During the 7.1 aftershock on April 7, water sloshed out of spent fuel pools in the plant’s No.1, No.2 and No.3 reactors, which had been shut down after the 9.0 magnitude quake on March 11, and had also leaked in three other locations in the No.3 reactor complex.
Tohoku Electric said on Friday two out of three lines supplying off-site power to the Onagawa site — in so-called cold shutdown since the March 11 quake — were lost last night in the strongest aftershock so far of the earlier quake.
Cooling operations of its spent pool fuels resumed after they stopped due to the quake, it said, and there was still an emergency backup generator to fall back on.
“We detected a small rise in radiation levels inside the reactor buildings, and are trying to find the locations of the leaks,” a Tohoku Electric official said. “We see no change in radiation levels outside the reactor buildings.”
According to Tohoku Electric, the cooling systems for the temporary storage pools for spent nuclear fuel rods at the Nos. 1 to 3 reactors in the Onagawa plant stopped temporarily after the aftershock hit the area and water spilled out from each pool.
The cooling systems were manually reactivated about an hour later. The fuel rods were not exposed and no radioactive substance had been found to have leaked outside, the company said.
Tohoku Electric said it’s possible the malfunction of a pressure gauge attached to a pump might have caused the cooling system for the No. 1 reactor’s storage pool to stop. The company is investigating the reason for the temporary shutdown of the cooling systems for the Nos. 2 and 3 reactors’ storage pools.
Tohoku Electric is also investigating the cause of the power cuts. A transformer station that supplies electricity to the plant suffered no failures.
The Onagawa Nuclear Power Plant is a nuclear power plant located on a 432 acres in Onagawa in the Oshika District and Ishinomaki city, Miyagi Prefecture, Japan. It is managed by the Tohoku Electric Power Company. It was the most quickly constructed nuclear power plant in the world.
The International Atomic Energy Agency said off-site power was lost at some other nuclear facilities in the country after Thursday evening’s quake, and that emergency power supply was operating.
A cooling system at the Higashidori nuclear power plant in Higashidorimura, Aomori Prefecture, stopped temporarily after the aftershock, plant operator Tohoku Electric Power Co. said Friday.
According to the company, two external power sources for the Higashidori plant, which was not operating at the time due to a regular inspection, were cut off as a result of outages throughout the prefecture following the aftershock. An emergency generator was used to supply power to the plant.
The cooling system for a temporary storage pool for spent nuclear fuel rods, where all the plant’s nuclear fuel was stored, stopped automatically, but restarted about one hour later.
No leakage of radioactive substances has been found, the company said. Another external power source, which was under inspection at the time of the aftershock, was restored at about 3:30 a.m. Friday.
Tokyo Electric Power Co. said Friday that the aftershock did not cause any new problems at its Fukushima No. 1 nuclear power plant. Nitrogen injections into the No. 1 reactor and the transportation of contaminated water continued smoothly despite some disturbances in restoration work, TEPCO officials said.
At Japan Nuclear Fuel Ltd.’s nuclear fuel reprocessing plant in Rokkashomura, Aomori Prefecture, two external power sources were cut off due to outages after the aftershock, but an emergency power source was activated. The plant was conducting test operations.
According to Japan Nuclear Fuel, spent nuclear fuel rods and a system to cool radioactive waste solution were unaffected.
According to the Economy, Trade and Industry Ministry’s Nuclear and Industrial Safety Agency, it is extremely rare for an emergency generator or power source to be activated at a nuclear power plant.
Such systems are a last resort, meant to supply power only after regular external power sources became unusable. An emergency power source was lost at the Fukushima No. 1 nuclear power plant after tsunami ravaged the facility on March 11, making it difficult to cool down nuclear reactors in a stable manner.
Three of the four external sources for the Nos. 1 to 3 reactors at Tohoku Electric Power’s Onagawa nuclear power plant in Onagawacho and Ishinomaki, Miyagi Prefecture, stopped supplying power late night Thursday. As of Friday morning, however, one of the three sources was working again, the company said.
Health Canada’s Fixed Point Surveillance Network Data
As part of routine operations, Health Canada’s Radiation Protection Bureau conducts 24/7 environmental monitoring across Canada using the Fixed Point Surveillance (FPS) network. Unlike the particulate and noble gas systems, these detectors do not concentrate radioactive material but rather report on the radiation levels from all gamma-emitting isotopes around the detector in near-real-time. As a result, they are less sensitive to low levels of radiation in air, but are better able to indicate changes in environmental radiation exposure. This information is being reported as a gamma radiation dose per day.
The following charts depict the average daily radiation dose from the environment for the month of April. In order to compare the actual daily values and account for normal fluctuations, the chart includes measured baseline values from 2010 (minimum, maximum and average over the entire year). Numbers are micro-Seiverts per day.
Canadian Comprehensive Nuclear-Test-Ban Treaty Station Radioisotope Concentrations
Under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS), Health Canada’s Radiation Protection Bureau operates four certified IMS particulate monitoring stations (Sidney, BC; Yellowknife, NWT; Resolute Bay, NU; and St John’s, NL), two experimental noble gas systems (Yellowknife and St. John’s), and one station in Ottawa, ON.
All systems collect air over 24-hour sampling periods and concentrate radioisotopes so that they can be measured by gamma radiation detectors. By concentrating the material, it is possible to detect much lower quantities of radioactivity in air than with a real-time monitoring system, such as the Fixed Point Surveillance Network. In fact, it was the monitoring station in Sydney BC that detected the first radioactive isotopes coming from Japan.
The measurements in the following charts show the concentrations of radioisotopes in the air.
All of the Canadian CTBT stations collect data for cesium-137, iodine-131, and tellurium-132. These data are reported in milliBecquerels per cubic meter (mBq/m3).
Only Yellowknife, Ottawa, and St John’s are equipped to collect data for xenon-133, which is a noble gas. Noble gases do not react with other elements in the environment. As a result, they do not settle out of the plume but travel long distances in the atmosphere and are detected in relatively high concentrations.
Because the activities are relatively high, xenon-133 measurements are reported in Becquerels per cubic meter.