JP5762840B2 - Subcriticality measuring method and apparatus - Google Patents

Description

  The present invention relates to a subcriticality measurement method and measurement apparatus for a neutron multiplication system.

  The subcriticality of the neutron multiplication system has useful information for ensuring the critical safety of the system. In the neutron multiplication system, the subcriticality in the reactor is often used for confirmation of reactor shutdown margin, safety analysis at the time of fuel change, and reactor physics analysis. It is also possible to measure a high degree of reactivity by bringing a load having a high degree of reactivity such as a control rod into a neutron multiplication system whose degree of subcriticality is known in advance and measuring the degree of subcriticality.

  Here, the subcriticality of the system is given by 1-keff when the effective multiplication factor is keff. When keff = 1, the subcriticality is zero, that is, the system is in a critical state.

  To date, exponential experiment method, negative period method, control rod drop method, compensation method, neutron source multiplication method, pulsed neutron source method, inverse characteristics method and reactor noise analysis method have been proposed as subcriticality measurement methods Has been. The negative period method and control rod drop method require that the system be made critical once in its application, and the application scene is limited to the measurement of subcriticality of reactors. Not applicable. The reverse motion method and compensation method require a change of subcriticality during measurement, and the pulse neutron source method requires neutrons to be shot into the system using a neutron generator, which complicates equipment and increases costs. There is. The furnace noise measurement method does not require complicated measurement equipment, but requires long-time measurement to obtain highly accurate measurement results.

  On the other hand, an exponential experiment method for bringing a neutron source into the system and determining the subcriticality from a change in the neutron count rate is a method that has been widely used from the standpoint of its simplicity (Patent Document 1, Patent Document 2). , See Patent Document 3). In general, the relationship between the neutron flux φ, the neutron source intensity S, and the effective multiplication factor k is expressed by the following equation where β is a constant.

φ = βS / (1-k) (1)
On the other hand, in the exponential experiment method, it is known that the ratio of the detector count rate C measured by changing the distance between the neutron source and the detector has a linear relationship with the effective multiplication factor keff. The detector count rate C can be expressed as C = Rφ by giving a certain reaction probability R to the neutron flux φ, and this reaction probability R is not greatly changed under the subcriticality measurement conditions. If the relationship is to be expressed by the above equation (1), the ratio of the count rates becomes constant regardless of k, resulting in a contradiction.

Therefore, in Patent Document 1, a correction factor for the real system measurement is added to the theoretical formula of Formula (1), and
φ = αS + βS / (1-k) (2)
And should be.

The above formula (2) is
φ = βS / (1-k) × [1 + α (1-k) / β] (2a)
When the ratio of the neutron fluxes φ1 and φ2 under the two measurement conditions is calculated using equation (2a),
φ1 / φ2
= Β1 / β2 × [1 + α1 (1-k) / β1} / {1 + α2 (1-k) / β2]
... (3)
It becomes.

Usually, α1 (1-k) / β1 and α2 (1-k) / β2 are sufficiently smaller than 1, so the equation (3) is approximated,
φ1 / φ2≈β1 / β2 × [1+ (1-k) (α1 / β1-α2 / β2)]
... (3a)
It can be.

Since it is clear that the second term in the right-hand side [] is sufficiently smaller than 1, when expanding by taking the natural logarithm of the equation (3a),
LN (φ1 / φ2) ≈LN (β1 / β2) + (1 + k) (α1 / β1-α2 / β2)
... (4)
It becomes.

  The above equation (4) indicates that the ratio of the neutron flux φ, that is, the ratio of the counting rate C is linearly related to the effective multiplication factor k and the subcriticality.

  In Patent Document 1, in order to effectively obtain the ratio of the counting rate C, two points having different measurement distances between the neutron source and the neutron detector are selected, and the fissil amount of the nuclear fuel material arranged in the measurement section is selected. Measurement with different count rate C is performed from the difference.

  However, in the method disclosed in the above-mentioned Patent Document 1, if the fissile amount is different for each of the two measurement sections, that is, if sections having different enrichment and burnup are set, the ratio between the count rate and the effective multiplication factor is linear. It has been confirmed that it is out of the relationship.

