Causes of Nuclear Wastewater

Nuclear wastewater refers to polluted liquid which contains radioactive elements. As we all know, water is essential for nuclear power plants to operate properly. The water required by nuclear power plants is mainly used for cooling, and the cooling method of coastal nuclear power plants generally adopts seawater directly. However, inland nuclear power plants not only require a large amount of water, but also need stable water supply for the normal operation. Therefore, the availability of water resource is one of the key factors in the site selection of nuclear power sector. Besides, the quality of water and water resource scheduling conditions in the drought period should be fully considered.

As for nuclear wastewater, the long-term discharge of low concentration radioactive wastewater will lead to the formation of radioactive element enrichment areas where the current exchange is weak. Also, these areas would be caused by sediment adsorption, food chain transfer, groundwater infiltration and so on, affecting downstream water quality. Therefore, the unexpected accidents of inland nuclear power plants have potential risks to the safety of downstream water supply. Once the radionuclide in the nuclear island is leaked into the downstream water, it will influence on the water quality safety seriously.

There are several ways to deal with radioactive wastewater produced by nuclear power plants under normal operating conditions to reduce the contamination within the standard range. In reality, engineers will adopt different treatment processes or a reasonable combination of processes according to water quality conditions, so as to achieve the ideal effect. After a series of treatments, the concentration of the radioactive waste liquid would be very low and could be harmlessly to the environment.

Fatal Nuclear Accidents in History

1. Three Mile Island Accident (TMI-2)

Unit 2 at three Mile Island, located 16 kilometers southeast of Harrisburg, Pennsylvania, reached critical value on March 28, 1978, a year before the worst accident in the U.S. commercial nuclear power plant history.

The accident was caused by the automatic activation of the plant’s cooling system after a water pump malfunctioned. However, a few days ago, after maintenance, the staff did not open the valve of the accident cooling system, resulting in the automatic start of the system, when the water was still in a state of interruption. In this case, when the temperature and pressure rose, the relief valve would automatically open to release some of the soda mixture in the core. Subsequently, the pressure relief valve failed to return to the seat due to a fault, causing the core coolant to continue to flow out, and the pressure dropped below the normal value. So, the emergency core cooling system automatically switched on, but the operators did not correctly discover that the pressure relief valve failed to return and he shut down the emergency core cooling system and stopped the injection of water into the core. Intertwined with equipment failures, a series of management and operational failures led to the serious accident in which the core melted.

Some of the radioactive material entered the relief chamber at the bottom of the containment vessel. Fifteen minutes later, the relief chamber overflowed and exploded, sending radioactive water into the pit and fissile gas into the containment vessel. Since then, part of the radioactive water was sent to the auxiliary plant in the drainage tank, causing part of the radionuclide overflow. In addition, the operators turned on the main drainage system and caused the release of radionuclides. During the accident, the main system produced a large amount of gas, overloading the degassing system and discharging the gas from the safety valve of the container control box.

The minimal amount of radioactive material released at Three Mile Island revealed the importance of containment. Although the containment vessel was not absolutely leakproof, it was still largely undamaged mechanically, and most of the iodine and cesium were trapped in the containment vessel. Also, gases escaping from the containment vessel passed through the auxiliary building, where most of the radioactive material was collected by filters. Although the consequences of the release of radioactivity to the environment and the radiation people were minimal, the accident, rated as a level 5, had a profound impact on the development of the world nuclear industry.

2. Chernobyl Disaster: Nuclear Power Plant Accident

In the early morning of April 26, 1986, a level 7 nuclear power plant accident, the highest level in the history of nuclear power, occurred at the Chernobyl No. 4 unit in the former Soviet Union. The reason of the serious accident was a transient criticality caused by power transient during the test of the reactor safety system. As a result, the reactor core, reactor building and turbine building were destroyed, and large amounts of radioactive material were released into the atmosphere.

The Chernobyl Nuclear Power Plant planned to build 8 nuclear power units with 1 million kilowatts of electric power. Units 1, 2, 3 and 4 had been put into use, units 5 and 6 were under construction, and units 7 and 8 were also ready to be started. These units have been operated under normal and safe conditions without any problems. But in the event of an accident, when a large amount of radioactive material was released, there was no safeguard to prevent it from entering the atmosphere. This was because the main cooling loop of the original design of the reactor body and the soda water separator were respectively placed in the concrete radiation protection shielding isolation chamber, and the adjacent isolation chamber was not sealed, which could neither bear the pressure or play the role of the containment.

It was concluded that the primary cause was the introduction of excess reactivity, but there were other factors made the accident developed into the most serious nuclear power plant accident in human history. First, management confusion had led to serious violations of rules and regulations. Second, the reactor was fatally flawed in design and had no inherent safety. In detail, the reactor has a reactivity coefficient as indicator. Although the combined power reactivity coefficient was negative at normal operation, the combined effect would be positive when the power was below 20% and the reactor was prone to great instability. Third, the plant has no containment vessel, which was also a significant factor that negatively influenced on the environment. After the Chernobyl accident, the local authorities took a number of measures to protect the region’s water resource, preventing the spread of radioactive materials and further contamination of the local water system.

