JP3075633B2 - Method and apparatus for removing and recovering tritium from gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for removing and recovering tritium from a gas containing tritium, and in particular, the concentration of tritium and its isotope hydrogen in a target gas is low. The present invention relates to a method and apparatus for removing and recovering tritium which is suitable in such a case. For example, the present invention can be suitably used for removing tritium from a cover gas covering a coolant of a fast breeder reactor using liquid metal as a coolant.

[0002]

2. Description of the Related Art In a fast breeder reactor using liquid metal as a coolant, the coolant temperature during operation is higher than in a light water reactor, and the body expansion coefficient of sodium as a coolant is larger than that of water. For this reason, when the reactor is started, the volume of the coolant and the cover gas in contact with the coolant expands as the coolant temperature rises, so that a part of the cover gas is systematized to keep the cover gas pressure at a predetermined value. When the pump is stopped, the coolant and the cover gas shrink, and conversely, a new cover gas is supplied from outside the system to keep the cover gas pressure at a predetermined value. Is common.

[0003] In a nuclear power plant, radioactive tritium is generated in fuel by three-body fission or the like. In a fast breeder reactor using liquid metal as a coolant, tritium generated in the fuel permeates the fuel cladding tube by thermal diffusion and moves into the coolant. This tritium moves to the cover gas covering the coolant, and is released outside the plant with the release of the cover gas. However, since tritium is a radioactive element, it is necessary to remove and recover it.

Conventionally, as a method of removing tritium in a fast breeder reactor, a hydrogen storage alloy that captures hydrogen and tritium is used for a mesh material of a cold trap for purifying a coolant, as described in Japanese Patent Application Laid-Open No. 2-243998. There has been a method of indirectly reducing the tritium concentration in the cover gas covering the coolant by reducing the tritium concentration in the coolant.

Japanese Patent Application Laid-Open No. 62-247294 discloses that in a fusion reactor, an oxidation catalyst is provided on the outer wall of a fuel gas circulation system pipe, and tritium is converted to water to form tritium water. A method for preventing leakage is described.

[0006]

SUMMARY OF THE INVENTION Tritium behaves chemically identically to hydrogen, its isotope. In a fast breeder reactor using liquid metal as a coolant, hydrogen is present in the coolant and in the cover gas at a concentration several hundred times that of tritium by weight. Therefore, tritium is also removed mainly by removing hydrogen. Is done. The hydrogen concentration in sodium, which is a coolant, is usually the saturated dissolved concentration at the operating temperature of the cold trap, and is not more than several ppm (weight concentration, the same applies hereinafter). The hydrogen concentration in the cover gas in contact with the coolant is described by the following evaluation formula known as Sieverts' law: HS = KH · HG ^1/2 (1) TS = KT ² · TG / HS Where HS is the hydrogen concentration in sodium, TS is the tritium concentration in sodium, HG is the partial pressure of hydrogen in the cover gas, TG is the partial pressure of tritium in the cover gas, and KH is the Sieverts constant of hydrogen. , KT is Si of tritium hydrogen
events constant. D. for the Sieverts constant. R. Measurement results of Viscers (Nucl. T
ech. 21, 1974), when the hydrogen concentration in the coolant is several ppm, the hydrogen partial pressure in the cover gas is about 10 ^-4 to 10 ^-5 torr. Since the pressure of the cover gas of the fast reactor using sodium as the coolant is at most about 2 atm, if the pressure is assumed to be 2 atm, the hydrogen concentration in the cover gas is as extremely low as about 1 to 10 ppb.

In the method for removing tritium described in JP-A-2-243998, the efficiency of recovering tritium in a coolant by a cold trap can be improved several times by using a hydrogen storage alloy. It has the effect of reducing the tritium concentration to a fraction, and it is believed that the amount of tritium release accompanying the release of the cover gas can be reduced to a fraction. However, since this method does not directly remove and recover tritium in the cover gas, there is a disadvantage in that the amount of tritium released outside the system cannot be significantly reduced.

