JP3345275B2 - Method and system for loading fissile material into storage container - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for storing a substance containing a fissile nuclide in a container and a technique for ensuring that the inside of the container is maintained in a sufficiently subcritical state.

[0002]

2. Description of the Related Art Conventionally, methods of evaluating subcriticality include a "pulse neutron method" and a "spontaneous neutron multiplication method".

[0003] In the "pulse neutron method", pulsed high-speed neutrons are injected from an external neutron source into an object to be measured (for example, a nuclear reactor in a subcritical state), and measurement is performed after the neutrons are injected until the state returns to the original state. The measurement is based on the fact that the temporal change of the thermal neutron density in the target varies depending on the subcriticality (Narihiro Yasu, "Reactor Theory and Design", The University of Tokyo Press, pp.154-156).

"Spontaneous neutron multiplication method" (JP-A-1-169398)
Publication) measures or evaluates the axial burnup distribution, dependent group constant, and spontaneous neutron source distribution for each loaded spent fuel assembly for storage of spent fuel, and The neutron flux is calculated by a three-dimensional fixed source calculation each time a single body fuel is loaded into the container based on the neutron detector, and measured by a neutron detector installed in the storage container. Both (calculated value and measured value)
Is calculated while changing the nuclear constant (for example, the rate of fission generation by fast neutrons) so that the ratio of
Is evaluated.

[0005]

[Problems to be solved by the invention]

Normally, the container containing fissile material (as compared to the pulse neutral method) is designed to have a sufficiently deep subcriticality even when the container is filled with the fissile material to be stored. . In a state of deep subcriticality, since the effective delayed neutron ratio β and the prompt neutron lifetime 1 change with respect to Keff, it is very difficult to accurately evaluate the subcriticality. In addition, since spent fuel itself emits neutrons, it is necessary to use a strong pulsed neutron source.

[0006] The fuel assembly (for spontaneous neutron multiplication) depends on its location in the core where it was placed and operating conditions.
It has various burnup distributions. For this reason, in order to accurately evaluate the subcriticality by calculation, the burnup distribution in the axial direction and circumferential direction of each spent fuel assembly is accurately measured, and then
It is necessary to calculate Keff for each loading step.

An object of the present invention is to provide a method for securing a subcriticality margin in a container when a fissile material is loaded into the container.

It is another object of the present invention to provide a method for preventing the inside of a container from exceeding the criticality during loading of fissile material.

Another object of the present invention is to provide a fissile material loading system which achieves the above two objects.

[0010]

[0011]

A feature of the first aspect of the present invention that achieves the object of the present invention is that a container containing a substance containing a fissile substance has a known concentration and composition,
A neutron detector capable of measuring the neutron count rate and a neutron source are installed at the same time, and the subcriticality in the process of loading the vessel with fissile material is determined by the neutron count rate and the neutron flux.
After confirming from either one of the changes, by removing the fissile material, neutron detector and neutron source whose concentration and composition are known, the neutron multiplication factor is less than the reference value . located in the kite to have a certain level of subcriticality margin.

A feature of the invention of claim 2 that achieves the object of the present invention is that, in the invention of claim 1, a substance containing a fissile substance to be stored in a container emits neutrons spontaneously. And the neutron source is not installed separately.

Another feature of the third aspect of the present invention that achieves another object of the present invention is that, in the first or second aspect, the contribution of the fissile material of known concentration and composition to the effective neutron multiplication factor is not provided. when a container to be measured of the critical degree becomes critical (effective neutron multiplication factor = 1), the subcriticality margin predetermined (1-effective neutron multiplication factor) than larger as the concentration of the fissile material , Composition and structure.

According to a fourth aspect of the present invention, there is provided a fuel assembly according to the first or second aspect, wherein the object containing fissile material to be stored is a spent fuel assembly of a nuclear reactor. In the case where a plurality of the fuel assemblies are loaded in one storage container, fission with a known concentration and composition is performed under conditions such that the storage container becomes critical (effective neutron multiplication factor = 1). contribution to the effective multiplication factor by sex material, the contribution to the effective neutron multiplication factor with the minimum burnup of fuel is allowed to loading to at least said container, subcriticality margin (1-effective predetermined Neutron multiplication factor), the concentration, composition or structure of the fission material is determined.

According to another aspect of the present invention, there is provided a neutron detector capable of measuring a fissile material having a known concentration, composition and distribution, a neutron counting rate, and the neutron detector. Means for recording the neutron count rate in the vessel, and means for confirming subcriticality based on whether or not the ratio between the recorded neutron count rate and the neutron flux in the container measured during the loading process satisfies the limit value. A neutron detector that can measure the fissile material whose concentration, composition, distribution is known, and the neutron counting rate,
Installed in a container containing a substance containing fissile nuclides,
The neutron detector that can measure the counting rate of fissile materials and neutrons with known concentrations, compositions, and distributions has a subcriticality in a container when all materials containing fissile nuclides to be loaded are loaded. When the margin is confirmed by the means for confirming subcriticality, the margin is removed from the container.

[0016]

[0017]

According to a sixth aspect of the present invention which achieves another object of the present invention, the time change of the neutron counting rate due to the loading of the nuclear material to be stored into the container is a predetermined upper limit. If the value is exceeded, the neutron absorber may be automatically inserted into the container.

