Fukushima Nuclear Accident Update Log

Updates of 24 March 2011

Chronology of Updates:
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Summary of Reactor Status

On Thursday, 24 March 2011, Graham Andrew, Special Adviser to the IAEA Director General on Scientific and Technical Affairs, provided the following information on the current status of nuclear safety in Japan.

1. Current Situation

As far as the reactors at the Fukushima Daiichi site are concerned, there is some good news to report from the last 24 hours, although the overall situation is still very serious.

With AC power connected, instrumentation continues to be recovered in Units 1, 2 and 4. Workers returned after being evacuated from Units 3 and 4 on March 23, following confirmation that black smoke emissions from Unit 3 had ceased.

Reactor pressure is increasing in Unit 1, pressure readings are unreliable in Unit 2, and stable in Unit 3 as water continues to be injected through their feed-water pipes. The temperature at the feed-water nozzle of the Reactor Pressure Vessel (RPV) is decreasing at Units 1 (243 °C) and 3 (about 185 °C), and stable at Unit 2 (about 102 °C).

Units 5 and 6 are still under cold shutdown, they are undergoing maintenance using off-site AC power and existing plant equipment.

Dose rates in the containment vessels and suppression chambers of Units 1 and 2 have decreased slightly.

2. Radiation Monitoring

The IAEA radiation monitoring team made additional measurements at distances from 21 to 73 km from the Fukushima nuclear power plant. At distances between 34 and 73 km, in a westerly direction from the site, the dose rate ranged from 0.6 to 6.9 microsievert per hour. At the same locations, results of beta-gamma contamination measurements ranged from 0.04 to 0.4 Megabecquerel per square metre.

At distances between 30 and 32 kilometers from the Fukushima Nuclear Power Plant, in a north westerly direction from the site, dose rates between 16 and 59 microsievert per hour were measured. At these locations, the results of beta-gamma contamination measurements ranged from 3.8 to 4.9 Megabecquerel per square metre. At a location of 21 km from the Fukushima site, where a dose rate of 115 microsieverts per hour was measured, the beta-gamma contamination level could not be determined.

The second IAEA monitoring team has started their work today in Fukushima and Tokyo. Measurements will be taken to determine more precisely the actual radionuclides that have been deposited.

On-site monitoring by the Japanese authorities at the Daiichi nuclear power plant produced new data on 23 March for radionuclide concentrations in the air from samples collected between 19 and 23 March. Of the six radionuclides monitored, only iodine-131 was found to be in excess of the limits set by Japan. Overall, the dose rates reported on the site have decreased from 1930 to 210 microsievert per hour between 21 to 23 March.

There continues to be considerable daily variation in the deposition of iodine-131 and caesium-137 reported in 10 Prefectures. Recent rainfall and the resulting wet deposition may help explain the increased deposition in Tokyo. As measured by the Japanese authorities for the Shinjuku district of Tokyo, iodine-131 deposition increased by 36 000 Becquerel per square metre from 22 to 23 March, and caesium-137 deposition by 340 Becquerel per square metre.

Monitoring of the marine environment by ships of the Japanese Ministry of Education, Culture, Sport, Science and Technology has now begun. Seawater and air samples were collected on 23 March in coastal waters, at distances of about 30 km off-shore. Dose rate measurements were also taken. Results from 24 March indicate surface seawater concentrations at eight locations ranging from 24.9 to 76.8 Becquerel per litre for iodine-131, and 11.2 to 24.1 Becquerel per litre for caesium-137. Radionuclide concentrations in dust in the air above the sea were also measured. The results are being assessed by experts from the IAEA's Environment Laboratories Monaco.

New data provided by the Japanese authorities has been made available concerning radionuclide concentrations in foodstuffs, milk and drinking water. Sampling has been most thorough and extensive in the Fukushima and Ibaraki Prefectures. Of the 11 varieties of vegetables sampled from 18 to 22 March iodine-131 and caesium-137 levels exceed limits set for food and drink ingestion. Permissible levels of iodine-131 and caesium-137 (one sample) were also exceeded in nearly all of the milk samples taken in Fukushima and Ibaraki Prefectures between 16 to 21 March. In addition, permissible levels of iodine-131 were exceeded in drinking water samples taken in the Fukushima and Ibaraki Prefectures and in Tokyo from 17 to 23 March. Further sampling and analysis will be carried out in the days ahead by the Japanese authorities. A joint FAO/IAEA mission to Japan will be undertaken to provide advice and assistance on sampling strategies, analysis and the interpretation of data collected by the Japanese authorities related to food contamination.

In summary, radioactivity in the environment, foodstuffs and water is moving more to the forefront, as some technical concerns related to the status of the reactors at the Fukushima Daiichi site appear to be slightly less acute in some respects. However, the overall situation on the Fukishima site remains very serious.



Japanese Seawater Samples Show Signs of Radioactive Materials

Japanese authorities today provided the IAEA with data on seawater samples they collected on 22 and 23 March, after detecting iodine and cesium in the water near the Fukushima Daiichi nuclear power plant. (See earlier update.)

A vessel from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) collected water samples at several points 30 kilometres from the coastline and found measurable concentrations of iodine-131 and cesium-137. The iodine concentrations were at or above Japanese regulatory limits, and the cesium levels were well below those limits.

The IAEA's Marine Environmental Laboratory in Monaco has received the data for review.



Japanese Workers Treated for Radiation Exposure

Japanese authorities today reported that three workers at the Fukushima Daiichi nuclear power plant were exposed to elevated levels of radiation. The three were working in the turbine building of reactor Unit 3 and have received a radiation dose in the range of 170-180 millisieverts.

