JP2018159669A - Method for measuring composition, subcriticality, delayed neutron ratio, neutron generation time, and prompt neutron lifespan of nuclear fissile material on the basis of only signals of neutron detector and the like - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and device using the method that allow reactivity to be properly measured even in a situation where information on an inner structure of a nuclear fuel body system such as, for example, a post-incident nuclear reactor, or an entire shape thereof is not obtainable.SOLUTION: Using that n(t)=αq(t)+nof an expression is established where the number of neutrons, to be detected by a neutron detector, at a time t in a nuclear fuel is denoted as n(t), the number of neutrons when the neutron remains stable after a change in reactivity and the like is denoted as n, a coefficient of subcriticality is denoted as α, and a weighted time differential n(t) of the number of neutrons n(t) is denoted as q(t), and a ratio of αq and n - n(a difference between n serving as a calculation rate deducting a noise component from a neutron detection signal and a value nat a stable time of the neutron stands at 1 regardless of the time, a delayed neutron ratio and a delayed neutron decay constant are determined.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for measuring dynamic characteristics of a subcritical nuclear fuel system whose internal structure and shape are unknown in advance, such as fuel debris in a nuclear reactor after an accident, based only on signals from a neutron detector.

  Nuclear fuel systems such as uranium 235 and plutonium 239 are fissile materials that absorb neutrons and cause fission or a mixture of non-fissile materials such as fissile materials and structural materials (for example, the entire core in a nuclear reactor) Call it. The main method for investigating the composition of fissile materials (100% uranium 235, 90% uranium 235 and the remaining plutonium 239) for nuclear fuel systems is a chemical analysis performed by collecting a small sample. Are known.

  In addition, as a technique for measuring the subcriticality of a nuclear fuel system, for example, in a conventional reactor, for example, as a typical subcriticality measuring method, for example, (1) negative period method, (2) Control rod drop method (Patent Document 1), (3) Compensation method, (4) Neutron source multiplication method (Patent Document 2), (5) Reverse motion characteristic method (Patent Document 3), (6) Reactor noise analysis method (7) The pulse neutron source method is known.

  Of the seven conventional methods described above, the first three methods need to be in a critical state as an initial state (negative period method, control rod drop method), or a control rod that has already been calibrated must already exist ( Compensation method). Moreover, the neutron source multiplication method requires a reference system with a known subcriticality. The inverse dynamic characteristics method requires accurate estimation of dynamic characteristics parameters, and should be used only in shallow subcriticality near the criticality (subcriticality where the shape of the neutron flux distribution can be regarded as equal to the critical shape). Can do. In addition, when the process noise occurs in the furnace noise method, the solution condition and prompt neutron lifetime change with time, so the alpha value determined to be abnormal fluctuates. Furthermore, the time tracking of the measurement system becomes a problem. It is difficult to remove the effects of higher order modes. In addition, the pulse neutron source method listed last requires a pulse neutron source with a fixed period, and in order to correct the influence of higher-order modes of the neutron flux distribution excited by incident neutrons, numerical analysis must be performed in advance. is necessary.

Known methods for measuring the effective delayed neutron ratio include the ²⁵² Cf neutron source method, Rossi-alpha, Nelson number method, improved Bennett method, and furnace noise method. These methods require detailed dimensions related to the internal structure of the nuclear fuel system in order to perform numerical analysis in advance. ^{The 252} Cf neutron source method and the Nelson number method assume that the nuclear fuel system is subcritical but close to criticality.

  Furthermore, the neutron generation time and prompt neutron lifetime are usually obtained by numerical analysis using detailed dimensions relating to the internal structure of the fissile material and the entire core containing it (in the case of a nuclear reactor).

