JP5826605B2 - Apparatus and method for detecting water level in spent fuel storage pool - Google Patents

Description

  The present invention relates to a technique for detecting the level of a spent fuel storage pool that cools spent nuclear fuel.

The spent fuel storage pool is monitored and operated so as not to drop below a predetermined reference water level, for example, a level that is more than twice the length of the spent fuel assembly, in order to ensure the radiation shielding effect of water. .
Conventionally, the water level of a spent fuel storage pool has been measured by installing a float type level switch at the upper end of the pool. Moreover, the temperature of pool water was measured with the thermometer installed separately from this float type level switch.

  In the spent fuel storage pool, a refueling crane is arranged at the upper part, and the installation space for the water level gauge and the thermometer is very limited in order to move the entire upper surface. Further, from the viewpoint of preventing leakage of pool water, it is not possible to provide a through hole in the pool wall surface portion, and it is not possible to employ a differential pressure type water level measurement method that is generally employed as a water level meter. Furthermore, since it is difficult to remove the foreign matter if it falls into the fuel storage pool, measures for preventing foreign matter from entering the pool must be taken into consideration.

  Under such circumstances, a sensor that detects a water level by arranging a heater in the vicinity of one of two junction points in a thermocouple has been proposed (for example, Patent Document 1). According to this technology, since the thermal diffusivity in water and in the air is different, based on the temperature difference (electromotive force difference) between the two junctions, whether the sensor unit is located in the water or in the air Judging.

Japanese Patent Laid-Open No. 10-153681

By the way, in the spent fuel storage pool, when the cooling function is stopped for a long time and water supply cannot be performed, the water temperature rises due to the radiation of the spent fuel and boils, and the water level is lowered due to evaporation. When the water level is lowered in this way, the radiation shielding effect is reduced and the radiation environment is deteriorated. Therefore, when the water level falls below a predetermined reference water level, it is required to accurately grasp the water level and evaluate the safety of the radiation environment.
However, in the technique of Patent Document 1, when the water temperature rises to the boiling temperature, it is difficult to stably measure the temperature difference (electromotive force difference) between the two junction points of the thermocouple. For this reason, we are anxious about the fall of the water level detection precision of a spent fuel storage pool.

  The present invention has been made in view of such circumstances, and even when the pool water boils and the water level falls, the water level detection of the spent fuel storage pool can reliably detect this water level. The purpose is to provide technology.

In a spent fuel storage pool water level detection device, a gas phase temperature sensor that detects a temperature above the reference water level of the pool water, and a water level detection temperature sensor that detects a temperature below the reference water level of the pool water, A heat supply unit for supplying thermal energy to each of the gas phase temperature sensor and the water level detection temperature sensor, and the water level of the pool water based on the detected temperatures of the gas phase temperature sensor and the water level detection temperature sensor. Each of the gas phase temperature sensor and the water level detection temperature sensor includes a thermocouple element for detecting the temperature contained in a sheath tube whose tip is closed. A pair, a heater that is disposed in the vicinity of the tip of the element wire and generates the thermal energy by energization, and a storage tube that houses the tip of the sheath thermocouple and the heater positioned at the end , And wherein the Rukoto to have a.

  The present invention provides a technique for detecting the level of a spent fuel storage pool that can reliably detect the water level even when the pool water boils and the water level drops.

(A) is a block diagram which shows embodiment of the water level detection apparatus of the spent fuel storage pool which concerns on this invention, (B) is a fragmentary sectional view of the front-end | tip of a temperature sensor. The flowchart which shows operation | movement of the water level detection apparatus of the spent fuel storage pool which concerns on 1st Embodiment. The flowchart which shows operation | movement of the water level detection apparatus of the spent fuel storage pool which concerns on 2nd Embodiment. The flowchart which shows operation | movement of the water level detection apparatus of the spent fuel storage pool which concerns on 3rd Embodiment. The flowchart which shows operation | movement of the water level detection apparatus of the spent fuel storage pool which concerns on 4th Embodiment.

