Research and Application of Digital Emergency Decontamination System

With the development of nuclear energy, tritium is extensively used in fusion reactors, nuclear power plants, and tritium-related systems. A digital Emergency Detritiation System (EDS) was developed by combining an ionization chamber, a dew point meter, a circulating pump, a drying bed, a liquid nitrogen condenser, etc. to monitor the system and the tritium gas in the industrial exhaust in real time as well as recover and remove the tritium gas in the exhaust. In the work of a factory, the online monitoring of tritium-containing gas in industrial tail gas was realized. A large amount of tritium gas in industrial tail gas was treated by catalytic oxidation of precious metal oxidation bed to form HTO, and finally, HTO was recovered by a liquid nitrogen cold trap. The complex process flow is visualized and automated by the system integration software to improve the efficiency of scientific research. The results demonstrate that the system can monitor the amount of tritium in tail gas instantaneously and accurately. The removal efficiency of tritium from the industrial exhaust can be up to 99%. After repeated regeneration and activation, the working efficiency of the molecular sieve can still reach 99%, ensuring that the composite emission standards of tritiation-related sites or fusion reactors and other plants are met.


Introduction
Tritium, an isotope of hydrogen, has a growing number of uses and prospects after being transferred from military to civilian fields.With the advancement of society, the energy demand will continue to rise, and nuclear power energy will be developed on a huge scale shortly.The processing system is utilized in a sealed container, and the exhaust gas or the tritium permeated and retained in the sealed container is purified to ensure the radiation safety of personnel in chemical research and process operation [1]~ [3] .The Tritium Process Research Laboratory of Hara Research Institute in Japan contains an inert gas purification system (GPS) for controlling the concentration of the protective atmosphere in glove boxes.It applies a precious metal catalyst to catalyze the oxidation of tritium gas, which is absorbed by a molecular sieve [4] .The tritium purification device TSTA of Los Alamos Laboratory in the United States may remove the tritium released into the room of the emergency air purification system [5]~ [6] , as well as tritium-containing waste gas treatment system TWT [7]~ [8] .
At present, tritium is a common component of nuclear power energy facilities.Besides, studies show that the concentration of tritium near nuclear-related facilities, both domestically and internationally, is significantly higher than that in other non-nuclear areas.Tritium primarily exists in the form of HT and HTO in the atmosphere, soil, and other environments.As a result, the concentration of tritium in the environment around the nuclear-related sites is relatively high.For instance, HTO concentrations in the atmosphere and soil reach 3822 mBq/m 3 and 5838 Bq/L, respectively [9]~ [10] .Since China lacks industrialized tail gas purification technology currently, it is urgent to develop a corresponding tritiated tail gas treatment system to guarantee the optimal principle of radiation protection.
This paper combined the Emergency Detritiation System (EDS) for real-time exhaust treatment.The ionization chamber was used for online monitoring of the tritium-containing gas in the industrial tail gas, and HTO was subsequently formed by catalytic oxidation of the precious metal oxidation bed.Finally, HTO was recovered through a drying bed and a liquid nitrogen cold trap, and the complex process was visualized and automated with integrated software, improving the efficiency of scientific research.

System Overview
The composition and process flow of the EDS system is shown in Figure 1.The EDS system mainly consists of a catalytic oxidation bed, a molecular sieve adsorption drying bed, a liquid nitrogen cold trap (low-temperature dryer, activated regenerative gas collection tank), a circulating pump, a mass flow controller, an ionization chamber, a pressure sensor, and other equipment components, as well as pipe interfaces with process circulating gas path, the process exhaust collection path, the exhaust and exhaust system, etc.

