JPH0525319B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains]

The present invention relates to a subcriticality monitoring device for nuclear fuel equipment that monitors the subcriticality in the nuclear fuel arrangement state of nuclear fuel equipment that loads or stores nuclear fuel.

[Prior art and its problems]

Criticality safety management is performed to ensure that the nuclear fuel in nuclear fuel equipment loaded or stored with nuclear fuel does not become critical. Criticality alarms are used in nuclear fuel reprocessing facilities as a safety control for this purpose, but this criticality alarm is a system that issues an alarm when it becomes critical, so it is possible to predict in advance that it will become critical. It is not possible. Therefore, it is necessary to understand the degree of subcriticality (effective multiplication factor in a subcritical state) and manage safety. Conventional methods for measuring subcriticality include the pulsed neutron method, which involves injecting pulsed neutrons, the reactor noise analysis method, which measures the fluctuation components of neutron density with two detectors, and the insertion of a neutron source. Neutron source multiplication methods are known, which are calculated from the gradient of neutron multiplication, but these methods require a special neutron source generator that generates pulsed neutrons, and a synchronization method that synchronizes detection by two detectors. This method requires a special circuit, such as a circuit, and also requires a reactivity calibration experiment near the criticality, which has the drawbacks of requiring complicated equipment and time.

[Purpose of the invention]

In view of the above-mentioned points, the present invention does not require a reactivity calibration experiment in the vicinity of criticality, and uses ordinary neutron counting means to monitor the subcriticality of the fuel arrangement state of nuclear fuel equipment. The purpose of the present invention is to provide a criticality monitoring device.

[Key points of the invention]