JP-A-8-313669 Japanese Patent Application Laid-Open No. 09-178887 Japanese Patent Laid-Open No. 2006-142623

  In chemical analysis to examine the composition of fissile material within the nuclear fuel system, it is necessary to collect a sample of interest. In addition, any of the conventional subcriticality measurement methods other than the pulse neutron source method described above depends on the accuracy of the analysis value of the dynamic characteristic parameter. The pulse neutron source method depends on the accuracy of numerical analysis for correction. In addition, these numerical analyzes require information on the internal structure and overall shape of the nuclear fuel system or the reactivity of control rods.

  In general, the conventional methods require detailed dimensions related to the internal structure of the nuclear fuel system for numerical analysis. Therefore, the evaluation results of subcriticality, effective delayed neutron ratio, neutron generation time, prompt neutron lifetime depend on the accuracy of their dimensions and the calculation accuracy of the calculation code used for analysis.

  The purpose of the present invention is not affected by the accuracy of the dimensions of the reactor and the accuracy of the numerical calculation code in a normal reactor in a wide range of subcriticality (subcriticality that does not limit the shape of the neutron flux distribution). Therefore, the object is to provide a method for measuring the composition, subcriticality, delayed neutron ratio, neutron generation time, and prompt neutron lifetime of a fissile material.

  For example, even in a situation where information on the internal structure and overall shape of the nuclear fuel system cannot be obtained, such as a nuclear reactor after an accident, the composition of fissile material, subcriticality, delayed neutron ratio, neutron generation time An object of the present invention is to provide a method capable of appropriately measuring the prompt neutron lifetime.

  The present invention eliminates a situation in which an accurate parameter cannot be obtained in advance by preliminary analysis because the internal structure and overall shape of the nuclear fuel system in a subcritical state are not known in advance, or an error derived from preliminary analysis. Situations, situations where pre-configured control rods cannot be used, situations where pre-criticality is not possible, situations where measurement is desired for the first time reached subcriticality, approaching or moving away from criticality by adding positive or negative reactivity and / Or, when the output of the external neutron source fluctuates, based on only the signal proportional to the fission rate from the neutron detector, etc., the composition of the fissile material, subcriticality, delayed neutron ratio, neutron generation time, prompt This is a method for measuring the neutron lifetime.

Specifically, if the number of neutrons at time t in the nuclear fuel system detected by the neutron detector is n (t), the subcriticality is ρ (t), and the neutron source intensity is S (t), When the fluctuation almost stops after either or both of the intensity and the neutron source intensity fluctuate (ρ (t) ≒ S (t) ≒ 0), it finally becomes stable and n (t) becomes The fact that the following equation holds until the constant value n _∞ is reached is used.

ここでα_yは、実効遅発中性子割合をβとすると、ドル単位の未臨界度ρ_$=ρ/βにより
α_y = 1/ρ_$−1と表され、q(t)はn(t)の時間的変化率n^・(t)と以下の式で表される時間の関数μ(t)の比（n(t)の重み付き時間微分）である。

Here, α _y is expressed as α _y = 1 / ρ _$ −1 by sub-criticality ρ _$ = ρ / β in dollar units, and q (t) is n (t ) Is the ratio of the time change rate n ^· (t) to the time function μ (t) expressed by the following equation (weighted time derivative of n (t)).

Ci ^· (t) represents the temporal rate of change at time t of the amount Ci (t) of the delayed neutron leading nuclei of the i-th group when n (t) is the number of neutrons. Normally, 6 groups are used, but the number of groups is not limited in the following measurements. λ _i is the decay constant of the delayed neutron precursor nucleus of group i.

In Equation 1, α _y is only the subcriticality ρ _{$ in} dollars, n _∞ is a function of the subcriticality ρ and the neutron source strength S, and β is a function of the subcriticality ρ. If the change is sufficiently small, α _y and n _∞ can be regarded as constants. At this time, Equation 1 can be regarded as a linear equation with α _y as a slope and n _∞ as an intercept. This is used in the following measurements.

  As the neutron source intensity, only those neutrons emitted from the neutron source that reach the target nuclear fuel system are considered (effective neutron source intensity).

  n (t) can be easily obtained by using a calibration constant obtained from a neutron detector signal (counting rate). If it cannot be calibrated in advance, the neutron detector signal is used. Make necessary and feasible corrections and conversions, such as dead time correction.

