JPH05297184A - Mixer-settler, method for measuring its neutron multiplication factor and calibrating method of neutron detecting device - Google Patents

Abstract

PURPOSE:To reduce the influence of neutron multiplication effect, evaluate the detecting efficiency of a neutron detector, and measure and calibrate the neutron multipication factor during campaign. CONSTITUTION:A plate neutron moderator 6 is arranged on the lower surface of a rectangular box vessel 1, and a neutron detector 15 is arranged in the neutron moderator with a distance from the bottom surface of the vessel 1. A neutron absorber 16 is arranged in plate form in the position on the output (lower side) of the bottom surface of the vessel 1 except a fixed range where the distance between the neutron detector 16 and the bottom surface on the vessel 1 is minimized. A neutron source is arranged in the position where the neutron absorber 12 is arranged within a fixed range having the shortest distance position to the bottom surface of the vessel 1 as a center in the inner part of the neutron moderator 6 or the outside of the neutron moderator 6 on the opposite side (lower side) to the vessel 1 in such a manner as to be capable being inserted and pulled out.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mixer-settler used when reprocessing spent nuclear fuel, a neutron multiplication factor measuring method for the mixer-settler, and a neutron detector calibration method.

[0002]

2. Description of the Related Art The concept of a mixer-settler for reprocessing spent nuclear fuel that has been conventionally used will be described with reference to FIG. FIG. 12 (A) is an elevation view seen from the direction of arrow AA in FIG. 12 (B), and FIG. 12 (B) is shown in FIG. 12 (A) and FIG.
FIG. 12C is a plan view seen from the direction of the arrows B-B in FIG. 12C, and FIG. 12C is a sectional view taken along the line C-C in FIGS. 12A and 12B.

As shown in FIG. 12 (C), the mixer-settler has a rectangular box-shaped container 1 in which a partition wall 2 is provided, and stirring blades 3 are provided in the partition wall 2 via a rotary shaft 4 to form a rectangular shape. Box-shaped container 1
A neutron moderator 6 is provided below the bottom plate 5 of the above, and a neutron detector guide tube 7 is inserted into the neutron moderator 6. In addition, as shown by the broken line in FIG.
A plate-shaped neutron absorbing material 12 is provided near the lower surface of the. However, the neutron absorbing material 12 is not directly arranged on the lower surface of the bottom plate 5 near the shortest distance between the bottom plate 5 and the neutron detector guide tube 7.

Individual mixer-settlers are connected in multiple stages as shown in FIGS. 12 (A) and 12 (B), and are used as a multi-stage solvent extraction device. In FIG. 3B, M is a mixer section and S is a settler section. In FIG. 12 (C), the left side is the mixer section M and the right side is the settler section S, and the water phase 8 and the oil phase 9 are mixed in the left mixer section M, for example, the nuclear fuel components are mixed almost uniformly.

The liquid in either the water phase 8 or the oil phase 9 entering the mixer M contains nuclear fuel. Among the liquids entering the mixer part M, the aqueous phase 8 contains the nuclear fuel, and the step of transferring the nuclear fuel to the oil phase 9 in the settler part S is called the extraction step, and the liquid entering the mixer part M is Among these, the process in which the oil phase 9 contains the nuclear fuel and the nuclear fuel is transferred to the water phase 8 in the settler portion S is called a back extraction process.

When the mixed phase 10 discharged from the mixer section M to the settler section S gently flows to the right in FIG. 12 (C), the mixed phase 10 almost disappears due to the difference in specific gravity, and a predetermined liquid level 11 is maintained. The oil phase 9 and the water phase 8 are separated. A liquid interface is formed between the phases 8 and 9 and stays at a predetermined position during normal operation. However, if some abnormality occurs, the interface may be biased or fluctuate in some way.

In the extraction process, the nuclear fuel components are mainly in the upper oil phase 9 and the mixed phase 10, and in the back extraction process, the nuclear fuel components contained in the aqueous phase 8 and the mixed phase 10, especially plutonium which needs to be managed, emit neutrons. discharge. Figure 12 (A) of the neutron
And as shown in (C), the neutron detector (not shown) inserted in the neutron detector guide tube 7 of the moderator 6 provided on the lower side of the rectangular box-shaped container 1 measures the flow state of the nuclear fuel liquid. Are to be monitored. Usually, one neutron detector guide tube 7 is provided for each stage.

[0008]

However, the neutron emission rate from the nuclear fuel changes depending on not only the nuclear fuel concentration but also the composition ratio, and the count rate of the neutron detector also changes depending on the height direction position of the nuclear fuel liquid. In addition, if the nuclear fuel concentration is high to some extent, neutrons associated with induced fission will also be measured. Therefore, it is extremely difficult to properly monitor the concentration of the nuclear fuel in the liquid, the subcriticality, and the state of the liquid flow by the neutron detector arranged below the bottom plate of the mixer-settler.

Since the neutron detector measures the neutrons emitted by plutonium, the concentration of the nuclear fuel in the liquid, the behavior of the flow, the subcriticality related to the criticality safety, etc. are evaluated by measuring and monitoring the counting rate of the neutrons. Development is about to proceed.

As described above, the neutron detector installed in the mixer-settler is expected to achieve important functions, but these techniques have not been developed so far.

As a first step to solve such a problem, it is necessary to confirm that the neutron detector is operating properly and determine the detection efficiency thereof. It is most reliable to perform these under the conditions where a geometrical relationship with the mixer-settler, neutron absorber, neutron moderator, etc. is given at the position where the neutron detector is used.

From such a background, it is preferable that the above operation can be performed not only when the nuclear fuel liquid is not present in the container but also when the nuclear fuel liquid is present.