  For this reason, in the measurement of fuel assemblies with a relatively large measurement system and enrichment distribution in the axial direction, spent fuel with different burnup history, etc., the result of subcriticality determination is large depending on the measurement position in the axial direction. In some cases, it was mis-evaluated, and it was necessary to pay close attention to the selection of the measurement position.

Japanese Patent Publication No. 3-17115 Japanese Patent Publication No. 2-51158 Japanese Patent Publication No. 4-58915

  In the method described in Patent Document 1, when the fissile amounts included in the two measurement intervals for obtaining the logarithmic count rate ratio are different, the relationship between the logarithmic count rate ratio and the subcriticality is not linear, In some cases, the criticality could not be determined. In particular, when the subcriticality is overestimated, it is necessary to increase the safety margin from the viewpoint of criticality safety, and there is a problem that economics and rationality are lost.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to be able to measure the subcriticality without being greatly affected by the selection of the measurement section in the subcriticality measurement for a nuclear reactor, a fuel assembly or the like.

In order to achieve the above-described object, the present embodiment is a subcriticality measurement method for automatically measuring the subcriticality of a neutron multiplication system, the step of arranging a neutron multiplication system as a measurement target in water, A step of arranging a plurality of neutron sources spaced apart from each other along one longitudinal side of the neutron multiplication system on one side of the neutron multiplication system, and the opposite of the neutron source across the neutron multiplication system A step of arranging a neutron detector on the side surface of the side, and a step of determining a neutron flux distribution from a neutron flux in a longitudinal direction of the neutron multiplication system measured by the neutron detector and positional information of the neutron detector If, from the determined neutron flux distribution, and the minimum value among the plurality of peaks caused by neutron source disposed, a smaller value of the peak adjacent to the peak having the smallest value Calibration curve read step of reading deriving the ratio of the minimum value, a calibration curve showing the relationship between the neutron flux distribution and the subcriticality in subcriticality known neutron multiplication system from the storage device between the peak When, and having the steps of: determining at least one of the effective multiplication factor and subcriticality of the neutron multiplication system from said calibration curve and the neutron flux distribution measurement results.

Further, this embodiment is a subcriticality measuring apparatus that measures the subcriticality of a neutron multiplication system, and is arranged on one side of the neutron multiplication system to be measured along the longitudinal direction of the neutron multiplication system. A plurality of neutron sources arranged at intervals, a neutron detector arranged on the side opposite to the neutron source across the neutron multiplication system, and the neutrons in the longitudinal direction of the neutron multiplication system A neutron source driving unit that drives the source, a neutron detector driving unit that drives the neutron detector in the longitudinal direction of the neutron multiplication system, and a control unit that enables positioning of the neutron source and the neutron detector A neutron source position detector for detecting the positions of the plurality of neutron sources, a neutron detector position detector for detecting the positions of the neutron detectors, the neutron source position detector and the neutron detector position detector Position from To confirm the neutron flux distribution in the longitudinal axis direction of the neutron multiplication system from the neutron flux signals from the signal and the neutron detector, and the minimum value among the plurality of peaks caused by the placement of the neutron source, of the minimum The above-mentioned ratio and subcriticality based on the neutron flux distribution in a neutron multiplication system of known subcriticality prepared from the ratio of the minimum value to the peak having the smaller value among the peaks adjacent to the peak having a value based on the calibration curve of the relationship between degrees, and having a data processing operation unit for obtaining the subcriticality of the neutron multiplication scheme.

  According to the present invention, in the subcriticality measurement for a nuclear reactor, a fuel assembly or the like, the subcriticality can be measured without being greatly affected by the selection of the measurement section.

It is a block diagram which shows the system of 1st Embodiment of the subcriticality measuring apparatus which concerns on this invention. It is a figure which shows the distribution shape of the neutron flux which can be measured in 1st Embodiment of the subcriticality measuring apparatus which concerns on this invention. It is a characteristic view which shows the relationship between the neutron flux ratio which can be measured in 1st Embodiment of the subcriticality measuring apparatus based on this invention, and subcriticality. It is a flowchart which shows the flow of a process in the data processing calculating part of 1st Embodiment of the subcriticality measuring apparatus which concerns on this invention. It is a figure which shows the system | strain in the case of PSD use which is 2nd Embodiment of the subcriticality measuring apparatus based on this invention. It is a figure which shows the system | strain in the case of multiple neutron source installation which is 3rd Embodiment of the subcriticality measuring apparatus which concerns on this invention.

  Embodiments of a subcriticality measuring apparatus according to the present invention will be described below with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

[First Embodiment]
FIG. 1 is a configuration diagram showing a system of a first embodiment of a subcriticality measuring apparatus according to the present invention.

  In this embodiment, a fuel assembly is taken as an example as a neutron multiplication system to be measured.

  As shown in FIG. 1, on one side of the fuel assembly 1, a plurality of neutron sources whose source intensity can be regarded as substantially the same, for example, a first neutron source 2 such as Cf-252 and a second neutron source 2a. Are arranged in the longitudinal direction of the fuel assembly 1 with a certain distance L2. On the other hand, the neutron detector 3 is arranged on the side surface opposite to the first neutron source 2 and the second neutron source 2a with the fuel assembly 1 interposed therebetween.

  Examples of the neutron detector 3 include a He3 proportional counter.

  The distance from each fuel assembly 1 of the first neutron source 2 and the neutron detector 3 is approximately 4 to 5 cm, and L2 is greater than or equal to L1 with respect to the distance L1 between the first neutron source 2 and the neutron detector 3. Arrange to secure.

  This apparatus includes a first neutron source driving unit 5 and a second neutron source driving unit 5a for driving the first neutron source 2 and the second neutron source 2a, and a neutron detector for driving the neutron detector 3. It has a drive unit 6, and further has a control unit 7 for positioning the first neutron source drive unit 5, the second neutron source drive unit 5 a, and the neutron detector drive unit 6.

  Further, the neutron flux data from the neutron detector 3 and the respective position data from the neutron detector driving unit 6, the first neutron source driving unit 5 and the second neutron source driving unit 5a are input to the data processing calculation unit 8. The

  The data processing calculation unit 8 outputs the result to the data storage unit 9, the recording unit 10, and the display unit 11 after processing and calculating the input data.

  With the above configuration, the neutron detector 3 is run in the longitudinal direction of the fuel assembly, and the neutron flux distribution in the longitudinal direction of the fuel assembly 1 is measured.

  In the measurement, the measurement range L3 of the neutron flux distribution in the longitudinal direction is set to be larger than the distance L2 between the first neutron source 2 and the second neutron source 2a.

  FIG. 2 is a diagram showing a distribution shape of neutron flux that can be measured in the first embodiment of the subcriticality measuring apparatus according to the present invention. By performing the above measurement, an axial neutron flux distribution as shown in FIG. 2 is obtained.

  FIG. 2 also shows the difference in the shape of the axial neutron flux distribution due to the difference in subcriticality. As shown in this figure, the axial neutron flux distribution has a relatively large axial neutron flux distribution as the effective multiplication factor keff of the system to be measured increases, that is, as the subcriticality decreases.

  This axial neutron flux distribution is characterized in that two peaks occur due to the presence of the first neutron source 2 and the second neutron source 2a.

  The count rate of the neutron detector 3 at these two peaks is (CPSmax), and the count rate of the neutron detector 3 at the portion showing the minimum neutron flux between the two peaks is the count rate (CPSmin). .

  From these two count rates, for example, a logarithmic count rate ratio of the ratio (CPSmin) / (CPSmax) is obtained.

  FIG. 3 is a characteristic diagram showing the relationship between the neutron flux ratio and the subcriticality that can be measured in the first embodiment of the subcriticality measuring apparatus according to the present invention. In actual measurement in a system with a known subcriticality, the correlation between the logarithmic count rate ratio and the subcriticality is shown.

  This figure shows the logarithmic count rate ratio obtained for a system with a known criticality and the effective multiplication factor (subcritical) in a relatively large subcriticality region where the effective multiplication factor (keff) is equivalent to 0.5 to 0.8. (Degree) shows a correlation close to a straight line.

  Therefore, when using a characteristic curve obtained for a system with a known criticality, a method of approximating with a straight line by autoregression or the like is realistic and may be used in this way.

  The neutron flux distribution in the longitudinal axis direction of the system with the known subcriticality may be obtained by measurement in a solid system or may be obtained by transport calculation.

  This correlation is based on the distance L1 between the first neutron source 2 and the second neutron source 2a and the neutron detector 3 in FIG. 1, and the distance between the first neutron source 2 and the second neutron source 2a. It changes according to L2. Therefore, the values of the distance L1 and the distance L2 in the system to be measured need to use the values of the distance obtained when the correlation is obtained in the subcriticality known system.

  FIG. 4 is a flowchart showing the flow of processing in the data processing operation unit according to the first embodiment of the subcriticality measuring apparatus according to the present invention.

  The neutron flux data from the neutron detector 3 and the position data from the neutron detector driving unit 6, the first neutron source driving unit 5 and the second neutron source driving unit 5a input to the data processing calculation unit 8 Based on this, the neutron flux distribution is determined (S1).

  Based on the neutron flux distribution obtained in step S1, CPSmax and CPSmin are derived. At this time, if the two peak values are different, after the respective values are derived as CPSmax1 and CPSmax2, the smaller one is selected as CPSmax (S2).

Next, from the CPSmax and CPSmin obtained in step S2, Δφ = (CPSmin) / (CPSmax)
Is required. Note that the logarithmic count rate ratio of Δφ may be obtained from the character of the numerical value of the neutron flux. (S3).

  In order to evaluate the keff of the measurement target based on Δφ obtained in step S3 or the logarithmic coefficient ratio, first, characteristic data relating to a system with a known subcriticality is read from the data storage unit 9 (S4).

  Then, based on the characteristic data read in step S4, Δφ obtained in step S3 or keff with respect to this logarithmic coefficient rate ratio is obtained, and the subcriticality is obtained from 1-keff (S5).

  Here, the method of obtaining keff based on Δφ or the logarithmic coefficient ratio may be performed by interpolating between the measurement points or calculation points of the calibration curve, or at the measurement points or calculation points of the calibration curve. An autoregressive curve based on it may be created and used.

  Finally, the result is output to the data storage unit 9, the recording unit 10, and the display unit 11 (S6).

  Using the correlation between the logarithmic count ratio in the system to be measured and the logarithmic count ratio in the system with the known subcriticality and the effective multiplication factor (subcriticality) obtained as described above, the subcriticality of the system to be measured Can be determined.

  When the subcriticality decreases, the ratio of the neutron flux generated by the neutron multiplication effect in the fuel assembly 1 that is the system to be measured is relative to the neutron flux that reaches the neutron detector directly from the neutron source. Therefore, the peak in the neutron flux distribution shape is not significant.

  In that case, measurement can be performed by increasing the distance L2 between the two neutron sources 2 and 2a.

  As described in the processing flow, when the fuel assembly 1 as the system to be measured has an axial concentration distribution or a different combustion history, 2 is used as in the case of keff = 0.86001 in FIG. Different results occur with the two peak values, in which case the smaller neutron count rate of the two peak values is used as (CPSmax).

  This is because in the case where the smaller one is used as CPSmax and the case where the larger one is used, Δφ is smaller in the former, and the keff is evaluated in the larger one, which is an evaluation on the safety side for criticality management. It is.

  The configuration of the measurement system in the above embodiment may be a method by changing the interval between neutron sources. In this case, the evaluation is made by paying attention to the correlation between the relative positions of the neutron detector and the plurality of neutron sources and the neutron flux distribution.

  According to the present embodiment, the subcriticality can be measured without being greatly affected by the selection of the measurement section while securing a sufficient counting rate by bringing the two neutron sources 2 into the measurement system.

  In addition, it is not necessary to consider the safety margin more than necessary, and an economic effect such as increasing the storage density of the fuel assembly can be obtained.