  Specifically, the present invention resides in the following method using the fact that Equation 1 holds.

(1) A method of determining the delayed neutron ratio and the delayed neutron decay constant so that the difference between the time series data of the ratio of α _y q and nn _∞ and 1 is minimized.

(2) Using the fact that time series data (q, n) is on the straight line n (t) = α _y q (t) + n _∞ , the subcriticality ρ _$ and n (t) are stable. How to find the value _n∞ .

(3) A method of obtaining a set of (n ^· , n) using n = exp (f (t)) in the above item (1) or (2).

(4) In the above item (2), if a = 1 / ρ ₀ -1 / ρ _k , b = 1-r, r = n _0∞ / n _k∞ , then ρ _{k $} → ρ _{0 $} A method for obtaining the effective delayed neutron ratio β by using a / b to converge to β at the subcriticality ρ ₀ .

(5) A method for obtaining l [s] and Λ [s] using the fact that Λ / β = l (−ρ) + l / β holds in the above item (2).

  For nuclear fuel in a subcritical state, the internal structure and overall shape are not known in advance, so accurate parameters cannot be obtained in advance by preliminary analysis, etc., or situations where it is desired to eliminate errors derived from preliminary analysis. In situations where control rods cannot be used, situations where criticality cannot be achieved in advance, or situations where measurement is desired at the first criticality level, and when approaching or moving away from criticality by adding positive or negative reactivity, and / or external neutron source When the output fluctuates, it is possible to obtain the delayed neutron ratio, delayed neutron decay constant, and reactivity in dollars based only on the signal from the neutron detector.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. A figure in which a straight line is fitted to the detector signal (blue marker) to obtain the values of slope α _y and intercept n _∞ (fitting results are α _y = −1.094, n _∞ = 6.902 × 10 ⁻⁵ ). When uranium 235 is 100%, the value of α _y q / (n−n _∞ ) is used every moment, and the value of uranium 235 is used as the value of delayed neutron ratio and delayed neutron preceding nuclear decay constant. The figure which showed the comparison of the case where the value of plutonium 239 was used (red) when it was (blue). The figure which showed the mode of change of the signal of a neutron detector after reactivity was added and subcriticality became shallow. The figure which processed the time series data of FIG. 4, and created the time series data of (n < _infinity > -n). When by processing the sequence data in Figure 5 (n _∞ -n) of natural logarithm, ln (n _∞ -n), the time series data created this by fitting a straight line y = at + b and the coefficients a The figure which calculated | required b. FIG. 7 is a diagram in which time series data of n = _n∞ + e ^{at + b} is created using the values of coefficients a and b obtained in FIG. 6 and compared with the original data in FIG. 4. For different reactivities ρ _{0 $} , ρ _{1 $} , ρ _{2 $} , ..., (1-r) is the x-axis and (1 / ρ _{0 $} -r / ρ _{k $} ) is the y-axis (r = n _0∞ / n _k∞ ), plotting the slope β using the origin and the point closest to the origin.

  FIG. 1 shows a basic configuration of a reactivity measuring device for a subcritical nuclear fuel system according to the present invention. A reactivity measuring device 10 includes a detection unit 1 composed of a neutron detector and the like, an input unit 2 for inputting data for numerical calculation, a calculation unit 3 for performing numerical calculation, and a numerical value connected to the calculation unit 3 It comprises a storage unit 4 for storing data necessary for calculation and an output unit 5 for transmitting measurement results to the outside. The signal from the detector 1 is calibrated in advance so as to correspond to the value of the output emitted by fission. The output unit 5 is configured to be able to transmit numerical data to an external device and display the result on a screen in an easy-to-understand manner.

  Of the reactivity measuring device 10 according to the present invention, the portion excluding the detection unit 1 can be basically constituted by a microcomputer. Therefore, by installing the apparatus according to the present invention in a container that is not critically controlled, it is possible to detect critical proximity due to erroneous transfer of nuclear fuel material.