When the neutron detector is arranged at a predetermined use position and the above-mentioned work is carried out when the nuclear fuel liquid is present in the container, the neutron detector causes neutron multiplication in the nuclear fuel liquid portion. Will be affected by the multiplication effect. Among the above-mentioned functions expected of the neutron detector of Mixer Setra, ensuring the criticality safety related to neutron multiplication is the most important issue.

In order to achieve the above-mentioned functions expected of the neutron detector of the mixer-settler, the neutron detector is properly arranged and operates properly, and the neutron count rate of the neutron detector and the plutonium concentration in the nuclear fuel liquid are required. It is indispensable that the correspondence with the above is correctly associated, that is, the neutron detector is calibrated.

The present invention has been made on the basis of such a background, and an object thereof is that a neutron detector is arranged at a predetermined use position, and a nuclear fuel liquid is present in a container. Even if the neutron multiplication effect cannot be ignored, it is an object of the present invention to provide a mixer-settler capable of evaluating the detection efficiency as well as confirming the operation of the neutron detector by arranging a neutron source to reduce the influence of the neutron multiplication effect.

Another object of the present invention is to provide a mixer-settler capable of measuring neutron multiplication factor during a campaign and a neutron multiplication factor measuring method thereof.

Further, according to the present invention, it is possible to perform various functions of the neutron detector using a neutron source and a calibration test prior to the campaign for actually processing nuclear fuel, and the calibration test requires a neutron count rate more than necessary even during the campaign. It is an object of the present invention to provide a calibration method for a mixer-settler and a mixer-settler neutron detector that can be performed without increasing the cost.

[0018]

A first aspect of the present invention relates to a mixer section for mixing an oil phase and an aqueous phase in a rectangular box-shaped container, and a settler section for separating the oil phase and the aqueous phase mixed in the mixer section. In a mixer-settler that includes and that performs a liquid extraction by flowing a nuclear fuel liquid in the container, a plate-like neutron moderator is arranged on the lower surface side of the container, and the moderator is separated from the bottom surface of the container by a certain distance. The neutron detector is placed inside the neutron detector, and the neutron absorber is placed in a plate shape on the outside (bottom side) of the bottom surface of the container except a certain range where the distance between the neutron detector and the bottom surface of the container is minimized. With the inside of the neutron moderator or the opposite side of the container (lower side) outside the neutron moderator on the outer side of the neutron moderator to the bottom surface of the container centered on the position of the neutron absorber at the position Place the neutron source freely It is characterized in.

A second aspect of the present invention comprises a rectangular box-shaped container having a mixer section for mixing an oil phase and an aqueous phase, and a settler section for separating the oil phase and the aqueous phase mixed by the mixer section. In a mixer-settler that performs liquid extraction by flowing a nuclear fuel liquid into the container, a neutron moderator is disposed on the lower surface side of the container, and a neutron detector is freely insertable and removable in the longitudinal direction of the container in the neutron moderator. A guide tube is arranged, and a neutron reflector is arranged on a part of an upper portion of the neutron detector guide tube on the upper surface of the container.

According to a third aspect of the present invention, a rectangular box-shaped container is provided with a mixer section for mixing an oil phase and an aqueous phase, and a settler section for separating the oil phase and the aqueous phase mixed by the mixer section. In a mixer-settler that performs liquid extraction by flowing a nuclear fuel liquid into the container, a neutron moderator is placed on the lower surface side of the container, a neutron detector is placed in the neutron moderator, and a neutron source guide tube is freely inserted and removed. Arranged in the upper surface of the container or in the neutron moderator, and insert and remove calibration neutron source guide tube for guiding the calibration neutron source is arranged in the moderator near the neutron detector To do.

A fourth aspect of the present invention is the mixer-settler according to the second aspect, in which the neutron reflector is disposed on the upper surface of the container and below the neutron reflector is not disposed near the position. Of the neutron detector guide tube to determine the ratio by measuring the neutron count rate, to determine the neutron multiplication factor at or near the position where the reflector is placed through the parameter obtained by theoretical calculation of this ratio Characterize.

According to a fifth aspect of the present invention, in the mixer-settler according to the third aspect, the neutron source is moved in the neutron source guide tube to measure the sensitivity and operating characteristics of the neutron detector prior to the campaign for treating nuclear fuel. At least one of the absorber and the neutron absorber including the neutron source is moved in the neutron source guide tube provided near the neutron detector to monitor the operating state of the neutron detector.

[0023]

According to the first aspect of the invention, the neutron source guide tube is arranged in the neutron moderator in the depth direction parallel to the neutron detector guide tube on the axis of symmetry. A neutron source (not shown) can be inserted / extracted in / from the neutron source guide tube, and is usually pulled out from the mixer-settler section except in the case of detector calibration.

Since the neutron absorbing material is arranged on the lower surface of the bottom plate between the neutron source guide tube and the bottom plate of the container, the neutrons emitted from the neutron source are decelerated and the neutrons converted into thermal neutrons enter the container. Almost no inflow.

As a result, the number of neutrons multiplied in the container based on the neutron source is significantly reduced, so that the multiplied neutrons emitted from the container flow into the neutron detector due to the existence of the neutron absorber. Significantly reduced compared to the case without.
That is, even if the nuclear fuel is present in the container, the neutron detector can be calibrated regardless of the multiplication effect.

According to the second aspect of the invention, a short neutron detector is arranged below one of the vessels constituting the mixer-settler,
A mixer-settler in which a neutron reflector is locally arranged on the upper side of the other opposing container is obtained.

According to the third invention, the neutron detector guide tube is arranged in the neutron moderator arranged under the vessel, the neutron source guide tube is arranged adjacent to the neutron moderator guide tube, and the neutron source guide for calibration is arranged. A mixer settra with tubes is obtained.

