Filtered ventilation?

January 22, 2013

News Navigator: What are filtered ventilation systems at nuclear power plants?

http://mainichi.jp/english/english/perspectives/news/20130122p2a00m0na014000c.html

The Mainichi answers common questions readers may have about filtered ventilation systems that are being installed at nuclear power plants.

Question: What is a "filtered ventilation system"?

Answer: "Vent" in English means "to release or discharge." In order to reduce the radioactive materials from a nuclear accident, electric companies are set to install filters in the ventilation system that releases steam and gas. The Tokyo Electric Power Co. (TEPCO) has initiated the filter installation at the Kashiwazaki-Kariwa Nuclear Power Plant in Niigata Prefecture from Jan. 15.

Q: So the system is designed to filter?

A: That's right. For the Kashiwazaki-Kariwa plant, a tank under construction that is designed to contain 50 tons of water just outside of the reactor building will be the filter. Dusty radioactive materials in steam will be filtered by going through the tank and being dropped in the water. By installing the system, the amount of radioactive iodine and cesium released in the air is expected to be reduced to one thousandth. The detailed design of the system and the time of completion are undetermined and the construction cost has not been disclosed to the public.

Although in theory the steam and gas should be kept inside the plant, the "nuclear reactor container" might explode in case of an accident that causes the pressure to rise inside the plant. In order to prevent an explosion that spreads large amount of radioactive materials, the steam needs to be released.

Q: What is a "nuclear reactor container"?

A: The nuclear plant's atomic fuel is covered with three layers of containers: From the outside they are the "reactor building," "nuclear reactor container," and "pressure vessel," which is the atomic reactor itself. When there's something wrong with the pressure vessel, the reactor container is supposed to prevent radiation from spreading. In the case of the Fukushima No. 1 Nuclear Power Plant, TEPCO attempted to vent the steam and gas from the reactor container.

Q: Did it work?

A: No. Although TEPCO tried to release steam from the No. 1 to No. 3 reactors some 13 times in total, the reactor containers and buildings were all damaged. This was due to a blackout that was caused by the earthquake and tsunami, leading to manual operation, which ultimately caused a delay. Also, the amount of steam that had been released appeared insufficient.

On top of that, despite a plan to filter the steam through the water in the "pressure suppression pool," TEPCO failed to do so at one time in the No. 2 reactor during the 13 attempts, and released radioactive materials into the environment without any filtration. From this experience, filters are thought to be necessary for nuclear plants' ventilation systems.

TEPCO estimated that 99 percent of the radioactive materials released in the accident were caused by the reactor container damage and the hydrogen explosion of the reactor building rather than from the ventilation. So it is clear that the prevention of damage or an explosion is essential.

Q: What about nuclear power plants in other countries?

A: After the 1986 Chernobyl accident, Britain, Germany, France, Switzerland and Sweden made it mandatory to have ventilation systems for their nuclear plants. Japan, however, continued to permit non-filtered ventilation.

Q: What about other electric companies?

A: The Federation of Electric Companies of Japan announced last year the need to install filtered ventilation systems in all nuclear power plants. Meanwhile, the Nuclear Regulation Authority is considering making it compulsory to have filtered ventilation in all nuclear power plants in new safety standards scheduled be released in July this year. (Answers by Shogo Takagi, Kashiwazaki Bureau)