Working Principle
The emergency decontamination system is mainly employed to cope with radionuclide tritium in ventilation systems and industrial exhaust.The radioactivity of exhaust is measured by online ionization chamber analysis.When the gas enters the system, the catalytic oxidation bed converts the hydrogencontaining gas into water vapor, and the molecular sieve suction bed or cold well efficiently captures and drys water vapor to purify and collect the active gas, ensuring that the process exhaust meets the limit requirements of the emission source item.In addition, an emergency purification treatment is performed for the process atmosphere when the exhaust gas of the process is released abnormally.(1) When the capacity of the catalytic oxidation bed diminishes, air can be passed through it to enhance the activation effect.When the capacity of the molecular sieve bed (drying bed) decreases, it can be activated by raising the temperature and opening the collection state of the low-temperature cold trap.Moreover, the molecular sieve bed (drying bed) can be heated to the activation temperature.The adsorbed water vapor will be released into the system at high temperature, and then be pursed by circulating gas to the cold well for collection.Only the condensing collector that needs to be replaced in practice can achieve system collection and reuse, which is long-term efficient.

Software Description
EDS adopts a programmable logic controller (PLC) and engineering control simulation programming software to integrate bed temperature increase and valve control into the system.It realizes rapid operation in various working situations with scripts, calculating the output data and displaying the temperature by a digital table.The device is simple, convenient, and intuitive, and its electronic control system can operate steadily and reliably.The system has a complete log recording function because the discharge of the system is related to the safety of the discharge of airborne effluents.Ultimately, complete communication between the EDS and measurement and control server is realized to ensure the integrity of data records and timeliness of measurement and control.Measurement and control data are uploaded to the server and the data recording interval is less than 1 s.It is possible to implement remote control, meaning that all control commands of the EDS system are completed at the computer operating end.

Running Mode
For the emission management requirements of test run or plant, the EDS measurement and control software is programmed to create a combination of function buttons that can automatically set valve status and equipment parameters to achieve a variety of operation modes, such as atmosphere monitoring and emergency treatment, process exhaust monitoring and purification treatment, gas storage tank purification treatment, catalytic oxidation bed regeneration activation treatment, molecular sieve drying bed regeneration activation treatment.In addition to different automated operating modes, software capabilities include multiple bed status monitoring, other parameter settings, real-time data recording, historical data recording, operation logs, and other functions.

Environmental monitoring and emergency treatment
An ionization chamber IC-1/ exhaust air monitor is utilized to monitor the activity concentration in the environment during the test process.If the activity concentration exceeds the specified limit value (alarm value, setting value) of the "operating limit condition", the alarm is triggered or the corresponding processing mode is selected.According to the working parameters set in the EDS emergency treatment, the staff starts the catalytic oxidation bed and drying bed.The recycling purification treatment on the process gas atmosphere is also performed based on the specified flow rate until the activity concentration meets the emission requirements.

Process exhaust monitoring and purification treatment
When the gas in the processing system has to be discharged, it is first released into the EDS buffer tank.Meanwhile, the ionization chamber is employed to monitor the activity concentration of the discharged gas.During the discharge process, changes in the system pressure are tracked.According to the amount of gas, the gas is first discharged into the 15 L buffer tank.When the system pressure exceeds 75 kPa, the gas is initially vented into the 70 L buffer tank.The buffer tank is purified after the process exhaust.During the treatment process, the ionization chamber is observed and the gas is discharged to the exhaust system when the discharge requirements are met.The two buffer tanks of the EDS should maintain sufficient capacity to handle various emergencies before they are put into use.Therefore, purification and exhaust should be conducted immediately to maintain the two buffer tanks at < 25 kPa after receiving gas from the buffer tanks.

Catalytic oxidation bed regeneration activation treatment
When the capacity of the catalytic oxidation bed declines, it is necessary to raise the temperature to activate the oxidation bed, checking the state of the low-temperature cold trap to ensure that the cold trap has been cooled for more than 0.5 h in liquid nitrogen.We set the temperature of the oxidation bed automatically, and observe as well as record the readings from the ionization chamber and dew point meter while focusing on the system flow.If the flow is reduced, it needs to check whether the lowtemperature cold trap is saturated.If so, it should be replaced promptly.Hydrogen and helium can be filled to the oxidation bed as needed to enhance the activation effect.When the dew point meter and ionization chamber readings are drastically low, the activation is completed and the operation mode is terminated.