[0019] Features of the invention of claim 7 to achieve the another object of the present invention, in claim 5, loading of the fissile material to be loaded subject to operate the control device for loading machine with loading machine is performed by, at the stage where it is determined that there is a possibility to exceed the critical by means for confirming subcritical, or to stop the loading of the fissile material in the loading, or control signals as taken to confirm subcritical It is to be sent to the unit or RaSo load machine.

[0020]

[0021] Features of the invention of claim 8 to achieve the another object of the present invention, in claim 5, the concentration and composition distribution are divisible known fissile material into a plurality of divided each part That it can be taken out independently.

The size of the neutron flux in the subcritical state is strongly affected by the intensity of the neutron source. For this reason, fissile materials that contain few nuclides that generate spontaneous neutrons due to spontaneous fission, etc., have no neutron source unlike spent fuel. The change is very small. Therefore, a neutron source is required to ensure a subcriticality margin with fissile materials that do not contain or contain only a small amount of spontaneous neutron emitting nuclides. At this time, the intensity of the neutron source does not need to be known, but the intensity that can be detected by the neutron detector loaded at the same time is a minimum.

A neutron multiplication factor (Keff) in the system is increased by loading a fissile material having a known concentration, composition, distribution, etc. together with a neutron detector into a nuclear material storage container to be measured. As shown in FIG. 2, generally, as fissile material is loaded into a container, Keff in the container increases and approaches a critical state. Therefore, when the fissile material to be stored is loaded in a state where Keff in the nuclear material storage container is raised and the subcriticality is confirmed, the concentration, composition, distribution, etc., are known. By removing the fissile material, the subcriticality margin in the storage container can be secured at least by the increase of Keff by the fissile material.

Further, the magnitude of the increase of Keff by a fissile material whose concentration, composition, etc. are known becomes smaller as the inside of the container approaches the criticality. Therefore, when the inside of the vessel becomes critical while Keff is raised, the concentration of the fissile material and the concentration of the fissile material are adjusted so that the magnitude of the raising becomes equal to or more than the required subcriticality margin (1-Keff). By setting the composition, it is possible to obtain at least a necessary margin of subcriticality simply by checking subcriticality in a raised state (regardless of the depth of subcriticality).

As described in the third aspect of the present invention, the loading of the fissile material into the container is completed, and it is confirmed that the inside of the container is subcritical, and the concentration, composition, and distribution are known. If a new fissile material is loaded after the fissile material is removed from the measurement system, the fissile material whose concentration, composition, and distribution are known is used to secure a subcriticality margin. In addition to raising Keff, it is necessary to have the fissile material concentration, composition, and distribution necessary for raising Keff by newly loaded fissile material.

When the inside of the container is subcritical, the size of the neutron flux inside the container reaches an equilibrium with the passage of time, but continues to increase with time after the criticality is exceeded. Therefore, the subcriticality can be easily confirmed by monitoring the time change of the count rate of the neutron detector when the fissile material is loaded and confirming that the neutron flux reaches the equilibrium.

On the other hand, the subcriticality becomes shallower as the fissile material is loaded into the container (the neutron multiplication factor approaches 1).
As a result, as shown in FIG. 18, the neutron flux greatly changes with a slight change in Keff. Therefore, if you want to ensure subcriticality with a sufficient margin for the critical conditions in the container: ・ In the case of step loading, the increase in the neutron count rate per unit loading step ・ In the case of continuous loading, the unit loading time An upper limit may be set for the increase in the neutron count rate per hit, and loading may be stopped when the upper limit is exceeded during the loading process, and the nuclear material being loaded may be withdrawn.

If the concentration, composition, and distribution of the fissile material to be loaded are known, Keff and neutron flux distribution in the container when the fissile material is loaded in the container are evaluated to some extent by neutron transport calculations. it can. If a fissile material with a different concentration, composition, and distribution is loaded due to misloading, the Keff in the container will differ from the calculated value, and the neutron flux will also differ. When Keff is small (subcriticality is deep), Keff
The neutron flux change is small even if it changes slightly, and if the loaded fissile material contains spontaneous neutron generating nuclides, it may be indistinguishable from the change in neutron flux due to spontaneous neutrons . However, Keff is large (0.95).
Degree), a slight change in Keff (for example, 0.02)
However, since a large change in neutron flux (about 1.5 times) is caused, a change in Keff due to misloading can be easily grasped by measuring the neutron flux in the container.

The maximum Keff in the container is at the final stage of loading. However, it is practically impossible to fill the container only with the fissile material having the reactivity of the critical design limit. Even in the final stage, Keff in the container is 0.9
It is kept much lower. In the present invention, Keff is raised (about 0.05 depending on conditions) by a fissile material whose concentration, composition, and distribution are known.
f increases, making it easier to detect misloading.

The infinite neutron multiplication factor k∞ possessed by an object containing fissile material is represented by four factors: a fast neutron fission increase rate ε, a resonance escape probability p, a thermal neutron utilization rate f, and a thermal fission neutron generation rate η. Expressed as the product of factors, at least one of each factor is affected by the type, concentration, distribution, or presence of neutrons in the fissile material. Then, the effective neutron multiplication factor Keff, which gives the infinite neutron multiplication factor the effect of neutron leakage, is also affected.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) An embodiment in which the method for securing a subcriticality margin of the present invention is applied to spent fuel storage of a nuclear reactor will be described with reference to FIGS. The method generally includes four steps as shown in FIG. Hereinafter, each step will be described.

