Fukushima Nuclear Accident Update Log

Fukushima Nuclear Accident Update (6 April 2011, 15:15 UTC)

Japanese authorities have informed the IAEA that TEPCO has been authorized to begin injection of nitrogen into the primary containment vessel (PCV) of Unit 1 at the Fukushima Daiichi nuclear power plant. Injection of nitrogen is intended to displace oxygen inside the containment vessel, thereby reducing a risk of explosion due to the combustible combination of hydrogen and oxygen.

Summary of Reactor Status

On Wednesday, 6 April 2011, the IAEA provided the following information on the current status of nuclear safety in Japan:

1. Current Situation

Overall, the situation at the Fukushima Daiichi plant remains very serious.

TEPCO has identified a possible leakage path from the Turbine building of Unit 2 to the sea via a series of trenches/tunnels used to provide power to the sea water intake pumps and supply of service water to the reactor and turbine buildings. On 4 April, a tracer was used in an attempt to determine where the water was coming from. The tracer was also injected into two new bore holes that had been drilled near the pit. On 5 April it was confirmed that the tracer was seen leaking from the crack into the sea. Coagulation agents (liquid glass) were injected into the holes drilled around the pits to block leakage of water. It was reported that the leakage has currently stopped at 20:38 UTC on 5 April. Work continues to prevent further releases to the sea.

According to the TEPCO Press Release of 4 April, approximately 10 000 T of water from the radioactive waste treatment plant and 1 500 T of subsurface waters stored in the sub drain pits of Unit 5 and 6 are being discharged to the sea to provide room to store water with higher levels of radioactivity in a safer manner. The discharges started at 10:00 UTC and 12:00 UTC respectively on 4 April. TEPCO has estimated that these discharges would increase the effective dose to a member of the public by 0.6 mSv, if he/she were to eat seaweed and seafood from the discharge area every day for a year. It should be noted however that the movements of all ships, including fishing boats, are restricted within a 30km zone from the NPP, based on the hazardous area set by the Maritime Safety Agency. Also, Fukushima prefecture reported that no fishing has started beyond a 30km zone from the NPP in this prefecture.

In Unit 1 fresh water is being continuously injected into the reactor pressure vessel through feed-water line at an indicated flow rate of 6 m3/h using a temporary electric pump with off-site power. Fresh water is being injected continuously into the RPVs through the fire extinguisher lines in Units 2 and 3 at indicated rates of 8 m3/h and 7 m3/h respectively using a temporary electric pump with off-site power.

In Unit 1 the indicated temperature at the feed water nozzle of the RPV decreased from 234 °C to 222 °C and at the bottom of RPV stable at 115 °C. Instrumentation "B" for Reactor Pressure indicates that the pressure in the RPV is increasing and instrumentation "A" indicates that it has stabilized. NISA has indicated that some instruments in the reactor vessel may not be working properly. Drywell pressure is stable. In Unit 2 the indicated temperature at the feed water nozzle of the RPV is stable at 141 °C. The temperature at the bottom of RPV was not reported. Indicated Drywell pressure remains at atmospheric pressure. The indicated temperature at the feed water nozzle of the RPV in Unit 3 is stable at 85 °C and at the bottom of the RPV is about 115 °C.

Additional water was injected via Spent Fuel Cooling System line to the spent fuel pool in Unit 2 by a temporary pump on 4 April.

Power is available to instrumentation in Unit 3.

There has been no change in status on Units 4, 5, 6 and the Common Spent Fuel Storage Facility.

2. Radiation Monitoring

On 5 April, low levels of deposition of both iodine-131 and cesium-137 were detected in 5 and 7 prefectures respectively. The values for iodine-131 ranged from 12 to 70, for cesium-137 from 3.6 to 41 becquerel per square metre.

Gamma dose rates reported for 6 April showed no significant changes compared to yesterday. Since 23 March, values have tended to decrease. Gamma dose rates were reported for 45 prefectures to be between 0.02 to 0.1 microsievert per hour. In one prefecture the gamma dose rate was 0.16 microsievert per hour. These values are within or slightly above the natural background of 0.1 microsievert per hour.

