JPH0579952B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a radioactive water monitoring device, and particularly to a device for measuring low-energy radioactivity contained in radioactive wastewater with high sensitivity.

[Prior Art] In recent years, as the use of nuclear power and radioactive isotopes (hereinafter referred to as RI) has become more popular, the amount of radioactive waste liquid has increased, and the number of RI nuclides has become diverse. Therefore, from the standpoint of safety management, it is necessary to measure and monitor the radioactivity concentration according to the properties of these radioactive wastewaters.

Conventionally, radioactive waste liquid was sampled and measured using a liquid scintillation counter, or
Measurement methods include flowing the waste liquid to be measured through the radiation detection window surface of a GM tube or the detection surface of a sheet plastic scintillator.

However, although the former sampling method has high detection sensitivity, it has the problem that it cannot perform real-time measurement. In addition, the latter method, in which the waste liquid to be measured is passed through the detection surface, has a low detection sensitivity for low-energy β-rays, so there is the problem that real-time measurement of low-concentration radioactive waste liquid that corresponds to the permissible concentration for tritium wastewater is difficult. be.

Therefore, at present, high-sensitivity measurement of low-energy β-rays such as tritium is not possible in real time, but the former sampling method is used and measurements are carried out at appropriate timings. on the other hand,
The latter type of water monitoring device is generally used for medium and high energy β-ray nuclides.

[Problems to be Solved by the Invention] In addition to the spread of water monitoring devices as described above, it is expected that the amount of waste liquid containing low-energy β-rays, particularly tritium, will increase in the future. Therefore, from the perspective of radiation safety management, the development of a water monitoring device that can actually measure low-energy β-rays in real time corresponding to the permissible tritium concentration of 3×10 ^-3 μCi/cm ³ in wastewater is awaited. In other words, nuclear power generation, which accounts for 27% of the current electricity supply, and heavy water reactors in particular, are about to produce a huge amount of tritium. In addition, in nuclear fusion research, the amount of tritium used just once is 100,000 Ci.
It is said that the amount will exceed 20%, and the amount is about to increase further. In addition, large amounts of tritium are used as targets for neutron generation and as tracers in biomedicine, which inevitably results in an increase in waste fluids.
Therefore, there is a need for a highly sensitive water monitoring device that can reliably, easily, and economically measure the allowable concentration of tritium in real time.

However, the tritium β ray to be measured is
The maximum range in water is only a few μm, and in air (1 atm) it is several mm. Therefore, the tritium that enters the radiation detector from the waste liquid is only present in a very small layer within a few micrometers. Furthermore, if the detector has a radiation entrance window, the thickness of the window is equivalent to several micrometers of water, so tritium β rays are absorbed and detected before they reach the sensitive part of the detector. I can't.

For these reasons, tritium measurement is carried out by directly introducing waste liquid into the detection section of a liquid scintillation counter as a detector, as in the sampling method described above.

Therefore, the liquid scintillation counter will be explained based on FIG. 3. This measurement uses a liquid scintillator that emits light depending on the energy when it is irradiated with radiation. First, the waste liquid to be measured is mixed in a scintillator, and the volume is
Pour an appropriate amount into a 20 ml (popular type) transparent glass bottle 10 (sometimes a milky white fluororesin bottle). Then, the scintillator emits light due to the radiation emitted from the radioactive material in the waste liquid. The light emitted from this scintillator is received by a photomultiplier tube 12, and after photoelectron conversion, the electrons are multiplied (10 ⁷ times). and,
The light emission is measured as an electric pulse through a simultaneous circuit 14, a linear amplifier 16, and a counting rate meter 18.

Further, the measurement result is displayed on the recorder 20, and when the measurement result is equal to or higher than a predetermined value, it is displayed on the alarm meter 22. Note that the photomultiplier tube 12 usually consists of two tubes, and distinguishes between thermionic noise of the photomultiplier tube 12 itself and radiation signals. That is, thermionic noise is generated independently in each photomultiplier tube 12. On the other hand, light emitted by radiation enters both photomultiplier tubes 12 simultaneously. Therefore, if the simultaneous circuit 14 measures only the signals obtained simultaneously from both photomultiplier tubes 12 as emission of radiation, thermionic noise that does not result in simultaneous signals can be removed, and the target radiation can be effectively measured.