According to the present invention, the above object is to provide a pair of detectors consisting of a detector with high sensitivity to thermal neutron energy and a detector with higher sensitivity to fast neutron energy; a counting circuit that calculates the counting rate ratio of , a memory having a calculation program that calculates the degree of subcriticality from the counting rate ratio, and a memory having a calculation program that calculates the degree of subcriticality from the counting rate ratio, and inputting the counting rate ratio from the counting circuit and the calculation program to calculate the current nuclear fuel arrangement. A computing device that determines the degree of subcriticality corresponding to the state and predicts the degree of subcriticality at the time of nuclear fuel loading, and a predetermined value for safety management and the degree of subcriticality at the time of loading the nuclear fuel obtained from this computing device. This is achieved by configuring the system with an alarm device that issues a warning based on the comparison.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a system diagram of a nuclear fuel equipment monitoring device according to an embodiment of the present invention. In the figure, reference numeral 1 denotes a paired detector consisting of two detectors having different neutron energy response characteristics. Pair detector 1 is a combination of detectors with different neutron energy response characteristics, one with high sensitivity to thermal neutron energy and the other with higher sensitivity to fast neutron energy (approximately 1 MeV or higher), such as ²³⁵ U Fission counter and ²³⁸ U fission counter, ¹⁰ B counter and ²³⁷ Np
Fission counter, a combination of a ²³⁵ U fission counter and a ²³⁷ Np fission counter. Note that the former of the above combination arrays is highly sensitive to thermal neutrons,
The latter is a highly sensitive detector for fast neutrons;
The pair detector 1 may be selected from a combination suitable for the nuclear fuel equipment. The counter-detector 1 is inserted into a counter-detector guide tube 2 and placed at a location where nuclear fuel is placed in a nuclear fuel facility. That is, as shown in FIG. 2, a plurality of pair detectors 1 are arranged in the nuclear fuel collection area 3 at the nuclear fuel arrangement location, or
As shown in FIG. 3, a plurality of them may be arranged in the surrounding area 4 of the gathering area 3, or a plurality of them may be arranged in both the gathering area 3 and the surrounding area 4. In addition, paired detector 1
The number of subcriticality may be arbitrary, but the greater the number, the better the accuracy of the determined degree of subcriticality. The counting circuit 5 is connected to the paired detector 1 in the paired detector guide tube 2.
The detector 1 counts the neutrons generated by the neutron source in the nuclear fuel collection area 3 or surrounding area 4 (see Figures 2 and 3), and calculates the count rate ratio. . Note that the neutron source may be an external neutron source using a normal neutron source generator, or an internal neutron source using spontaneous fission neutrons of an isotope contained in the nuclear fuel itself. 7 is a microcomputer as a calculation unit, which inputs the count rate ratio inputted from the counting circuit 5 and a calculation program 10 consisting of a calculation program 8 from a memory 11 and a database 9, which will be described later, and calculates the paired detector 1. Calculate, determine, and predict the degree of subcriticality of the nuclear fuel arrangement state of the nuclear fuel equipment. Note that 12 is a printer that prints the output of the microcomputer 7, and 13 is an alarm that issues a warning by comparing it with a predetermined value for criticality safety management. The memory 11 includes a calculation program 8 that calculates the subcriticality (effective multiplication factor in a subcritical state) of the nuclear fuel arrangement state of the nuclear fuel facility equipped with the paired detector 1 from the count ratio of neutrons counted by the paired detector 1, and this calculation. It stores an arithmetic program 10 consisting of a database 9 that holds data such as absorption cross sections that are sometimes required. The calculation program 10 calculates the degree of subcriticality from the count rate ratio R, and the calculation method will be explained next. The behavior of neutrons in a subcritical state system is usually described by a neutron flux distribution function φ(γ, E). Here, φ is a distribution function that depends on the spatial coordinate γ and the energy E, and is expressed by the well-known Boltzmann equation. Solving the following Boltzmann equation Hφ=S ...(1), which is expressed using operators. It is determined by Here, H is the Boltzmann operator and S is the source term due to the external neutron source. In addition, the neutron flux distribution function φ is generally
The eigenfunction φ _0o (n _{= 0} , 1, 2,
...) is expressed as a linear combination of That is, if a _o is an arbitrary coefficient, the neutron flux distribution function φ is φ= 〓 ⁿ a _o φ _0o ……(3). If the eigenfunction belonging to the minimum eigenvalue among the positive eigenvalues in equation (2) is represented by φ ₀ , only the minimum eigenvalue state φ ₀ is realized in the critical state, and in this case, equation (2) is called a critical equation. Furthermore, equations (1) and (2) are equivalent, and φ=φ ₀ (critical state)...(4). By the way, if the response functions of the two detectors of paired detector 2 are Σ ₁ and Σ ₂ , respectively, then paired detector 2
The count rate ratio R of the neutrons counted is the ratio of the count ratios <Σ ₁ φ and <Σ ₂ φ> by the respective detectors.
That is, R=<Σ ₂ φ>/<Σ ₁ φ> (5) Here, the symbol <> represents an integral quantity over the phase space by γ and E. The count rate ratio R in equation (5) is a quantity that can be measured in a subcritical state, and is usually called a spectral index. On the other hand, in order to connect this quantity to the critical state, we use the minimum eigenvalue state φ ₀ , which is the solution to the eigenvalue equation (2), and write R ₀ =<Σ ₂ φ ₀ >/<Σ ₁ φ, exactly as in equation (5). Consider the unique count rate ratio R ₀ defined by ₀ > ...(6). Here, using this unique count rate ratio R ₀ , a new quantity R' is introduced as follows: R'=R/R ₀ (7). Note that R' is called the effective count rate ratio. Therefore, in the critical state when the effective multiplication factor k=1, by applying the relationship (φ=φ ₀ ) in equation (4) to equations (5) and (6), the effective count rate ratio R′ becomes R′=1. ...(8) becomes. Therefore, the effective count rate ratio R' corresponds to the effective multiplication factor k. From the above relationship, the effective count rate ratio R' can generally be expressed as follows using an arbitrary functional form F(x). R'=C ₀ -F(1-k)...(9) Here, C ₀ is a constant and k is an effective multiplication factor. However, when F(0)=0 and theoretically the critical state k=1, from equation (8), C ₀ =1 (10). In other words, in general, after determining the arrangement of a pair of detectors and the nuclear fuel in the fuel equipment, the relationship when bringing the nuclear fuel equipment to a critical state by the near-criticality procedure by loading nuclear fuel is expressed by equation (9). Equation (9) is a basic relation between the effective count rate ratio R' and the effective multiplication factor k (reactivity is (k-1)/k). Therefore, the effective multiplication factor k in the subcritical state of the nuclear fuel equipment can be obtained via the effective counting rate ratio R' based on equation (9). Here, R′ is in the form of a ratio of counting rate ratios as shown in equation (7), so it is not directly related to the detector counting efficiency, so it is difficult to include counting errors, but in addition, R′ In order to improve the evaluation accuracy of the value of , input data such as the neutron cross section in the Boltzmann equation is corrected so that the measured data of the count rate ratio R and the count value agree well. The calculated value of the corrected effective multiplication factor k of this input data is also corrected at the same time. The set of (R', k) values obtained by adjusting in this way will be obtained for the number of nuclear fuel loading state systems for which the count rate ratio was measured, and this set of values can be expressed as (9)
When applied to the equation, the functional form F( _1-
k) counting parameters can be determined.
In this case, (10) guarantees the certainty of the above adjustment.
This is the condition of C ₀ =1 when k=1 in the equation. When determining the above counting parameters, the normal regression analysis procedure is performed using the counting rate ratio normalization constant C _R as another parameter in addition to the neutron cross section, and the coefficient parameters of equation (9) are determined. Confirm. Note that the count rate ratio normalization constant C _R mentioned above is defined as the following equation. C _R =R _c /R _e (11) where R _e is a measured value for the count rate ratio of the detector to the detector, and R _c is a calculated value. The above-mentioned regression analysis procedure is so-called reactivity calibration and does not need to include measurement data near the critical level. With the above configuration, the paired detector 1 is arranged at the nuclear fuel installation location of the nuclear fuel facility, and the counting circuit 5 calculates the counting rate ratio of neutrons multiplied by the neutron source, and the calculation program 8 and the database 9 are stored in the memory 11. The degree of subcriticality is determined by taking out the calculation program 10 and performing calculations on the minicomputer 7. Furthermore, since the evaluation accuracy of the effective count rate ratio R' in the process of determining the degree of subcriticality mentioned above is extremely good,
Effective count rate ratio for upcoming nuclear fuel loading conditions
The R′ value can also be predicted with high accuracy. Therefore, by applying this R' value to the calibrated equation (9), a predicted value of the effective multiplication factor k can be obtained.

