JP2014137259A - Subcriticality measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately measure a subcriticality of a container even when a shape and a dimension of the container including a fissile material is undecided.SOLUTION: A subcriticality measuring device includes: a neutron detector 11 disposed on a side face of a nuclear fuel assembly 1; a neutron absorber 12 possible to be disposed facing another face which forms a right angle with one face on which the neutron detector 11 is disposed in the nuclear fuel assembly 1; an attaching/detaching mechanism 13 for allowing the neutron absorber 12 to approach to or separate from the nuclear fuel assembly 1; and calculation unit including time counting means, neutron source intensity calculation means, and effective multiplication factor calculation means. The neutron source intensity calculation means calculates a neutron source intensity of the nuclear fuel assembly 1 from a temporal change of a neutron counting rate detected by the neutron detector 11 in a state that the neutron absorber 12 approaches the nuclear fuel assembly 1. The effective multiplication factor calculation means is configured to calculate an effective multiplication factor of the neutron in a state that the neutron absorber 12 is separated from the nuclear fuel assembly 1 based on the neutron source intensity.

Description

  Embodiments of the present invention relate to a subcriticality measuring apparatus for obtaining an effective multiplication factor of neutrons in a container (eg, nuclear fuel assembly) containing a fissile material and measuring the subcriticality thereof.

  In criticality management, in order to improve maintainability, a design is made to handle even burned fuel as new fuel, which is called a new fuel assumption. On the other hand, a rationalized design that considers a decrease in reactivity due to fuel combustion is called burnup credit.

  When the burnup credit is adopted in the reprocessing plant, the burnup of the spent nuclear fuel assembly is confirmed by a confirmation device called a burnup monitor. In some cases, the effective multiplication factor of neutrons and the amount of fissile material in the nuclear fuel assembly are measured by attaching / detaching a neutron absorber instead of the burnup (Patent Documents 1 to 3, Non-Patent Document 1).

  In Non-Patent Document 1, cadmium (Cd) is used as a neutron absorber, and from the respective neutron count rates detected when Cd is around the spent nuclear fuel assembly and when Cd is not present, A method for obtaining an effective multiplication factor of a spent nuclear fuel assembly and estimating a concentration of a fissile material contained in the spent nuclear fuel assembly is shown.

That is, the relationship between the initial neutron count rate (Φ0) detected in the absence of Cd and the proportionality coefficient (α0), effective multiplication factor (k0), and neutron source strength (S) in this case, and Cd. The relationship between the detected neutron count rate (φ) and the proportionality coefficient (α), effective multiplication factor (k), and neutron source intensity (S) in this case is expressed by the following equations (1) and (2), respectively. become. Here, since the presence or absence of Cd around the nuclear fuel assembly does not affect the neutron source intensity S, the neutron source intensity S is not changed.
φ0 = α0 · S / (1-k0) (1)
φ = α · S / (1-k) (2)

From these equations (1) and (2), the following equation (3) is obtained.
(Φ / φ0) = (α / α0) · (1-k0) / (1-k) (3)
By transforming this equation (3), the effective multiplication factor k0 of the nuclear fuel assembly in the absence of Cd is expressed by the following equation (4).

  Here, (α / α0) changes depending on the presence or absence of Cd, but is relatively close to 1, and can be calculated in advance by analysis. Further, (k / k0) is equal to the ratio of the probability that thermal neutrons do not leak from the nuclear fuel assembly, and if the shape of the nuclear fuel assembly is known, it can be calculated in advance by analysis. Accordingly, the concentration of the fissile material contained in the nuclear fuel assembly can be estimated from (φ / φ0) obtained by detection, and the effective multiplication factor k0 in the absence of Cd is obtained by using Equation (4). The subcriticality of the nuclear fuel assembly can be measured from the effective multiplication factor k0.

JP 54-155393 A JP-A-62-2192 JP-A-62-47590

TOSHIBA CORPORATION TLR-R001 Burnup Measurement System at Reprocessing Facility Ohmsha Nuclear Textbook Reactor Dynamics and Plant Control H. Taninaka, et al. , “An Extended Rod Drop Method Applicable to Subscriber Reactor System Driven by Neutral Source, Journal of Nuclear Science.” 47, no. 4, p. 351-356 (2010)

  The measurement of the effective multiplication factor with or without a neutron absorber as described above is possible when the dimensions and shape are known and the values of α / α0 and k / k0 can be calculated, as in the case of nuclear fuel assemblies, or This is effective when a calibration test using a nuclear fuel assembly whose content and effective multiplication factor are known in advance is possible.

  However, when the shape, dimensions, etc. of the nuclear fuel assembly are undecided, such as when the fuel melts due to severe accidents, the calculation and calibration as described above are difficult, and the effective multiplication factor k0 of the nuclear fuel assembly is set. Therefore, the subcriticality of this nuclear fuel assembly cannot be measured.

  The object of the embodiment of the present invention is made in consideration of the above-mentioned circumstances, and the subcriticality of the container is reliably measured even if the shape, size, etc. of the container containing the fissile material is undetermined. An object of the present invention is to provide a subcriticality measuring apparatus that can be used.

  A subcriticality measuring apparatus according to an embodiment of the present invention is disposed so as to face one surface of a container containing a fissile material, and detects a neutron detector emitted from the container as a neutron count rate, A neutron absorber that is disposed so as to be opposed to another surface of the container that is perpendicular to the one surface of the container in which the neutron detector is disposed, and a detachable move to move the neutron absorber closer to or away from the container An attaching / detaching mechanism, and a calculation unit including a time counting means, a neutron source intensity calculating means, and an effective multiplication factor calculating means, wherein the time counting means has elapsed since the neutron absorber approached the container. The time is measured, and the neutron source intensity calculating means is configured such that the neutron count rate detected by the neutron detector in a state where the neutron absorber is close to the container and the time count. The neutron source intensity of the container is calculated from the time output from the stage, and the effective multiplication factor calculating means detects an initial neutron measurement rate detected by the neutron detector in a state where the neutron absorber is separated from the container. And the neutron source intensity, the neutron absorber is configured to calculate an effective multiplication factor of the neutron when the neutron absorber is separated from the container.

  According to the embodiment described above, even if the shape, size, etc. of the container containing the fissile material is undecided, the subcriticality of the container can be reliably measured.

The side view which shows 1st Embodiment of a subcriticality measuring apparatus. The arrow line view visually observed from the II-II line of FIG. The block diagram which shows the arithmetic unit of FIG. The graph which shows the time change of the neutron count rate from the time of inserting a control rod into the system in which fission has occurred. The graph which shows the straight line P obtained by plotting each value of the neutron number N (t) and the delayed neutron number Q (t). The side view which shows 2nd Embodiment of a subcriticality measuring apparatus.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.
[A] First embodiment (FIGS. 1 to 5)
FIG. 1 is a side view showing a first embodiment of a subcriticality measuring apparatus. The subcriticality measuring apparatus 10 shown in FIG. 1 and FIG. 2 has a nuclear fuel assembly 1 as a container containing a fissile material, in particular, the fuel has melted due to severe accidents at a nuclear power plant, and its shape, dimensions, etc. are undecided. The effective multiplication factor of the neutron emitted from the nuclear fuel assembly 1 is obtained by applying the control rod insertion method described in Non-Patent Documents 2 and 3, for example, and the subcriticality of the nuclear fuel assembly 1 is measured. And includes a neutron detector 11, a neutron absorber 12, an attachment / detachment mechanism 13, and an arithmetic unit 14. As shown in FIG. 3, the calculation unit 14 includes a time counting unit 15, a neutron source intensity calculation unit 16, and an effective multiplication factor calculation unit 17.

In the control rod insertion method, the dynamic characteristic equation of the reactor by the one-point reactor approximation with delayed neutrons as one group is expressed as follows: the number of neutrons, the number of delayed neutrons, and the number of delayed neutrons as a function of time t. N (t), Q (t), C (t), prompt neutron production time Λ, delayed neutron preceding nuclear decay constant λ, reactivity ρ after insertion of control rod, delayed neutron ratio β, neutron source strength S When the measurement time interval Δt is used, the following equations (5) to (8) are obtained.

Further, the reactivity ρ is defined as the following equation (9) using the effective multiplication factor k.
ρ = (k−1) / k (9)
Here, the effective multiplication factor refers to the ratio between the number of neutrons generated per unit time and the number of neutrons consumed per unit time in a system in which fission occurs. A system in which fission has occurred is in a critical state when the effective multiplication factor is 1, and can be said to be in a subcritical state when the effective multiplication factor is less than one.

  When a control rod comprising a neutron absorber is inserted into a system in which fission has occurred, the effective multiplication factor k changes, so the reactivity ρ also changes from equation (9). Further, the time change of the neutron number N (t) (that is, the neutron count rate φ (t)) detected when the control rod is inserted into the system in which fission is occurring is shown in FIG. It changes slowly when the effective multiplication factor of the system in which the occurrence is high, and changes abruptly when the effective multiplication factor is low.

Using this time change in the number of neutrons, the values of the number of neutrons N (t) and the number of delayed neutrons Q (t) that change over time are shown in the graph shown in FIG. A straight line P is obtained by plotting the number Q (t) and the vertical axis with the number of neutrons N (t). The following expressions (10) and (11) are obtained from the read value A of the slope of the straight line P, the read value B of the intercept, and the expression (5).
A = −Λ / (ρ−β) (10)
B = −Λ · S / (ρ−β) (11)

  Since the delayed neutron ratio β and the prompt neutron production time Λ can be calculated if the type of fissile material is known, the reactivity ρ is first calculated from the equation (10), and then the calculated reactivity ρ Is substituted into the equation (11) to calculate the value of the neutron source intensity S.

Then, the measured value of the initial neutron number N (0) before insertion of the control rod in the system in which fission occurs and the value of the neutron source intensity S calculated as described above are substituted into the following equation (12). Then, the effective multiplication factor k0 of the neutron before the insertion of the control rod in the system in which fission is occurring is calculated. From the calculated value of the effective multiplication factor k0, the subcriticality of the system in which fission has occurred can be measured.
N (0) = S / (1-k0) (12)

  Here, Expression (12) is an expression representing the relationship between the effective multiplication factor k0 of the neutrons before the control rod is inserted and the initial number of neutrons N (0) in the system in which fission occurs. When the initial neutron count rate φ0 shown in the equation (1) is used as the initial neutron number N (0), S in the equation (12) is equivalent to α0 · S in the equation (1).

  By the way, the subcriticality measuring apparatus 10 shown in FIG. 1 is described above with respect to a nuclear fuel assembly 1 (particularly a nuclear fuel assembly 1 whose shape and size are undecided due to melting of the fuel) as a system in which fission occurs. The control rod insertion method is applied, and a neutron absorber 12 is used in place of the control rod to be inserted.

  As shown in FIGS. 1 and 2, the neutron detector 11 of the subcriticality measuring apparatus 10 is located near at least one of the four side surfaces of the nuclear fuel assembly 1, in the vicinity of the two opposite side surfaces in this embodiment. Be placed. These neutron detectors 11 detect the number of neutrons emitted from the nuclear fuel assembly 1 as a neutron count rate, and this neutron count rate is connected to the neutron source intensity calculation means 16 and the effective multiplication factor of the calculation unit 14 via the cable 18. It is output to the computing means 17. In the figure, the neutron detector 11 is shown in a columnar shape, but is not limited to this, and for example, the detection surface may be a cube having a planar shape. In the figure, the neutron detector 11 is shown in a shape that is long in the vertical direction (vertically long), but may be in a shape that is long in the horizontal direction (horizontally long).

  The constituent metal of the neutron absorber 12 is cadmium, boron, gadolinium, or hafnium, and absorbs neutrons emitted from the nuclear fuel assembly 1. As shown in FIG. 2, the neutron absorber 12 is opposed to another surface of the nuclear fuel assembly 1 perpendicular to one surface of the nuclear fuel assembly 1 where the neutron detector 11 is arranged in a plan view of the nuclear fuel assembly 1. In this embodiment, a pair is provided so that at least one can be arranged.

  When the neutron absorber 12 is arranged all around the nuclear fuel assembly 1, the proportionality coefficient α0 of α0 · S in the equation (1) equivalent to the neutron source intensity S in the equation (12) is the neutron absorber 12 When the nuclear fuel assembly 1 is attached or detached (described later), the amount of change to the proportional coefficient α increases. Therefore, by disposing the neutron absorber 12 in a direction perpendicular to the neutron detector 11 in a plan view of the nuclear fuel assembly 1, the amount of change in the proportional coefficients α0 and α can be reduced. This is because correction by calculation to obtain the effective multiplication factor k0 of neutrons is unnecessary or minimized.

  In the nuclear fuel assembly 1 whose shape and dimensions have become undecided due to the melting of the fuel, correction by the analysis as described above becomes difficult, so the neutron absorber 12 is orthogonal to the neutron detector 11 in a plan view of the nuclear fuel assembly 1. It is particularly effective to arrange in the direction formed.

  As shown in FIGS. 1 and 2, the attachment / detachment mechanism 13 moves the neutron absorber 12 so as to be attached to or detached from the nuclear fuel assembly 1. This attachment / detachment mechanism 13 is connected to two storage containers 20 that store a pair of neutron absorbers 12 and a connecting rod 22 that connects two operating rods 21 connected to each neutron absorber 12. A wire 23, a hoisting machine 24 that can wind up the wire 23, and a neutron absorber 12 that is built in the container 20 and is separated from the nuclear fuel assembly 1 in the container 20 to the nuclear fuel assembly 1. And a spring 25 that generates an urging force to approach. 1 is a wire guide tube that guides the wire 23 between a pulley 27 that is fixed on the container 20 side and winds the wire 23 and the hoisting machine 24.