Step 1 An apparatus comprising a fissile substance 12 having a known concentration, composition, and concentration distribution and a neutron detector 11 in a vessel 2 containing a fissile substance such as a cask or canister to be measured for subcriticality. 1 (hereinafter referred to as a subcritical monitor). If the container 2 is empty before loading the subcritical monitor 1, the effective neutron multiplication factor Keff in the container 2 increases from 0 by the contribution of the fissile material 12 built in the subcritical monitor 1, The value is indicated by a in FIG. This value varies depending on the concentration and composition of the fissile material 12, the concentration distribution, the shape of the container, and the like.

In this step, it is not necessary that the container 2 is empty. If the subcriticality in the container 2 is sufficiently deep (Keff is much smaller than 1), the spent fuel assembly 3 may already be loaded in the container 2.

Step 2 While the spent fuel assembly 3 is loaded in the container 2, a change in the neutron counting rate accompanying the loading is measured by the neutron detector 11 provided in the subcritical monitor 1. If the subcriticality in the container 2 is maintained even by loading, the neutron flux in the container 2 converges to a constant value, so that it is confirmed that the inside of the container 2 is subcritical. Spent fuel assembly 3
As the number of loaded bodies increases, Keff increases. Further, as the loading of the spent fuel assembly 3 progresses and Keff increases, the Kef caused by loading the subcritical monitor 1 increases.
The rise in f becomes smaller.

Step 3 When the loading of the spent fuel assemblies 3 into the container 2 is completed, it is confirmed by the same method as in step 2 that the inside of the container 2 is subcritical. At this time, Keff is assumed to be the value of b in FIG. 2 (the value of b depends on the amount of fissile material contained in the subcritical monitor 1, the infinite multiplication factor of the spent fuel loaded, and the like).

Step 4 The subcritical monitor 1 loaded in the container 2 is taken out. As a result, the subcriticality in the container 2 is reduced by the increase of Keff due to the fissile material 12 built in the subcriticality monitor. As described above, the increase in Keff due to the fissile material 12 contained in the subcritical monitor becomes smaller as the Keff in the container 2 increases. Inside is critical (Keff
= 1), Keff by subcritical monitor 1
Subcriticality margin ΔK is determined in advance by regulations, etc.
If the concentration, composition and distribution of the fissile material 12 contained in the subcritical monitor 1 are determined so as to be 0 or more,
At the time of step 4, the subcriticality margin is surely secured.

(Embodiment 2) Another embodiment in which the method of securing a subcriticality margin according to the present invention is applied to spent fuel storage of a nuclear reactor will be described with reference to FIGS. In the present embodiment, the measurement is performed by inserting the subcriticality monitor 1 into the spent fuel assembly loading position in the container. The method of the present embodiment is roughly divided into FIG.
Consists of five steps as shown in FIG. Steps 1 to 4 are the same as in the first embodiment, and a description thereof will not be repeated. This embodiment differs from the first embodiment in that the spent fuel assembly 3 is loaded even after the subcritical monitor 1 is taken out in step 5. For this reason, taking into account the increase in Keff due to the spent fuel assemblies inserted after loading the subcritical monitor 1, when the last loaded one is the spent fuel assembly with the assumed maximum reactivity, However, in order to secure the subcritical margin ΔK0, under the condition that the inside of the container 2 reaches the critical state (Keff = 1) with the subcritical monitor 1 loaded, the increase of Keff by the subcritical monitor 1 is ΔK0 and ΔK1. If the concentration, composition, and distribution of the fissile material 12 contained in the subcritical monitor 1 are determined so as to be equal to or greater than the sum, it is ensured that the subcriticality margin is secured at the time of Step 5. Become.

In this embodiment, the criticality is almost exceeded during the loading of a certain number of spent fuel assemblies 3 for some reason (misloading of the fuel assemblies, changes in the neutron deceleration conditions, etc.). However, if ΔK1 is sufficiently large (for example, a value obtained assuming that the last used fuel assembly has reactivity equivalent to that of the new fuel assembly), If the loading is stopped once and the subcritical monitor 1 is taken out, a subcritical margin of ΔK0 or more can be secured even if the fuel assembly is loaded.

In connection with Embodiments 1 and 2, an example of effective loading of the spent fuel assembly 3 will be described. As shown in FIG. 5, the spent fuel assemblies 3 are loaded near the neutron detector 11. The numbers shown in the square frames in FIG. 5 showing the spent fuel assemblies 3 indicate the loading order of the spent fuel assemblies. The spent fuel assemblies 3 are loaded in the container 2 sequentially from around the subcriticality monitor. Since the spent fuel assembly 3 is generally loaded into the container 2 in water, the generated neutrons are decelerated and absorbed in the water, and the neutron flux decreases exponentially as the distance from the spent fuel assembly 3 increases.
Therefore, when the spent fuel assembly 3 is loaded from a position away from the fissile material 12 and other loaded spent fuel assemblies, in the initial stage, the neutrons that disappear without contributing to fission are removed. Since the ratio becomes large, Kef
f becomes an undervalue, and in the later stage, the loaded spent fuel assemblies are filled with newly loaded spent fuel assemblies, so that neutrons are easily used for fission,
Keff sharply increases (neutron flux also sharply increases) (see FIG. 25). On the other hand, in this embodiment, the arrangement in which neutrons emitted from the loaded spent fuel assemblies 3 reach the fissile material 12 most effectively is always maintained. If the final arrangement of the spent fuel assemblies 3 is the same, the value of Keff will be the same regardless of the loading process, but the K
The maximum value of the rate of change of eff is minimum in this embodiment.