It is conceivable to directly remove and recover hydrogen and tritium in the cover gas by using a hydrogen storage alloy. However, as described above, since the hydrogen concentration in the cover gas is extremely low, hydrogen that can be used in engineering is considered. The storage alloy can hardly absorb hydrogen and tritium in the cover gas.

On the other hand, the technique described in Japanese Patent Application Laid-Open No. 62-247294 does not use a hydrogen storage alloy but oxidizes hydrogen and tritium with an oxidation catalyst to fix it as water and tritium water. Thus, a method of removing water and tritium water with water and tritium water using a dehumidifier or the like is disclosed. However, this method is for a fusion reactor, and when this method is diverted to the removal of tritium from the cover gas of a liquid metal cooled fast breeder reactor, the hydrogen concentration in the cover gas is low. The partial pressure of the generated water and tritium water is low and can hardly be removed.

Therefore, a first object of the present invention is to efficiently remove and recover tritium from a low hydrogen concentration gas containing tritium, such as a cover gas of a liquid metal cooled fast breeder reactor, and a second object of the present invention. It is an object of the present invention to achieve the first object and to greatly reduce the amount of hydrogen as waste.

[0011]

The first object of the present invention is to remove tritium from a gas according to any one of claims 1 to 3 or 6 to 9 of the claims. Achieved by a recovery method or device,
Further, the second object is achieved by a method and an apparatus for removing and recovering tritium from a gas according to claim 4 or claim 10 or 11 of the present invention.

[0012]

According to the present invention, hydrogen is added to a gas having a low hydrogen concentration containing tritium to reduce the water concentration in the gas to several tens torr.
After the pressure is increased to at least r, hydrogen and tritium in the gas are occluded and recovered by the hydrogen storage alloy, so that the efficiency of tritium removal and recovery is greatly improved.

In this case, for example, assuming that the cover gas of the fast breeder reactor is released several thousand m ³ per year, in order to remove and recover tritium from the cover gas, a large amount of hydrogen gas of several hundred m ³ per year is added. And the amount of hydrogen removed with tritium is enormous.

In order to avoid this problem, it is particularly effective to adopt the configuration described in claim 4 or claim 10 or claim 11. That is, the hydrogen storage alloy stores hydrogen at a low temperature and releases hydrogen at a high temperature. Therefore, two devices with a built-in hydrogen storage alloy are installed, and the temperature of the upstream device is made higher than that of the downstream device so that hydrogen is released and stored in the downstream device. After the downstream device has absorbed a predetermined amount of hydrogen and the upstream device has released a predetermined amount of hydrogen,
By exchanging the upstream side and the downstream side, it becomes possible to release hydrogen again from the upstream side device and occlude it with the downstream side device.

[0015] In such an operation, there is further an action by an isotope exchange reaction. That is, the hydrogen-containing alloy-containing device on the upstream side stores hydrogen that does not contain tritium in advance, and the downstream device starts operation in a state where hydrogen is not stored. Since the isotope ratio of tritium in the cover gas is higher than that in the device, tritium is transferred into the device by the isotope exchange reaction. This allows
Tritium can be stored in the upstream device. Since hydrogen equivalent to the tritium transferred into the apparatus moves from the apparatus to the passing gas, the isotope ratio of tritium in the passing gas at the apparatus outlet becomes smaller than that in the cover gas. The isotope ratio of tritium stored in the downstream device is smaller than that in the cover gas. Therefore, even after the upstream device and the downstream device are exchanged, the tritium moves from the cover gas into the device again by the isotope exchange reaction on the upstream side.

The tritium concentration in the gas that has passed through the tritium removal / recovery device is lower than that in the cover gas due to the two effects of increasing the partial pressure of hydrogen in the cover gas and the isotope exchange reaction as described above. As described above, by alternately using the upstream side and the downstream side, hydrogen is not added from outside the system, and therefore, an increase in the amount of hydrogen as waste can be prevented.