(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
As shown in FIG. 1A, the water level detection device 20 of the spent fuel storage pool 1 according to the first embodiment detects a temperature above the reference water level c of the pool water 4 as a gas phase temperature sensor 10 ₀ ( k = 0), a water level detection temperature sensor 10 _k (k = 1 to n) for detecting a temperature lower than the reference water level c of the pool water 4, a gas phase temperature sensor and a water level detection temperature sensor 10 _k. (k = 0 to n) heat supply unit 22 supplies heat energy to each of the detected temperature T _k of the gas-phase temperature sensor and a water level detector for temperature sensor _{10 k (k = 0~n) (} k = 0~ and a determination unit 23 that determines the water level of the pool water 4 based on n).

  In the spent fuel storage pool 1 (hereinafter referred to as “storage pool 1”), a rack 2 that houses a plurality of spent fuel assemblies 3 is arranged. Further, the storage pool 1 is provided with a circulation cooler (not shown) for cooling the pool water 4 that is heated by the decay heat of the spent fuel assembly 3.

For example, when the length a of the spent fuel assembly 3 is about 4.5 m and the height b of the rack 2 is about 5 m, the spent fuel storage pool 1 having a depth d of about 12 m is required. The water level of the pool water 4 is maintained so that the reference water level c = about 11 m.
As a result, high-level radiation emitted from the spent fuel assembly 3 is blocked by the pool water, and leakage from the storage pool 1 to the outside is suppressed.

Vapor temperature sensor 10 ₀ can be installed so that the tip is above the reference level c, the water level detection temperature sensor 10 _{k (k} = 1~n) is such that its tip is below the reference level c Installed. Vapor temperature sensor 10 ₀ and the water level detection temperature sensor 10 _k (k = 1 to n), the basic structure (see FIG. 1 (B)), because of the same, when there is no need to distinguish between them, hereinafter It is simply described as temperature sensor 10 (10 _k ).
In the water level detection device 20 shown in FIG. 1, a plurality of water level detection temperature sensors 10 _k (k = 1 to n) are arranged at different heights in the vertical direction. In some cases, 10 _k (k = 1).

  As shown in FIG. 1B, the temperature sensor 10 includes a copper-constantan sheath thermocouple 12, a heater 14 for supplying thermal energy to the vicinity of the temperature measuring contact 15 of the sheath thermocouple 12, and the sheath thermocouple. 12 and a housing tube 11 that houses the heater 14.

The detected temperature T _k (k = 0 to n) is output from the sheath thermocouple 12 of the temperature sensor 10 configured as described above. The Joule heat generated by passing an electric current through the heater 14 has a different thermal diffusivity depending on whether the surroundings of the temperature sensor 10 is a gas or water, and therefore the detected temperature T _k (k = 0) of the sheath thermocouple 12. ~ N) make a difference.

The sheath thermocouple 12 is a copper-constantan thermocouple strand 13 accommodated in a sheath tube whose tip is closed. And between this strand 13 and a sheath pipe | tube, it fills with magnesium oxide as an insulating material.
At the temperature measuring contact 15, the copper strand and the constantan strand are welded. Then, the opposite ends of the strands 13 are led to the temperature detection unit 21 and the ambient temperature of the temperature measuring contact 15 is measured based on the thermoelectromotive force detected at the opposite end.
The temperature detector 21 constantly acquires all the detected temperatures T _k (k = 0 to n) of the temperature sensor 10 _k (k = 0 to n) in real time regardless of the operation of the heat supply unit 22. .

  In order to detect the water level of the pool water in the deep position of the storage pool 1, it is necessary to install the thermocouple element 13 in a long state. However, in this case, since a large load is applied to the strand 13 of the thermocouple, excellent mechanical characteristics are required for the strand 13 itself. Furthermore, since the noise of the detected thermoelectromotive force increases as the wire 13 of the thermocouple becomes longer, it is necessary to employ a thermocouple having a large thermoelectromotive force in order to increase the S / N ratio.

