By Akira OIKAWA /
via asia.nikkei.com /
September 14, 2015 /
The Sendai No.1 reactor, Japan’s first active reactor in about two years, has resumed full-scale commercial operations. As safety screening progresses, other suspended reactors are expected to follow, but mostly in the western part of Japan.
All 43 reactors in Japan are light-water models. “Light water” simply means normal water. Light-water reactors are further categorized into boiling-water and pressurized-water types.
Boiling-water reactors generate steam inside the reactor by boiling water inside, while pressurized-water reactors heat water inside the reactor by adding pressure and sending the heated water to a steam generator.
Nuclear plants in western Japan, including the Sendai plant operated by Kyushu Electric Power, typically have pressurized-water reactors. These include the Takahama Nos. 3 and 4 reactors in Fukui Prefecture, operated by Kansai Electric Power, and the Ikata No. 3 reactor in Ehime Prefecture, run by Shikoku Electric Power. All of these have also passed the new safety screening process.
In contrast, many nuclear facilities in eastern Japan have boiling-water reactors, including the Fukushima Daiichi nuclear power plant that suffered three meltdowns after the earthquake and tsunami of 2011.
Currently, 10 boiling-water reactors at eight plants across Japan are being screened.
Japan’s Nuclear Regulation Authority in July 2013 began reviewing what safety measures suspended reactors would have to take before being cleared to start up again. Nuclear plants with boiling-water reactors were generally slow to apply for reviews because of the time it took to bring their containment vessels up to the new standards.
Reactors reside in these vessels, which are designed to keep radioactive substances inside. After the earthquake and tsunami, the fuel rods inside the Fukushima Daiichi reactors could not be cooled. As a result, they began to melt, releasing massive amounts of radioactive substances.
If the containment vessel does its job during an accident, these substances will not leak into the surrounding environment. Since it is an important job, the nuclear power regulator is making sure new measures to protect the vessels are up to the task.
Containment vessels are made of solid steel and concrete. Even these building materials, however, can give way to extreme inner pressure. When fuel rods cannot be cooled for a prolonged period, the heat they generate turns surrounding water into vapor, a process that generates hydrogen as a byproduct. If enough hydrogen accumulates, it can break through the containment vessel.
Containment vessels for pressurized-water reactors are generally much larger than those around their boiling-water cousins. For example, the containment capacity of the Mihama No. 3 pressurized-water reactor in Fukui Prefecture is 10 times that of the Fukushima No. 3 boiling-water reactor.
Both reactors put out similar amounts of power.
Bigger containment buildings can tolerate the buildup of more inner pressure. Since boiling-water reactors have less containment ability, the regulator has asked their operators to take preventive measures.
As a result, many operators of boiling-water reactors are mulling whether to install filtered vents — special pieces of equipment that can release gas to reduce pressure. When steam is released through these vents, it is filtered to remove most radioactive substances.
For the Nos. 6 and 7 reactors at the Kashiwazaki-Kariwa plant in Niigata, filters are expected to be able to remove more than 99.9% of radioactive particles such as cesium and more than 98% of radioiodine gas.
Filtered vents are only opened in worst-case, last-resort scenarios.
“Basically, we do not expect to use the measure,” said Tadayuki Yokomura, who is in charge of operating the Kashiwazaki-Kariwa plant.
The plant is also installing a worst-case, second-to-last-resort system — one meant to turn steam back into water, which would help to reduce the inner pressure. Pipework for the system is now being prepared.
The regulator seems to be completing the screening of some boiling-water models. But even after passing the tests, a suspended reactor cannot restart without the consent of the municipal government where it resides and other relevant bodies.
And that could prove to be an even taller hurdle than passing the new tests. More than four years after the meltdowns, many Japanese remain skeptical about nuclear reactor safety.