  The operation rod 21, the connecting rod 22, the wire 23, the hoisting machine 24, and the like constitute the separation mechanism portion 13A. When the hoisting machine 24 winds up the wire 23 manually or conductively, the pair of neutron absorbers 12 are synchronously moved away from the nuclear fuel assembly 1 in the container 20 as shown by a two-dot chain line in FIG. Move in the direction. At this time, the spring 25 is pressed and contracted by the neutron absorber 12 separated from the nuclear fuel assembly 1 in the storage container 20, and accumulates an urging force that causes the neutron absorber 12 to approach the nuclear fuel assembly 1. The spring 25, the operating rod 21, the connecting rod 22, and the like constitute the approach mechanism portion 13B. By opening the wire 23 operated by the hoisting machine 24, the neutron absorber 12 approaches the nuclear fuel assembly 1 at high speed (instantly) by the urging force of the spring 25.

  The time measuring means 15 of the arithmetic unit 14 shown in FIGS. 1 and 3 measures (counts) the elapsed time t from the time when the neutron absorber 12 approaches the nuclear fuel assembly 1, and calculates the measured value as the neutron source intensity. Output to means 16. Further, the neutron detector 11 detects a neutron count rate φ (t) that changes with time t from the time when the neutron absorber 12 approaches the nuclear fuel assembly 1, and the neutron absorber 12 becomes a nuclear fuel assembly 1. The initial neutron count rate φ (0) in the separated state before approaching is detected. The detected neutron count rate φ (t) is output to the neutron source intensity calculating means 16, and the initial neutron count rate φ (0) is output to the neutron source intensity calculating means 16 and the effective multiplication factor calculating means 17.

  The neutron source intensity calculating means 16 calculates the neutron source intensity S from the neutron count rate φ (t) detected by the neutron detector 11 and the elapsed time t measured by the time counting means 15. That is, the neutron source intensity calculation means 16 uses the detected value of the neutron count rate φ (t) by the neutron detector 11 after the neutron absorber 12 approaches the nuclear fuel assembly 1 as the number of neutrons N ( t). The neutron source intensity calculation means 16 sets the detected value of the neutron count rate φ (t) as the value of the neutron number N (t) and sets the detected value of the initial neutron count rate φ (0) as the initial neutron number N (0 ), The delayed neutron leading number C (t) is calculated from the formulas (7) and (8), and the delayed neutron leading number C (t) is calculated from the calculated value of the delayed neutron leading number C (t) using the formula (6). The number of neutrons Q (t) is calculated.

In order to improve accuracy, delayed neutrons can be multigrouped. In general, the equation of the delayed neutron 6 group in which the delayed neutron leading nuclear decay constant is divided into 6 groups is often used. In that case, the expression (6)-(8) becomes the following expression (6) ′-(8) ′.

  The neutron source intensity calculating means 16 further plots the value of the neutron number N (t) and the value of the delayed neutron number Q (t) obtained as described above in a graph as shown in FIG. And the reading A of the slope of the straight line P and the reading B of the intercept are obtained. The neutron source intensity calculating means 16 obtains the expressions (10) and (11) from these readings A and B and the expression (5), and obtains the neutron source intensity S of the nuclear fuel assembly 1.

  The effective multiplication factor calculation means 17 calculates the detected value of the initial neutron count rate φ (0) detected by the neutron detector 11 in the state where the neutron absorber 12 is separated from the nuclear fuel assembly 1 as the initial neutron number N (0). The initial neutron count rate φ (0) and the value of the neutron source strength S of the nuclear fuel assembly 1 calculated by the neutron source strength calculation means 16 are substituted into the formula (12), and the neutron absorber 12 The effective multiplication factor k0 of the neutron of the nuclear fuel assembly 1 in a state where is separated from the nuclear fuel assembly 1 is calculated. The subcriticality of the nuclear fuel assembly 1 is measured using this effective multiplication factor k0.

With the configuration as described above, the first embodiment has the following effects.
The number of neutrons N (t) (that is, the neutron count rate φ (t)) and the number of delayed neutrons Q (t) that change with the passage of time from the time when the neutron absorber 12 approaches the nuclear fuel assembly 1 The effective multiplication factor k0 of the neutron in the nuclear fuel assembly 1 in the separated state before the neutron absorber 12 approaches the nuclear fuel assembly 1 is obtained, and the subcriticality of the nuclear fuel assembly 1 is determined from the effective multiplication factor k0. taking measurement. As a result, even if the shape, dimensions, etc. of the nuclear fuel assembly 1 are not yet determined, the subcriticality of the nuclear fuel assembly 1 can be reliably measured.