Furthermore, this method is effective for measuring low-energy β-rays because there is no energy loss in the waste liquid or at the radiation entrance window that would impede measurement, and the entire energy of the scintillator light is effectively used.

However, although such a liquid scintillation counter is expected to have high sensitivity, it cannot be used as a water monitoring device that can perform continuous measurements because it cannot perform real-time measurements. Therefore, a method is used in which the liquid scintillator section is replaced with a sheet-like plastic scintillator or a granular plastic scintillator, and real-time measurement is carried out while flowing waste liquid on the surface of the scintillator. This method allows measurement of low-energy β-rays because the waste liquid is in direct contact with the scintillator. However, if the waste liquid layer is thick, as it enters the waste liquid side from the scintillator surface, it will be self-absorbed by the waste liquid and the detection efficiency will decrease. That is,
Regarding tritium, as mentioned above, the thickness of the waste liquid layer effective for measurement is several μm, which corresponds to the maximum range, and will not exceed this.

Therefore, in this method, when measuring low-energy radiation such as measuring the permissible concentration of tritium wastewater, the structure of each part of the scintillator must be specially adapted, and the radiation count can be carried out over a long period of time (for example, about 30 minutes). It is necessary to take measures such as reducing statistical fluctuations through integration. These results show that it is necessary to improve the sensitivity in real-time measurement.

Furthermore, when the sheet scintillation method is used for long-term measurements, it is accompanied by the adhesion of limescale, deposition of sludge, and contamination by radioactive materials. and,
This causes drawbacks such as formation of a low-energy β-ray absorption layer, increased background, and decreased detection sensitivity, resulting in a loss of reliability in measurement results. In addition, it becomes necessary to take appropriate measures such as removing contaminants, etc., which increases the burden of maintenance and management of the water monitoring device.

On the other hand, in real-time monitoring of gaseous tritium, it is necessary to introduce tritium into the detection unit and measure it, just like in a water monitor. For this purpose, a ventilated ionization chamber as shown in FIG. 4 is used. In measurements using such a vented ionization chamber, the ionization charge (hereinafter referred to as ionization current) due to tritium generated in the ionization chamber 30 is amplified by gas, and the cylindrical external electrode 32 and collector electrode 34 are connected to prevent recombination. This is done by applying a voltage so that an electric field is created between them and collecting the ionizing current. Then, this ionization current is measured with an electrometer or the like to determine the tritium concentration.

Here, the relationship between the ionization current and the tritium concentration N is expressed by the following equation.

I=η・eE/W・NV = η{(1.6×10 ¹⁹ ×5.7×10 ³ ×3.7×10 ⁴ /34}・NV =9.6×10 ^-13・NV(A) ...(1) η: Efficiency e: Charge of one electron 1.6×10 ^-19 C E: Energy lost by one tritium β ray in the ionization chamber (average energy of tritium
5.7KeV) W: Average energy to generate one electron pair in the measurement gas (air 34eV) V: Effective volume of the ionization chamber (cm ³ ) N: Concentration (μCi/cm ³ ) And the maximum allowable concentration of tritium is 5×10 ⁻⁶ μCi/cm ³ (average over 8 hours) in air. The ionization current I generated in the 10 effective volumes of the ionization chamber at this concentration is determined by the above equation (1), and is approximately 5×10 ⁻¹⁴ A when the efficiency η=1. This ionization current I is measured by an electrometer 36, and the detection sensitivity of this electrometer 36 is usually about 10 ^-14 A. Therefore, it is also possible to detect tritium concentrations below the maximum allowable concentration.

However, this ionization current I is weak, and the insulator 38 separating the collector electrode 34 and the external electrode 32 is
Must be at least 10 ¹⁶ Ω. Therefore, the insulation resistance changes depending on the humidity in the air flowing through the air inlet/outlet pipe 40. At saturated humidity, moisture condenses and insulation resistance decreases, making concentration measurement impossible. Regarding this point, empirically, the appropriate operating humidity of the ionization chamber 30 is 60% or less,
Used by up to 90%.