[0017]

FIG. 1 shows a system configuration of a preferred embodiment of the present invention. In the present system configuration, the cover gas 3 in contact with the sodium 2 in the reactor vessel 1 of the fast breeder reactor using liquid sodium as a coolant is removed after the sodium vapor is removed by the vapor trap 4 during the operation of the reactor. A pair of hydrogen storage alloy-containing devices 5 and 6, each containing a hydrogen storage alloy, are connected in series by valves 8, 9, 10 and 11 so that one of them is on the upstream side and the other is on the downstream side. Flow through the closed loop back to the reactor vessel 1 by the blower 7.

Now, consider the point in time when the valves 8 and 11 are opened, the valves 9 and 10 are closed, and the device 5 with built-in hydrogen storage alloy is switched to be on the upstream side of the device 6 with built-in hydrogen storage alloy. The hydrogen storage alloy in the upstream device 5 has stored hydrogen and tritium, while the hydrogen storage alloy in the downstream device 6 has released hydrogen and tritium. At this time, the switch 14 of the heater 12 provided in the device 5 on the upstream side is turned on, and the hydrogen storage alloy in the device 5 is heated to a predetermined temperature to release the stored hydrogen and tritium. . The cover gas 3 containing tritium that has migrated from within the sodium 2 in the reactor vessel 1 enters the upstream device 5 from the reactor vessel 1 through the vapor trap 4 and the valve 8, where the gas is After being mixed with the hydrogen containing tritium released from the hydrogen storage alloy in the device 5, it is sent to the hydrogen storage alloy built-in device 6 on the downstream side. The switch 15 of the heater 13 provided in the device 6 on the downstream side is turned off, the hydrogen storage alloy in the device 6 is kept at a lower temperature than that in the device 5 on the upstream side,
It absorbs tritium with hydrogen from the gas sent there.

As described above, the device 5 with built-in hydrogen storage alloy on the upstream side functions as a hydrogen addition device, and the device with built-in hydrogen storage alloy 6 on the downstream side functions as a hydrogen absorption device.

The concentration of hydrogen at the outlet of the device having a built-in hydrogen storage alloy on the downstream side depends on the material used as the hydrogen storage alloy.
When a metal such as uranium or magnesium is used, the pressure is about 0.1 torr at room temperature, and the amount of hydrogen brought into the reactor vessel 1 by the circulating gas from the apparatus 6 via the blower 7 is approximately equal to the circulating gas flow rate. If it is 10 Nm ³ / h, it is about 400 g. Therefore, even if the entire amount of hydrogen dissolves in the coolant and is collected in the cold trap as an impurity in the coolant, the increase in the impurity load is not a serious problem. The hydrogen storage alloy in the device 6 can be cooled to room temperature or lower in order to further improve the removal efficiency in the device 6 in order to further reduce the amount brought into the reactor vessel 1.

By continuing the above operation and closing the valves 8 and 11 and opening the valves 9 and 10 before the stored hydrogen in the hydrogen storage alloy built-in device 5 is completely discharged, The system configuration is rearranged so that 6 is on the upstream side of the device 5, and the switch 14 is turned off, and at the same time, the switch 15 is turned on. As a result, the hydrogen storage alloy built-in device 6 functions as an upstream hydrogenation device for releasing the hydrogen and tritium stored therein and adding it to the cover gas, while the hydrogen storage alloy built-in device 5 has the hydrogen storage alloy built-in device. The operation is switched from the outlet gas of the device 6 to a function to function as a downstream hydrogen absorbing device for storing hydrogen and tritium. Then, after this operation is continued, the operation is switched again to the operation in which the hydrogen storage alloy built-in device 5 functions as the upstream hydrogen addition device and the hydrogen storage alloy built-in device 6 functions as the downstream hydrogen absorption device. Hereafter, this is repeated.

