JPH0525320B2 - - Google Patents

Description

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

The present invention relates to a method for measuring the degree of subcriticality of nuclear fuel equipment, which measures the degree of subcriticality in the state of arrangement of nuclear fuel in nuclear fuel equipment in which nuclear fuel is loaded or stored.

[Prior art and its problems]

In a collection area where nuclear fuel is collected, for example, a nuclear fuel facility that stores nuclear fuel, criticality safety management is carried out to prevent it from becoming critical. In this case, it is necessary to understand the degree of subcriticality (effective multiplication factor in a subcritical state) and perform safety management. 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 method that uses a neutron source. There are methods such as neutron source multiplication method that calculates from the scent of neutron multiplication.
These require special neutron source generators that generate pulsed neutrons, special circuits such as synchronization circuits that synchronize the detection of two detectors, and reactivity calibration experiments near criticality. The disadvantage is that it requires 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, but measures the number of neutrons using a normal neutron counting means, and determines the degree of subcriticality in the nuclear fuel arrangement state of nuclear fuel equipment. The purpose of this study is to provide a method for measuring the subcriticality of nuclear fuel equipment and predicting the degree of subcriticality.

[Key points of the invention]

According to the present invention, the energy distribution of neutrons multiplied from the neutron source in at least one of the nuclear fuel collection area and the surrounding area of the area is such that one area has a high energy distribution with respect to thermal neutron energy. The neutron count rate ratio is measured by a pair of detectors consisting of a sensitive detector and a detector more sensitive to fast neutron energy, and the subcriticality of the nuclear fuel equipment is determined from this count rate ratio. Find the corresponding effective count rate ratio, perform regression analysis using a functional formula between this effective count rate ratio and the degree of subcriticality, determine the degree of subcriticality when the count rate ratio is measured from the regression analysis result, and This is achieved by predicting the degree of subcriticality at the time of fuel loading by reactivity calibration of the value of the effective count rate ratio.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings. FIGS. 1 and 2 are layout diagrams in which a paired detector consisting of two detectors for counting neutrons during subcriticality measurement according to the present invention is arranged. In FIG. 1, a plurality of paired detectors 2 are arranged in an accumulation area 1 where nuclear fuel is collected, and in FIG. 2, a plurality of paired detectors 2 are arranged in a peripheral area 3 of the accumulation area in FIG. The paired detector 2 is placed in at least one of the collection area 1 and the peripheral area 3 to measure the number of neutrons,
Determine and predict the subcriticality (effective multiplication factor of subcritical state) of nuclear fuel equipment. Pair detector 2 consists of two detectors that exhibit different counting responses to the neutron energy distribution, e.g. ²³⁵ U
Fission counter and ²³⁸ U fission counter, ¹⁰ B counter and
It consists of a combination of a ²³⁷ Np fission counter, ^{a 235} U fission counter, a ²³⁷ Np fission counter, etc. This combination is one (the former in the above combination)
has a high sensitivity to thermal neutron energy, and the other (latter) consists of a detector that has a higher sensitivity to fast neutron energy (approximately 1 MeV or more), and this combination is selected appropriately for each nuclear fuel facility. Neutrons are generated into nuclear fuel equipment by neutrons from an external neutron source by a normal neutron source generator disposed in the nuclear fuel collection area 1 or peripheral area 3 of the nuclear fuel equipment, or by neutrons from an internal neutron source by spontaneous fission neutrons of isotopes contained in the nuclear fuel itself. is multiplied, and the counting rate ratio R is measured by the paired detector 2. Next, a method for determining the degree of subcriticality from this count rate ratio R will be explained. 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 found by solving the following eigenvalue equation H ₀ φ _0o =0 ...(2) corresponding to equation ₍ 3). …
...) is expressed as a linear combination of That is, if a _o is an arbitrary system number, the neutron flux distribution function φ becomes 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 〓 ₂ , respectively, then paired detector 2
The counting rate ratio R of the neutrons counted is the ratio of the counting rate <〓 ₁ φ> and <〓 ₂ φ> by each detector. That is, R=<〓 ₂ φ>/<〓 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 eigenvalue equation (2), and write R ₀ =<〓 ₂ φ ₀ ＞／＜〓 ₁ φ in exactly the same way 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, the effective count rate ratio R ¹ becomes R′=1 by applying the relationship (φ=φ ₀ ) of equation (4) to equations (5) and (6). ...(8) becomes. Therefore, the effective count rate ratio R' corresponds to the effective multiplication factor k. From the above function, the effective count rate ratio R' is generally expressed as follows using an arbitrary function form F(x). R'=C ₀ -F(1-k)...(9) Here, C ₀ is a constant and k is an effective multiplication factor. However, F(0) = 0, and theoretically the critical state k
When =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 within 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 relational expression between the effective count rate ratio R' and the effective multiplication factor k (the 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 detection counting efficiency, so it is difficult to include calculation error, but R' In order to improve the evaluation accuracy of the value of R, input data such as the neutron cross section in the Boltzmann equation is corrected so that the measured data and the calculated value of the count rate ratio R match well. This modification of the input data also modifies the calculated value of the effective multiplication factor k at the same time. As many sets of (R′, k) values as are obtained by adjusting in this way are 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= in the equation. When determining the above coefficient parameters, the normal regression analysis procedure is performed using the count rate ratio normalization constant C _R as another parameter in addition to the neutron cross section, and the count parameters in equation (9) are Determine. In addition,
The count rate ratio standard constant C _R mentioned above is defined as the following equation. C _R =R _c /R _e (11) where R _e is the measured value for the count rate ratio of the detector to the detector, and R _c is the calculated value. The above regression analysis procedure is what is called reactivity calibration and does not need to include measurement data near the critical level. As described above, according to the present invention, the effective multiplication factor k in the subcritical state of nuclear fuel equipment is the effective count rate ratio
Using R′ as the main concept, we use the neutron cross section and the count rate ratio normalization constant C _R as parameters.
It is determined by regression analysis using equations (9) and (10), that is, reactivity calibration. Note that since the obtained R' is extremely accurate, the value of R' for the upcoming nuclear fuel loading state can also be predicted with high accuracy. When this value of R' is applied to equation (9) calibrated by the reactivity calibration means, a predicted value of k can be obtained. Note that only one curve of equation (9) is determined for each pair of detectors, so if there are many pairs of detectors, the final test value and predicted value of the k value will be the average of them. Therefore, the greater the number, the better the accuracy. Figure 3 is a graph showing the curve of equation (9) after the reactivity calibration procedure is completed using the count rate ratio measurement data of paired detectors, with the effective count rate ratio R' on the vertical axis and the horizontal axis. The effective multiplication factor k is calculated and shown. In the figure, the solid line curve P is the part where measurement data exists, and the broken line curve Q is the part predicted to be plotted by the next nuclear fuel addition step, and the neutron cross section and count rate ratio after correction. If the value of the effective count rate ratio R' of the next nuclear fuel loading step is predicted using the normalization constant, the predicted value of the effective multiplication factor k can be read from the curve Q.

