2.1

Personnel radioactive decontamination methods

Radioactive decontamination refers to the process of using various methods to remove radioactive pollutants from contaminated surfaces, in order to reduce the radiation dose to personnel. In the event of a severe accident at a nuclear power plant, radioactive pollutants in the form of aerosols and fine dust will form and diffuse with the wind, known as “radioactive plumes”. Under the influence of gravity and other factors, radioactive plumes continuously settle onto personnel, facilities, buildings, and the ground, causing radioactive hazards to evacuated personnel. In addition, personnel close to nuclear power plants may also be contaminated with other radioactive substances, which can have an impact on the public and the environment. Decontaminating personnel contaminated with radioactivity is an important measure to ensure their health and safety. When personnel are found to have radioactive contamination, they should quickly use toilet equipment locally decontamination. If the level still cannot reach the specified level, further treatment should be carried out at a specially set washing and disinfecting station. If the level of radioactive dose rate still exceeds the standard, further treatment should be carried out at a specially designed decontamination station.

The off-site nuclear emergency decontamination station is generally deployed upwind of the nuclear power plant, and its main function is to decontaminate personnel affected by radioactive plumes in the emergency plan area of the nuclear power plant². The evacuees were mainly contaminated with radioactive plumes. Evacuees in nuclear emergency situations need to undergo radiation testing. If the radiation dose rate exceeds the standard, decontamination must be carried out.

(1) Partial decontamination

There are wiping methods and water or aqueous solution decontamination methods for partial decontamination of personnel. If partial contamination is found on the skin of evacuees during testing. First, we rinse the contaminated area with tap water, then use a clean towel, cotton, cloth, etc. dipped in water to wipe the exposed skin. At the same time, we rinse the mouth, and wipe the eye sockets, earholes, and nasal cavity. If there are still contaminated areas, they can be carefully scrubbed with 5% EDTA-2Na aqueous solution or special cleaning soap. Attention should be paid not to use organic agents that promote the entry of contaminated radioactive isotopes into the human body, high-concentration acid-base solutions, and solvents that strongly irritate the skin. For eye and head contamination, first, we rinse the eyes and face with flowing water sprayed upwards. Next, we rinse the eyes with physiological saline. 3% citric acid or 2% sodium bicarbonate can be used to rinse the mouth, and if necessary, 3% hydrogen peroxide solution can be used to rinse the throat for contamination. We wipe the nasal cavity with a cotton swab, we cut off nasal hair if necessary, and we spray vasoconstrictor into the nasal cavity to reduce the absorption of radioactive isotopes. For severely contaminated personnel whose radioactive dose still exceeds the standard after being decontaminated, different chemical reagents can be used for targeted decontamination according to different types of radioactive contamination. Reagents and methods for decontamination under severe nuclear accidents are shown in Table 1.

Table 1.

Reagents and methods for decontamination under severe nuclear accident severe accident.

In addition, foam scrubbing technology can also be used. After foam treatment, the cleaning agent and disinfectant can be covered on the surface. After a certain period of time, the purpose of decontamination can be achieved. This method effectively improves the technical level of decontamination. The partial decontamination method is highly targeted and requires a high level of decontamination medium. This method is not suitable as the main method of large-scale personnel decontamination in nuclear emergency situations. Partial decontamination can be used in conjunction with whole-body decontamination as a decontamination method for severely radioactive contaminated individuals, special populations, or injured individuals.

(2) Large-scale decontamination methods

In the event of a severe accident at a nuclear power plant, with a large number of evacuees, it is necessary to carry out large-scale decontamination at off-site nuclear emergency decontamination stations. Fixed decontamination facilities cannot be used in emergency situations if they are located downwind of the accident nuclear power plant due to their immovable position. At the same time, during the large-scale decontamination process, the interior of fixed decontamination facilities is prone to contamination by radioactive substances, which affects the decontamination efficiency. In addition, the infrastructure and daily maintenance costs of fixed decontamination facilities are relatively high. At present, domestic scholars mainly focus on the research of mobile decontamination stations, such as decontamination vehicles or large integrated decontamination equipment. At present, domestic scholars mainly focus on the research of mobile decontamination stations, such as decontamination vehicles or large integrated decontamination equipment. Although they can be maneuvered according to the meteorological conditions around nuclear power plants, there are also problems such as high cost, low usage frequency, low decontamination efficiency, and susceptibility to radioactive wastewater contamination. According to the guidelines released by the Edgewood Chemical Biological Center (ECBC), it is recommended to use the Layer Pipe Decontamination System (LDS) for large-scale decontamination³. The system will park multiple fire trucks parallel to each other at a distance of 20 feet (about 6 meters). The nozzles will be arranged as shown in Figure 1 to form a surround-free spray channel, with a length of about 40 feet (about 12 meters). The water pressure should be adjusted to about 50 to 60 psi.