Molecular sieve drying bed regeneration activation treatment
The molecular sieve bed needs to be heated for activation when its capacity diminishes.Dry gas is used to continuously purge the molecular sieve bed (drying bed).The temperature of the bed is set to the preset value.Additionally, the ionization chamber, dew point value, and system pressure indicator are monitored and noted.The system flow rate should be examined simultaneously.If the flow is lowered, it needs to check whether the low-temperature trap is saturated.If the gas release rate in the bed body is too fast and the pressure is too high (> 100.0 KPa) during the regeneration activation, it must stop heating the bed body and restart heating once the pressure drops.The water vapor content in the system is brought down to a very low level and activation is finished.

Environmental monitoring and verification
The main task of the emergency decontamination device, which is an important component of the tail gas treatment unit, is to monitor the environmental tritium-specific activity in daily operation environment and industrial operation.In a stable operation state, the environmental tritium-specific activity is basically within the detection limit of the ionization chamber, which is less than 1 MBq/m 3 .When the tritium-specific activity value exceeds the operating limit value, the emergency decontamination device will be treated per the emergency treatment mode.The gas will subsequently be discharged after treatment.The following Figure 2 depicts the monitoring status of the tritiumspecific activity in a unit of industrial operation after about 180 hours.

Results of the process exhaust gas purification treatment
Figure 3 illustrates the exhaust gas emitted by the process during the operation of a plant, which contains a significant quantity of tritium gas.The exhaust gas is treated by the process of exhaust gas purification mode of the exhaust gas treatment unit.Firstly, the gas is discharged into the EDS buffer tank, and its activity concentration is measured by an ionization chamber.This type of gas can only be discharged after undertaking recycling treatment to reach the background value of 0.1 MBq/m 3 because of the high

Molecular sieve drying bed regeneration activation treatment
The water vapor drying bed achieves a particular saturation state and the water vapor absorption efficiency decreases after the completion of the above work and experiments.The water vapor drying capacity of the water vapor drying bed is restored by utilizing the molecular sieve regeneration activation treatment mode.The water vapor content in the system achieves its maximum level of about 1600 ppm by continuously increasing the temperature of the water vapor drying bed.When the liquid nitrogen condenser is turned on, the water vapor content at the outlet rapidly drops from 1600 ppm to around 200 ppm.The water vapor components released from the water vapor drying bed are collected once the gas carrier is blown away.When the water vapor content at the outlet of the drying bed drops to about 10 ppm, the water vapor release of the drying bed is deemed to be accomplished.At this time, the regeneration activation of the molecular sieve drying bed is finished.The outcomes of processing experimental data are displayed in Figure 4.After the water vapor drying bed has been activated, it can continue to work even after the bed returns to normal temperature.The residual water vapor reduced by the catalytic bed is discharged into the system and cycled continuously at this point.The change in water vapor content is indicated in Figure 5.The water vapor content is currently about 1~2 ppm, suggesting that the drying efficiency of a water vapor drying bed may still reach 99%.

Conclusion
The emergency decontamination system, which is essential equipment for the tritium-related sites or conversions reactors and other plants, as well as the exhaust treatment process of the system, is mainly applied to discharge residual tritium-containing exhaust after oxidation is completed to meet the emission standards of radioactive pollutants and avoid air pollution.In this paper, the emergency decontamination system is taken as an example and the working principle of the system is presented.The emergency monitoring of the tritium amount in the exhaust gas by the emergency decontamination system is verified to be real-time and accurate based on field tests of the working environment monitoring, industrial exhaust gas treatment, and molecular sieve activation during the actual operation of the exhaust gas treatment device.The removal efficiency of tritium from the industrial exhaust can approach 99%.After repeated regeneration and activation, the working efficiency of a molecular sieve can still reach 99%, ensuring the composite emission standard of tritiation-related sites, fusion reactors, and other plants.

Figure 1
Figure 1 Emergency Detritiation System

Figure 2
Figure 2 Environmental tritium-specific activity monitoring data within 180 hours concentration.While the software is running, it automatically logs all parameters and can process data automatically to produce the results of the graph.

Figure 3
Figure 3 Test results of tritium removal from the industrial exhaust gas

Figure 4
Figure 4 Variation of water vapor with time during the catalytic oxidation of the drying bed

Figure 5
Figure 5 Water vapor trapping test results in water vapor drying bed