  Next, the calculation procedure of the reactivity measurement method for the subcritical nuclear fuel system, which is performed by the calculation unit 3 of the reactivity measurement apparatus 10 described above, will be described.

The equation used in the present invention is derived from a one-point furnace dynamic characteristic equation based on the following concept. When an external disturbance such as a change in subcriticality ρ or a change in neutron source strength S is given to the nuclear fuel system, the change in the number of neutrons in response to it should be closely related to the subcriticality ρ. In particular, when the rate of change in time of ρ and S (dρ / dt and dS / dt) is very small, the fluctuation of the number of neutrons n (value of n and its rate of change dn / dt) is subcritical. To neutrons supplied by decay of delayed neutron precursor nuclei (characterized by delayed neutron precursor nucleus concentration Ci, delayed neutron precursor nuclear decay constant λi, delayed neutron ratio βi) Since it is considered to be dominated, the equation was derived by applying a condition with very small rate of change of ρ and S (dρ / dt and dS / dt) to the one-point reactor dynamics equation. In this specification, for the sake of convenience, for example, time differentiation of n is indicated as n ^· . Similarly, Ci is indicated as Ci ^· .

<(1) Reactivity measurement method>
The signal from the detector is n, and its time derivative is represented by n ^· . From n, μ is obtained by the following equation.

Where β is the effective delayed neutron ratio, β _i is the delayed neutron ratio of group i, Λ is the neutron generation time, λ _i is the delayed neutron leading nuclear decay constant of group _i , and C _i is group i. Of late neutrons. Ci ^· represents the time derivative of C _i.
q is obtained from n ^· and μ by the following equation.

データの組（q, n）を用いて自己回帰分析によりα_yとn_∞ を求める。α_yよりドル単位の反応度ρ_$を次式で求める。

Α _y and n _∞ are obtained by autoregressive analysis using the data set (q, n). The reactivity ρ _$ in dollar units is obtained from α _y by the following equation.

<(2) Delayed neutron ratio measurement method>
Please refer to FIG. In (1), the ratio between α _y q and n −n _∞ is obtained as time series data. The values of (β _i , λ _i ) are adjusted so that this ratio is as close to 1 as possible in the time range to be calculated. A multivariate regression analysis method may be used. If there are presumed fissile materials, prepare a set of values (β _i , λ _i ) for each material type (for example, U-235, Pu-239, etc.) according to their mixing ratio (β _i, λ _i) determine the interpolated values, the set of obtained values (β _i, λ _i) by applying the alpha _y q and n - calculate the ratio between n _∞. The mixing ratio when this ratio is closest to 1 is the mixing ratio of the fissile material.

<(3) Method of obtaining a set of (n ^· , n) values at time t>
Data obtained by taking the logarithm of time series data of n (FIG. 4) or n −n _∞ or n _∞ -n (FIG. 5) is defined as n ₁ (FIG. 6). The obtained n ₁ data is approximated by a function of time (FIG. 7), and a set of (n ^· , n) values at time t is obtained based on the function.

<(4) Method for estimating values of α _y and n _{∞ at} a new subcriticality>
In (3), after performing (1) once or more, the first estimated values of α _y and n _∞ can be obtained as follows. Using the following value p ₀ obtained using the subcriticality ρ ₀ (= ρ _{$ 0} × β) obtained _previously and the number of stable neutrons n _0∞ ,

未臨界度がそれほど大きく変わらなければ、前述の式１は常に点（p₀, −p₀）を通るので、あらたな未臨界度ρ₁におけるα_1y とn_1∞の最初の推定値を次のようにして得ることができる。

If the subcriticality does not change so much, Equation 1 above always passes through the point (p ₀ , −p ₀ ), so the first estimate of α _1y and n _{1∞ at} the new subcriticality ρ ₁ It can be obtained as follows.