Figure 1.

Schematic diagram of LDS decontamination system.

Evacuees should enter the decontamination channels one by one after removing their clothes. During decontamination, it is necessary to extend the head backward, arms, and legs, exposing the armpits and groin. Water should be avoided from entering the eyes, nose, or mouth, and sufficient distance should be maintained between individuals to avoid secondary contamination. The comparison of characteristics between the LDS channel decontamination system and other facilities is shown in Table 2.

Table 2.

Characteristics of LDS decontamination system, fixed decontamination facilities, and mobile decontamination facilities.

2.2

Radioactive detection methods

After the evacuees arrive at the decontamination station, they need to conduct radioactivity detection to determine whether they need decontamination. After decontamination, they also need to conduct radioactivity detection again to test the decontamination effect. The personnel radiation detection of large-scale decontamination usually uses the surface contamination detector to conduct the whole-body detection on the evacuees and record the contamination amount. In order to prevent radioactive substances from contaminating the surface contamination detector, the probe should be wrapped with plastic film. The probe should be 0.5-1 cm away from the skin and move at the speed of 5 cm/s. The measurement should be carried out from top to bottom, from left to right, and from front to back. When detecting surface contamination, for α contamination, the detection distance should be 0.5 cm, for β contamination, the detection distance should be 2.5-5 cm⁴.

When the body surface contamination of evacuees is more than twice the natural background dose rate, they shall be regarded as radionuclide-contaminated personnel and shall be further measured and decontaminated. When the body surface of evacuees is contaminated by more than 10 times the natural background dose rate, or γ dose rate greater than 0.5 μSv/h, which should be regarded as personnel with severe radionuclide contamination, should be given rapid decontamination treatment⁵.

2.3

Large-scale decontamination process

After the evacuees arrive at the decontamination station, they should first carry out radioactivity detection. If the radioactive dose rate exceeds the standard, they should immediately remove the clothes and place them in plastic bags for sealing. When removing clothes, the evacuees must raise their heads, put their hands and arms on the inside of the clothes, pull the opening of the clothes away from the face and head as far as possible with their hands, and prevent the head, face, and eyes from being contaminated. Then enter the decontamination channel for decontaminating. After decontamination, the radioactivity detection shall be carried out again. If the dose rate is lower than the standard, enter the isolated observation area. If the dose rate exceeds the standard, decontamination shall be carried out again. After decontamination, the evacuees leave the decontamination station from the isolated observation area, and the injured can enter the infirmary in the clean area for medical help. The large-scale decontamination process is shown in Figure 2.

Figure 2.

Large-scale decontamination process.

3.1

Decontamination objects

The service object of the decontamination station is the personnel affected by the radioactive plume in the plume emergency planning area of the nuclear power plant, including the emergency relief personnel for nuclear accidents, the staff evacuated from the nuclear power plant and the residents around the nuclear power plant. According to the nuclear accident emergency plan and environmental impact assessment documents, the number of evacuees in the event of a severe nuclear accident can be estimated. Generally, when a nuclear accident occurs, evacuation is not required to occur in the whole area of the plume emergency planning area. In the event of a nuclear accident, the radioactive substances released by the nuclear power plant will move with the wind rather than evenly spread around. The wind direction will determine the affected sector in the emergency planning area. Generally, the personnel in the sector area within the plume emergency planning area, the sector area in the downwind area and its adjacent sectors on both sides need to be evacuated⁶. Whether the personnel in the ingestion emergency planning area needs to be evaluated depends on the progress of the accident, the diffusion range of radioactive substances and dose evaluation. Generally, the wind direction is divided into 16 sectors, and the number of personnel to be evacuated under accident conditions is calculated as follows.

In the above equation:

N_t —Total number of people to be evacuated;

N_w —Number of staff in the nuclear power plant;

N_e —Number of other people in the nuclear power plant;

Ni—Number of people in the internal plume emergency planning area;

N₀—Number of people in the external plume emergency planning area;

δ—Kronecker function;

N_oi—Number of residents in sector i of the external plume emergency planning area;

к—Sector corresponding to downwind direction of accident nuclear power plant;

k – 1 and k + 1—Adjacent sectors of the sector corresponding to the downwind direction of the accident nuclear power plant.

Assuming that the number of residents is evenly distributed outside the plume emergency planning area and the downwind direction does not change, it is estimated that the total number of people to evacuate is about 10000 to 40000, as shown in Table 3. In a nuclear emergency, some residents will evacuate spontaneously due to fear of nuclear accidents, and the actual number of evacuees may be more.

Table 3.

Number of people that may be evacuated under emergency conditions in some nuclear power plants in China (Unit: Person).

Note: * The numbers outside the brackets indicate the diameter of the area inside the plume emergency plan area, and the numbers inside the brackets indicate the diameter of the area outside the plume emergency plan area.

