JPS63108298A - Boiling transition generation monitor for nuclear-reactor in-core channel - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is directed to the boiling of channels in the core of a nuclear reactor! The present invention relates to a boiling transition occurrence monitoring device for a channel in a nuclear reactor core, which monitors the occurrence of transition.

(Prior Art) In the thermal design of the reactor core, in order to effectively remove the heat generated within the fuel frame, a nucleate boiling state (nucleate boiling state) is used.
Operating conditions such as core flow rate and fuel assembly output are set so as not to exceed When a boiling transition that exceeds the nucleate boiling state occurs, the heat transfer coefficient on the fuel rod surface decreases and the boiling transition exceeds the nucleate boiling state.
(transition boiling) or film boiling state is reached and the surface temperature begins to rise. If this state continues for a long time and the high temperature state persists, fuel failure occurs due to oxidation of the fuel cladding tube and decrease in material strength, that is, thermal damage occurs.

Under normal operating conditions, the thermo-hydrodynamic state of the fuel assembly, that is, the fuel assembly inlet temperature, dosing flow rate, core pressure, twisted assembly output, etc., are determined from measured values and values calculated by a process computer. A limit output correlation formula (GEXL correlation formula, etc.) is applied to confirm that interconductor aS does not occur. As an indicator, the minimum critical power ratio (MCPR
) is used to evaluate that this minimum critical output ratio is not less than 1.0. This minimum critical output ratio is defined by the following 0 formula.

CPR: Limit output ratio Qcp: Limit output Q: Output during fuel assembly operation MCPR= win (CPR)...■
However, since this evaluation method of the minimum critical power ratio is accompanied by errors, it is necessary to confirm that the thermal integrity is not compromised during operation, or to confirm that a boiling transition will occur.
It is desirable to be able to measure A.

Furthermore, even if boiling 1115 occurs, it is expected that the aforementioned thermal damage will not occur within a certain period of time, so if the occurrence of boiling transition can be constantly monitored, it is possible to This makes it possible to operate the vehicle in a manner that allows the vehicle to move freely, leading to an expansion of the operating range, which also has a large economic effect.

(Problems to be Solved by the Invention) The present invention has been made in view of the above circumstances, and its purpose is to solve the problem up to the boiling of the channels in the reactor core during operation of the nuclear reactor. An object of the present invention is to provide a system for monitoring the occurrence of boiling transition in a channel in a nuclear reactor core, which ensures the integrity of fuel by constantly monitoring the margin and the occurrence of boiling am transfer.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention statistically analyzes detection signals obtained from a plurality of in-core neutron detectors installed along the flow of cooling water in the reactor core. From this, the propagation velocity of the disturbance wave propagating on the fuel rod surface liquid film is calculated, and when this propagation velocity decreases as the reactor power increases, it is determined that a boiling transition has occurred. The present invention relates to a system for monitoring the occurrence of boiling transition in a channel in a nuclear reactor core, which is characterized by:

Next, the measurement principle of the present invention will be explained.

The propagation time of the disturbance wave on the liquid film on the surface of the fuel rod is determined as follows.

Two signals X1. detected by the in-core neutron detector in the same string. From the time series data of X2, the cross-correlation function φ1
2(τ) is obtained using the following equation 0.

Fourier transform φ,2(τ) to obtain the mutual power spectral density φ□2(f) using equation (a) below.Due to the nature of the mutual power spectral density φ1□(f), there is a simple transport delay between the signals. In the case, the cross-correlation function φ1□(τ
), a time-delayed peak appears, and the mutual power spectral density φ1. A linear trend proportional to the frequency f appears in the phase of (1□πf).

In addition, the coherency c, which represents the cross-correlation coefficient between signals,
oh(f) is determined by the following formula.

Coherency represents the strength of the coupling between propagating signals; in the case of uncorrelation, Icoh(f)I = O, and in the case of signals propagating without attenuation, Icoh(f)l
= 1, and takes a value between O and 1.