ここで、t₀は未臨界度と中性子源強度の変動が停止した直後（未臨界度はρ₁）以降の任意の時刻である。

Here, t ₀ is an arbitrary time immediately after the change of the subcriticality and the neutron source intensity stops (subcriticality is ρ ₁ ).

<(4) Method for obtaining execution delayed neutron ratio β>
Two or more sets of data with different subcriticality (ρ _{k $} , n _k∞ ) obtained by (1) above (of which the data set for the reactivity for which the effective delayed neutron ratio is to be _calculated is (ρ _{0 $} , with n _{0∞) to), a = 1 / ρ 0} $ -r / ρ k $ and b = 1 - Request r _k (and where _{r k = n 0∞ / n k∞} ) . Β is obtained using the fact that the ratio a / b converges to the effective delayed neutron ratio β as the data set (a, b) approaches the origin.

  Hereinafter, a data processing procedure using a signal (neutron count rate n [number / s]) from a neutron detector installed in the nuclear fuel material will be described in more detail.

The signal of the neutron detector or a signal obtained by calibrating and processing the neutron detector is obtained at a constant or arbitrary time interval (Δt), and the obtained data string n ₁ , n ₂ , n ₃ ,. Hereinafter, _j is referred to as time-series data. Below, these are collectively written as n (t).

The physical quantity handled here will be described.
Among the fission fragments produced by fission such as uranium, the nuclide that decays with a certain time constant and emits neutrons is called delayed neutron precursor nucleus. In an actual system with a finite size, the ratio of delayed neutrons to the total number of neutrons (the sum of prompt neutrons and delayed neutrons) is the effective delayed neutron ratio (expressed as β). There are many delayed neutron precursor nuclei, but usually the ones with close decay constants are collectively treated as one group, and their concentration (number per unit volume) is called the delayed neutron precursor nucleus concentration, Ci (r, t) i is a group number, and usually 6 groups are used. Ci (t, t) is the result of spatial integration of Ci (r, t) over the whole system. This represents the total amount of the delayed neutron precursor nuclei of group i at time t. The ratio of delayed neutrons generated from Ci (t) (the delayed neutron ratio of the i-th group) is represented by bi. bi satisfies Equation 9. Let λi be the decay constant of the delayed neutron precursor nucleus of group i. For the set of 6 (bi, λi), different values have been proposed for each fissile material (uranium 235, plutonium 239, etc.) (literature James J. Duderstadt and Louis J. Hamilton, “Nuclear Reactor Analysis”. , John Wiley & Sons, ISBN 0-471-22363-8).

The physical quantities to be evaluated are, in order, the coefficient rate n [number / s] excluding the noise component from the neutron detector signal, the time change rate n ^· (= dn / dt) [number / s ² ] of this neutron count rate, Neutron predecessor equivalents Ci (t), time variation rate Ci (t) of Ci (t), variable μ (t) [1 / s], variable q (t) [1 / s], subcriticality ρ Function α (ρ) [-] and the value of n at stability n _∞ (ρ) [number / s], subcriticality ρ _$ (= ρ / β) [-] in dollars, delayed i group Neutron leading nuclear decay constant λi [1 / s], group i delayed neutron ratio bi [-], effective delayed neutron ratio β [-], prompt neutron lifetime l [s], neutron generation time Λ [s] It is. Variables that are functions of time including n and n ^· are time-series data. The procedure uses procedure A when the neutron detector signal rises due to a change in subcriticality or neutron source strength, and procedure B when it falls.

<Procedure 1-A (Evaluation of n and n ^· excluding noise components)>
After the detector signal has stabilized, calculate x = ln (n _∞ -n) using the measured value of n _{∞ and} the appropriate predicted value if it is changing, and this time series data x (t ) Is approximated by a function f (t) of an appropriate time. For example, x (t) is approximated by f (t) = at + b by fitting y = at + b to the data of x (t) to obtain constants a and b. Using this f (t), n and n ^· are expressed by the following equations 10 and 11, respectively.