FIG. 6 shows an example of the structure of the subcritical monitor described in the first and second embodiments. In the present embodiment, the structure of the BWR fuel assembly (STEP II) is used, and the neutron detector 11 (fission counter, BF
3), and the fuel rod is filled with a fissile material 12 having a known concentration, composition, and concentration distribution. According to the present embodiment, the subcriticality monitor can be installed at the fuel loading position without providing a special support structure or the like in the container, and application to an existing canister is easy. In addition, the distribution of fissile material (especially in the axial direction), which has a large effect on Keff, can be made similar to that of fuel assemblies. Easier to do. In this embodiment, parts having little effect on criticality evaluation do not necessarily have to have the same structure and the same material as the fuel assembly. For example, spacers, upper and lower tie plates, and the like can be simplified. Further, the material of the channel box may not be Zircaloy.

FIG. 7 shows another embodiment of the subcriticality monitor.
In the present embodiment, the outside of the pipe containing the neutron detector 11 is surrounded by a neutron moderator 14, and the surrounding area is further surrounded by fissile material. Most of the fast neutrons generated by fissile material are thermalized by the moderator 14 and incident on the neutron detector 11, and the sensitivity of the neutron detector is generally high for thermal neutrons. Changes can be detected with high sensitivity. In the present embodiment, the neutron detector 11, the moderator 14, and the fissile material 12 are all cylindrical, but high-speed neutrons generated by fission are thermally converted by the moderator and incident on the neutron detector. Then, a similar effect can be obtained in an arbitrary shape such as a square or a sphere. In addition, the moderator thickness, fissile material concentration, composition,
The distribution may be freely changed according to the required margin of subcriticality, the structure of the container, the type of fissile material contained in the container, and the like.

FIG. 8 shows another embodiment of the subcriticality monitor.
This embodiment is different from the previous embodiment of the subcritical monitor in that a plurality of detectors are arranged in the axial direction (a) or the detectors are movable in the axial direction (b), and the neutron flux distribution in the axial direction can be measured. It is what it was. From the measurement results at each axial position, by selecting the measurement value at the axial position where the rate of change of the neutron flux size due to the loading of fissile material is the highest, and evaluating the subcriticality, It is possible to secure a certain subcritical margin.

FIGS. 14 and 15 show another embodiment of the subcriticality monitor. In the present embodiment, as shown in FIG. 6, the fissile material 12 having a known concentration, composition, and distribution is formed in a form (fuel rod) divided into a large number like a fuel assembly of a light water reactor. In the criticality monitor, each part (fuel rod) can be replaced. Then, according to the composition of the fissile material to be stored and the required subcritical margin, fuel rods having different concentrations, compositions, and distributions are replaced so that the required fissile material concentration, composition, and distribution can be obtained. In the embodiment shown in FIG. 14, the fuel rods around the neutron detector 11 are changed from the fuel rods 12A having the enrichment of A to the fuel rods 12B having a higher enrichment of the B. The amount of increase is increasing. Even if the type of the fissile material to be stored or the type of the storage container changes due to such replacement of the fuel rods, it is possible to cope with the change with one subcritical monitor.

FIG. 9 shows an example of an arrangement of a subcritical monitor in a container 2 for accommodating a cylindrical fissile material. In this embodiment, a subcritical monitor 1 is provided in a gap portion in a container 2 for storing fissile materials (spent fuel, transuranium element waste, etc.).
(A neutron detector 11 and a fissile material 12 having a known concentration, composition, and concentration distribution are inserted). This embodiment is applicable to the invention of claim 2, and the stored spent fuel assembly 3 needs to contain spontaneous neutron emitting nuclides. In FIG. 9, as the subcritical monitor 1, a fissile material 12 having a known concentration, composition, and concentration distribution and a neutron detector 11 are integrated. However, this is not an essential condition, and FIG.
As described above, the fissile material whose concentration, composition, and distribution are known and the neutron detector 11 may be inserted at different positions. When the fissile material 12 and the neutron detector 11 whose concentration, composition, and concentration distribution are known in the arrangement as shown in FIG. 10 are inserted, a sharp change in Keff is avoided, so that a spent fuel assembly to be stored is stored. The loading of 3 starts around the fissile material of known concentration, composition and concentration distribution, or the fissile material of known concentration, composition and concentration distribution and the neutron detector 11
It is desirable to load to fill the gap.

FIG. 11 shows an example of the arrangement of the subcritical monitor 1 in the cylindrical container 2. This embodiment is different from the embodiment shown in FIG. 9 in that it does not contain a spontaneous neutron generating nuclide or has a subcriticality of a fissile material having a very low spontaneous neutron emission rate (for example, a reactor fuel before combustion). The present invention is applied to secure a margin. For this reason, Californium (Cf) and Americium (Am) -Beryllium (Be)
A neutron source 4 for generating neutrons by spontaneous fission or nuclear reaction as shown in FIG. Neutrons generated from this neutron source are loaded into the spent fuel assembly 3 to be stored and the fissile material 12 constituting the subcritical monitor.
And generate neutrons according to the effective multiplication factor in the vessel. These neutrons are captured by the neutron detector 11 constituting the subcritical monitor 1 and used for subcritical determination. The neutron source 4 is taken out together with the subcritical monitor 1 when the loading progresses and the subcritical determination is no longer necessary.