According to the fourth invention, by measuring the neutrons emitted from the nuclear fuel while moving the short neutron detector in the longitudinal direction of the container, the neutrons can be obtained without artificially introducing a neutron source from the outside. The effective multiplication factor can be obtained and the criticality safety can be ensured.

According to the fifth aspect of the present invention, prior to the campaign, the neutron detector guide tube is inserted and withdrawn the calibration neutron source to perform an operation test and a calibration test of the neutron detector. It is possible to confirm that the neutron detector is operating normally by inserting or removing the neutron absorbing material or a neutron absorbing material equipped with a weak neutron source so as not to cause an increase in the rate.

In particular, since the increase in the neutron count rate is suppressed during the campaign, no alarm or scrum signal is generated due to the abnormal increase in the count rate, and therefore, the neutron detection is performed without lowering the operation rate of the mixer-settler. The vessel can be constantly monitored.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first to fourth embodiments according to the first invention will be described with reference to FIGS. In each drawing, (A) is a vertical cross-sectional view in the width direction of one stage of the mixer-settler corresponding to FIG. 12, and is a vertical cross-sectional view in the direction of arrow AA in each drawing (B). Each figure (B) is a mixer-settler 1 corresponding to FIG. 12 (C).
It is a longitudinal cross-sectional view of a step, and is a vertical cross-sectional view taken along the line BB of FIG. The reference numerals in the figure are the same as those in FIG. 12, and the description of the overlapping portions will be omitted.

FIG. 1 shows a first embodiment, which is a relatively large mixer-settler. If the width of the one-stage container (left-right direction in FIG. 1 (A)) is larger than about 30 cm, almost no neutron information can be obtained at the end in the width direction. Therefore, in this embodiment, the two neutron detector guide tubes 7, 7 are arranged symmetrically in parallel with the axis of symmetry 14 in the depth direction at intervals of about 30 to 50 cm. In addition,
The dimensions of the mixer-settler will be described later. As the neutron detector, a long object covering most of the length of the settler portion S is shown, but a plurality of neutron detectors may be arranged in series.

Here, as an example, the structural material such as the side portion and the bottom plate of the container 1 is, for example, a stainless steel plate, and each has a thickness of 5
Assume mm. The neutron absorbing material 12 is a cadmium (Cd) metal plate having a thickness of 1 mm. Further, the thickness of the container 1 (vertical direction in the figure) is about 15 cm, the thickness of the neutron moderator 6 on the lower side of the container 1 is about 15 cm, the container 1 is 60 cm in the width direction (horizontal direction in the figure),
It is assumed that the total length in the depth (left and right direction in Fig. 1 (B)) is 150 cm and the depth of the settler part S is 120 cm. Roughly speaking, a mixer-settler of this size can be considered as a typical size.

In FIG. 1 (A), the neutron absorbing material 12 per one-stage container 1 is 12a, 12b, 12c from the left, respectively.
15cm, 13a and 13b without neutron absorber
Assume 7.5 cm. The container 1 is filled with a nuclear fuel liquid containing plutonium fuel, and has a constant neutron effective multiplication factor (generally written as k _eff , but in the present specification, it is simply denoted as k except _eff ). Shall be.

If the spontaneous neutron emission rate emitted by spontaneous fission from nuclear fuel and (α, n) reaction between oxygen and the like and alpha rays is S ₀ , neutrons based on spontaneous neutrons at the position of the neutron detector 16 The bundle (or may be thought of as the neutron count rate) φ ₀ is given by equation (1) via the proportionality coefficient γ. φ ₀ = γS ₀ / (1-k) (1)

By the way, Californium (Cf) 252
Based on the external neutron source when an artificial neutron source such as that is externally placed at the position of the symmetry axis 14 as the external neutron source, the active neutron flux φ _S (also referred to as neutron source multiplication neutron flux) is a constant a , B is given by the equation (2). φ _S = a + (b / (1-k)) (2)

Regarding the relationship between a and b, for the case of a fuel assembly placed in water, the journal "Nuclea of the Atomic Energy Society" is published.
r Techuology ”, January 1992 issue (Fig. 8). If the intensity of the external neutron source is S, the constants a and b may be written as a = αS and b = βS via the proportional coefficients α and β.

In the present embodiment, in order to enable calibration of the sensitivity (detection efficiency) of the neutron detector 16 even when a nuclear fuel such as plutonium that emits spontaneous neutrons is present in the container 1, the φ _S value is φ. It should be considerably larger than the ₀ value. One of the methods is to set S >> S ₀ , but if the neutron source is indiscriminately arranged, an error due to the dead time of the neutron detector 16 may occur or the neutron multiplication effect may be affected.

From FIG. 8 of the above paper, it can be seen that the a / b ratio increases near the neutron source. This means that it is not affected by the neutron multiplication effect. This paper discusses the case of arranging the fuel assembly in water.However, even in the mixer-settler system targeted in this example, the neutron moderation / diffusion / absorption and multiplication characteristics are mainly controlled by hydrogen atoms. The behavior of neutrons is similar in both cases.

In the present invention, as is clear from FIG. 1A, the neutron absorbing material 12 is provided in the lower surface of the container 1 where the distance between the neutron source guide tube 15 and the lower surface of the container 1 is the shortest. Since the cadmium (Cd) sheet is placed,
Most of the neutrons emitted from the neutron source (not shown) in the neutron source guide tube 15 are absorbed by Cd and do not enter the container 1, so that the neutron flux φ represented by the formula (2) based on the neutron source _{With S} , the multiplication effect becomes even smaller.