<Procedure 1-B (Evaluation of n and n ^• excluding noise components)>
After the detector signal stabilizes, calculate x = ln (nn _∞ ) using the measured value of n _{∞ and} the appropriate predicted value if it is changing, and calculate this time series data x (t) Approximate with a function f (t) of appropriate time. For example, x (t) is approximated by f (t) = at + b by fitting y = at + b to the data of x (t) to obtain constants a and b. Using this f (t), n and n ^· are expressed by the following equations 12 and 13, respectively.

In the above steps 1-A and 1-B, if the predicted value is used for n _∞ , the time series data of n obtained and the time series data of the neutron detector signal are compared, and the square of all data It evaluates the sum, which is to adjust the value of n _∞ so becomes minimum, to perform the procedure 1-a, 1-B again. Repeat this procedure until the difference is minimized.

In Step 1, if the step 5 after it has carried out more than once, the initial estimate of n _∞ can be obtained as follows. Using the following value p ₀ obtained using the subcriticality ρ ₀ (= ρ _{$ 0} × β) obtained _previously and the number of stable neutrons n _0∞ ,

未臨界度がそれほど大きく変わらなければ、式１は常に点（p₀, -p₀）を通るので、あらたな未臨界度ρ₁におけるn_1∞の最初の推定値を

If the subcriticality does not change so much, Equation 1 always passes through the point (p ₀ , -p ₀ ), so the first estimate of n _{1∞ at} the new subcriticality ρ ₁ is

ここで、t₀は未臨界度と中性子源強度の変動が停止した直後（未臨界度はρ₁）以降の任意の時刻である。

Here, t ₀ is an arbitrary time immediately after the change of the subcriticality and the neutron source intensity stops (subcriticality is ρ ₁ ).

In the conventional method, since it is based on the one-point reactor dynamic characteristic equation, it was necessary to consider a function form such as n = n _∞ -ΣAie ^{f (ti)} in the case of procedure 1-A from the form of the solution. . However, n = α _y q - In n this _{technique ∞} based is true that, n = α _y q - n the form of _∞ than solution ^{_{n = n ∞ - e f (}} t) because it is shown comprising The values of n and n ^· with no noise component can be obtained more easily.

<Procedure 2 (Evaluation of Ci (t), Ci ^· (t))>
For bi, λi, and Λ, Ci ^· (t) is obtained by the following equation using the assumed nuclear fuel material value or a provisional value that can be assumed to be closest. Here, βi = β × bi.

Δt後のCi(t+Δt)を以下により求める。

Ci (t + Δt) after Δt is obtained as follows.

このような数値積分はルンゲクッタ法や陰解法など多数存在するので、どれを用いてもよい。Ciの初期値は、中性子検出器信号が安定している状態のときに

There are many such numerical integrations such as the Runge-Kutta method and the implicit method, and any of them can be used. The initial value of Ci is when the neutron detector signal is stable

として求めておく。

I ask for it.

<Procedure 3 (Evaluation of μ)>
The value of μ (t) at time t is evaluated by the following calculation.

<Procedure 4 (Evaluation of q)>
The value of q (t) at time t is evaluated by the following calculation.

<Procedure 5 (Evaluation of α _y , n _∞ , ρ _$ )>
The time series data of q (t) obtained in the above procedure 4 and the time series data of n (t) obtained in procedure (1) are used. A point (q (t), n (t)) is plotted for each time with q (t) on the horizontal axis and n on the vertical axis. At this time, the value of the slope obtained by fitting a linear equation to the locus of the point is α _y , and the value of the intercept is n _∞ . Calculate ρ _$ by the following formula.

<Procedure 6 (Evaluation of λi [1 / s], bi [-])>
The following time series data y (t) is calculated using the values of α _y and n _∞ obtained in the above procedure 5.