Two of the workers have been hospitalized for treatment of severely contaminated feet, which may have suffered radiation burns. The workers had been working for about three hours in contact with contaminated water.

The IAEA is seeking additional information.



Summary of Reactor Status

The IAEA today released updated summary slides on reactor conditions at Fukushima Daiichi nuclear power plant.



Spent Fuel Pools at Fukushima Daiichi Nuclear Power Plant - Updated

Spent fuel removed from a nuclear reactor is highly radioactive and generates heat. This irradiated fuel needs to be stored for one to three years in pools that cool the fuel, shield the radioactivity, and keep the fuel in the proper position to avoid fission reactions. If the cooling is lost, the water can boil and fuel rods can be exposed to the air, possibly leading to severe damage and a large release of radioactive materials.

Nuclear power plants must replace fuel every one to two years, and the Fukushima Daiichi reactors typically remove about 25 percent of the reactor's fuel - to be replaced with fresh, or unirradiated, fuel - during each refuelling outage. The spent fuel, which is hottest immediately after it is removed from the reactor, is placed in the spent fuel pool until it is cool enough to be moved to longer-term storage.

The concern about the spent fuel pools at Fukushima Daiichi is that the capability to cool the pools has been compromised. See diagram below for location of the pool in each reactor building.

Number of Fuel Assemblies in Cooling Pools at Fukushima Daiichi
(Reported 17 March by Japan's Ministry of Economy, Trade and Industry)

Spent Fuel Pool


  Capacity Irradiated Fuel Assemblies Unirradiated Fuel Assemblies Most Recent Additions of Irradiated Fuel
Unit 1 900 292 100 March 2010
Unit 2 1240 587 28 September 2010
Unit 3 1220 514 52 June 2010
Unit 4 1590 1331 204 November 2010
Unit 5 1590 946 48 January 2011
Unit 6 1770 876 64 August 2010


Here is a summary of spent fuel conditions at Fukushima Daiichi nuclear power plant, based on documents and confirmed by Japanese officials (new information in bold):

Unit 1

Unit 1 experienced an explosion on 12 March that destroyed the outer shell of the building's upper floors. No precise information has been available on the status of the spent fuel pool.

Unit 2

Precise information on the status of the spent fuel pool was unavailable in the days following the earthquake, but Japan's Nuclear and Industrial Safety Agency began to release temperature data on 20 March:

20 March, 23:00 UTC: 49 °C
21 March, 05:25 UTC: 50 °C
21 March, 21:20 UTC: 51 °C
22 March, 02:20 UTC: 53 °C
22 March, 06:30 UTC: 50 °C
22 March, 19:20 UTC: 51 °C
23 March, 00:00 UTC: 51 °C
23 March, 06:00 UTC: 51 °C
23 March, 16:00 UTC: 52 °C
24 March, 00:00 UTC: 47 °C

Workers conducted an operation to spray 40 tonnes of seawater to the spent fuel pool on 20 March, and they added another 18 tonnes on 22 March.

Unit 3

Unit 3 experienced an explosion on 14 March that destroyed the outer shell of the building's upper floors. The blast may have damaged the primary containment vessel and the spent fuel pool. To address these concerns, authorities began spraying water into the building, first by helicopter on 17 March and then by fire trucks and other vehicles through 22 March. Starting 23 March, seawater was injected into the spent fuel using the cooling and purification line. By midday 24 March, 4-5 tonnes of seawater had been injected through this line.

Unit 4

This reactor was shut down 30 November 2010 for routine maintenance, and all the fuel assemblies were transferred from the reactor to the spent fuel pool, before the 11 March earthquake. The heat load in this pool is therefore larger than the others.

On 14 March, the building's upper floors were severely damaged, possibly causing a reduction of cooling capability in the spent fuel pool. Emergency workers began spraying water into the building on 20 March, and have continued daily since then. On 22 March, workers began using a concrete pump truck that can deliver water more effectively, placing 150 tonnes of water on 22 March and 130 tonnes on 23 March.

Units 5 and 6

Instrumentation at these reactors began to indicate rising temperatures at their spent fuel pools starting on 14 March. Three days later, Japanese technicians successfully started an emergency diesel generator at Unit 6, which they used to provide power to basic cooling and fresh-water replenishment systems. Workers created holes in the rooftops of both buildings to prevent any hydrogen accumulation, which is suspected of causing earlier explosions at Units 1 and 3.

A second diesel generator came online on 18 March, and the next day, the higher-capability Residual Heat Removal (RHR) system recovered full function. External power was restored to Units 5 and 6 on 22 March. Temperatures in the spent fuel pools of Units 5 and 6 have gradually returned to significantly lower temperatures, although the Unit 5 pool temperature increased somewhat on 23 March after pumps for the RHR system were stopped when the diesel generators were removed from service.

Common Use Spent Fuel Pool

In addition to pools in each of the plant's reactor buildings, there is another facility - the Common Use Spent Fuel Pool - where spent fuel is stored after cooling at least 18 months in the reactor buildings. This fuel is much cooler than the assemblies stored in the reactor buildings. Japanese authorities confirmed as of 18 March that fuel assemblies there were fully covered by water, and the temperature was 57 °C as of 20 March, 00:00 UTC. Workers sprayed water over the pool on 21 March for nearly five hours, and the temperature on 23 March was reported to be 57 °C.

Last update: 27 May 2015