Although the absolute intensity of the neutron source 4 to be used does not need to be known, the neutron flux can be measured by the neutron detector 11 constituting the subcritical monitor 1 from the initial stage of loading the fuel to be stored. A certain degree of strength is required. In this embodiment, the neutron source and the subcritical monitor 1 are separately provided.
Fissile material 1 constituting neutron source 4 and subcritical monitor 1
2 may be integrated.

FIG. 12 shows an example of the arrangement of a fissile material 12 having a known concentration, composition, and concentration distribution in the cylindrical container 2 and a neutron detector 11 (in this embodiment, 40 fissile materials are used). Assume storage container). In the present embodiment, the neutron detector 11 is placed at the center of the vessel 2 with the least leakage of neutrons, and the fissile material 12 having a known concentration, composition, and concentration distribution is divided into four parts and loaded in another part. . In particular, in a large storage container, since the effect of a single fissile material on Keff is small, the amount of fissile material 12 of known concentration, composition, and concentration distribution loaded in order to raise Keff is known. You need to make more. To realize this in the space of one fissile material to be loaded,
It is necessary to increase the concentration, which makes production and management extremely difficult. On the other hand, if the fissile material 12 having a known concentration, composition, and concentration distribution is divided into a plurality and loaded as in this embodiment,
The method of the present invention can be applied to a large-capacity storage container without increasing the concentration per one body (for example, even at a concentration that is conventionally used for fuel). Noting that the fissile material 12 having a known concentration, composition, and concentration distribution is divided and loaded, the neutron detector 11 shown in FIG.
And fissile material 12 of known concentration, composition, and concentration distribution
The same effect can also be obtained by loading a plurality of subcritical monitors 1 in which is integrated into the container 2. FIG. 13 shows an example of the arrangement. At this time, the neutron flux is measured by the neutron detector 11 of each subcritical monitor 1 and the change in the counting rate of each detector accompanying the loading of the spent fuel assembly 3 is compared. It is possible to more accurately estimate the change in the neutron flux due to the change in the neutron multiplication factor Keff (the portion where the change in the count rate is the smallest reflects the change in Keff the best. The neutron flux is more strongly affected by the decrease in the spontaneous neutrons and the neutron leakage effect of the fuel assembly 3). FIG. 16 shows an example of a system in which the spent fuel assemblies 3 are loaded while ensuring the subcriticality in the container 2 and a subcritical margin of a certain level or more is secured at the end of the loading. The system according to the present embodiment includes a neutron detector 11 and a fissile material whose concentration, composition and distribution are predetermined, for example, a subcritical monitor 1 as shown in FIG.
Amplifies the signal generated by neutrons incident on the
A count rate measuring means 15 for converting into a count rate; a count rate recording means 16 for recording the measured count rate; a subcriticality determining means 17 for determining subcriticality from the obtained count rate data; It comprises an alarm generating means 18 for generating an alarm when it is determined that the criticality is approached by the means. Of these, the sub-criticality monitor 1 performs the measurement of the container 2 before loading of the spent fuel assembly 3 to be stored in the container to be measured.
It is installed in. Here, the concentration, composition, and concentration distribution of the fissile material contained in the subcritical monitor 1 are the same as those of the subcritical monitor 1 and the spent fuel assembly 3 to be loaded. The subcritical monitor 1 is taken out of the container 2 so that the subcritical margin (1-Keff) becomes a constant value (for example, 0.05) under the condition that the subcritical monitor 1 becomes critical when filled with the substance. .

The operation of this embodiment will be described with reference to FIGS. First, the subcritical monitor 1 is inserted into the container 2 to be measured (101), and the neutron counting rate C0 in the container is measured by the counting rate measuring means 15 (102). This neutron count rate was recorded by the count rate recording means 16 such as a floppy disk, a magnetic card, and an optical disk (103).
Thereafter, the neutron counting rate C is measured while loading one spent fuel assembly 3 to be stored in the container (104).
05). At this time, the current neutron count rate C sent from the count rate measuring means 15 and the current neutron count rate C remaining in the count rate recording means 16 (the neutron count rate C0 before the used fuel currently loaded is not loaded). When the difference divided by C0 is calculated (106), the value is calculated as shown in FIG. 18 for the fuel (as long as the infinite multiplication factor of the spent fuel assembly to be loaded is 1 or more). As the loading depth of the spent fuel assemblies 3 into the container 2 increases, the average rate of change with respect to the change in the loading depth also decreases in the subcriticality (Keff
Becomes closer to 1). When one fissile material 12 is completely loaded, it is determined by the subcriticality determining means 17 whether (C−C0) / C0 is within a predetermined limit value (107), and the limit value is determined. If it is within the range, it is confirmed whether there is a place in the container 2 for further loading the spent fuel assembly 3 (108), and if loading is possible, the latest neutron counting rate C at that time is set to C0. With (1
09) C0 is recorded in the count rate recording means (103), and loading is proceeded in the same procedure.

On the other hand, if (C-C0) / C0 is equal to or greater than a predetermined limit value in step 107 for determining subcriticality, a signal is sent from subcriticality determination means 17 to alarm generation means 18. Then, an alarm is issued to the worker performing the loading operation (111). When an alarm is issued, the loaded spent fuel assembly 3 is taken out of the container 2 (112), and the subcritical monitor 1 is taken out of the container 2 (110) to obtain a predetermined value (for example, 0.05) or more.