縦軸をy、横軸をtとして点(t,y(t))をプロットする。このとき、y(t)が直線y=1に最も近くなるように、λi[1/s]、bi[-]を調整する。このとき、biの全群の総和は1となるようにする。

Plot the point (t, y (t)) with y on the vertical axis and t on the horizontal axis. At this time, λi [1 / s] and bi [−] are adjusted so that y (t) is closest to the straight line y = 1. At this time, the sum of all groups of bi is set to 1.

  More simply, depending on the type of fissile material, there are (λi [1 / s], bi [-]) pairs (i = 1, ..., 6), such as uranium 235 and plutonium 239, respectively. Is given in the literature. In the following, it is assumed that uranium 235 is λ5i [1 / s] and b5i [−], and plutonium 239 is λ9i [1 / s] and b9i [−]. For example, it is assumed that uranium 235 and plutonium 239 are present in the target nuclear fuel material, but when λi [1 / s] and bi [-] are adjusted when the ratio is unknown, the fissile material If the number ratio of uranium 235 in the whole is R, λi [1 / s] and bi [−] are expressed by the following equations.

この計算をi=1〜6のすべてで同時に行う。λi[1/s]、bi[-]を個別に変更しない。Rを変えて上述の手順2から手順6を繰り返し、(y(t)-1)²の総和が最も小さくなったときのRが、対象とする核燃料物質におけるウラン２３５の個数割合である。

This calculation is performed simultaneously for all i = 1 to 6. Do not change λi [1 / s] and bi [-] individually. Steps 2 to 6 described above are repeated while changing R, and R when the sum of (y (t) -1) ² becomes the smallest is the number ratio of uranium 235 in the target nuclear fuel material.

<Procedure 7 (Evaluation of β)>
It is assumed that there is a set of values of n _∞ and ρ _$ obtained in the above procedure 5. This is expressed as n _∞0 and ρ _{$ 0} . When the subcriticality changes to a new subcriticality ρ _$ , n _∞ is obtained by applying the above procedure 1 to procedure 6. When the two stable output ratio of the _{r (= n ∞ / n ∞0} ), β is given by the following equation.

別の例としては、複数の値の組（n_∞i、ρ_$i）を用いて、点（n_∞0、ρ_$0）に対してyi = 1/ρ_$0-r/ρ_$i とxi = 1 - ri (ri=n_∞0／ n_∞i)を求め、これらの点（xi, yi）をプロットする。これらの点に原点を通る直線をフィッティングしたとき、この直線の傾きからβを得る。ここで得られたβの値を手順２で用いることにより、仮の値であったβの値を正確な値とすることができるため、手順２を安定的に実施することができる。

As another example, yi = 1 / ρ _{$ 0} -r / ρ _$ i and xi _for a point (n _∞0 , ρ _{$ 0} ) using a set of values (n _∞i , ρ _{$ i} ) = 1-ri (ri = _n∞0 / _n∞i ) and plot these points (xi, yi). When a straight line passing through the origin is fitted to these points, β is obtained from the slope of this straight line. By using the value of β obtained here in the procedure 2, the value of β, which was a temporary value, can be set to an accurate value, so that the procedure 2 can be stably performed.

<Procedure 8 (Evaluation of l [s], Λ [s])>
A state in which the subcriticality is ρ _$ and a constant state continues, and a state where the neutron detector signal has a stable value n _∞ is defined as an initial state. The external neutron source intensity is S. Turn off or shield the external neutron source (such as Pulsatron) to instantly set S = 0. The maximum value ω of n ^· / n is obtained from n and n ^· obtained in the procedure 1-B. Λ / β is obtained by Λ / β = ρ _$ / ω. Two or more data sets (−ρ _{$ k} , Λk / β) with different subcriticality are plotted. From a and b obtained by fitting a straight line y = ax + b to these points, l is evaluated by l = a or l = b × β. Λ is obtained by Λ = βρ _$ / ω or Λ = l / keff (keff = 1 / (1−ρ)). By using the value of Λ obtained here in the above-described procedure 2, the value of Λ, which was a temporary value, can be set to an accurate value, so that the above-described procedure 2 can be stably performed. .