  The strand 13 of the copper-constantan thermocouple is superior in terms of obtaining a large thermoelectromotive force and suitable for low-temperature measurement, but inferior in mechanical properties as compared with a commonly used chromel alumel thermocouple. Therefore, a copper-constantan sheathed thermocouple 12 was employed to ensure mechanical properties.

  The copper-constantan sheathed thermocouple 12 is manufactured by simultaneously pulling both the strands of the copper-constantan thermocouple before the tensile processing in the sheath tube before the tensile processing. Since it is housed in the sheath tube, the long sheathed thermocouple 12 can be created without applying an excessive load to the element wire 13 of the copper-constantan thermocouple.

  The accommodating tube 11 accommodates the sheath thermocouple 12 and the heater 14 inside, is further filled with magnesium oxide having a high thermal conductivity, and the outside is in contact with the pool water 4 and the atmosphere. The sheath thermocouple 12 measures the temperature of the pool water 4 and the atmosphere through the storage pipe 11 and magnesium oxide, and the heat energy from the heater 14 passes through the magnesium oxide and the storage pipe 11 to cause the pool water 4 and Released into the atmosphere.

The heat supply unit 22 energizes the heater 14 to generate Joule heat over a set time Δt, and supplies a predetermined amount of heat energy around the temperature measuring contact 15 of the temperature sensor 10 _k (k = 0 to n).
Alternatively, the heat supply unit 22 can apply Joule heat while turning on / off the current applied to the heater 14 in order to intermittently supply thermal energy to the periphery of the temperature measuring contact 15.

The determination unit 23 instructs the heat supply unit 22 to supply controlled thermal energy to each of the temperature sensors 10 _k (k = 0 to n), and detects the temperature sensor 10 _k acquired from the temperature detection unit 21. The water level of the pool water 4 is determined based on the temperature T _k (k = 0 to n).
The terminal 24 transmits various setting items necessary for the determination to the determination unit 23 by an input operation of the operator, information on each temperature sensor 10 (“in the air”, “underwater”, “failure”, etc.), and the determination result of the water level. Is displayed.

In the first embodiment, the determination unit 23 determines the temperature difference T ₀ (Δt) −T _k between the gas phase temperature sensor 10 ₀ and the water level detection temperature sensor 10 _k (k = 1 to n) after the thermal energy is supplied. Water level determination is executed based on (Δt).

Based on the flowchart in FIG. 2 (see FIG. 1 as appropriate), the operation of the water level detection device for the spent fuel storage pool according to the first embodiment will be described.
In a state where the detected temperature T _k (k = 0 to n) of the temperature sensor 10 _k (k = 0 to n) is acquired in real time by the temperature detection unit 21, the heat supply unit 22 is connected to the temperature sensor 10 _k (k = 0 to n) is energized for a set time Δt (S11).

The detected temperature T ₀ (Δt) of the gas phase temperature sensor 10 ₀ changed by the predetermined amount of heat energy supplied in this way and the detected temperature T _k (Δt) of the water level detecting temperature sensor 10 _k (k = 1). ) Is acquired (S12, S13, S14).
Then, when the absolute value of the vapor temperature sensor 10 ₀ and the water level detection temperature sensor 10 the temperature difference _{k T 0 (Δt) -T k} (Δt) is less than the set value α is (S15; Yes), the temperature sensor It is determined that 10 _k is located in the air (S16).

On the other hand, when the temperature difference T ₀ (Δt) −T _k (Δt) between the gas phase temperature sensor 10 ₀ and the water level detection temperature sensor 10 _k is equal to or larger than the set value α (S15; No, S17; Yes), It is determined that the temperature sensor 10 _k is located in water (S18).
And, if the gas phase temperature sensor 10 ₀ and the water level detection temperature sensor 10 the temperature difference _{k T 0 (Δt) -T k} (Δt) is less than -α (S17; No), the temperature sensor 10 _k failure (S19).