The opening / closing switching operation of the valves 8, 9, 10, 11 and the heater switches 14, 15 for alternately switching the series connection relationship of the hydrogen storage alloy built-in devices 5, 6 as described above and alternately switching their roles. ON / OFF switching operation is performed by a control device (not shown).

In this way, the two hydrogen storage alloy built-in devices 5, 6 are alternately operated at appropriate time intervals as upstream and downstream hydrogen addition devices and hydrogen absorption devices, respectively. The tritium in the cover gas can be removed and recovered together with the hydrogen without adding methane.

When it is necessary to release the cover gas outside the system, the valve 16 of the outside system is opened in FIG. 1 and the outside gas is released by the waste blower 21 in FIG. As described above, tritium and hydrogen are extremely small.

In this embodiment, the vapor trap 4 is installed on the upstream side of the hydrogen storage alloy containing devices 5, 6.
When the pipes from the reactor vessel 1 to the hydrogen storage alloy built-in devices 5 and 6 are sufficiently long, the cover gas is sufficiently cooled on the way, and sodium vapor in the cover gas is removed by the pipes on the way. No vapor trap 4 is required. The vapor trap 4 has an effect of preventing the performance of the hydrogen storage alloy from being deteriorated due to the transfer of sodium vapor to the hydrogen storage alloy in the hydrogen storage alloy built-in device 5 or 6.

FIG. 2 is an explanatory diagram showing the relationship between the hydrogen storage amount of a hydrogen storage alloy suitable for use in the present invention and the hydrogen equilibrium partial pressure in the gas in contact with the alloy. The hydrogen storage alloy has hysteresis between the case where hydrogen is stored and the case where hydrogen is released. In the case of storing hydrogen, only the hydrogen in the gas entered at a hydrogen partial pressure equal to or higher than the equilibrium pressure P1 in FIG. 2 is stored, and the hydrogen partial pressure of the gas at the outlet becomes P1. Therefore, even when uranium or the like having a low equilibrium pressure P1 during storage at room temperature is used for the hydrogen storage alloy, the hydrogen partial pressure in the gas needs to be several torr or more in order to make the hydrogen removal efficiency 90% or more. . (However, when the hydrogen storage amount exceeds a certain amount Q1, the storage capacity is rapidly reduced. Therefore, when hydrogen is stored, the hydrogen storage amount should not exceed Q1.) From this, the gas to be treated is used. Hydrogen is not absorbed when the hydrogen concentration in the gas is lower than a certain value. Therefore, in order to occlude hydrogen in a gas having a very low hydrogen concentration such as a cover gas for a fast reactor, it is essential to add hydrogen to the gas. It can be seen that it is. After the hydrogen has been absorbed, if the hydrogen partial pressure in the surrounding gas is made equal to or lower than the equilibrium pressure P2 in FIG. 2 while keeping the temperature constant, hydrogen starts to be released, and the hydrogen partial pressure at the outlet becomes P2. The equilibrium pressures P1 and P2 increase when the temperature is increased, and decrease when the temperature is decreased.

As can be seen from the above, when the temperature of the hydrogen storage alloy in the upstream device used for releasing hydrogen is the same as the temperature of the hydrogen storage alloy in the downstream device used for absorbing hydrogen, The partial pressure at the outlet of the side unit does not reach the partial pressure of hydrogen required at the downstream unit. Accordingly, in the present invention, the temperature of the upstream device is higher than that of the downstream device, and the hydrogen equilibrium pressure P2 in the upstream device becomes at least 10 times or more the equilibrium pressure P1 required for hydrogen absorption in the downstream device. It is desirable to set the temperature in such a manner.