On the other hand, the signal X1(t) from the neutron detector includes a component corresponding to a minute disturbance moving along the flow, and this component is delayed in the signal XZ(t) from the neutron detector located downstream of the same string. will appear later. Therefore, the propagation velocity Ud(t) of the minute disturbance is expressed by the following equation 0.

Ud(t) = L1□/τ(f)
...0 However, the distance between the neutron detectors is -Ltd:2+1.In addition, the following two types of noise are well-known and observed as minute disturbances on the flow that exist in a gas-liquid two-phase flow state. There is a source.

■ Droplets flowing in the main stream of steam ■ Disturbing waves propagating on the liquid film on the surface of the fuel rod Among the noise sources mentioned above, ■ propagates at a speed equivalent to the vapor velocity, and ■ propagates at a slightly slower speed than ■. It is known. Therefore, in the region where the above two noise sources are dominant, there exists a transfer characteristic between the signals measured by the two neutron detectors placed in the same string in the axial direction, which can be simplified by the first-order 0 equation. do.

However, in Sit '12, the vibration component I nil n2 corresponding to the above ■■ is the noise component other than S and s2 observed by each detector, and α1. α8 is the signal attenuation rate, τ0. τ2 is the signal transmission time.

Therefore, the mutual power spectrum φx x of X□, X2
(f) is expressed by the following formula. However, n l l
It was assumed that n2 was uncorrelated.

φx x (f)=l Gx x I”
−(e where Gxx(f)=cx, Gs, s, (
f) e '10 α, G52s, (f) e 2+
The coherency between two const signals is determined by the above
In the vicinity of the frequency f2 for the propagation time τ2 of .

It can be written as follows, and the coherency is proportional to the signal attenuation rate α2 described in (2) above.

In reality, Gx x (f) cannot be expressed by a simple equation such as the above equation due to the finite data length and complex frequency characteristics, so τ1. Filtering processing for estimating τ2 is important.

Inside the fuel channel, a subcooled state (
Water (below the saturation temperature) is heated from the fuel rods and
Boiling begins and changes as follows as the void fraction or steam weight fraction increases.

Bubble flow ↓ Slag flow ↓ Annular flow During normal operation of a boiling water reactor (BWR), it is thought that the upper half of the fuel channel is in an annular flow state, and the above The noise becomes noticeable, causing two fluctuation components with different frequencies in the neutron flux fluctuation.

The fluctuation component of the neutron flux, which is the object of the present invention, is determined by the wt
This is the fluctuation induced in the measured component.

The annular two-phase flow has the following flow pattern.

■ Disturbance wave crest
region) ■ Ripple region (ripple re
ion) ■ Liquid film rupture area (non-wetting
region) The above (1) → ■ corresponds to the transition of the flow pattern that occurs when the steam quality increases, and in the above ■,
The liquid film ruptures and a dry heat transfer surface is exposed.
) corresponds to a boiling transition event.

By the way, in the case (2) above, the annular flow mode starts, and as the vapor velocity increases, a stirring wave with a high wave height propagates at a high frequency on a relatively thick liquid film.

In the above case (2), a ripple-shaped wave with a small wave height propagates at a high speed for a vapor velocity higher than that in the above case (2). Therefore, in the fluctuation of the neutron detector corresponding to such a flow pattern, a decrease in the propagation time τ is observed according to the above-mentioned ■→the above-mentioned ■. When the vapor quality further increases, it shifts to the region (3) above, and the liquid film breaks, resulting in the cross-correlation component caused by the disturbance waves on the liquid film between the two neutron detectors seen in (■) above. disappears.

The present invention uses the principle of detecting the occurrence of a boiling transition by the disappearance of such a disturbance velocity. That is, the propagation velocity Ud of the disturbance wave given by the above equation 0 is constantly monitored, and as shown in Fig. 2, when the propagation velocity Ud disappears due to the increase in output, it is determined that a boiling transition has occurred. It is. In addition.