【Effect of the invention】

As is clear from the above description, according to the present invention, the counting rate ratio is measured by a pair of detectors at each nuclear fuel loading step of the nuclear fuel equipment, and the current and future nuclear fuel placement status of the nuclear fuel equipment is determined from this counting rate ratio. By determining and predicting the degree of subcriticality by calculating it with a calculator, it is possible to set a predetermined alarm value for criticality safety management of the degree of subcriticality in advance. In addition, the subcriticality prediction function can prevent future criticality accidents due to nuclear fuel loading.
Criticality safety management standards enable safer criticality management by predicting and issuing warnings in advance, giving us time to take preventive measures. In addition, the determination and prediction of the degree of subcriticality described above does not require a separate reactivity calibration experiment unlike the conventional technology, and is performed using spontaneous fission neutrons and neutron counting circuits from a normal neutron source generator or the nuclear fuel itself. This also has the effect that the cost of the monitoring device is low and its maintenance is easy.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of a subcriticality monitoring device for a nuclear fuel facility according to an embodiment of the present invention, FIG. 2 is a layout diagram showing the arrangement of a pair of detectors in the subcriticality monitoring device of FIG. 1, and FIG. FIG. 6 is a layout diagram showing different arrangements of paired detectors. 1: Pair detector, 5: Counting circuit, 7: Arithmetic unit, 1
0: Arithmetic program, 11: Memory.

Claims (1)

[Claims] 1. A pair of detectors consisting of a detector with high sensitivity to thermal neutron energy and a detector with higher sensitivity to fast neutron energy, and a counting circuit that calculates the count rate ratio of neutrons detected by this pair of detectors. and a memory having a calculation program for calculating the degree of subcriticality from the count rate ratio, and inputting the count rate ratio from the counting circuit and the calculation program to determine the degree of subcriticality corresponding to the current arrangement state of the nuclear fuel. At the same time, a computing device predicts the degree of subcriticality at the time of nuclear fuel loading, and an alarm device that issues an alarm by comparing the degree of subcriticality at the time of nuclear fuel loading obtained from this computing device with a predetermined value for safety management. A subcriticality monitor device for nuclear fuel equipment, characterized by comprising: