JPS5883287A - Sampling method for radioactivity - Google Patents

Abstract

PURPOSE:To provide a sampling method which can capture krypton and xenon which are rare gases inexpensively and safely by liquefying the oxygen in air with liquid nitrogen, cooling activated carbon for capturing of rare gases with said liquid oxygen and capturing the rare gases. CONSTITUTION:When air is supplied to a hose 17 through an air supply line 18, the hose is immersed substantially in liquid nitrogen; therefore oxygen which is high in b.p. liquefies therein and accumulates as liquid oxygen on the lower side of an inside vessel. After the formation of the liquid oxygen, a blower 10 is operated to introduce sampling air through the duct of a ventilation air conditioning system, whereby sampling is started. After the sampling air is captured of iodine in an iodine trap, the air is captured of rare gases in a rare gas trap using the activated carbon cooled with the liquid oxygen and is released through an air releasing line by an air releasing blower. Upon completion of the sampling for a specified time, radioactivity is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a radioactivity sampling method, and more particularly to a radioactivity sampling method suitable for efficiently, inexpensively, and safely collecting krypton, which is a rare gas.

Radioactivity in the air passing through ventilation air conditioning systems and exhaust stacks is extremely dilute, so it is difficult to measure it as it is, and a concentration operation is required. Of these, gaseous radioactivity such as iodine and rare gases are concentrated by being adsorbed onto activated carbon or the like. At this time, rare gases such as krypton and xenon have small adsorption equilibrium constants, so activated carbon is cooled with liquid nitrogen or the like to adsorb and concentrate them. Normally, nuclear power plants use 01 semiconductor detectors, which require liquid nitrogen at all times, as radiation measuring instruments.

Therefore, liquid nitrogen is always stored within the power plant and can be obtained at a relatively low cost. Conventionally, activated carbon was directly cooled with this liquid nitrogen to collect rare gases such as krypton and xenon.

A conventional method of directly cooling activated carbon and sampling a rare gas will be explained with reference to FIG. The sampling device is packed with a sampling hose 2 for sucking sampled air from a ventilation air conditioning system duct 1, an iodine trap 3 at room temperature for collecting iodine, a pressure reducing valve 4, a pressure gauge 5 for monitoring the pressure at that time, and activated carbon 7. A rare gas trap 6, liquid nitrogen 8 for cooling the trap, a liquid nitrogen container 9 insulated from the atmosphere, a vacuum pump 10 for sucking sampling air, an exhaust pipe 11 for exhausting air, and liquid nitrogen. It consists of a laboratory jack 19 for raising and lowering the container. In this method of blowing sampling air through an activated carbon layer directly cooled with liquid nitrogen, oxygen in the air may be liquefied by the liquid nitrogen and react with the activated carbon, causing rapid combustion (explosion). In order to prevent this, the above-mentioned sampling device uses a method of reducing the pressure until the liquefaction temperature of oxygen is lower than the boiling point of liquid nitrogen in order to prevent liquefaction of oxygen. In other words, in conventional equipment, iodine is first collected at room temperature and pressure, and then the pressure of the sampling air is reduced to a pressure of approximately 70 Hg, which is the pressure at which the liquefaction temperature of oxygen is lower than the boiling point of liquid nitrogen -196C using a pressure reducing valve. Depressurize. The velocity of the sampling air in the activated carbon adsorption layer needs to be constant, so it is 1/10 of the flow rate when sampling at normal pressure. Therefore, the lower limit of detection also deteriorates, and in conventional measurement examples, measurements are almost never made below the lower limit of detection.

The purpose of the present invention is to improve the above-mentioned problems and inexpensively,
This provides a sampling method that can safely collect rare gases such as krypton and xenon.

The present invention provides that if activated carbon for adsorbing rare gases is cooled with liquid oxygen, the oxygen in the sampled air will not be liquefied, and that liquid nitrogen can be obtained much more cheaply and reliably than liquid oxygen at nuclear power plants. Focusing on this, the oxygen in the air is liquefied using liquid nitrogen, and the activated carbon used to collect rare gases is cooled by the liquid oxygen and the rare gases are collected.

An embodiment of the radioactivity sampling method according to the present invention will be described below using a flow sheet of a radioactivity sampling apparatus for a ventilation air conditioning system shown in FIG.

In FIG. 2, the sampling equipment for sampling from the ventilation and air conditioning system duct is a ventilation and air conditioning system duct 1, a sampling hose 2, an iodine trap 3, a noble gas trap 6 filled with activated carbon 7, an exhaust proar 10, an exhaust pipe 11. and a cooling tank to cool the rare gas trap. The cooling tank is
an outer container 9 insulated from the atmosphere for containing liquid nitrogen 8 and an uninsulated inner container 15 for filling liquid oxygen 16;
Thermocouples 13 and 14 that monitor liquid nitrogen, liquid oxygen,
Thermocouple 12 for monitoring, air supply pipe 18 for creating liquid oxygen, cooling corrugated pipe 17, laboratory jack 19 for raising and lowering the cooling tank, supply pipe 20 for supplying liquid nitrogen
It becomes more. In this radioactivity sampling device, the sampling air taken in from the ventilation air conditioning system duct is used to collect iodine in an iodine trap, and then the rare gas is collected in a rare gas trap using activated carbon cooled with liquid oxygen. , is released from the exhaust line at the exhaust prower. After a certain period of sampling is completed, the rare gas trap is returned to room temperature, removed, and radioactivity is measured using a radioactivity measuring device installed in the measurement room. Since this sampling device uses liquid oxygen to cool the air, the oxygen contained in the sampled air liquefies in the noble gas trap, causing rapid combustion with activated carbon.
There is no risk of causing an explosion. Therefore, sampling can be performed at normal pressure.

Liquid oxygen for cooling the rare gas trap may be purchased, but it is expensive and cannot be obtained reliably, so it is prepared as follows. At nuclear power plants, Gl (
Li) Semiconductor detectors are used. Therefore, liquid nitrogen is always on hand and can be obtained reliably at low cost. When producing liquid oxygen, liquid nitrogen is first injected into the outer container from the liquid nitrogen supply pipe to the level shown by the solid line in FIG.

At this time, the level of liquid nitrogen is monitored by thermocouples 13 and 14. When the air is injected to the solid line level, air is supplied from the air supply line 1B to the flexible pipe 17. The corrugated tube is fully immersed in liquid nitrogen, and oxygen, which has a high boiling point, liquefies inside and accumulates as liquid oxygen at the bottom of the inner container. That is, since the boiling point of liquid nitrogen is lower than -196C, the liquefaction temperature of oxygen, -1830C, 20% of the oxygen contained in the air is completely liquefied in the corrugated tube together with argon gas, etc., and becomes liquid oxygen.

The remaining 80 inches of nitrogen gas is not liquefied, so it passes through the gap near the sampling pipe and is released into the air as a gas. Since liquid nitrogen is consumed in the process of creating liquid oxygen, it is assumed that it is supplied at a constant level. Air supply is stopped when a certain amount of liquid oxygen is generated. Since the inner container is not insulated, the liquid oxygen in the outer container is cooled down to below its boiling point by the liquid nitrogen in the outer container, so the consumption of liquid oxygen is minimal. The concentration of organic matter in the air is several ppm, and the amount of liquid oxygen produced is at most 2 ppm.
t, there is almost no risk of explosion during the process of producing liquid oxygen.

After the generation of liquid oxygen is completed, the supply of liquid nitrogen is temporarily stopped, and the liquid level is lowered to the position shown by the broken line in Figure 2.
Prevents the formation of new liquid oxygen.

Even in this state, liquid nitrogen is constantly vaporized.

Since the outer container is fitted with a lid to sufficiently isolate it from the outside air, vaporized nitrogen gas constantly flows upwards, and there is no fear that oxygen or organic matter in the atmosphere will naturally accumulate in the inner container.

When liquid oxygen is generated, the proa 10 is activated, sampling air is introduced from the ventilation air conditioning system duct, and sampling is started. First, iodine is collected at room temperature, and then rare gas is collected. The sampling air contains nitrogen,
Includes oxygen, argon, krypton, xenon, etc.