FIG. 3 shows an embodiment in which a tritium removing device is installed in a system for releasing the cover gas 3 of the fast breeder reactor outside the system. In this embodiment, the cover gas 3 is a vapor trap 4
After passing through the rare gas hold-up device 23, the blower 7 returns the reactor gas to the reactor vessel 1 and is circulated. The rare gas hold-up device 23 is an adsorption tower filled with activated carbon, and is a device that adsorbs fission products and attenuates the radioactivity. Thereby, the cover gas 3 is purified. However, tritium is hardly adsorbed on activated carbon and has a long half-life of about 12 years, so that it is not purified by the rare gas hold-up device 23. The tritium removing device is installed in a line in which the cover gas 3 is discharged out of the system by the waste blower 21 via the valve 16 branched from the outlet of the vapor trap 4 in the circulation line.
Note that the valve 16 is opened only when the cover gas 3 is discharged, and the tritium removing device operates only when the valve 16 is opened. The configuration and function of the tritium removing device are the same as those of the embodiment shown in FIG. In the case of the present embodiment, since the cover gas 3 is not returned to the reactor vessel 1, hydrogen that could not be occluded by the tritium removal device dissolves in the sodium 2 in the reactor vessel 1 and becomes impurities. There is no. Therefore, there is an effect that the selection range of the operating temperature of the upstream hydrogen addition device and the downstream hydrogen absorption device is large.

FIG. 4 shows an apparatus 19 containing a hydrogen storage alloy.
In this example, only one was used. In this embodiment, as in FIG. 3, a hydrogen storage alloy built-in device 19 is connected to a cover gas discharge line branched from a cover gas circulation line via a valve 16 as in FIG.
However, similar to the embodiment shown in FIG. 1, the same effect can be obtained by installing this in the circulation line of the cover gas 3. In this embodiment, the hydrogen storage alloy built-in device 19 is used.
Is added via a valve 18 from a hydrogen tank 17 to the cover gas 3 passing therethrough. The hydrogen storage alloy built-in device 19 removes and collects tritium from the cover gas flowing therethrough together with hydrogen in the hydrogen storage alloy. The cooler 20 serves to cool the hydrogen storage alloy built-in device 19 and reduce the partial pressure of hydrogen at the outlet thereof. In the present embodiment, the provision of the cooler 20 has the effect of reducing the amount of hydrogen supply from the hydrogen tank 17.

Also in the embodiment shown in FIGS. 1 and 3, the hydrogen storage alloy built-in devices 5, 6 are provided not only with the heater 12 and the heater 13 but also with a cooler so as to function as a hydrogen absorption device on the downstream side. By using a cooler, the efficiency of removing and recovering tritium can of course be further increased.

[0031]

According to the present invention, the removal and recovery of tritium from a low hydrogen concentration gas containing tritium can be performed with high efficiency.

Further, in the case of a configuration in which the upstream hydrogen storage alloy is used for hydrogen addition to the gas and the downstream hydrogen storage alloy is used for hydrogen absorption and the roles are switched so that the roles are sometimes reversed, a certain number By storing 10 m ³ of hydrogen in the hydrogen storage alloy on the upstream side, subsequent addition of hydrogen is unnecessary, and an increase in the amount of hydrogen as waste can be prevented. If the present invention is applied to the removal of tritium from a cover gas of a fast breeder reactor, it is possible to remove tritium in the cover gas with high efficiency and at low cost.

[Brief description of the drawings]

FIG. 1 is a system configuration diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating hydrogen storage and hydrogen release of a hydrogen storage alloy.

FIG. 3 is a system configuration diagram of another embodiment of the present invention.

FIG. 4 is a system configuration diagram of still another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor vessel 2 ... Sodium 3 ... Cover gas 4 ... Vapor trap 5 ... Hydrogen storage alloy built-in apparatus 6 ... Hydrogen storage alloy built-in apparatus 7 ... Blower 8 ... Valve 9 ... Valve 10 ... Valve 11 ... Valve 12 ... Heater 13 ... Heater 14 ... Switch 15 ... Switch 16 ... Valve 17 ... Hydrogen tank 18 ... Valve 19 ... Hydrogen storage alloy built-in device 20 ... Cooler 21 ... Waste blower 22 ... Valve 23 ... Rare gas hold-up device