Step 108 for confirming whether or not there is a place in the container for further loading the spent fuel assembly 3.
In the above, even when it is determined that there is no place to be loaded (full loading), the subcritical monitor 1 is taken out of the container 2 (1).
10), a predetermined value (for example, 0.05
) The above subcritical margin is added.

In this embodiment, the change in the count rate with respect to the loading depth of the fuel assembly during the loading operation is determined and used based on the count rate before the loading of the fuel assembly for the determination of subcriticality. The absolute value C of the neutron counting rate may be used as a criterion for determination. In this case, it is desirable to determine the criterion in consideration of the neutron flux due to spontaneous neutrons emitted from the loaded fuel assembly.

Although the present embodiment has been described with reference to a spent fuel assembly as an example, the present invention can be applied to the case where all materials containing fissile nuclides are loaded into a container.

In this embodiment, a spent fuel assembly that generates spontaneous neutrons is taken as an example. However, if a neutron source is added to the container in addition to the configuration of this embodiment, new fuel It can also be applied to fissile materials that generate few spontaneous neutrons such as aggregates. At this time, it is desirable that the neutron source is installed in the container together with the subcritical monitor, and is taken out together with the subcritical monitor taken out of the container.

FIG. 20 shows another embodiment of the system for loading the spent fuel assemblies 3 while ensuring the subcriticality in the container 2 and securing a subcritical margin of a certain level or more at the end of the loading.
Shown in The difference between the present embodiment and the previous embodiment (FIG. 16) is that in the previous embodiment, when the sub-criticality judging means 17 judges that the criticality is approached, a signal is sent to the alarm generating means 18 to issue an alarm. On the other hand, in this embodiment, a signal is sent to the loader controller 5 of the spent fuel assembly in place of the alarm generating means in this embodiment, and the controller 5 stops the loader 6 in response to this signal. Alternatively, the control signal is sent to remove (pull up) the loaded fuel assemblies. In this case, a warning is issued to the worker that the criticality is approaching, and even if the worker who is performing the loading operation attempts to continue the loading operation (dangerous side), the warning is issued. It is desirable not to accept.

FIG. 21 is a flowchart showing the operation of the system of this embodiment. 19 is different from FIG. 19 in that the load stop warning step 111 is replaced with a step 113 for instructing the loader to stop the loader or to remove the fuel assembly being loaded.
Is the point where

In the process of loading the spent fuel assembly into the container using the neutron detector installed in the container,
FIG. 22 shows an example of the operation flow of the over-criticality detection means for determining whether or not the inside of the container has exceeded the criticality. First, the neutron counting rate Ct1 after a certain time (t = t1) has elapsed after loading the spent fuel assembly into the container is measured by the neutron detector installed in the container and the counting rate measuring means provided outside the container. (202), and after a certain period of time (t = t
2) The neutron count rate Ct2 is measured (203), and the ratio Ct2 / Ct1 of both is obtained. As shown in FIG.
If the inside of the container does not exceed the criticality, the neutron flux rapidly increases with the loading of the spent fuel assembly, but soon becomes equilibrium and approaches a constant value, so that Ct2 / Ct1 becomes 1 or a value close thereto. On the other hand, when the criticality is exceeded, the neutron flux continues to increase, so that Ct2 / Ct1 becomes larger by an order of magnitude than in the subcritical case. Therefore, a limit value is provided for the value of Ct2 / Ct1, and if it exceeds this value, it is determined that the criticality has been exceeded, and if it has not been exceeded, it is determined to be in a subcritical or critical state (204).

FIG. 23 shows an example of a device for inserting a negative reactivity when the inside of the container 2 exceeds the criticality during loading of the spent fuel assembly 3 to be stored. The apparatus of this embodiment is installed in a tank 7 kept under reduced pressure or vacuum and a boron water tank 8 filled with boron water and a pipe connecting the two, and is opened by a signal from the supercritical detection means 9. Tank 7 which is composed of a valve 10
Are inserted into the container 2. Incidentally, the amount of boron water filled in the boron water tank 8 is larger than the capacity of the tank 7, and the pipe from the boron water tank 8 to the tank 7 is connected from the bottom of the boron water tank 8 to the top of the tank 7. .
Further, the installation position of the tank 7 is preferably near the center of the container 2.

Normally, since the valve 10 is always closed and the tank 7 is kept under reduced pressure or vacuum, incident neutrons are hardly decelerated or absorbed, and have a great effect on Keff in the vessel. Do not give. However, when the inside of the container 2 exceeds the criticality due to the loading of the spent fuel assembly and is detected by the supercritical detecting means 9, a signal to open the valve 10 is issued from the supercritical detecting means 9, and the valve 10 is opened. You. Since the pressure in the tank 7 installed in the container 2 is reduced, the pressure difference between the boron water tank 8 and the tank 7
(At this time, if the boron water tank 8 is located at a higher position than the tank 7, it can carry a larger amount of boron water than the capacity of the tank 7.) When boron water flows into the tank 7, the neutrons incident on the tank 7 are decelerated by the water, and are mainly absorbed by 10B, so that Keff in the vessel is reduced and the supercritical state is resolved.

If the tank 7 of this embodiment is built in a subcritical monitor as shown in FIG. 6, the effect of reducing Keff when the criticality is exceeded is large, and the labor for installation can be reduced.

This embodiment shows an example in which the present invention is applied to a means for preventing a spent fuel assembly of a nuclear reactor from being misloaded into a transport container.