3.2

Decontamination area division and facility layout

The decontamination station shall be provided with a cleaning area, transition area and contamination area according to the upwind direction to the downwind direction. The cleaning area is the exit of the decontamination station, where the staff preparation area and infirmary shall be set. The staff preparation area is the area where the staff wear protective equipment and make corresponding preparations before decontamination. Independent dressing rooms, toilets, material storage rooms, etc. can be set up for the staff. The minor wounded whose radioactivity detection is lower than the radiation dose rate standard can be treated in the infirmary⁷.

The contaminated area is the place where the evacuees arrive and the entrance of the decontamination station. A pre-decontamination area should be set up. The pre-decontamination area is a radioactive contaminated area, and the evacuees must remove the clothes and articles contaminated by radioactivity in this area, wrap them in sealed bags and place them in the collection barrel, and set up a temporary storage room for radioactive contaminated articles in this area as appropriate.

The transition area is located between the cleaning area and the contamination area. It is the area for decontamination and radioactive detection. The radioactive detection area, decontamination area, isolation observation area and sewage storage area should be set up respectively. Large-scale decontamination facilities and equipment shall be arranged in the decontamination area, radioactive sewage collection channels shall be laid on the ground, and radioactive sewage shall be transported to the sewage storage facilities through the pipe network to prevent impact on the environment and the public. The isolation and observation area is the area for temporary placement of personnel who meet the standards of radioactive detection, which is a relatively clean area. Evacuees need to pass through three areas in turn from arriving at the decontamination station to leaving the decontamination station and carrying out radioactive dose rate detection, decontamination and isolation observation^8,9.

4.1

Characteristics of nuclear emergency decontamination station

Different from the general biochemical accident decontamination facilities, the nuclear accident emergency decontamination station should have the following characteristics. (1) Due to a severe nuclear accident will produce radioactive plumes, causing a wide range of radioactive contamination, resulting in the need for decontamination of more people. In addition, considering the public’s fear of radioactive substances, the public who are not actually contaminated by radioactivity will also go to the decontamination station for monitoring and decontamination, so the emergency decontamination facilities for nuclear accidents need to meet the needs of large-scale decontamination. (2) Severe nuclear accidents cause great harm, which means the necessity of the existence of large-scale nuclear emergency decontamination facilities and the high requirements for their availability. However, the probability of a severe nuclear accident is extremely low, which means that the frequency of the use of large-scale decontamination facilities is very low, and facilities with simple structure, convenient use, convenient maintenance and repair should be selected as far as possible. (3) Radioactive contamination caused by severe nuclear accidents is difficult to remove. If fixed decontamination facilities are built, decontamination is required once the fixed decontamination and decontamination facilities are contaminated. Therefore, nuclear emergency decontamination facilities should generally be arranged in the outdoor site with good ventilation conditions. (4) In order to avoid contamination caused by radioactive plume, the site selection of the nuclear accident emergency decontamination station and the layout of decontamination facilities need to change flexibly according to the wind direction, so the decontamination facilities need mobile features, in order to make timely adjustment in case of transfer. (5) The wastewater produced by decontamination may be radioactive, so it needs to be collected completely and cannot be discharged directly. Therefore, the decontamination station also needs to have the characteristics of no overflow and no leakage of wastewater^10,11.

4.2

Construction strategy

Based on the above characteristics, the construction strategy of “Combination of Peacetime and Wartime” is suitable for the construction of a nuclear emergency decontamination station. The main facilities of the nuclear accident emergency decontamination station shall adopt mobile equipment, so as to transfer according to the meteorological conditions in case of emergency. The decontamination facilities shall have the characteristics of convenient deployment, simple operation, and durability so that they can be quickly deployed in an emergency state to form the decontamination capacity. In addition, due to the extremely low frequency of use, the main facilities of the nuclear emergency decontamination station should be able to be used for society in peacetime or easy for daily maintenance without affecting the decontamination capacity. The construction strategy of “Combination of Peacetime and Wartime” considers the practical problems of nuclear emergency decontamination station construction on the basis of the current research, taking into account the availability of decontamination station and the maintenance of decontamination capacity. For large-scale decontamination caused by nuclear accidents, the LDS decontamination channel system can be used in terms of decontamination methods, which can be deployed according to the number of evacuees from the nuclear power plant. It is efficient and convenient and can form large-scale decontamination ability in a short time without daily maintenance. In terms of detection methods, the more common surface contamination detector can be used, and the detection standard can be quickly determined according to the environmental background dose rate. The radioactive sewage generated by decontamination can be collected by a sewage collection system consisting of a collection channel, portable water pump and large water bag, and then treated in a centralized manner. According to the large-scale decontamination principle of ECBC, time is the first factor to save more lives. The above strategy can form a closed loop of decontamination, radioactive detection and sewage collection in a short time, and quickly form the decontamination capacity in an emergency state.