That is, as shown in FIG. 1 (A), a neutron source for calibration is provided in the neutron moderator 6 below the vessel 1 or below the neutron moderator 6 in the vicinity of the neutron detector 16. When the Cd sheet is placed as shown in 12b, the neutron detector is hardly affected by neutron multiplication by nuclear fuel.
16 calibrations can be done.

FIG. 2 is a modified example of FIG. 1, in which the neutron detector
16 are two in series (16a, 16b) in the depth direction of the mixer-settler
(Indicated by) is located. As shown in FIG. 12 (C), the nuclear fuel liquid discharged from the mixer section M to the settler section S is phase-separated in the settler section S. However, at the time of abnormality, the height of each phase may be shifted vertically. is there.

Two neutron detectors as in the embodiment of FIG.
When two 16a and 16b are arranged in the depth direction, the neutron response characteristics of the neutron detector on the side of the mixer M and the neutron detector on the opposite side change relatively, so the neutron count rates of both neutron detectors 16a and 16b Abnormality information can be obtained from the ratio.

FIG. 3 shows a second embodiment according to the first invention. In the case of a mixer-settler where the concentration of nuclear fuel is high, it is necessary to make it significantly smaller than that in the case of FIG. In such a case, if two neutron detector guide tubes 7 are arranged in the width direction as shown in FIG. 1, neutrons emitted from the adjacent vessels of other stages will be detected by the neutron detector, which is inconvenient. And FIG.
As described above, only one is arranged at the center in the width direction.

According to this embodiment, the neutron source is arranged not at the center of the stage to be measured but at the boundary with another stage, and the same effects as those of the first embodiment can be obtained. In FIG. 3, there is an advantage that the neutron detectors in the two neutron detector guide tubes 7, 7 located below the two stages can be simultaneously calibrated.

In this embodiment, as in the case of FIG.
The neutron detectors 16a and 16b are arranged in series in the neutron detector guide tube 7, and the same effects as those of the modification can be obtained.

FIG. 4 shows a third embodiment according to the first invention, which is the case of a relatively small mixer-settler as in the case of FIG. For the above-mentioned reason, also in this embodiment, the neutron detector guide tubes 7a and 7b are arranged on the central axis surface in the width direction of the container 1 as in the case of FIG. 3 to avoid unnecessary adverse effects from other stages.

The neutron source guide tube 15 is the neutron detector guide tube 7
It is located on the middle central axial plane between a and 7b. As a result, the detectors housed in the four neutron detector guide tubes (7a, 7b, 7a, 7b) arranged below the two adjacent vessels can be calibrated.

As shown in FIG. 4 (B), two neutron detectors are arranged in series in the neutron detector guide tube 7a.
One long neutron detector is arranged in the neutron detector guide tube 7b.

The action and effect of each neutron detector causes the above-mentioned action and effect of each neutron detector arrangement in a composite manner. The neutron detector guide tubes 7a and 7b are arranged adjacent to each other in the vertical direction in this example, but naturally, they can also be arranged in the horizontal direction.

Next, embodiments according to the second and fourth inventions will be described with reference to FIGS. FIG. 5 is a first embodiment, FIG. 5 (A) is a longitudinal cross-sectional view of one stage of the mixer settra corresponding to FIG. 12 (A), and is a vertical cross-sectional view taken along the line AA of FIG. 5 (B). FIG. 5 (B) is a longitudinal cross-sectional view of one stage of the mixer-settler corresponding to FIG. 12 (C), and FIG.
It is a -B sectional view.

The rectangular box-shaped container is a rectangular box-shaped container partitioned by the ceiling plate, the partition wall 2 and the bottom plate 5.
As shown in (C), the nuclear fuel liquid or the like is flowing. A neutron moderator 6 is arranged on the lower side of the container 1, and three neutron detector guide tubes 7 are provided in the container 1 in parallel with the longitudinal direction of the container 1.
(7a, 7c, 7b) are arranged.

Of the neutron detector guide tubes, 7a and 7b are provided with neutron detectors (not shown) that also monitor the normal process. During the campaign, neutrons leaking from the nuclear fuel liquid are generally kept without moving. I'm measuring.

In order to increase the neutron count rate as a process monitor, the neutron detector guide tubes 7a and 7b are arranged so that the neutron absorbers 12a, 12b and 12c shown by the broken line avoid the container near the nearest contact point. .. A cadmium sheet is often used as the neutron absorber.

On the other hand, in the neutron detector guide tube 7c, as shown in FIG. 1 (B), a short length whose detection range is shorter than the width of the neutron reflector (also simply referred to as a reflector) R in the longitudinal direction of the container. Type neutron detector can be freely moved during the campaign to measure neutrons.

Since it is known that the change in the neutron multiplication factor (k) of the mixer-settler is generally gentle even in an abnormal condition, the neutron measurement time may take several minutes per point. It does not matter if the neutron detector is shortened and the detection efficiency is lowered to some extent. Further, even if a sheet of the neutron absorbing material 12c is arranged between the container and the neutron detector guide tube 7c, it is generally acceptable in many cases.

The reflector R is arranged in the settler section S at a position relatively close to the mixer section M. The width of the reflector in the longitudinal direction of the container is about 15 to 30 cm, the thickness (up and down direction) is about 5 to 15 cm, and the width in the container width direction (the left and right direction in FIG. 5A) is at least 10 cm or more, usually 15 cm. That is all. Although it may be wider than the width of one container, it is usually about 15 to 30 cm.

If these dimensions are made smaller than 15 cm × 15 cm × thickness of 5 cm, the function as a hydrogen-containing reflector such as polyethylene is significantly reduced. The use of a hydrogen-containing substance is optimal because the action as a reflector will be reduced unless a material such as heavy water or graphite is made larger and thicker.