In this way, it is determined for all the water level detection temperature sensors 10 _k (k = 1 to n) whether the position is in the air or in water or is malfunctioning (S20; Yes, No).
Then, the water level determination of the pool water is performed based on the information that each of the water level detection temperature sensors 10 _k (k = 1 to n) belongs to “in the air”, “underwater”, or “failure” ( S21).

(Second Embodiment)
The configuration of the water level detection device for the spent fuel storage pool according to the second embodiment is the same as that of the first embodiment shown in FIG.
In the second embodiment, the determination unit 23 determines the water level based on the temperature difference T _k (t + Δt) −T _k (t) between the detected temperatures T _k (k = 0 to n) before and after the thermal energy is supplied. Run.

Based on the flowchart of FIG. 3 (see FIG. 1 as appropriate), the operation of the water level detection device for the spent fuel storage pool according to the second embodiment will be described.
In a state where the detected temperature T _k (k = 0 to n) of the temperature sensor 10 _k (k = 0 to n) is acquired in real time by the temperature detecting unit 21 (S31, S32), the heat supply unit 22 is the temperature sensor. The heaters 10 _k (k = 0 to n) are energized for a set time Δt (S33).

The detected temperature T _k of the temperature sensor 10 _k which is changed by the thermal energy of the thus supplied a predetermined amount (t + Δt), and acquires the detected temperature T _k of the temperature sensor 10 _k before heater energization (t) ( S34).
When the temperature difference T _k (t + Δt) −T _k (t) between the temperature sensor 10 _k after the heater energization and before the heater energization is equal to or larger than the set value β (S35; Yes), the temperature sensor 10 _k is in the air. (S36).

On the other hand, when the temperature difference T _k (t + Δt) −T _k (t) between the temperature sensor 10 _k after the heater energization and before the heater energization is a positive number smaller than the set value β (S35; No, S37; Yes) ), It is determined that the temperature sensor 10 _k is located in water (S38).
When the temperature difference T _k (t + Δt) −T _k (t) between the temperature sensor 10 _k after the heater energization and before the heater energization is 0 or less (S37; No), the temperature sensor 10 _k is in failure. It is determined (S39).

In this way, it is determined for all the water level detection temperature sensors 10 _k (k = 1 to n) whether the position is in the air or in the water or is malfunctioning (S40; Yes, No).
Then, the water level determination of the pool water is performed based on the information that each of the water level detection temperature sensors 10 _k (k = 1 to n) belongs to “in the air”, “underwater”, or “failure” ( S41).

(Modification of the second embodiment)
In the second embodiment, the example in which the water level is determined based on the temperature change after the heater energization is started has been described. However, the water level determination can also be performed based on the temperature change after the heater energization is completed. .
In this case, the determination unit 23 determines the temperature difference T _k (t + Δt) − between the detected temperatures T _k (k = 0 to n) before and after the supply of the thermal energy continuously supplied from the heat supply unit 22 is stopped. The water level is determined based on T _k (t).

(Third embodiment)
The configuration of the water level detection device for the spent fuel storage pool according to the third embodiment is the same as that of the first embodiment shown in FIG.
In the third embodiment, the determination unit 23 is a temperature difference between two water level detection temperature sensors 10 _k and 10 _{k + 1} (k = 1 to n−1) adjacent to each other in the vertical direction after the thermal energy is supplied. The determination is executed based on T _k (Δt) −T _{k + 1} (Δt).

Based on the flowchart of FIG. 4 (see FIG. 1 as appropriate), the operation of the water level detection device for the spent fuel storage pool according to the third embodiment will be described.
In a state where the detected temperature T _k (k = 0 to n) of the temperature sensor 10 _k (k = 0 to n) is acquired in real time by the temperature detecting unit 21, the heat supply unit 22 is connected to the temperature sensor 10 _k (k = 0 to n), each heater 14 is energized for a set time Δt (S51).

