JPH0219919B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for monitoring the axial power of a nuclear reactor core outside the core.

In order to operate a nuclear reactor safely, it is necessary to constantly monitor the power distribution of the reactor core.

Conventionally, the axial power distribution of a nuclear reactor core for this purpose has been determined by, for example, calculations using a one-dimensional diffusion calculation code. The power distribution of the reactor under various steady-state conditions is determined by the reactor power,
Once the coolant temperature, burnup, and control rod position are determined,
It has been confirmed that one-dimensional diffusion calculations can be used to accurately grasp this phenomenon. In addition, during xenon transient fluctuations such as output fluctuations, if the one-dimensional diffusion calculation code is incorporated to track temporal changes in iodine and xenon by calculation, the output distribution can be grasped with high accuracy. Are known. However, the calculations for understanding using this one-dimensional diffusion calculation code are quite complex, requiring a lot of repeated calculations, requiring a lot of input data, and requiring a lot of computer calculation time. It was difficult to use it directly for power plant operation. Therefore, in order to easily calculate the reactor power distribution using a one-dimensional diffusion calculation code at the nuclear power plant site, it is possible to reduce the accuracy and shorten the processing time by reducing the number of nodal points in the calculation of the axial core. It's possible, but even if you make it possible to calculate it with the site's calculator,
It was still too time-consuming and unsuitable for immediate use in predictive calculations during various output fluctuations. For this reason, it is desired to develop an axial power distribution monitoring method that can easily and manually compare changes in power distribution during various output fluctuations without spending a lot of time on calculations.

This invention was made in view of the above-mentioned circumstances, and is an axial power distribution that allows on-site operators to easily, quickly, and accurately grasp changes in the power distribution of a nuclear reactor when output fluctuates. The purpose is to provide a monitoring method.

Corresponding to this purpose, the axial power distribution monitoring method of the present invention includes a power range neutron detector that detects the power in the axial upper half of the reactor core, and a power range neutron detector that detects the power in the axial lower half of the reactor core. a detector is arranged outside the reactor core, and an axial offset signal representing the difference between the relative output of the upper half of the reactor core and the relative output of the lower half of the reactor core is extracted based on the output detection by each of the power range neutron detectors, And when the axial position of the bottom of the core is Zb, the axial position of the power peak in the lower half of the core is Z1, the axial position of the power peak in the upper half of the core is Z2, and the axial position of the top of the core is Zt. , the core is axially
Zb≦Z≦Z1, Z1≦Z≦Z2 and Z2≦Z≦
When dividing Zt into three parts and using the axial offset AO signal for each part to derive and monitor the axial power distribution in a functional form, the lower axial half of the core and the axial upper half of the core The axial output distribution is derived by setting the change in the axial offset ΔA.O. to ΔA.O.=g1・ΔKeff+g2 when the reaction difference due to half the xenon is ΔKeff, and g1 and g2 are constants. There is.

The details of this invention will be explained below with reference to one embodiment.

In FIG. 1, reference numeral 1 denotes a reactor core, which is disposed within a pressure vessel 2. As shown in FIG. Four power range neutron detectors 3 are arranged symmetrically with respect to the axis of the reactor core 1 on the outside of the pressure melt vessel 2, surrounding the reactor core 1. As shown in FIG. 2, the power range neutron detectors 3 are each composed of an upper power range detector 3a and a lower power range detector 3b, and the upper power range detector 3a is located in the upper half of the reactor core 1. The power of the upper half of the reactor core 1 can be detected by being arranged correspondingly,
Further, the lower power region detector 3b can detect the power of the lower half of the reactor core 1. If the output of the upper output area detector 3a is q _T and the output of the lower output area detector 3b is q _B , then the axial offset (A.
O.) is defined as AO=q _T −q _B /q _T +q _B ×100(%)…(1). In this invention, first, this axial offset is taken out and the axial output distribution is derived. In other words, as shown in Figure 2, the height of the reactor core is
1.0, and P(z): Axial output Z: Axial height (0 to 1.0) Z ₁ : Lower peak position Z ₂ : Upper peak position Fz ₁ : Lower peak output Fz ₂ : Upper peak output Ai ( i = 1 to 4): Assuming the display formula for the power distribution in each region in the axial direction, (1) 0ZZ ₁ , (2) Z ₁ in the axial direction of the core
ZZ ₂ , and (3) Z ₂ Z1.0, divided into three regions, (1) At 0ZZ ₁ , P (z) = A ₁ = Fz ₁ {cos [2π (Z - Z ₁ ) × a ₁ ] +c ₁ (Z−Z ₁ )} …(2) (2) In Z ₁ ZZ ₂ , P(z)=A ₂ +A ₃ A ₂ =Fz ₁ e ^-(zz ₁ ^)b1 ×cos [−2π(Z − Z ₁ ) × a ₂ ] …(3) A ₃ = Fz ₂ e ^(zz ₂ ^)b2 × cos [2π (Z − Z ₂ ) × a ₃ ] …(4) (3) Z ₂ P at Z1.0 (z)=A ₄ =Fz ₂ {cos[−2π(Z−Z ₂ )×a ₄ ] +c ₄ (Z ₂ −Z)} …(5). Here, a ₂ and a ₃ are a ₂ = a ₃ = 1/4 (Z ₂ - Z ₁ ).