Neutron measurement is performed at points d ₁ , d ₂ and d ₃ (see FIG. 5B) corresponding to points i _l , i and i _r in FIG. 6 and reflected from the equation (7) described later. When the body R does not exist, the neutron effective multiplication factor (k) above the d ₂ point in the presence of the reflector R can be calculated from the equation (7a) described later.

The mixer-settler of this embodiment has a rectangular box-shaped container having a mixer part for treating nuclear fuel and a settler part,
Place the neutron moderator under the container, and arrange the neutron detector guide tube that allows the neutron detector to be inserted and removed in the longitudinal direction of the container inside the neutron moderator, and the neutron detector guide tube on the upper surface of the container. Horizontal dimension at least 15x on top of part of
It has a neutron reflector of 15 cm and a thickness of at least 5 cm.

The neutron detector freely moves in the neutron detector guide tube, and the primary neutrons (its strength is S) emitted by the nuclear fuel, particularly plutonium, and the secondary neutrons emitted by the induced fission The neutron flux (φ) formed by neutrons is measured.

Since the obtained count rate is proportional to φ, neutron count rate and neutron flux will be used as synonyms here. The effective multiplication factor k _eff is targeted as the neutron multiplication factor. _Typical k _eff values are used for near critical reactors.

As a common knowledge in reactor physics, the neutron multiplication system is to be expressed by one k _eff value, but the layer of the nuclear fuel liquid is thin and the probability of neutron leakage is large like mixer-settler, Moreover, k (k _eff ) value is 1.0 (critical)
When the value is much smaller, the so-called neutron-like coupling action spreads only in the vicinity, so that the k value has a physical meaning only when it is considered to locally change.

Against this background, the neutron flux (= neutron count rate) φ, the neutron source intensity S, and the effective multiplication factor k are proportionally divided by α, and φ = αS / (1- k) It is well known that the relationship (3) is derived from the theory of the single-point reactor model for the subcritical neutron multiplication system.

From the equation (3) as a starting point, the spontaneous neutrons emitted by the nuclear fuel itself are used as an internal neutron source without artificially introducing an artificial neutron source from the outside, and its intensity is not known. Also, a method for obtaining the "local" k value at or near the measurement position will be described with reference to simple diagrams and equations.

FIG. 6 shows a simplified vertical cross section of the mixer-settler in the longitudinal direction. A neutron reflector is locally arranged on the upper side of the container 1. A neutron moderator 6 is arranged below the container 1, and a neutron detector guide tube 7 is arranged in the neutron moderator 6. As the neutron detector 16, one having a width shorter than the longitudinal width of the reflector is generally used.

Letting i be the case where the neutron detector is arranged below the reflector, and i _l and i _r be the cases on the left side and the right side apart from the underside of the reflector, from equation (1), i _l , i,
At the i _r position, the S value is constant and can be written as equations (3a), (3b), and (3c). The "°" mark indicates that there is no reflector.

[0068]

[Equation 1]

The case where the neutron reflector does not exist at the i position is shown by the equation (3d).

[Equation 2]

[0070] If the longitudinal width of the reflector was approximately 15~30cm includes i _l point of φ and k in the absence of the reflector i
_{Since the} difference from the value at the _r point is small, the φ and k values at the i point can be represented (approximated) by a simple average. That is,

[Equation 3]

Therefore, the expressions (4a) to (4c) are substituted into the expression (3d) for use. That is,

[Equation 4]

The following equation is obtained by taking the ratio of both equations. That is,

[Equation 5]

Here, Is the measured quantity, Either one of k i and k i is the required amount.

[0074] Is relatively close to 1.0 and almost constant because the reflector is placed on the opposite side (upper side) from the neutron detector. Therefore, it can be accurately calculated by theoretical calculation. Formula (5)
On the right side Since there are two unknowns, i.e. and ki, either one will be deleted. Now tentatively Will be asked.

Now, when Ri is defined,

[Equation 6]

It can be seen from the critical equation of the one-point reactor model that this value is the ratio of neutron leakage probabilities. With or without the reflector, the neutron spectrum in the container hardly changes, so the infinite multiplication factor hardly changes. Therefore, equation (6)
It is a ratio of probabilities of not leaking, and it is known from theoretical calculation that the Ri value is constant regardless of the composition of the nuclear fuel. However, the Ri value may change depending on the position of the mixer-settler.
Value is required.

Substituting equation (6) into equation (5) Solving for gives the formula shown in Eq. (7). That is,

[Equation 7]

On the contrary, You may solve with ki instead of. Then, formula (7a) is obtained.

[Equation 8]

That is, according to equation (7) , (7a), the “local” effective neutron multiplication factor (k) in the case of ki (with a reflector) can be obtained. Equation (7a) is used when the mixer-settler approaches criticality.
Since it approaches the criticality quickly, it is convenient to generate a warning near the criticality.

In the present embodiment, the reflector R is arranged relatively near the mixer section M. However, as can be understood from FIG. 5C, the vertical fluctuation of the flow is unlikely to occur, and the accuracy is relatively high. This is because it is expected to be high. However, it does not matter if the reflector R is placed at a position greatly apart from the mixer section.

FIG. 7 shows a second embodiment of the second invention and is a widthwise vertical sectional view corresponding to FIG. 5A. The difference from the first embodiment shown in FIG. 5 is that this mixer-settler is small, and only one neutron detector guide tube 7a as a process monitor for routine is arranged. Therefore, the neutron detector guide tube 7c for carrying out the present invention is arranged on the lower side of 7a, and the neutron effective multiplication factor (k) is expressed by the formula (7) as in the first embodiment.
And equation (7a).

FIG. 8 shows a third embodiment of the second invention, and is a widthwise vertical sectional view corresponding to FIG. 5 (A). As in the case of FIG. 5, this configuration is suitable for application to a relatively large mixer settler. The reflector R is arranged above the neutron detector guide tube 7c, and the reflector R is not arranged above the neutron detector guide tube 7b.