The detected temperature T _k of this way of the temperature sensor 10 _k which is changed by the thermal energy of the supplied predetermined amount detected temperature T _k (Delta] t), the temperature sensor 10 k _{+ 1} under that stage (k = 0) _{+ 1} (Δt) is acquired (S52, S53, S54).
When the absolute value of the temperature difference T _k (Δt) −T _{k + 1} (Δt) between the temperature sensor 10 _k and the temperature sensor 10 _{k + 1} is smaller than the set value γ (S55; Yes), the temperature sensor 10 It is determined that _{k + 1} is located in the air.

In this case, the determination of S53 to S55 is repeated for the next combination of temperature sensors 10 _k , 10 _{k + 1} (k = 1...), And the temperature difference T _k (Δt) −T _{k + 1} (Δt) is set to the set value γ. When it is above (S55; No, S56; Yes), it is determined that the temperature sensor 10 _{k + 1} is located in water, and the water level is also determined (S57).
If the temperature difference T _k (Δt) −T _{k + 1} (Δt) is equal to or less than −γ (S55, S56; No), it is determined that the temperature sensor 10 _{k + 1} is in failure (S58). .

(Fourth embodiment)
Since the configuration of the water level detection device for the spent fuel storage pool according to the fourth embodiment is the same as that of the first embodiment shown in FIG. Is omitted.
In the fourth embodiment, the heat supply unit 22 continuously supplies thermal energy, and the determination unit 23 is based on the change rate ΔT _k (t) / Δt of the detected temperature detection temperature T _k (k = 0 to n). Execute water level judgment.

Based on the flowchart of FIG. 5 (see FIG. 1 as appropriate), the operation of the water level detection device for the spent fuel storage pool according to the fourth embodiment will be described.
In a state where the detected temperature T _k (k = 0 to n) of the temperature sensor 10 _k (k = 0 to n) is acquired in real time by the temperature detecting unit 21, the heat supply unit 22 operates in the temperature sensor 10 _k (k = 0 to n) is continuously energized to each heater 14 (S61).

As described above, since the heat energy is continuously supplied, when there is no change in the water level, the temperature detected by the temperature sensor 10 _k is in a steady state regardless of whether in the air or underwater, and the rate of change ΔT _k (t) / Δt is ideally 0.
For this, the absolute value if less than the set value η of the rate of change ΔT _k (t) / Δt of the detection temperature (S63, S64; Yes), the change remains the temperature sensor 10 _k is positioned in water or air It is determined that there is no (S65).

And when there is no water level change, the loop of S66; No-S63 and S66; Yes-S62 is repeated. However, if the water level continues to drop, the detected temperature of one of the temperature sensors 10 _k becomes a transient state, and the change rate ΔT _k (t) / Δt of the detected temperature becomes equal to or higher than the set value η (S67; Yes, S68).
On the other hand, when the water level continues to rise, the detected temperature of one of the temperature sensors 10 _k becomes a transient state, and the change rate ΔT _k (t) / Δt of the detected temperature becomes −η or less (S67; No, S69). .
Thereby, it is possible to observe in real time how the water level is updated (S70).

  According to the spent fuel storage pool water level detection apparatus of at least one embodiment described above, even when the water level of the spent fuel storage pool falls below a predetermined reference water level, the water level and the water temperature are accurately detected. It becomes possible.

Further, in each embodiment, the water level determination accuracy can be further improved by reducing noise of the detected temperature T _k caused by temperature fluctuation due to a void in the vicinity of the sensor or a time change of the water surface by an appropriate filter. .
Further, the temperature detection unit 21 can further improve the water level determination accuracy by reducing the temperature fluctuation noise in a short time by smoothing the data of the detected temperature T _k by moving average or the like.