The boiling points of nitrogen are -196C, oxygen -183C, and argon -186C.

Lipton-1530, Xenon-108C.

Therefore, when the sampling air is cooled with liquid oxygen in the inner container, only krypton and xenon are liquefied, and oxygen,
Nitrogen and argon are not liquefied.

Therefore, it is safe to use activated carbon in rare gas traps. Oxygen and nitrogen that have not been liquefied are exhausted in the proa. During this time, the liquid oxygen in the inner container is cooled by the liquid nitrogen in the outer container, so the consumption amount is small even if it is heated by the sampling air.

The liquid nitrogen in the outer container is consumed over time, so it is monitored with a thermocouple and supplied through the inlet.

Even when sampling continues, vaporized nitrogen gas flows from the outer container through the gap in the lid and into the atmosphere, so there is no risk of oxygen or organic matter in the atmosphere flowing back into the liquid oxygen inside the container.

If the liquid oxygen level drops during sampling,
Can be detected with a thermocouple. At this time, the outer container is filled with liquid nitrogen to a level higher than the flexible pipe, and air is vented into the flexible pipe to create liquefied oxygen.

After a certain period of sampling is completed, the Proa is stopped and both the inner and outer containers are lowered using a lab jack to remove the rare gas trap from the liquid oxygen and return to room temperature. The trap outlet side is open to the atmosphere in order to release the gas adsorbed on the trap. After returning to room temperature, it is removed and radioactivity is measured using a radiation detector prepared in the measurement room.

When such a sampling device is used, there is no risk that the oxygen in the sampled air will be liquefied, so normal pressure sampling becomes possible, the sampling flow rate is more than 10 times higher than vacuum sampling, and the detection limit is more than 10 times higher. improves. In addition, there is no fear of explosion between liquid oxygen and activated carbon, and rare gases, especially krypton, which has been difficult to measure in the past, can be safely sampled. Additionally, since it uses liquid nitrogen, which is normally used at power plants, sampling can be done reliably at low cost.

According to the present invention, it is possible to sample rare gases, especially krypton, at normal pressure, which increases the sampling flow rate and improves the lower limit of detection by more than one order of magnitude compared to conventional vacuum sampling methods. Since it uses liquid nitrogen, which is always in use, it can be operated at low cost.

In the present invention, a flexible pipe is provided in a liquid nitrogen container to create liquid oxygen, but the same effect can be obtained even if a liquid oxygen supply device is installed outside the container containing liquid nitrogen.

[Brief explanation of drawings]

FIG. 1 shows a flow sheet of a conventional rare gas sampling method using a reduced pressure method, and FIG. 2 shows a flow sheet of a rare gas sampling method according to the present invention. 1... Ventilation air conditioning system duct, 2... Sampling hose 3... Iodine trap, 4... Pressure reducing valve, 5...
Pressure gauge, 6... Rare gas trap, 7... Activated carbon, 8
Liquid nitrogen, 9...Liquid nitrogen container, 10...Exhaust prower, 11...Exhaust pipe, 12, 13.14...Thermocouple, 15...Inner container for liquid oxygen, 16... Liquid oxygen, 17...Stork tube, 18...Air injection tube, 19...
Labo Jatsuki, 20...Figure 1 Part Z

Claims (1)

[Claims] 1. In a method for concentrating and sampling radioactivity from ventilation and air conditioning system ducts of nuclear power plants, a double outer container is filled with liquid nitrogen and an inner container is filled with liquid oxygen. A radioactivity sampling method characterized by cooling a rare gas trap filled with activated carbon and collecting the rare gas. 2. Claim 1 is characterized in that oxygen in the air is liquefied by introducing air into liquid nitrogen, and the rare gas trap is cooled with this liquid oxygen to collect the rare gas. radioactivity sampling method. 3. Percentage of Claims The radioactivity sampling method according to claim 1, characterized in that a liquid oxygen generator or the like is separately provided, and the rare gas trap is cooled with the liquid oxygen produced thereby to collect the rare gas.