The spent fuel assemblies having a burnup of 30 GWd / t are sequentially loaded in the container 2 capable of storing 10 fuel assemblies. The change is shown by a dashed line in FIG. Here, assuming that the new fuel assembly is loaded at the time of the fifth loading, the neutron flux at the center of the container changes as shown by the solid line in FIG. 24, and it is assumed that the subsequently loaded fuel assembly is correct. However, the value of the neutron flux will be far away from the neutron flux when all are loaded correctly. Therefore, the neutron flux is evaluated from the counting rate of the neutron detector 11 placed in the container, the change tendency accompanying the loading is compared with the calculated value of the neutron flux when the container is loaded correctly, and the difference between the two is compared. The presence / absence of misloading is confirmed from the presence / absence, and when a difference occurs between the two, the position of the occurrence of misloading can be specified by searching for a part where the comparison value of both begins to change.

[0062]

Effects of the Invention According to the present invention, containing no spontaneous neutron emission nuclide, or a low emissivity nuclear material can be easily ensured subcriticality margin when loaded into the container.

[0063] According to the invention of claim 2, the subcriticality margin when loaded with fissile material containing spontaneous neutron-emitting nuclides in the container can be easily ensured.

According to the third and fourth aspects of the present invention,
In the invention of claim 1 or 2, the subcriticality which is not less than a certain value
To keep the degree margin to ensure the concentration and composition of the fissile material for raising the effective neutron multiplication factor Keff, it can be determined and distributed.

[0065] According to the fifth aspect of the present invention, in the invention of claim 1 or 2, and easily implement the method of securing the subcriticality margin when loaded with fissile material container.

[0066]

[0067]

According to the invention of claim 6, in the invention of claim 5, even if the inside of the container exceeds the critical state while loading the fissile material to be stored, the inside of the container is kept in a subcritical state. It is possible to return to.

According to the invention of claim 7 , according to the invention of claim 5,
In the container while loading the fissile material to be stored
A situation exceeding the criticality can be prevented beforehand .

According to the eighth aspect of the present invention, the same system can be used even if the type of fissile material loaded in the storage container or the required size of the subcritical margin changes in the fifth aspect of the present invention. You can easily respond while using it.

[Brief description of the drawings]

FIG. 1 is a diagram showing a procedure of an embodiment of the present invention when applied to loading of a spent fuel assembly into a container for storing the spent fuel assembly.

FIG. 2 is a characteristic diagram showing a change in an effective neutron multiplication factor associated with loading of a spent fuel assembly and removal of a subcritical monitor when the procedure of FIG. 1 is adopted.

FIG. 3 is a diagram showing a procedure of an embodiment of the present invention (additional fuel after taking out a subcritical monitor) when applied to loading of a spent fuel assembly into a container for storing a spent fuel assembly. is there.

FIG. 4 is a characteristic diagram showing a change in an effective neutron multiplication factor associated with loading of a spent fuel assembly and removal of a subcritical monitor when the procedure of FIG. 3 is performed.

FIG. 5 is a schematic view showing an example of an order of loading spent fuel assemblies into containers.

FIG. 6 is a perspective view showing an example of a subcritical monitor in which a neutron detector and a fissile material having a known concentration, composition, and concentration distribution are integrated.

FIG. 7 is a perspective view showing another example of a subcritical monitor in which a neutron detector and a fissile material whose concentration, composition, and concentration distribution are known are integrated.

FIG. 8 is a schematic diagram showing an example in which a plurality of neutron detectors are installed or movable in the axial direction in a subcritical monitor in which a neutron detector is integrated with a fissile material having a known concentration, composition, and concentration distribution. It is.

FIG. 9 is a schematic view showing an example in which a subcritical monitor is arranged at a position other than a position where a spent fuel assembly is loaded in a container.

FIG. 10 is a schematic diagram showing an example in which a neutron detector and a fissile material having a known concentration, composition, and concentration distribution are arranged at different locations in a storage container.

FIG. 11 is a schematic view showing an example in which a neutron source is arranged in a spent fuel assembly container in addition to the subcriticality monitor.

FIG. 12 is a schematic view showing an example in which fissile materials having known concentrations, compositions, and concentration distributions are dispersed and arranged in a spent fuel assembly container.

FIG. 13 is a schematic view showing an example in which a plurality of subcritical monitors are dispersed and arranged in a spent fuel assembly storage container.

FIG. 14 is a cross-sectional view showing an example of a subcritical monitor capable of replacing a part of a fissile material having a known concentration, composition, and concentration distribution.

FIG. 15 is a sectional view taken along line AA of FIG. 14;

FIG. 16 is a configuration diagram illustrating an example of a subcritical margin monitoring system used in the embodiment of FIG. 1;

FIG. 17 is an explanatory diagram showing an operation flow of the subcritical margin monitoring system of FIG. 16;

FIG. 18 is a characteristic diagram showing a change in a neutron counting rate ratio in the storage container with respect to a loading depth of the spent fuel assembly in the storage container.

FIG. 19 is a characteristic diagram showing a change in a neutron counting rate in the container with respect to an increase in the number of loaded fuel assemblies in the storage container.

FIG. 20 is a configuration diagram of another embodiment of the subcritical margin monitoring system applicable to the embodiment of FIG. 1;

FIG. 21 is an explanatory diagram showing an operation flow of the subcritical margin monitoring system of FIG. 20;

FIG. 22 is an explanatory view showing the operation of the supercritical detecting means in the process of loading the spent fuel assembly into the container.