Next, normalize as ∫ ^1.0 ₀ P(z)dz=1.0 (6). The axial offset (AO) in equation (1) is expressed as follows.

AO=∫ ^1.0 _z2 A ₄ dz+∫ ^z2 _0.5 (A ₂ +A ₃ )dz −∫ ^0.5 _z1 (A ₂ +A ₃ )dz−∫ ^z1 ₀ A ₁ dz …(7) On the other hand, the output at each point in the axial direction is As illustrated in FIG. 3, there is a tendency to form a straight line with respect to changes in axial offset. Therefore, in general AO
and the upper peak Fz ₂ can be expressed as:

Fz ₁ = α ₁ × (AO) + α ₀ Fz ₂ = β ₁ × (AO) + β ₀ …(8) The previous equations (2) to (5) are transformed into (6), (7), (8) When combined with , the constants a ₁ , a ₄ , b ₁ , and b ₂ in equations (2) to (5) are determined.

On the other hand, the constants c ₁ and c ₄ are determined by the following equations using the assumption that the extrapolated distances λ ₁ and λ ₂ at the upper and lower ends of the core are constant values.

As mentioned above, each constant in equations (2) to (5) is automatically determined by providing the following data, and the output distribution can be calculated synthetically.

Upper peak position: Z ₂ Lower peak position: Z ₁ Coefficients of equation (7): α ₁ , α ₀ , β ₁ , β ₀ Extrapolated distance at the upper end: λ ₂ Extrapolated distance at the lower end: λ ₁ These If you calculate the data in advance,
An axial power distribution corresponding to an arbitrary axial offset is calculated using equations (2) to (4).

The above technology is described in the specification and drawings of Patent Application No. 182107 filed in 1982.

By the way, in order to further improve the accuracy of the required axial output distribution during xenon transient fluctuations, it is necessary to incorporate the effects of time changes in xenon and iodine into the axial output distribution. be.

The temporal change of xenon and iodine is expressed by the following formula.

dI(t, Z)/dt=λ _I I(t, Z)+Y _I (
)…(10) dx(t, Z)/dt=λ _I I(t, Z)+Yx(
) −(λx+())・x(t,Z)…(11
) However, I (t, Z): Number density of iodine at axial position Z at time t x (t, Z): Number density of xenon at axial position Z at time t λ _I : Decay constant of iodine λx: Decay constant of xenon Y _I : Fission yield of iodine Yx: Fission yield of xenon (): Number of fission at axial position Z, calculated as follows.

() = (Σf ₁ φ ₁ + Σf ₂ φ ₂ ) × P (z) × P _REL … (12) ^x (): Calculated as follows using the product of the absorption cross section of xenon and the neutron flux at the axial position Z be done. ^x () = (σa ₁ ^x φ ₁ + σa ₂ ^x φ ₂ ) × P (z) × P _REL … (13) Σf ₁ , Σf ₂ : Core average fission cross section of high-speed group and thermal group φ ₁ , φ ₂ : Core average neutron flux of high-speed set and thermal set σa ₁ ^x , σa ₂ ^x : Micro absorption cross-section of xenon in high-speed set and thermal set P _REL : Relative core power of the reactor P(z): At axial position Z The average in the output
Standardized to 1.0. Same as P(z) mentioned above.

It is.

Solving equations (10) and (11) yields the following.

However, I ₀ (z): Initial value of the number density of iodine at the axial position Z. X ₀ (z): Initial value of the number density of xenon at the axial position Z. ΔT: Elapsed time.

If P(z) is expressed in the functional form of equations (2) to (5) above, the xenon axial distribution at any point in time can be found from the axial offset (for iodine, This is ignored because the neutron absorption cross section is small.) Once the xenon distribution is determined, the difference in reactivity due to xenon between the upper half of the core and the lower half of the core can be calculated as follows.

ΔKeffx＝lnKeff _T ／Keff _B ×100 …(17) However, D ₁ , Σa ₁ , Σ _R , νΣf ₁ : Core average nuclear constant D of high-speed group; Diffusion coefficient Σa; Absorption cross-section Σ _R ; Removal cross-section νΣf ; Product of fission cross section and number of neutrons generated D ₂ , Σa ₂ , νΣf ₂ : Core average nuclear constant of thermal set B ₂ : Buckling Σa ^x _1T , Σa ^x _2T : High-speed set of xenon half core,
Absorption cross section of thermal group Σa ^x _1B , Σa ^x _2B : Absorption cross section of xenon high speed group and thermal group in the lower half of the core. Here, the relationship between the reactivity difference ΔKeffx and the axial offset change is calculated and graphed as shown in FIG. 4, and the relationship between the two is a good straight line. That is, it can be assumed that the following relationship exists between the difference in reactivity between the upper and lower halves of the core and the change in axial offset.