Here, the air 10 containing moisture up to the saturated water vapor pressure at room temperature (35° C.) contains 0.4 g of water. Therefore, assuming that dielectric breakdown does not occur, the concentration of hetritium in the ionization chamber 30 is 3 × 10 ^-3 μCi/
cm ³ (maximum permissible concentration) is mixed into air up to the saturated water vapor pressure and the ionization current I is measured.The ionization current I, which can be determined using equation (1), is 1.2
×10 ^-15 A.

It is understood that this ionization current I is below the detection sensitivity of the electrometer 36 and cannot be measured even if the waste liquid contains water vapor up to its saturated vapor pressure. Therefore, in order to measure using this method, the volume of the ionization chamber must be 85 or more. Furthermore, if the humidity is set to 60% at a room temperature of 20°C, approximately 2.5 times as much volume is required, which results in a volume of approximately 210°C, which has the drawback of increasing the size of the water monitoring device.

Purpose of the Invention The present invention has been made in view of the above-mentioned conventional problems, and its purpose is to reliably, easily, and economically measure the radioactive concentration in waste liquid, such as low-energy β-rays, which are below the permissible concentration for tritium wastewater. The purpose of this invention is to provide a highly sensitive water monitoring device that measures water in real time.

[Means for Solving the Problems] In order to achieve the above object, the radioactive water monitoring device according to the present invention includes: a steam generating section that heats the radioactive waste liquid to be measured into high-temperature heated steam; A gas mixing section that takes in and mixes heated steam from one source and measurement gas from another source at a predetermined ratio; It is characterized by having a vented ionization chamber;

[Operation] The radioactive waste liquid is introduced into the steam generation section, where it is turned into high-temperature heated steam. Then, this heated steam is mixed with a measurement gas such as nitrogen or air in a volume ratio of, for example, about 10% in a gas mixing section. Then, this mixed gas is supplied to the ionization chamber in a high-temperature heated state.

Therefore, it is possible to increase the concentration of wastewater in the mixed gas while keeping the relative humidity low, and it is possible to reliably measure in real time the concentration of low-energy radioactivity, which is below the permissible concentration for tritium wastewater. . Furthermore, if it is provided on a display or alarm, it is possible to display and record the concentration, and to issue an alarm at a constant concentration.

[Example] Hereinafter, an example of the present invention will be described based on the drawings.

FIG. 1 shows a system of an example of an apparatus for carrying out the present invention. The ionization chamber 50 is a high temperature resistant, high pressure version of the ionization chamber 30 shown in FIG. Furthermore, radioactive waste liquid to be measured is stored in the waste liquid tank 52 . And this waste liquid tank 5
The waste liquid No. 2 is introduced into a steam generating section 54 equipped with a heating function, where it is converted into heated steam. Next, measurement gas is mixed into the heated steam in the gas mixing section 56 . Here, the measurement gas is injected from the measurement gas container (high-pressure gas cylinder) 58 into the gas mixing section 56 at a pressure higher than the pressure of the heated steam. The gas mixing section 56 has a heating mechanism to withstand high pressure and maintain an appropriate temperature. Also,
It is preferable to provide a control mechanism for maintaining the temperature at a predetermined temperature.

In this way, a high-temperature, high-pressure mixed gas containing the measurement waste liquid as water vapor is obtained in the gas mixing section 56. Next, this high temperature, high pressure mixed gas is injected into the high temperature resistant, high pressure ionization chamber 50. and,
The ionization current I in this ionization chamber 50 is measured by an electrometer 60. The measurement results by this electrometer 60 are appropriately recorded and displayed on a recording display section 62. Moreover, this ionization current I
is larger than a predetermined value, the alarm section 64 generates an alarm.

The gas after measurement in the ionization chamber 50 is cooled and moisture removed by a moisture remover 66. The removed water is then returned to the waste liquid tank 52, and the gas after water removal is released into the atmosphere from the exhaust duct 68.