A method for determining that the cross-correlation component corresponding to the above-mentioned region (■) disappears due to the occurrence of the region (■) is to calculate the coherency 1 at the frequency corresponding to the disturbance wave propagation, as shown in FIG.
It is conceivable to plot coh(fz)l and monitor whether the coherency decreases discontinuously or falls below a threshold with respect to an increase in output.

(Function) According to the boiling transition occurrence monitoring device of the present invention, it is possible to immediately determine that a boiling transition has occurred when the propagation velocity of the disturbance wave disappears due to an increase in the furnace output. Operation that allows transitions becomes possible, and the operating range can be expanded.

(Example) An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention, in which data detected by a plurality of in-core neutron detectors 1 and 2 installed along the flow of cooling water in the reactor core is sent to a data collection device 3. collected. The in-core neutron detection signal from the data collection device 3 passes through a filter 4 and a mutual power spectral density calculation device 5. It is inputted to the boiling transition occurrence determination device 8 via the propagation time calculation device 7. In addition, the aggregate output is the process computer 6
is inputted to the boiling transition occurrence determination device 8 via. Also,
The coherency at the frequency corresponding to the disturbance wave propagation is calculated using the coherency calculation device f! 9 to the boiling transition occurrence determination device 8. When the boiling transition occurrence determining device 8 determines that the boiling transition has occurred, it outputs a determination signal to the 1 display device 10.

Next, the operation of this embodiment will be explained.

A neutron detector signal in the reactor core is collected by a data collection device 3 and sent to a filter 4. Of the neutron detector fluctuation components, the filter 4 filters out components that fluctuate throughout the core (glo
bal component), and extract spatially varying components (local component).

The mutual power spectral density calculation device 5 calculates the mutual power spectral density from the neutron detector signals at two upper portions of the channel belonging to the same string, and sends it to the propagation time calculation device 7.

The propagation time calculation device 7 estimates the propagation time from the frequency characteristics of the phase of the mutual power spectral density and the component corresponding to the propagation of the disturbance wave on the liquid film.

The coherency calculation device 9 evaluates the coherency corresponding to the disturbance wave propagation time.

Then, the boiling transition occurrence determination device 8 determines the occurrence of a boiling transition based on the propagation time and the coherency and aggregate output corresponding to the propagation time, and sends it to the display device 10.

The display device 10 receives boiling transition occurrence determination signals for all fuel channels in the core and displays channels in which boiling transition has occurred in the core.

〔Effect of the invention〕

As explained above, according to the present invention, the integrity of the fuel can be ensured by monitoring the boiling transition in the channel in the reactor core, and furthermore, the operation that allows the boiling transition within a certain period of time is also possible. This has the excellent effect of expanding the operating range and improving the operating efficiency of the reactor.

[Brief explanation of the drawing]

Figure 1 is a block diagram of an embodiment of the present invention, Figure 2 is a diagram showing the relationship between disturbance wave propagation speed and aggregate output, and Figure 3 is coherency and aggregate output at frequencies corresponding to disturbance wave propagation time. FIG. 1.2... In-reactor neutron detector 3... Data collection device 4... Filter 5... Mutual power spectral density calculation device 6... Process computer 7... Propagation time calculation device 8. ... Boiling I14 transition occurrence determination device 9 ... Coherency calculation device 10 ... Display device

Claims (1)

[Claims] (1) Evaluate the mutual power spectral density between multiple in-core neutron detectors installed along the flow of cooling water in the reactor core, and use this to determine the propagation velocity of the disturbance wave propagating on the liquid film on the surface of the fuel rods. occurrence of a boiling transition in a channel in a reactor core, characterized in that it is determined that a boiling transition has occurred when coherency with respect to a frequency corresponding to a disturbance wave propagation time between the detectors decreases. Monitoring equipment.