[Second Embodiment]
FIG. 5 is a diagram showing a system in the case of using PSD which is the second embodiment of the subcriticality measuring apparatus according to the present invention.

  As a method for measuring the neutron flux distribution in the longitudinal direction in the first embodiment, a method using a long position sensitive neutron detector 4 as shown in FIG. 5 is also possible. In FIG. 5, the display of the drive unit, the control unit, the data processing unit, and the like is omitted.

  An example of the position sensitive neutron detector 4 is a position sensitive He3 proportional counter (PSPC). According to this method, scanning by the detector of the measurement system is not required, and the installation position of the neutron detector is determined. Since a single location can be used, the measurement time can be dramatically saved.

  As a result, the subcriticality can be measured without being greatly affected by the selection of the measurement section.

[Third Embodiment]
FIG. 6 is a diagram showing a system in the case of installing a plurality of neutron sources, which is the third embodiment of the subcriticality measuring apparatus according to the present invention.

  In the first and second embodiments, the number of neutron sources is two. However, as shown in FIG. 6, three or more neutron sources may be arranged in the longitudinal axis direction. In FIG. 6, the display of the drive unit, the control unit, the data processing unit, and the like is omitted.

  In this method, the length of the fuel assembly 1 to be measured is long in the longitudinal direction, and it is difficult to change the relative positions of the first neutron source 2 and the second neutron source 2a. This is effective.

  A logarithmic count rate ratio between a neutron count rate at any point of a plurality of neutron flux peaks corresponding to the neutron source and a minimum count rate corresponding to the minimum neutron flux appearing between the adjacent peaks is obtained. From the relationship between the logarithmic count rate ratio and the subcriticality in the system with known criticality, the local subcriticality at each location of the system to be measured can be obtained.

  As a result, the subcriticality can be measured without being greatly affected by the selection of the measurement section.

[Other Embodiments]
Each embodiment described above is merely an example, and the present invention is not limited thereto.

  For example, in the measurement of the neutron flux distribution, when the range that can be covered by the system of FIGS. 1, 5 and 6 is only a part of the length in the longitudinal axis direction of the fuel assembly, the following range is measured after the measurement. Therefore, a method of moving the entire neutron source and neutron detector may be used.

  Alternatively, the neutron source and the neutron detector may be fixed and the fuel assembly may be moved in the longitudinal direction. Alternatively, a method of moving the fuel assembly in the longitudinal direction and changing the relative positions of the neutron source and the neutron detector in the respective ranges may be used.

  The relative position of the neutron source and the neutron detector may be changed by a method of changing the position of the neutron source, a method of changing the position of the neutron detector, or a method of changing both.

  As for the neutron detector, as described above, in addition to the method of moving one in the longitudinal axis direction and the method of using a position sensitive neutron detector, a method of arranging a plurality of neutron detectors in the longitudinal axis direction. But you can.

  A method of fixing the neutron detector and moving the neutron source may be used.

  In each of the embodiments, the neutron source and the neutron detector are all on the same plane with the fuel assembly sandwiched therebetween. If it is a certain system, it is possible to obtain the effect of subcriticality by the same method.

  These embodiments are presented as examples, and are not intended to limit the scope of the invention. For example, the features of the embodiments may be combined.

  Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention.

  These embodiments and modifications thereof are included in the invention described in the claims and equivalents thereof as well as included in the scope and gist of the invention.

1 ... Fuel assembly (neutron multiplication system)
2 ... 1st neutron source 2a ... 2nd neutron source 2b ... 3rd neutron source 3 ... neutron detector 4 ... position sensitive neutron detector 5 ... 1st Neutron source driving unit 5a ... second neutron source driving unit 6 ... neutron detector driving unit 7 ... control unit 8 ... data processing operation unit 9 ... data storage unit 10 ... Recording unit 11 ... Display unit 20 ... First neutron source position detector 20a ... Second neutron source position detector 30 ... Neutron detector position detector

Claims (8)