3. Fukushima Nuclear Power Plant Accident

Unit 1 of Daiichi Nuclear Power Plant in Japan’s Fukushima Nuclear Power Plant was put into commercial operation in March 1971. It had been in service for 40 years and showed many signs of aging, including embrittlement and corrosion of the reactor pressure vessel and corrosion of the exhaust gas treatment system in the heat exchange area. In February 2011, the Nuclear Safety and Security Agency approved a request to extend the life of the unit and officially decommissioned the unit by 2031.

However, in the afternoon of March 11, 2011, accompanied by a magnitude 9.0 earthquake, the power grid system in Northern Japan was severely damaged, resulting in the power outage that area. The operating reactor units at Fukushima Daiichi Nuclear Power Plant had taken emergency measures to shut down the nuclear fission reaction inside the reactors in response to the earthquake and power failure, but a large amount of radioactive fission products still released energy in the form of decay. If cooling water was not used to quickly deliver the heat, the reactor core would quickly rise and melt down. Thus, reactors need to be cooled long after they were shut down.

Despite a series of emergency measures have been set in advance, the tsunami triggered by the earthquake flooded the emergency diesel generators and ran out of batteries, leaving units 1 to 3 without cooling. Then, the water level in the reactor core vessel dropped, exposing the fuel rods to water vapor. Therefore, hydrogen explosions occurred in units 1 to 3, blowing off the roof and walls of the upper part of the building. Meanwhile, radioactive gases were released into the environment, forming radioactive contamination. Fortunately, the structure beneath the reactor was very strong, so the explosion did not damage the structure below, the pressure vessel and containment vessel did not suffer serious damage, and most of the radioactive material remained shielded inside the reactor.

At the time of the earthquake, no. 4 to 6 units were shut down for maintenance, but still needed cooling. For the same reason, there was also a hydrogen explosion in unit 4, after which the amount of radioactive material around the explosion increased. In order to contain the further deterioration of the accident, The Tokyo Electric Power Company has used mobile pumps to inject seawater into the reactor core, the containment vessel and even the entire building, which was a last resort emergency measure. However, a large amount of radioactive waste liquid with high concentration was produced at the same time. Improper storage or improper treatment of these wastewater would produce secondary disasters and create obstacles for subsequent accident treatments.

Ten years later, in 2021, the Japanese government announced that it would release nuclear wastewater stored at the Fukushima Daiichi plant into the sea. Tokyo Electric Power Co Inc promises that most radioactive elements in the wastewater can be removed, and the concentration of tritium in the wastewater will be diluted to 1/40 of the National standard in Japan and 1/7 of the standard for drinking water set by the World Health Organization, without contaminating the sea. However, ordinary people are not convinced since it is difficult to effectively monitor and comprehensively evaluate the nuclear wastewater discharged into the sea due to the nature of the Marine ecosystem.

Summary: Current Development Based on Accidents and Experiences

First of all, under the condition of normal operation process of the nuclear power plant, the radioactive wastewater discharged will have some predictable impacts on the surrounding ecological environment. For a nuclear power plant in normal operation, even if it will discharge radioactive wastewater into the surrounding water, the radioactive level is low and will not cause great damage to the nearby environment and ecosystem. After all, any wastewater discharged from a nuclear plant has to go through a series of procedures to reduce its radioactive concentration to below a standard level before it is allowed to be released.

Next, for the vast majority of the population, the low level of radioactive waste from the plant is almost harmless to the human body. By using the numerical simulation methods of tidal current contamination diffusion, the detection agency can calculate the concentration distribution of liquid emissions from nuclear power plant in the nearby sea area, and then calculate the dose rate caused by the low radioactive waste liquid to various Marine organisms. It was concluded that the discharge of low-level radioactive waste liquid during the normal operation of the nuclear power plant would not cause harm to the health of the majority of residents and would not have harmful effects on aquatic organisms, which was later confirmed by the long-term monitoring and investigation results of the actual operation of the nuclear power plant.

Both the occurrence of nuclear accidents and the actual radioactive hazards in history have deepened people’s awareness of nuclear safety and promoted the continuous development of nuclear safety technology. What’s more, nuclear power developing countries and international organizations have established corresponding nuclear energy safety systems, including nuclear reactor safety technology, radioactive waste storage and management standards, radioactive material discharge technical standards, plant and off-site nuclear accident emergency plans, etc. These safety technical specifications for nuclear power mainly prevent the harm caused by nuclear power plant accidents from the perspective of reducing radionuclide leakage and protecting the public.