Continuation of the front page (56) References JP-A-5-19095 (JP, A) JP-A-60-61020 (JP, A) JP-A-58-210598 (JP, A) JP-A-60-94102 (JP, A) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G21F 9/02 G21D 1/02

Claims (12)

(57) [Claims] 1. A method for removing and recovering tritium from a gas having a low hydrogen concentration containing tritium, wherein the hydrogen concentration in the gas is increased by adding hydrogen to the gas, and then the hydrogen in the gas is increased. And a method for removing and recovering tritium from gas by storing and removing tritium in a hydrogen storage alloy. 2. The method for removing and recovering tritium from gas according to claim 1, wherein the addition of hydrogen to the gas is performed by releasing hydrogen from a hydrogen tank into the gas. 3. The removal and recovery of tritium from a gas according to claim 1, wherein the addition of the hydrogen to the gas is performed by releasing the stored hydrogen into the gas from a hydrogen storage alloy which has previously stored hydrogen. Method. 4. A method for removing and recovering tritium from a gas having a low hydrogen concentration containing tritium, wherein the stored hydrogen is released into the gas from a first hydrogen storage alloy which has previously stored hydrogen gas. After increasing the hydrogen concentration in the gas, hydrogen and tritium in the gas are stored in the second hydrogen storage alloy, and a predetermined amount of hydrogen is released from the first hydrogen storage alloy and the second hydrogen storage is performed. A method for removing and recovering tritium from gas, characterized in that the roles of the first and second hydrogen storage alloys are changed after storing a predetermined amount of hydrogen in the alloy. 5. The gas according to claim 1, wherein the low hydrogen concentration gas containing tritium is a cover gas for a fast breeder reactor using liquid metal as a coolant.
A method for removing and recovering tritium from gas. 6. A device for removing and recovering tritium from a gas having a low hydrogen concentration containing tritium, wherein a hydrogen addition device is provided upstream in a line in which the gas flows, and a hydrogen storage device is provided downstream thereof. Hydrogen absorber with built-in alloy
A device for removing and recovering tritium from a gas, wherein the devices are arranged in a series connection relationship with each other. 7. The hydrogen absorbing device includes a cooler for cooling the built-in hydrogen storage alloy.
The apparatus for removing and recovering tritium from gas described above. 8. The apparatus for removing and recovering tritium from gas according to claim 6, wherein the hydrogenation apparatus comprises a hydrogen tank for releasing hydrogen. 9. The hydrogen storage device according to claim 1, wherein the hydrogen storage device includes a hydrogen storage alloy having a built-in hydrogen storage alloy preliminarily storing hydrogen and a heater for releasing the stored hydrogen from the hydrogen storage alloy. The apparatus for removing and recovering tritium from gas according to claim 6 or 7. 10. A device for removing and recovering tritium from a low hydrogen concentration gas containing tritium, wherein each of the heaters has a built-in hydrogen storage alloy and releases the stored hydrogen from the hydrogen storage alloy. The two hydrogen storage alloy built-in devices provided for connecting the hydrogen storage alloy built-in devices serially and alternately so that the connection relationship between the upstream side and the downstream side can be alternately switched in a line in which the gas flows. The upstream hydrogen storage alloy built-in device functions as a hydrogen addition device by releasing the hydrogen stored in the hydrogen storage alloy contained therein by the operation of the heater, and serves as a downstream hydrogen storage device. The device for removing and recovering tritium from a gas, wherein the device containing a storage alloy functions as a hydrogen absorption device by storing hydrogen in a hydrogen storage alloy contained therein. 11. The hydrogen storage alloy-containing device according to claim 1, further comprising a cooler for cooling the built-in hydrogen storage alloy.
The apparatus for removing and recovering tritium from gas according to claim 10, wherein the cooler is operated. 12. The gas according to claim 6, wherein the low hydrogen concentration gas containing tritium is a cover gas of a fast breeder reactor using liquid metal as a coolant. Tritium removal and recovery equipment from inside.