In addition, the power source connected to the heat supply unit 22 includes an abutting circuit that can switch between an AC power source and a battery.
In addition, it is also possible to perform water level determination or sensor failure determination by applying spent fuel as a heat source of the thermal energy supplied to the temperature sensor 10 and capturing temperature changes due to the gamma heating.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

1 ... spent fuel storage pool, 10 (10 _k) ... Temperature _{sensor, 10 k (k = 0)} ... vapor temperature _{sensor, 10 k (k = 1~n)} ... level detection temperature sensor, 11 ... housing pipe , 12 ... sheath thermocouple, 13 ... strand, 14 ... heater, 15 ... temperature measuring contact, 2 ... rack, 20 ... water level detection device, 21 ... temperature detection unit, 22 ... heat supply unit, 23 ... determination unit, 24 ... terminal, 3 ... used fuel assembly, 4 ... pool water, c ... reference water level.

Claims (8)

A gas phase temperature sensor for detecting a temperature above the reference water level of the pool water;
A water level detection temperature sensor that detects a temperature below the reference water level of the pool water;
A heat supply section for supplying thermal energy to each of the gas phase temperature sensor and the water level detection temperature sensor;
A determination unit that determines a water level of the pool water based on a detection temperature of the gas phase temperature sensor and the water level detection temperature sensor,
Each of the gas phase temperature sensor and the water level detection temperature sensor,
A sheathed thermocouple housing the thermocouple element for detecting the temperature in a sheath tube whose tip is closed;
A heater that is arranged in the vicinity of the tip of the strand to generate the thermal energy by energization;
A water level detection device for a spent fuel storage pool, comprising: a housing tube for housing the sheath thermocouple at a distal end and a heater. The water level detection device for a spent fuel storage pool according to claim 1,
A water level detection device for a spent fuel storage pool, wherein the plurality of water level detection temperature sensors are arranged at different heights in the vertical direction. In the spent fuel storage pool water level detection device according to claim 1 or 2,
The determination unit performs determination of the water level of the pool water based on a temperature difference between the gas phase temperature sensor and the water level detection temperature sensor after the thermal energy is supplied. Storage pool water level detection device. In the spent fuel storage pool water level detection device according to claim 1 or 2,
The said determination part performs the determination of the water level of the said pool water based on the temperature difference of the said detection temperature before and after the said thermal energy is supplied, The water level detection apparatus of the spent fuel storage pool characterized by the above-mentioned. In the spent fuel storage pool water level detection device according to claim 1 or 2,
The heat supply unit stops supplying the thermal energy that has been continuously supplied,
The determination unit performs determination of the water level of the pool water based on a temperature difference between the detected temperatures before and after the supply of the thermal energy is stopped. The water level detection device for a spent fuel storage pool according to claim 2,
The determination unit performs determination of the water level of the pool water based on a temperature difference between two temperature sensors for water level detection adjacent to each other up and down after the thermal energy is supplied. Pool water level detector. In the spent fuel storage pool water level detection device according to claim 1 or 2,
The heat supply unit continuously supplies the thermal energy,
The determination unit performs a determination of the water level of the pool water based on the rate of change of the detected temperature, the spent fuel storage pool water level detection device. Detecting a temperature above the reference water level of the pool water with a gas phase temperature sensor;
Detecting a temperature below the reference water level of the pool water with a water level detection temperature sensor;
Supplying thermal energy to each of the gas phase temperature sensor and the water level detection temperature sensor;
Determining the water level of the pool water based on the detected temperatures of the gas phase temperature sensor and the water level detection temperature sensor, and
Each of the gas phase temperature sensor and the water level detection temperature sensor,
A sheathed thermocouple housing the thermocouple element for detecting the temperature in a sheath tube whose tip is closed;
A heater that is arranged in the vicinity of the tip of the strand to generate the thermal energy by energization;
A method for detecting a water level in a spent fuel storage pool, comprising: a housing pipe that houses a sheath thermocouple at a distal end and a heater positioned at a distal end.

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