According to this embodiment, the waste liquid is turned into heated steam, and the measurement gas is mixed with the heated steam. Therefore, it becomes possible to measure low-energy β-rays whose concentration is below the permissible concentration associated with tritium wastewater. Furthermore, by setting the amount of waste liquid to be less than the vapor pressure of water at the heating temperature, tritium in the waste liquid can be measured in real time without condensed water adhering to the insulator and causing dielectric breakdown.

Here, the temperature, pressure, and mixing ratio of the mixed gas with the measurement gas will be explained.

First, the vapor volume of 1 g of water is 1.24×10 ³ cm ³ at 0°C.
It is. Then, the volume V at the temperature T is obtained by the following equation.

V=(273+T)·V ⁰ /273 Here, V ⁰ is the gas volume at 0°C.

From the above formula, at 200℃, 1g of water is 100% water vapor.
It becomes 2.15 (1 atm). From the relationship between temperature and water vapor pressure shown in Figure 2, this steam is approximately
It is clear that condensation will not occur unless the pressure is 15.34 atmospheres (vapor pressure) or higher. In addition, in FIG. 2, 0.10132MPa=1 atmosphere.

Therefore, the ionization chamber 50 is set to 30, and the temperature is 3 atm and 200℃.
If the volume ratio of water vapor and measurement gas is 1:9, approximately 4 g of measurement waste liquid will be mixed.

Then, assuming that the tritium concentration in this waste liquid is the permissible tritium concentration for wastewater, the ionization current I obtained in this ionization chamber 50 can be found from equation (1),
Approximately 10 ^-14 A. Since this current value is greater than the detection sensitivity of the electrometer 60, it is understood that it is possible to measure the permissible concentration of tritium in waste water. Furthermore, since there is no dielectric breakdown due to condensed water in the water vapor pressure at this time, the reliability of the measured value can be increased. Furthermore, since the steam from the waste liquid is vented, it is possible to eliminate the adhesion of limescale and the deposition of sludge, which are problems with water monitoring devices, and it also reduces the contamination and contamination of RI other than tritium.
It can be used for real-time measurement, and can be made into a highly sensitive radioactive water monitoring device that is reliable, simple, and economical.

[Effects of the Invention] As described above, according to the radiation water monitoring device according to the present invention, the radioactive waste liquid is converted into heated steam and is supplied to the ionization chamber in a high-temperature heated state mixed with the measurement gas. Therefore, it is possible to increase the concentration of waste liquid in the mixed gas while keeping the relative humidity low, and it is possible to reliably measure in real time the concentration of low-energy radioactivity, which is below the permissible concentration for tritium wastewater. . Furthermore, if a display or an alarm is provided, concentration display, recording, and alarm at a constant concentration can be performed.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the overall configuration of a radiation water monitoring device according to an embodiment of the present invention, Fig. 2 is a characteristic diagram showing the relationship between water temperature and water vapor pressure, and Fig. 3 is a conventional liquid scintillation system. FIG. 4 is a block diagram showing an example of a counter, and FIG. 4 is a block diagram showing an example of a conventional ionization chamber. 50... Ionization chamber, 52... Waste liquid tank, 54...
...Steam derivation section, 56... Gas mixing section, 62... Record display section, 64... Alarm section.

Claims (1)

[Scope of Claims] 1. A steam generation section that heats radioactive waste liquid to be measured to produce high-temperature heated steam, and a steam generation section that takes in heated steam from this steam generation section and measurement gas from another supply source at a predetermined ratio. A radioactive water monitoring device comprising: a gas mixing section for mixing; and a high temperature resistant ventilated ionization chamber that receives a high temperature mixed gas from the gas mixing section and measures radioactivity. 2. The radioactive water monitoring device according to claim 1, further comprising a display for displaying the measurement results by the ionization chamber. 3. The radioactive water monitoring device according to claim 2, further comprising an alarm that issues an alarm when the measurement result by the ionization chamber exceeds a certain value.