ΔA.O.=g ₁・ΔKeff+g ₂ …(18) However, g ₁ and g ₂ are constants. Therefore, if we know the difference in reactivity due to xenon between the upper and lower halves of the core, we can find the range of change in axial offset from equation (18) and calculate a new power distribution. Changes in axial offset during xenon transient motion can be grasped by performing calculations sequentially over time in this manner. Changes in axial offset due to changes in output level and changes in axial offset due to insertion of control rods can be calculated in advance and inputted in a table format for use.
The output distribution in xenon transient states such as during output fluctuations can be easily grasped as described above.

The detection of the axial output distribution through the operations and calculations described above is performed by the apparatus shown in FIG. That is, in FIG. 5, 11 is an axial power distribution monitoring device, and the axial power distribution monitoring device 11 includes an upper power region neutron detector 3a and a lower power region neutron detector 3b.
a and 3b are connected to an axial offset calculation device 13, and the output signal of the axial offset calculation device 13 is connected to an axial power distribution calculation device 14. The detectors 3a and 3b detect the upper half output and the lower half output of the core, respectively, and send these signals to the axial offset calculation device 13, where the initial axial offset signal shown in equation (1) is created. ,
This is sent to the axial power distribution calculation device. The axial power distribution calculation device 14 uses the input axial offset signal, the output level change, control rod position signal, and time width ΔT that are input separately, and calculates Z that has been prepared and input in advance. ₁ , _Z2 ,
Based on P(z) calculation data such as α ₁ , α ₀ , β ₁ , β ₀ , λ ₁ , λ ₂ , xenon, iodine data, 2-group nuclear constant, axial offset change data, etc. ,
Perform calculations using equation (18) to calculate changes in axial output distribution. This synthetically calculated axial output distribution can be used as an input signal to other equipment as required.

With the axial power distribution monitoring method configured as described above, the axial power distribution in a xenon transient state such as when the output fluctuates can be accurately and easily derived.
That is, Fig. 6 compares the change in axial offset during xenon oscillation after creating a disturbance by inserting and withdrawing the control rod bank D from the full power state with the predicted value using the one-dimensional diffusion calculation and the method of this invention. It shows. It can be seen that the output values derived by the method of this invention are in good agreement with the one-dimensional diffusion calculation results. Also, in Figure 7, 50
The predicted value of the axial offset during xenon oscillation after the output is reduced to the % output state using the method of the present invention is compared with the calculation result using the one-dimensional diffusion calculation method. It can be seen from FIG. 7 that in this case as well, the predicted values according to the present invention agree well with the one-dimensional diffusion calculation results. Moreover, according to the method of this invention, the calculation time required by the conventional one-dimensional diffusion calculation code is approximately 1/1/1.
It only requires about 100, and is suitable as a method for site operators to use.

As is clear from the above description, according to the present invention, it is possible to obtain an axial power distribution monitoring method that allows operators on site to easily, quickly, and accurately grasp changes in the power distribution of a nuclear reactor when the power fluctuates. can.

[Brief explanation of drawings]

Fig. 1 is a plan view showing the arrangement of the neutron detectors, Fig. 2 is a side view showing the arrangement position of the neutron detectors, and Fig. 3 shows the relationship between the output of each part in the core axial direction and the axial offset signal. Fig. 4 is a graph showing an example of change in axial offset due to reactivity difference, Fig. 5 is a block diagram showing the configuration of the axial output distribution detection device, and Fig. 6 is obtained by one-dimensional diffusion calculation. A graph showing a comparison between the axial offset change and the axial offset change determined by the method of the present invention, and FIG. 7 show the axial offset change determined by the one-dimensional diffusion calculation and the axial offset change determined by the method of the present invention. 7 is a graph showing another example of comparison with axial offset change. 3 (3a, 3b)...output region neutron detector,
4... Intermediate region neutron detector, 11... Axial power distribution monitoring device, 13... Axial offset calculation device, 14... Axial power distribution calculation device.

Claims (1)

[Claims] 1. A power range neutron detector that detects the output of the upper half of the core in the axial direction and a power range neutron detector that detects the output of the lower half of the core in the axial direction are arranged outside the core. , extracts an axial offset signal representing the difference between the relative power of the upper half of the core and the relative power of the lower half based on the output detected by each of the power range neutron detectors, and determines the axial position of the bottom of the core as Zb, The axial position of the power peak in the lower half of the core is Z1, the axial position of the power peak in the upper half of the core is Z2, and the axial position of the top of the core is
When Zt, Zb≦Z≦Z in the axial direction of the core
1. When dividing into three parts, Z1≦Z≦Z2 and Z2≦Z≦Zt, and deriving and monitoring the axial output distribution by functional form display using the axial offset AO signal for each part. , the reaction difference due to xenon between the axially lower half of the core and the axially upper half of the core is ΔKeff, and when g1 and g2 are constants, the change in the axial offset ΔA.
An axial power distribution monitoring method characterized in that the axial power distribution is derived by setting O. to ΔA.O.=g1・ΔKeff+g2.