The neutron detector arranged in the neutron detector guide tube 7b is for routinely performing a process monitor, and the neutron detector arranged in the neutron detector guide tube 7c is a short length. Medium If necessary, the neutron detector guide tube 7c is moved to obtain the effective neutron multiplication factor (k) by the equations (7) and (7a). During the period when it is not necessary to move, it is routinely used as a process monitor by placing it at a predetermined position. In FIG. 8, Sn indicates a neutron source inserted in the neutron source guide tube.

In recent years, position-sensi
A tive counter has been developed, and it is possible to measure the neutron flux at an arbitrary position when a short neutron detector is arranged in series by discriminating the electric waveform.
If it is inserted into the neutron detector guide tube 7c, the routine process monitor and the measurement of the effective multiplication factor according to the present embodiment can be performed at the same time.

Further, in this figure, the operation characteristics and detection sensitivity calibration of the neutron detector can be performed by using the neutron source (Sn) as required.

Next, embodiments of the third and fifth inventions will be described with reference to FIGS. 9 to 11. FIG. 9 shows the first embodiment, FIG. 9 (A) is a longitudinal cross-sectional view of one stage of the mixer settra corresponding to FIG. 12 (A), and FIG.
-A vertical sectional view, FIG. 9 (B) is a longitudinal longitudinal sectional view of one stage of the mixer-settler corresponding to FIG. 12 (C), and is a BB vertical sectional view of FIG. 9 (A).

The rectangular box-shaped container 1 includes a ceiling plate and partition walls (side plates).
2 and the bottom plate 5 to form a mixer-settler body. In the container 1, as shown in FIG.
And the oil phase 9 is flowing. Either water phase 8 or water phase 9 is a nuclear fuel liquid.

A neutron moderator 6 is arranged below the container 1, and the neutron moderator 6 has two neutron detector guide tubes 7a and 7b along the longitudinal direction of the container 1 and neutrons in the present embodiment. An absorber / neutron source guide tube 15 (hereinafter referred to as a radiation source absorber guide tube) is arranged in parallel and adjacently.

Among these guide tubes, 7a and 7b are provided with a neutron detector (not shown) for monitoring a normal process, and generally measure neutrons leaking from the nuclear fuel liquid without moving during the campaign. ..

In order to increase the neutron count rate as a process monitor, the neutron absorbing materials 12a and 12b are removed in the vicinity of the closest portion between each guide tube and the container, and a window is formed especially for thermal neutrons. ing.

On the other hand, during the campaign, a neutron absorber, a neutron absorber fitted with a weak neutron source, or a neutron detector fitted with a neutron absorber can be freely inserted into and removed from the neutron source guide tube 15 for the purpose of this embodiment. Can be achieved. The neutron detector equipped with the neutron absorber can be used for another purpose because it can detect neutrons in addition to the purpose of this embodiment.

In the mixer-settler of the above-mentioned embodiment, as described above, a rectangular box-shaped container composed of a mixer part for treating nuclear fuel and a settler part, a neutron moderator underneath is arranged, and a neutron detector is arranged inside thereof. Along with the placement, a neutron absorber guide tube is provided within the neutron moderator within approximately 5 cm of the neutron source guide tube and neutron detector. Therefore, it is possible to easily perform the calibration test of the neutron detector using the neutron source prior to the campaign.

During the campaign, a neutron absorber containing a weak neutron source was used as a thermal neutron diffusion distance (about 2.5 cm) from a neutron detector in a normally used hydrogen-containing neutron moderator, or a neutron moderator usually made of polyethylene. ), It is possible to perform a calibration test of the neutron detector under the condition that the neutron count rate does not increase much from the count rate when counting neutrons from plutonium. ..

If the neutron count rate increases significantly,
Depending on the mixer / settler, if an alarm or scrum signal is generated and the campaign has to be interrupted, a bypass key must be specially installed to avoid it, which may cause an operation error. According to the embodiment, these expected troubles can be avoided.

It should be noted that the distortion of the thermal neutron flux due to the neutron absorbing material becomes almost negligible at a position about twice or more the thermal neutron diffusion distance, but this distortion is used in the present embodiment, so neutron detection is performed. The neutron absorber is configured to be movable at a position within twice the thermal neutron diffusion distance of the neutron moderator from the vessel.

Next, the operation of this embodiment will be described. In the calibration test using a neutron source performed before the campaign, a neutron source of known strength is used, and the sensitivity of the neutron detector is evaluated with the help of detailed neutron transport calculation. Since the expert of can be carried out according to the target conditions, the calibration test during the campaign will be mainly described here.

As the neutron detector, a so-called thermal neutron detector which is particularly sensitive to thermal neutrons is usually used. Although the thermal neutron detector has some sensitivity to epithermal neutrons, the epithermal neutron component is lower than the normal thermal neutron component at the measurement position of the mixer-settler, and as a result, the ratio of detecting epithermal neutrons is considerably small, so here For simplicity, we will consider only thermal neutrons.

Here, (1) campaign (regular data C
( ₁ )), (2) If the neutron absorber is placed near the neutron detector during the campaign and for calibration,
(3) Consider three cases where a neutron absorber with a weak neutron source is placed near the neutron detector.