As of 4 April, iodine-131 and cesium-134/137 was detectable in drinking water in a few prefectures. All values were far below levels that would initiate recommendations for restrictions of drinking water. As of 6 April, one restriction for infants related to I-131 (100 Bq/l) remains in place as a precautionary measure in only one village of the Fukushima prefecture.

On 6 April the IAEA monitoring team made measurements at 7 locations at distances of 23 to 39 km South and Southwest of the Fukushima nuclear power plant. The dose rates ranged from 0.04 to 2.2 microsievert per hour. At the same locations, results of beta-gamma contamination measurements ranged from 0.03 to 0.36 megabecquerel per square metre.

Since our written briefing of yesterday, data related to food contamination was reported on 5 April by the Japanese Ministry of Health, Labour and Welfare. These reported analytical results covered a total of 41 samples taken on 24 March (1 sample), 30 March (1 sample), 1 April (1 sample), 2 April (9 samples) and 4 April (29 samples). Analytical results for 40 of the 41 samples for various vegetables, spinach and other leafy vegetables, fruit (strawberries), chicken, poultry eggs, unprocessed raw milk and seafood in eight prefectures (Chiba, Fukushima, Gunma, Ibaraki, Kanagawa, Niigata, Saitama and Tokyo) indicated that iodine-131, caesium-134 and/or caesium-137 were either not detected or were below the regulation values set by the Japanese authorities. One sample of seafood (sand lance) taken on 4 April (offshore) in Ibaraki prefecture was above the regulation value set by the Japanese authorities for caesium-134/caesium-137.

TEPCO is responsible for near-shore sampling, taking samples of surface seawater. The near shore sampling point for Daiichi Units 1 - 4 is located 330 m south of their common discharge point. The near-shore sampling point for Daiichi Units 5 and 6 is located 30 m north of their common discharge point.

Samples near discharge areas are collected daily. Until 3 April a general decreasing trend was observed. However, after the discharge of contaminated water at 4 April, an increase from about 11 kBq/l at 09:00 UTC to 41 kBq/l at 14:00 UTC for I-131, and from 5.1 kBq/l at 09:00 UTC to 19 kBq/l for both, Cs-134 and Cs-137 at 14:00 UTC was detected.

There were no new data for off shore monitoring compared to yesterday's briefing.

3. IAEA Activities

The two agency experts in BWR technology in Japan are continuing their mission and will be joined by a third agency expert to have additional meetings with TEPCO at the end of the week. The marine expert from the IAEA Environment Laboratories Monaco, who joined the sampling campaign on the research vessel MARAI, returns to Vienna today.

Leakage of Highly Contaminated Water into Sea

According to Japanese authorities, the leak of highly contaminated water from the cable storage pit located next to the Unit 2 inlet point at Fukushima Daiichi nuclear power plant has stopped as of 20:38 UTC on 5 April.

Workers had employed measures to stop the flow of water directly to the sea since 2 April, when the leak was first observed.

On April 2, concrete was poured into the pit in an attempt to stop the water leakage to the ocean, but no significant decrease in leakage was observed.

From 4:47 UTC to 5:30 UTC on 3 April, the top of the trench was broken open and polymer was poured into the trench to stop the leakage of water, but this measure was not successful.

Approximately 13 kg of liquid tracer was injected into the pit at 22:08 UTC, 3 April. The tracer was also injected into two new bore holes that had been drilled near the pit.

At 4:15 UTC, 5 April it was confirmed that the tracer was seen leaking from the crack into the sea.

At 6:07 UTC, 5 April coagulation agents (liquid glass) were injected into the holes drilled around the pits. The leakage was reported to have ceased at 20:38 UTC on April 5. Work continues to prevent further releases to the sea.

The photographs below represent the status of leakage before and after:

2 April 2011:

5 April 2011, 20:38 UTC:

Related Resources

Last update: 12 November 2014