JPH0469759B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Purpose of the Invention [Field of Industrial Application] This invention is a nuclear reactor that changes the boron concentration in the primary coolant by adding dilution water like in a pressurized water reactor. This invention relates to a method for monitoring the approach of a nuclear reactor to achieve criticality.

[Prior art] In order to maintain a nuclear reactor in a critical state, it is necessary to balance the generation and absorption of neutrons and maintain the nuclear fission reaction in a constant state. In other words, by withdrawing the control rod from the core and changing the boron concentration in the primary coolant (lowering the boron concentration in the primary coolant by adding dilution water). A critical state is achieved.

In pressurized water reactors, a method is adopted in which the boron concentration in the primary coolant is gradually diluted and reduced from the boron concentration at reactor shutdown by adding dilution water, bringing the reactor closer to a critical state. ing. In this case, in order to understand to what extent the reactor is in a subcritical state at each point in time when it approaches a critical state, and how much dilution water must be added to make it critical, neutron detection in the source area is used. Nuclear measurements (detection of neutrons) are carried out using instruments. The detection of neutrons by this source area neutron detector is carried out, for example, by the source area neutron detector (NE-3) shown with reference numeral 1 in FIG.
1 and NE-32), and the approach of criticality is monitored by the signal from this detector.

That is, the total number of neutrons generated by the neutron source indicated by reference numeral 2 in Fig. 3 is S ₀ /(1-K _eff )...(1) where the neutron source intensity is S ₀ (n/sec). It is known that the formula is

Here, K _eff is the effective multiplication factor of the reactor, in the critical state K _eff = 0.1, and in the subcritical state K _eff <1.0
It is.

Further, the number of neutrons measured by the source area neutron detector 1 is proportional to the above equation (1), and therefore is inversely proportional to 1- _Keff . Generally, when approaching criticality, the following data processing (1/M) is performed to monitor criticality.

That is, (1/M)=(source area neutron detection value in reference state)/
(Source area neutron detection value at the time of measurement)...Equation (2) However, the source area neutron detection value in the reference state is the value measured by the source area neutron detector in a stable state before the start of critical approach. This refers to the number of neutrons, and the same value is used throughout the (1/M) plot.

In this way, the value of 1-K _eff is monitored by measuring (1/M) data over time, and therefore,
The criticality of the reactor is monitored by monitoring that the (1/M) data approaches zero.

However, since 1- _Keff is actually proportional to the amount of dilution water, (1/M) is in a decreasing relationship with the amount of dilution water.

As mentioned above, in the original method (calculation formula), the (1/M) plot is supposed to be a straight line that monotonically decreases with respect to 1-K _eff or the amount of dilution water, but this is not actually the case. The reason for this is that the neutron source 2 is located near the outer boundary of the reactor core 3, as shown in Figure 3, and is not located deep inside the reactor core. This is because the neutrons that are generated include not only neutrons generated by nuclear fission in the reactor core 3 but also neutrons that enter straight from the neutron source 2. Therefore, the shape of the (1/M) plot varies depending on the neutron source position and core configuration, and these positional relationships do not necessarily result in a linear relationship.

An example of this is shown in FIG. 4 for an actual pressurized water reactor. Fig. 4 shows a critical proximity record diagram (vs. dilution water volume) based on the (1/M) plot, and the curve indicated by numeral 6 in the figure is based on the detector NE-
The curve shown at 7 is the (1/M) plot of detector NE-32. In this way, it can be seen that (1/M) does not form a simple straight line for the amount of dilution water that is proportional to 1- _Keff . In particular, it can be seen that at the point approaching the criticality, there is a deviation from the straight line (dotted chain line) indicated by reference numeral 9 in the figure.

For the purpose of actual criticality control, it is important to predict how much dilution water remains to reach criticality when approaching criticality. Therefore, in the conventional method shown in Figure 4, (1/ M) There was a problem that the deviation of the plot from the straight line was not accurate in predicting the criticality.

[Problems to be solved by the invention] This invention has been made in view of the above-mentioned circumstances. M)
1-The objective is to overcome the difficulty of predicting the final critical point due to the non-linear relationship with K _eff and to provide a method for monitoring the approach of criticality of a nuclear reactor that can accurately predict the critical point. It is.

(b) Structure of the invention [Means for solving the problem] In response to this objective, the method for monitoring criticality approach of a nuclear reactor of the present invention changes the control rod operation and the boron concentration in the primary coolant. In the method for managing criticality at the time of criticality approach using the (1/M) plot defined by (source area neutron detection value in the reference state)/(source area neutron detection value at the time of measurement), Obtain the (1/M) plot at least three times over time, and for (1/M) at each time point, ((1/M)) = K ₁ √ _T −+K ₂ (V _T −v) +K ₃ (V _T - v) ^2However , V _T ...Amount of dilution water required up to the critical point v...Amount of dilution water up to the relevant measurement point _K1 , _K2 , _K3 ...(1/M) Constant formula to reproduce the plot The present invention is characterized in that it further predicts the approach of a nuclear reactor to criticality using V _T derived from the plurality of simultaneous equations assuming v = 0 and (1/M) = 1.0 as initial conditions.

Hereinafter, details of the present invention will be explained with reference to the drawings showing one embodiment.

Figure 1 is a one-dimensional diffusion calculation code (1/M)
An example of predictive calculation of the plot (an example of a change in the (1/M) plot due to 1-K _eff ) is shown.