【Effect of the invention】

As is clear from the above explanation, from the shape rate ratio of neutrons measured by the neutron counting detector,
By calculating the effective multiplication factor in the subcritical state (degree of subcriticality) based on the relational expression between the effective multiplication factor in the subcritical state and the effective counting rate ratio whose counting parameters were determined by performing reactivity calibration. , it becomes possible to calibrate the reactivity in a deep subcritical state, which was extremely difficult in the past, so it is possible to calculate the effective multiplication factor in the subcritical state without conducting reactivity calibration experiments in the critical state, which requires stricter criticality control. You can ask for it. Furthermore, since the relationship between the effective count rate ratio and the effective multiplication factor from a deep subcritical state to the critical state is determined, the effective multiplication factor in a state close to the critical state can also be predicted. Furthermore, when counting neutrons, the neutron source can be a normal nuclear reactor startup neutron source or the spontaneous fission neutrons of isotopes contained in the nuclear fuel itself, and the counting detector can be a normal detector and counting circuit. This has the advantage of lower costs and easier maintenance.

[Brief explanation of drawings]

FIG. 1 is a layout diagram showing the arrangement of paired detectors according to the present invention, FIG. 2 is a layout diagram showing a different layout of the paired detectors in FIG. 1, and FIG. 3 is an effective count rate ratio and effective count rate ratio according to the present invention. It is a graph showing the relationship with the multiplication factor. 1: Nuclear fuel collection area, 2: Detector counter, 3: Nuclear fuel surrounding area.

Claims (1)

[Claims] 1. Detection with high sensitivity to the energy distribution of neutrons multiplied from the neutron source in at least one of the nuclear fuel collection area of the nuclear fuel facility and the surrounding area of this collection area, while the other is highly sensitive to thermal neutron energy. The count rate ratio of neutrons is measured by a pair of detectors consisting of one detector and a detector more sensitive to fast neutron energy, and from this count rate ratio, an effective count corresponding to the subcriticality of the nuclear fuel equipment is determined. Find the rate ratio, perform regression analysis using the relational expression between this effective count rate ratio and the above-mentioned subcriticality, determine the subcriticality when the count rate ratio is measured from the regression analysis result, and calculate the above-mentioned effective count rate ratio. A method for measuring subcriticality of nuclear fuel equipment, characterized by predicting subcriticality at the time of fuel loading by calibrating the value of reactivity.