[B] Second Embodiment (FIG. 6)
FIG. 6 is a side view showing a second embodiment of the subcriticality measuring apparatus. In the second embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and the description is simplified or omitted.

  The difference between the subcriticality measuring apparatus 30 of the second embodiment and the first embodiment is the configuration of an attachment / detachment mechanism 31 that moves the neutron absorber 12 so as to approach or separate from the nuclear fuel assembly 1.

  That is, the attachment / detachment mechanism 31 includes a wire 32 that is connected to the neutron absorber 12 and extends in the vertical direction, and a manual or electric hoisting machine 33 that winds the wire 32. The wire 32 is a wire guide tube. 34 is guided in the vertical direction. The neutron absorber 12 is raised by the winding operation of the wire 32 by the hoisting machine 33, and the neutron absorber 12 is moved away from the arrangement position of the neutron detector 11 in the nuclear fuel assembly 1. Moreover, the neutron absorber 12 is dropped by opening the wire 32 that has been operated for winding, and the neutron absorber 12 is brought close to the arrangement position of the neutron detector 11 in the nuclear fuel assembly 1.

  In the second embodiment, in addition to the same effect as that of the first embodiment, since the gravity is used when the neutron absorber 12 approaches in the attachment / detachment mechanism 31, the spring 25 can be omitted, and the attachment / detachment mechanism The structure of 31 can be simplified.

  As mentioned above, although embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. It is included in the scope and gist of the invention, and is included in the invention described in the claims and the equivalent scope thereof. For example, the nuclear fuel assembly 1 that is the measurement target of the subcriticality may be a nuclear fuel assembly in which the fuel is not melted.

1 Nuclear fuel assembly (container containing fissile material)
DESCRIPTION OF SYMBOLS 10 Subcriticality measuring apparatus 11 Neutron detector 12 Neutron absorber 13 Attachment / detachment mechanism 13A Separation mechanism part 13B Approach mechanism part 14 Calculation unit 15 Time counting means 16 Neutron source intensity calculation means 17 Effective multiplication factor calculation means 23 Wire 24 Hoisting machine 25 Spring 30 Subcriticality measuring device 31 Attachment / detachment mechanism 32 Wire 33 Hoisting machine k, k0 Effective multiplication factor S Neutron source strength t Time φ (t) Neutron count rate φ (0) Initial neutron count rate

Claims (4)

A neutron detector arranged to face one surface of a container containing a fissile material and detecting the number of neutrons emitted from the container as a neutron counting rate;
A neutron absorber provided so as to be disposed opposite to another surface of the container perpendicular to one surface of the container in which the neutron detector is disposed;
An attachment / detachment mechanism for attaching / detaching the neutron absorber to approach or leave the container;
Having a time counting means, a neutron source intensity calculating means and an effective multiplication factor calculating means,
The time counting means measures an elapsed time from the time when the neutron absorber approaches the container,
The neutron source intensity calculating means calculates the neutron source intensity of the container from the neutron count rate detected by the neutron detector and the time output from the time counting means in a state where the neutron absorber is close to the container. Calculate
The effective multiplication factor calculating means is configured such that the neutron absorber is separated from the container from the initial neutron measurement rate detected by the neutron detector and the neutron source intensity in a state where the neutron absorber is separated from the container. A subcriticality measuring apparatus configured to calculate an effective multiplication factor of neutrons in a state. The subcriticality measuring apparatus according to claim 1, wherein the metal constituting the neutron absorber is cadmium, boron, gadolinium, or hafnium. The attachment / detachment mechanism for bringing the neutron absorber close to or away from the container containing the fissile material is a separation mechanism that separates the neutron absorber from the container by a wire connected to the neutron absorber and a hoist that winds the wire. 3. The structure according to claim 1, further comprising: a mechanism portion; and an approach mechanism portion that causes the neutron absorber to approach the container by a biasing force of a spring pressed against the neutron absorber. Subcriticality measuring device. The attachment / detachment mechanism for bringing the neutron absorber close to or away from the container containing the fissile material includes a wire connected to the neutron absorber and a hoisting machine that winds the wire, and winding the wire by the hoisting machine. The neutron absorber is raised by the above operation to move away from the neutron detector arrangement position of the container, or the neutron absorber is dropped by opening the wire that has been hoisted, and the neutron detector arrangement position of the container The subcriticality measuring apparatus according to claim 1, wherein the subcriticality measuring apparatus is configured to approach to the subcriticality.

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