In the subcriticality measurement method that automatically measures the subcriticality of the neutron multiplication system,
Placing the neutron multiplication system to be measured in water;
Disposing a plurality of neutron sources spaced apart from each other along one longitudinal side of the neutron multiplication system on one side of the neutron multiplication system;
Placing a neutron detector on the side opposite to the neutron source across the neutron multiplication system;
Determining a neutron flux distribution from the neutron flux in the longitudinal direction of the neutron multiplication system measured by the neutron detector and the positional information of the neutron detector;
From the determined neutron flux distribution, the minimum value between the minimum value among a plurality of peaks generated by the neutron source arrangement and the peak having the smaller value among the peaks adjacent to the peak having the minimum value. Deriving a ratio to and
A calibration curve reading step for reading from a storage device a calibration curve indicating the relationship between neutron flux distribution and subcriticality in a neutron multiplication system with a known subcriticality;
Obtaining at least one of effective multiplication factor and subcriticality of the neutron multiplication system from the neutron flux distribution measurement result and the calibration curve;
A subcriticality measuring method characterized by comprising: Arranging the plurality of neutron sources and arranging the neutron detector,
The relative position of the plurality of neutron sources or the relative position of the plurality of neutron sources and the neutron detector, or the relative position of the plurality of neutron sources and the relative position of the plurality of neutron sources and the neutron detector Including a step of changing the position;
The subcriticality measuring method according to claim 1.   3. The subcriticality measurement according to claim 1, wherein the step of arranging the neutron detector includes a step of moving the neutron detector in a longitudinal direction of the neutron multiplication system. Method.   2. The step of arranging the neutron detector includes a step of performing by a plurality of neutron detectors arranged at intervals of a certain distance or more along a longitudinal axis direction of the neutron multiplication system. Item 3. The subcriticality measuring method according to Item 2.   The step of arranging the neutron detector includes a step of arranging a position sensitive neutron detector capable of measuring a neutron flux at each position in a longitudinal axis direction of the neutron multiplication system. The subcriticality measuring method according to claim 1 or 2. Prior to the calibration curve reading step, the method further comprises a calibration curve preparation step of obtaining the calibration curve and storing the calibration curve in a storage device;
The calibration curve preparation step includes
Obtaining a calibration curve based on a measurement result in a neutron multiplication system with a known subcriticality;
Obtaining a calibration curve based on the analysis result of the neutron multiplication system with known subcriticality;
Any one of these is included, The subcriticality measuring method of any one of Claim 1 thru | or 5 characterized by the above-mentioned.   The calibration curve prepared in advance in the step of reading from the storage device the characteristic data indicating the relationship between the neutron flux distribution and the subcriticality in the neutron multiplication system with known subcriticality prepared in advance is calculated in advance by neutron transport calculation. 6. The subcriticality measuring method according to claim 1, wherein the subcriticality measuring method is based on a calculated result. In the subcriticality measuring device that measures the subcriticality of the neutron multiplication system,
A plurality of neutron sources arranged on one side of the neutron multiplication system to be measured and spaced apart from each other along the longitudinal direction of the neutron multiplication system;
A neutron detector disposed on the side opposite to the neutron source across the neutron multiplication system;
A neutron source drive unit for driving each neutron source in the longitudinal direction of the neutron multiplication system;
A neutron detector driver for driving the neutron detector in the longitudinal direction of the neutron multiplication system;
A controller that enables positioning of the neutron source and the neutron detector;
A neutron source position detector for detecting positions of the plurality of neutron sources;
A neutron detector position detector for detecting the position of the neutron detector;
A neutron flux distribution in a longitudinal direction of the neutron multiplication system is determined from a position signal from the neutron source position detector, a position signal from the neutron detector position detector, and a neutron flux signal from the neutron detector, and the neutron source The subcriticality prepared from the ratio of the minimum value among the plurality of peaks generated by the arrangement of the peak and the minimum value between the peaks adjacent to the peak having the minimum value and the smaller value Based on the calibration curve of the relationship between the ratio and the subcriticality based on the neutron flux distribution in the neutron multiplication system, a data processing operation unit for obtaining the subcriticality of the neutron multiplication system,
A subcriticality measuring device characterized by comprising:

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