The count rates of the neutron detector corresponding to these three cases are C ₁ , C ₂ and C ₃ . Further, _assuming that the C ₂ / C ₁ ratio is R and the increase in neutron count rate associated with a weak neutron source is C _s , the following two equations hold. C ₂ = RC ₁ (8) C ₃ = RC ₁ + C _s (9)

Assuming that the neutron absorbing material added with a weak neutron source does not become larger than the neutron count rate C ₁ in the campaign even if it is brought close to the neutron detector, C ₁ -C ₃ = C ₁ (1-R)- C _s ≧ 0 (10). The value of R can be obtained by theoretical calculation.
Since C ₁ can be evaluated against the actual operating conditions, the value of C _s and thus the intensity of the weak neutron source can be determined to satisfy the above conditions. Also (8),
(9) from the two equations, written as _{C 3 -C 2 = C s ...} (11).

Here, it is assumed that an abnormality occurs in the neutron detector. That is, considering the case where a gamma ray for which noise is to be measured due to a gain variation of the amplifier and a disc reset deviation is measured, a bias with a constant count rate is generated in C ₁ and C ₂ , and as a result, in Equation (8), An abnormality can be detected because the R value changes.

Considering the case where no noise is generated and the gain is lowered or the disk reset value is raised, C ₁ and C
The ratio with ₂ does not change, so it is not possible to detect anomaly by the R value of equation (8), but the C _s value of equation (11) can be used as a reference value before the anomaly occurs, for example, before the campaign. If it is measured, the value changes at the time of an abnormality, so that the abnormality can be found.

When carrying out the calibration tests of cases (1) and (3), it can be written that C ₁ -C ₃ = (1-R) C ₁ + C _s (12). Considering the case where noise or gamma ray counting occurs as described above, if the measurement value before the campaign is used for C _s , the same bias counting rate (background counting rate) δ is obtained for both C ₁ and C _3. Join, equation (5)
_{Is, C 1 -C 3 = (1} -R ') become _{(C 1 + δ) -C s} ... (12a).

Since the left sides of the equations (12) and (12a) are equal, R
Must be changed to R'to make both equations equal. That is, since the R value changes from the value obtained by theoretical calculation in advance, such an abnormality can be found. Also, formula (13) is created by dividing formula (12) by C ₁ . _{(C 1 -C 3) / C} 1 = (1-R) - (C s / C 1) ... (13)

Here, considering the case where the noise is not generated as described above, the gain is decreased and the disk reset value is increased, the left side of Expression (6) is constant and C _s / C ₁
Since the ratio changes, equation (6) does not hold unless the R value is changed. Since the R value is not a value that should originally change, C _s
It is understood that if / C ₁ changes, it is abnormal.

As described above, it is most preferable to carry out the calibration test in the above-mentioned three cases, but the same objectives can be achieved by (1) and (3). If the increase in neutron count rate due to the use of neutron sources is acceptable in the calibration test during the campaign, the neutron absorber can be excluded in (3). Further, the objects of the present invention can be achieved to some extent by (1) and (2).

FIG. 10 shows a second embodiment of the third invention, and FIG. 10 (A) is a vertical sectional view taken along line AA of FIG. 10 (B).
10B is a vertical cross-sectional view taken along the line BB of FIG.
9A corresponds to FIG. 9A and FIG. 10B corresponds to FIG. 10B.

In the second embodiment, the two neutron detector guide tubes 7a and 7b are arranged parallel to the longitudinal direction of the container 1 with a distance of, for example, 10 cm or more. Neutron source guide tube
15a and 15b are adjacent to each other corresponding to the neutron detector guide tubes 7a and 7b. That is, 7a is 15a, 7b is 15b
Are adjacent to each other.

In the neutron detector guide tubes 7a and 7b, as shown in FIG.
As shown in (B), two neutron detectors 16a and 16b are arranged in series, but in this embodiment, a calibration neutron source guide tube 17 is further provided.

Prior to the campaign, if the calibration neutron source is inserted / removed in / from the calibration neutron source guide tube 17, the operation test and the calibration test can be performed while the neutron detector is placed in the neutron detector guide tubes 7a and 7b. It can be carried out. During the campaign, the state of the nuclear fuel liquid in the longitudinal direction can be monitored by inserting and removing the neutron detector in this.

When the calibration neutron source guide tube 17 is used as a normal neutron source guide tube, the guide tube 17 may be arranged above the container 1.

Such a configuration is useful when the mixer-settler is relatively large. The purpose of the present embodiment can be achieved by the neutron source guide tubes 15a and 15b as in the case of the first embodiment.

FIG. 11 shows a third embodiment of the third invention. FIG. 11 (A) is a vertical sectional view taken along the line AA of FIG. 11 (B).
FIG. 11B is a vertical cross-sectional view taken along the line BB of FIG. 11A, and FIG. 11A is FIG. 10A and FIG. 11B is FIG. 10B.
It corresponds to.

The difference between FIG. 11 and FIG. 10 is that the width of the mixer-settler is narrow, so that one neutron detector guide tube 7a is arranged in the longitudinal direction for each stage, and the neutron source guide tube 15 is adjacent to it.
a is arranged, which achieves the purpose of this embodiment. Further, a calibration neutron source guide tube 17 is provided below the boundary between the two adjacent vessels as in the case of FIG.

Prior to the campaign, a neutron detector is placed in two adjacent neutron detector guide tubes 7a by inserting and removing the calibration neutron source in the calibration neutron source guide tube 17. The operation test and the calibration test can be performed as they are. There is no large requirement for the vertical height of the calibration neutron source guide tube 17, and the calibration neutron source guide tube 17 can be arranged above the container 1 or below the neutron moderator.

[0116]

According to the present invention, even if a nuclear fuel liquid is present in the mixer-settler, a neutron detector can be calibrated with almost no influence of neutron multiplication accompanying it. Also, the neutron multiplication factor can be measured during the campaign without using the neutron source. Furthermore, the neutron detector can be calibrated at a predetermined position prior to the campaign, and the neutron detector can be tested and calibrated during the campaign.