The straight line indicated by numeral 12 in the figure is an example in which there is a proportional relationship to 1-K _eff , and the dashed line indicated by numeral 11 is an example in which the radiation source is approximately 35 cm inside the core outer boundary, and the line indicated by numeral 10 The dotted line shown is an example where the source is approximately 20 cm inside the outer core boundary. 1st
The curves indicated by reference numerals 10 and 11 in the figure have shapes similar to the curves 6 and 7 in FIG. 4 described above, indicating that the shape of the (1/M) plot changes depending on the position of the radiation source. A feature of the present invention is to reproduce the (1/M) plot using the following calculation formula from these calculation results and the actual (1/M) plot.

In other words, (1/M)=K ₁ √ _T −+K ₂ (V _T −v) +K ₃ (V _T −v) ^2However , V _T ...The amount of dilution water required until the point of criticality (K _eff = 1.0) v... Amount of dilution water up to the relevant measurement point K ₁ , K ₂ , K ₃ ... (1/M) Constant to reproduce the plot The above calculation formula is applied to the reactor at least three times during the criticality approach process by adding dilution water. (1/
M), that is, equation (2), and calculate the required dilution water volume V _T up to the critical point from these multiple sets (simultaneous equations). In this way, once V _T is determined, the amount of dilution water (v) up to the measurement point (in the case of three measurements over time, the last measurement point) is a known value that has already been measured, so the V _T from v
By subtracting , the amount of dilution water required to reach criticality can be known from the measurement point in time, and therefore, the approach of criticality can be predicted with high accuracy.

The fact that the (1/M) plot in the form of equation (3) can be reproduced by the above-mentioned method of the present invention is indicated by the x point in FIG. In general, the (1/M) plot can be reproduced by equation (3), so when actually approaching criticality, the measured data of (1/M) is put into equation (3) and each constant K ₁ , K By determining ₂ and _K3 , the value of _VT (the amount of dilution water) can be calculated. Also, (1/M)
It is also possible to easily configure a device that automatically substitutes changes in measured data over time into equation (3) to estimate V _T . Also, once V _T is calculated, K _eff in the reference state
By giving in advance the value at the time of measurement,
K _eff can be calculated using the following formula.

K _eff = 1-(v/V _T )×(1-K _eff0 ) (4) In other words, it is possible to predict the degree of subcriticality at the time of measurement.

For example, FIG. 2 is a block diagram showing the configuration of a critical proximity monitoring device for implementing the method of the present invention, in which the neutron detection value measured by the source region neutron detector 1 is indicated by the symbol 8 (1/ M) It is input to the arithmetic unit and the calculation of (1/M) is executed at specified time intervals. The (1/M) plot prediction device indicated by reference numeral 9 uses the (1/M) data output from the (1/M) calculator 8, the amount of dilution water (v) already added, and the standard K. _eff is input and each constant K ₁ , K ₂ , K ₃ is calculated so that it becomes the form of equation (3). In other words, equation (3) includes four unknowns K ₁ , K ₂ , K ₃ and V _T Therefore, the (1/M) and v data can be given at multiple time points over time, and at least three sets of (1/M) and v data sets up to the measurement time point are given. , further assuming that v=0 and (1/M)=1.0 as initial conditions, 4
Since there are four simultaneous equations for two unknowns, the unknowns are determined and the constants K ₁ , K ₂ , and K ₃ are calculated. As a result, the dilution water volume V _T up to criticality, K _eff based on equation (4), and the conventional (1/M) plot prediction are output.

Furthermore, Figure 2 shows that the future (1/M) plot prediction, K _eff at the relevant measurement point, and the required amount of dilution water are output, but these are determined for each neutron detector in the source region. Therefore, two values are obtained for each (for example, like the curves 6 and 7 in FIG. 4).

(c) Effects of the invention According to this invention, K _eff at the relevant measurement point and the amount of dilution water up to the critical point can be determined from the (1/M) plot, and therefore criticality control with improved criticality prediction accuracy It is possible to provide a method for monitoring the approach to criticality of a nuclear reactor. Furthermore, the automatic processing device makes it possible to automate the criticality control using the (1/M) plot without manual intervention.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of calculating the change due to 1-K _eff in the (1/M) plot, FIG. 2 is a block diagram showing the configuration of a critical approach monitoring device for implementing the method of the present invention, and FIG. The figure shows the positional relationship between the neutron source, the source area neutron detector, and the reactor, and the fourth
The figure is an example of criticality approach recording using a (1/M) plot in a pressurized water reactor. 1... Source region neutron detector, 2... Neutron source, 3... Reactor core, 6... (1/M) plot by detector NE31, 7... (1/M) plot by detection NE-32 , 8...(1/M) arithmetic unit, 9...(1/M) plot prediction device, ×...
(1/M) plot reproduced by the method of the present invention.

Claims (1)

[Scope of Claims] 1 Defined by (source area neutron detection value in the reference state)/(source area neutron detection value at the time of measurement) by changing the control rod operation and the boron concentration in the primary coolant In the method of controlling criticality at the time of criticality approach using the (1/M) plot described above,
M) Obtain the plot at least three times over time, and for (1/M) at each time point, (1/M)=K ₁ √ _T −+K ₂ (V _T −v) +K ₃ (V _T −v) ² However, V _T ...The amount of dilution water required up to the critical point v... The amount of dilution water up to the relevant measurement point K ₁ , K ₂ , K ₃ ... (1/M) Give a constant equation that reproduces the plot, and further, A method for monitoring near-criticality of a nuclear reactor, comprising predicting near-criticality of a nuclear reactor using V _T derived from the plurality of simultaneous equations assuming v=0 and (1/M)=1.0 as initial conditions.