FIG. 23 is a schematic diagram showing the configuration and operation of a supercritical prevention device that operates when the supercritical state in the container is exceeded.

FIG. 24 is a characteristic diagram showing that a neutron flux in a container deviates from a predicted value due to erroneous loading when a spent fuel assembly having a known burnup is loaded into a storage container.

FIG. 25 is a characteristic diagram showing a difference in a change in the effective neutron multiplication factor Keff with an increase in the number of loaded bodies, in a case where the order of loading the spent fuel assemblies into the storage container is not optimized;

FIG. 26 shows a state where a spent fuel assembly is loaded into a storage container.
It is a characteristic diagram which shows the difference of the time change of the neutron count rate in a container when the inside of a container exceeds the criticality, and when it does not exceed.

[Explanation of symbols]

1 ... subcritical monitor, 2 ... container, 3 ... spent fuel assembly,
4: neutron source, 5: loader controller, 6: loader,
7 ... tank, 8 ... boron water tank, 9 ... supercritical detection means, 10 ... valve, 11 ... neutron detector, 12 ... fissile material with known concentration, composition and concentration distribution, 13 ... channel box, 14 ... deceleration Material, 15 ... counting rate measuring means, 16
... counting rate recording means, 17 ... subcritical judgment means, 18 ... alarm generation means.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Hideaki Kurokawa 7-2-1, Omika-cho, Hitachi City, Ibaraki Pref. Hitachi, Ltd. Power & Electric Equipment Development Division (72) Inventor Hiromi Maruyama Omika-cho, Hitachi City, Ibaraki Prefecture 7-2-1, Hitachi, Ltd. Electric Power & Electric Equipment Development Division (72) Inventor Masao Yamaguchi 3-2-1, Saimachi, Hitachi, Ibaraki Pref. Hitachi Engineering Co., Ltd. (72) Inventor Yoyasu Yamanaka Ibaraki 3-1-1, Sachimachi, Hitachi-shi Hitachi, Ltd. Hitachi Plant (58) Field surveyed (Int. Cl. ⁷ , DB name) G21C 17/00 G21C 17/06

Claims (8)

(57) [Claims] A container containing an object containing a fissile material, a fissile material having a known concentration and composition, a neutron detector capable of measuring a neutron counting rate, and a neutron source are simultaneously installed. Subcriticality in the process of loading the container with fissile material, after confirming from the change of either the neutron counting rate or neutron flux, the concentration and composition of the fissile material and neutron detector with known by removing the neutron source, nuclear fraction, characterized in that it gave at least a certain subcriticality margin as the container neutron multiplication factor is equal to or less than a reference value
A method for loading a fragile substance into a storage container. 2. The object according to claim 1, wherein the object containing a fissile substance to be stored in the container contains a substance that spontaneously emits neutrons, and a neutron source is not separately provided. loading method into the storage container of nuclear fissile material characterized by. 3. The method according to claim 1, wherein the contribution of the fissile material having a known concentration or composition to the effective neutron multiplication factor becomes critical when the container whose subcriticality is to be measured becomes critical. predetermined said to be greater than the subcriticality margin, the concentration of the fissile material, composition, loading method into the storage container of nuclear fissile material, characterized in that defining the structure. 4. An object according to claim 1, wherein the object containing fissile material to be stored is a spent fuel assembly of a nuclear reactor, and a plurality of the fuel assemblies are contained in one storage container. when loaded, under conditions such that the critical in the storage container, the concentration and composition contribution to effective multiplication factor with known fissile material, the minimum allowed the loading to at least said container fission characterized the contribution to the effective neutron multiplication factor with the combustion of the fuel, the concentration of the fissile material to be larger than the sum of the subcriticality margin predetermined that defined composition or structure loading method into storage container sexual material. 5. A fissile material having a known concentration, composition and distribution, a neutron detector capable of measuring a neutron count rate, means for recording a neutron count rate at said neutron detector, and said recorded neutrons. It consists of a means for confirming subcriticality based on whether the ratio between the counting rate and the neutron flux in the container measured during the loading process satisfies the limit value, and counts fissile materials and neutrons whose concentration, composition, and distribution are known. Neutron detectors with measurable rates
Installed in a container containing a substance containing fissile nuclides,
The neutron detector that can measure the counting rate of fissile materials and neutrons with known concentrations, compositions, and distributions has a subcriticality in a container when all materials containing fissile nuclides to be loaded are loaded. A system for loading fissile material into a container, wherein the container is removed from the container when it is confirmed by the subcriticality confirming means that the margin is secured. 6. A neutron absorber according to claim 5, wherein a neutron absorbing material is automatically inserted into the container when the time change of the neutron counting rate accompanying the loading of the nuclear material to be stored into the container exceeds a predetermined upper limit. A system for loading fissile material into a container. 7. The method according to claim 5, wherein the loading of the fissile material to be loaded is performed by operating the control device of the loading machine using the loading machine, and the possibility of exceeding the criticality by means of confirming the subcriticality is obtained. in step it is determined that there is nuclear fissile material, characterized in that to stop the loading of the fissile material in the loading, or control signals as retrieving is sent from the means to ensure subcritical to loading machine Loading system in the storage container. 8. A nucleus according to claim 5, wherein the fissile material having a known concentration, composition, and distribution can be divided into a plurality of parts, and each of the divided parts can be independently taken out.
Loading system into storage container of fissile material.