  For nuclear fuel in a subcritical state, the internal structure and overall shape are not known in advance, so accurate parameters cannot be obtained in advance by preliminary analysis, etc., or situations where it is desired to eliminate errors derived from preliminary analysis. When the control rods cannot be used, and when approaching or moving away from the criticality due to the addition of positive or negative reactivity and / or when the output of the external neutron source fluctuates, the neutron detector etc. It became possible to extract a noise-free signal from the signal proportional to the fission rate. The weighted time derivative of the number of neutrons can be calculated by steps 2 to 4 above. Procedure 5 makes it possible to determine the subcriticality in dollars based only on signals proportional to the fission rate from neutron detectors. Step 6 makes it possible to obtain the delayed neutron ratio and the delayed neutron leading nuclear decay constant, and further calculate the above steps 1 to 5 using the delayed neutron ratio and the delayed neutron leading nuclear decay constant obtained here. It is possible to determine the subcriticality in dollars based only on the signal proportional to the fission rate from the neutron detector etc., which does not depend on the value of the delayed neutron ratio etc. given as the initial value. became. Furthermore, the effective delayed neutron ratio can be obtained by the above-mentioned procedure 7, and it is possible to obtain the subcriticality not in dollars by multiplying this by the reactivity in dollars. Procedure 8 made it possible to measure the neutron generation time and prompt neutron lifetime.

  In the present invention, the ratio of fissile material (U-235, Pu-239, 241 etc.) can be measured by irradiating neutrons when receiving spent fuel even in a reprocessing facility. Can be used for measurement. Further, by installing the apparatus according to the present invention in a container that is not critically controlled, it is possible to detect critical proximity due to erroneous transfer of nuclear fuel material. At that time, even if the type of nuclear fuel material flowing in changes, it is possible to continue to calculate the neutron effective multiplication factor by automatically correcting. By irradiating materials with unknown contents with neutrons, fissionable materials can be identified. It is possible to automate the critical proximity of nuclear reactors. Further, the present invention is a promising candidate for a subcriticality measurement method necessary for operation control in an ADS (accelerator-driven subcritical reactor).

DESCRIPTION OF SYMBOLS 1 ... Detection part 2 ... Input part 3 ... Calculation part 4 ... Memory | storage device 5 ... Output part

Claims (6)

The detection signal of the neutron detector or the calibration of the detection signal when it becomes stable after fluctuations in n (t), reactivity, etc. N _∞ , the function of subcriticality α _y , the detection signal or the weighted time derivative n ^・ (t) of the neutron number n (t) obtained by calibrating this is q ( t)
Using the fact that n (t) = αq (t) + n _∞ holds, α _y q and n − n _∞ (the count rate excluding noise components from the neutron detection signal, n and its stable The method of determining the delayed neutron ratio and the delayed neutron decay constant so that the ratio of the difference of the value _n∞ is closest to 1 at any time during the measurement period. The subcriticality ρ _{$ according to} claim 1, wherein the time series data (q, n) obtained based on the detection signal of the neutron detector is on the straight line n = α _y q + n _∞. And obtaining a stable value n _∞ of n (t). 3. The method according to claim 1 or 2, wherein n = exp (f (t)) is used to determine a set of (n ^· , n).
Here, f (t) is a function of t determined so that n (t) reproduces the detector signal. According to claim 1 or 3, a method of estimating the n _∞ in new subcriticality with previous measurement values (subcriticality [rho _{0 $,} stable value n _0∞ neutron detector). In claim 2, when a = 1 / ρ ₀ -r / ρ _k , b = 1-r, r = n _0∞ / n _k∞ , a / b is subcriticality when ρ _k → ρ ₀ A method for obtaining the effective delayed neutron ratio β by using convergence to β at ρ ₀ .   3. The method for obtaining prompt neutron lifetime l [s] and neutron generation time Λ [s] using the fact that Λ / β = l (−ρ) + l / β holds in claim 2.

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