a rated power of N ₀ ;

- время прохода теплоносителя по необогреваемым участкам ТВС при текущем уровне мощности N₁,

- the time of passage of the coolant in unheated sections of the fuel assembly at the current power level N ₁ ,

N ₀ - rated power value;

N ₁ - the current value of power (at the time of filtering).

Further, the frequency spectrum of the power signal is filtered with shifted filtering bands relative to the nominal values by the calculated value Δf _G. Then, in the spectrum of the power signal, its noise part is suppressed, which has two components - the first, previously known (systematic), arising during normal operation of the reactor, the value of which is determined experimentally during normal operation of the reactor, and the second (variable) component, arising from various kinds of electromagnetic hardware interference and interference due to unpredictable reasons (for example, changes in the operating modes of the equipment, switching power supplies, etc.). If after the indicated filtering steps the spectrum of the power signal has harmonics with non-zero amplitudes (which is possible only if the spectrum of the neutron power signal has harmonics [of one or more] series f ⁿ _N ), the diagnostic signal “The occurrence of inter-channel instability” is generated and passed to the input of the reactor control system.

The method is illustrated by the following illustrations: figure 1 shows the dependence of the amplitude of the harmonics of the neutron spectrum on the frequency of the spectrum, illustrating the selection of the harmonic series against the background of noise by the method of amplitude-frequency discrimination; figure 2 is a functional block diagram of a device for diagnosing inter-channel instability in a reactor with water under pressure, with which the patented method can be implemented.

The device contains a neutron detector 1, the output of which is connected to the input of the power calculation unit 2, the outputs of which are connected to the control input of the calculation unit for the shift of the filtering bands 5 and the input of the spectrum signal calculation unit of the power signal 3. The output of the latter is connected to the input of the frequency discrimination filter 4, and the output of the unit calculating the shift of the filtering bands 5 is connected to the control input of the frequency discrimination filter 4, the output of which is connected to the input of the amplitude discrimination filter 7. The output of the variable detection channel the noise signal 8 is connected to the first control input of the amplitude discrimination filter 7, and the output of the discrimination level determination unit of the systematic noise signal 6 is connected to the second control input of the amplitude discrimination filter 7. The output of the latter is connected to the input of the comparison unit 9, and the output of the comparison unit 9 is connected to the input the control system of the reactor 10, the information output of which is connected to the control input of the unit for determining the level of discrimination of the systematic noise signal 6.

The device operates as follows:

the signal from the output of the neutron detector 1 is fed to the input of the power calculation unit 2, where the formation of the neutron power signal occurs, which in block 3 is decomposed into the frequency spectrum. The resulting neutron power spectrum passes through a filter 4, which selects frequency ranges where harmonics characterizing inter-channel instability can be located (a harmonic series f ⁿ _N with a fundamental frequency equal to the frequency of inter-channel flow oscillations f _G is highlighted). At the same time, in the filter 4, the filtering bands are shifted relative to the nominal values in proportion to the change in the time the coolant passes through unheated sections of the fuel assemblies by the value Δf _G calculated from the known dependence of f _G on the power level. Further, the filter 7 in the spectrum of the power signal suppresses the noise part of the spectrum of the power signal, which has two components - the first, known (systematic) in advance, which occurs during normal operation of the reactor, in the general case it can be different for different modes of operation of the reactor - this is taken into account in the unit for determining the discrimination level of a systematic noise signal 6, the control input of which receives a sign of the current operating mode of the reactor from the information output of the control system 10, the value of the first (sys the thematic) component of noise is determined experimentally during normal operation of the reactor, and the second (variable) component of noise arising from various kinds of electromagnetic hardware interference and interference due to unpredictable reasons — changes in the operating modes of the equipment, switching of power supplies, etc., whose value is determined by a known method in the channel for determining the discrimination level of the variable noise signal 8 (which combines the entire measuring path except the detector itself neutrons 1). At the output of the comparison unit 9, a diagnostic signal is generated, “The occurrence of inter-channel instability,” when a non-zero signal (that is, non-zero amplitudes in the spectrum of the power signal) appears at the output of the filter 7, which after all stages of amplitude-frequency discrimination is possible only if the spectrum the neutron power of one or more harmonics of the harmonic series f ⁿ _N.

Thus, the introduction of new operations in the method for diagnosing the occurrence of interchannel instability in a pressure water reactor, revealing a harmonic series with a known fundamental harmonic frequency in the spectrum of a neutron power signal that characterizes the occurrence of interchannel instability, makes it possible to compress the frequency band of the harmonic series to values limited by errors in computation of the spectrum and the calculation of the function f _G capacity, to allow for higher sensitivity of the method and izhaet probability of false positives, using only ex-core neutron flux detectors.

Claims (1)

A method for diagnosing the occurrence of inter-channel instability in a pressurized water reactor, including recording signals from off-band neutron flux sensors (ionization chambers), characterized in that the recorded neutron flux signal is converted into a neutron power signal, then expanded into a frequency spectrum, and the harmonic series is detected with the known fundamental harmonic frequency, which characterizes the occurrence of inter-channel instability, by amplitude-frequency discrimination with automatic shift m of discrimination frequency bands is proportional to the change in the time the coolant passes through unheated sections of the fuel assemblies, according to a previously known dependence on the power level of the reactor and the automatic change in the amplitude discrimination level is proportional to the level of spectral noise in the measuring path, if a non-zero signal is detected (i.e., non-zero amplitudes in the signal spectrum power) form the diagnostic signal "The emergence of inter-channel instability" and transmit it to the input of the control system reactor reactor.

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