[Brief description of drawings]

FIG. 1A is a first mixer-settler according to the first invention.
Is a longitudinal sectional view showing the embodiment of FIG.
Sectional drawing which cuts and shows the B arrow direction.

FIG. 2A is a vertical cross-sectional view showing a modification example of FIG.
(B) is sectional drawing which cuts and shows the BB arrow direction in (A).

FIG. 3A is a second mixer-settler according to the first invention.
Is a longitudinal sectional view showing the embodiment of FIG.
Sectional drawing which cuts and shows the B arrow direction.

FIG. 4 (A) is a third mixer-settler according to the first invention.
Is a longitudinal sectional view showing the embodiment of FIG.
Sectional drawing which cuts and shows the B arrow direction.

FIG. 5 (A) is a first mixer-settler according to a second invention.
Is a longitudinal sectional view showing the embodiment of FIG.
Sectional drawing which cuts and shows the B arrow direction.

FIG. 6 is a vertical cross-sectional view for explaining the operation of the mixer-settler in FIG.

FIG. 7 is a vertical cross-sectional view showing a second embodiment of the mixer-settler according to the second invention.

FIG. 8 is a vertical sectional view showing a third embodiment of the mixer-settler according to the second invention.

FIG. 9 (A) is a first mixer-settler according to a third invention.
Is a longitudinal sectional view showing the embodiment of FIG.
Sectional drawing which cuts and shows the B arrow direction.

FIG. 10A is a vertical sectional view showing a second embodiment of the mixer-settler according to the third invention, and FIG.
-B is a cross-sectional view taken along the arrow B direction.

FIG. 11A is a longitudinal sectional view showing a third embodiment of the mixer-settler according to the third invention, and FIG.
-B is a cross-sectional view taken along the arrow B direction.

FIG. 12 (A) is an elevation view schematically showing a conventional mixer-settler, (B) is a plan view of (A), and (C) is C of (B).
-C is a vertical cross-sectional view showing the direction of the arrow.

[Explanation of symbols]

1 ... Container, 2 ... Compartment wall, 3 ... Stirring blade, 4 ... Rotation axis, 5
... Bottom plate, 6 ... Neutron moderator, 7, 7a, 7b ... Neutron detector guide tube, 8 ... Water phase, 9 ... Oil phase, 10 ... Mixed phase, 11 ... Liquid level, 12, 12a, 12b, 12c ... Neutron Absorber, 13, 13a,
13b ... part without absorbent material, 14 ... symmetry axis, 15,15a, 15
b ... Neutron source guide tube, 16, 16a, 16b ... Neutron detector, 17 ... Calibration neutron source guide tube.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location G21F 9/04 D 9216-2G

Claims (5)

[Claims] 1. A nuclear fuel in a rectangular box-shaped container, comprising a mixer section for mixing an oil phase and an aqueous phase, and a settler section for separating the oil phase and the water phase mixed in the mixer section. In a mixer-settler that performs liquid extraction by flowing a liquid, a plate-shaped neutron moderator is arranged on the lower surface side of the container, and a neutron detector is arranged inside the moderator at a constant distance from the bottom surface of the container. The outer side of the bottom surface of the vessel (lower side), the neutron detector and the bottom surface of the vessel except for a certain range where the distance between the bottom surface of the vessel is minimized and the neutron absorber is arranged in a plate shape, and the inside of the neutron moderator Alternatively, the neutron source is arranged so that the neutron source can be inserted and removed at the position where the neutron absorbing material is arranged around the position of the shortest distance to the bottom surface of the container outside the neutron moderator on the opposite side (lower side) to the container. Mixet which is characterized by La. 2. A nuclear fuel is provided in a rectangular box-shaped container, comprising a mixer section for mixing an oil phase and an aqueous phase, and a settler section for separating the oil phase and the aqueous phase mixed in the mixer section. In a mixer-settler that performs liquid extraction by flowing a liquid, a neutron moderator is arranged on the lower surface side of the container, and a neutron detector guide tube is freely inserted and removed in the longitudinal direction of the container in the neutron moderator. A mixer-settler, characterized in that a neutron reflector is arranged on a part of an upper part of the neutron detector guide tube on the upper surface of the container. 3. A nuclear fuel is provided in a rectangular box-shaped container, comprising a mixer section for mixing an oil phase and an aqueous phase, and a settler section for separating the oil phase and the aqueous phase mixed in the mixer section. In a mixer-settler for performing liquid extraction by flowing a liquid, a neutron moderator is arranged on the lower surface side of the container, a neutron detector is arranged in the neutron moderator, and the neutron source guide tube is freely insertable and removable. A mixer-settler, characterized in that a calibration neutron source guide tube that is arranged on the upper surface or in the neutron moderator and that guides the calibration neutron source in a removable manner is arranged in the moderator near the neutron detector. 4. The mixer-settler according to claim 2,
Measure the neutron count rate in the neutron detector guide tube below the position where the neutron reflector is not arranged below the position where the neutron reflector is arranged on the upper surface of the container and in the vicinity of that position. A method for measuring a neutron multiplication factor of a mixer-settler, characterized in that a neutron multiplication factor is obtained at a position where a reflector is arranged or in the vicinity thereof, by obtaining a ratio and using a parameter obtained by theoretical calculation or the like. 5. The mixer-settler according to claim 3,
Prior to the campaign to process nuclear fuel, the neutron source is moved in the neutron source guide tube to measure the sensitivity and operating characteristics of the neutron detector, and at least one of the neutron absorber and the neutron absorber containing the neutron source is measured during the campaign. A method for calibrating a neutron detector of a mixer-settler, characterized in that the operating state of the neutron detector is monitored by moving the inside of the neutron source guide tube provided near the neutron detector.