Situation Report: Fukushima

March 16, 2011 | Jesse Jenkins,

Last update to post at March 29 at 7:00pm, Pacific time; please check timestamps for individual sections below to find out when information was last updated.

Live Updates via Twitter

Track live updates and breaking news relayed via Twitter below. Breakthrough Director of Energy and Climate Policy Jesse Jenkins has been covering the crisis in Japan since it began @JesseJenkins. See this "Nuclear Crisis" list for a curated feed of other sources of news on the nuclear crisis at the damaged Fukushima nuclear power station.





Note: The Twitter widgets have at times been unreliable and if the widgets above do not load properly, click on the links to the direct Twitter pages in the first paragraph above


Situation Report: the Fukushima Daiichi Nuclear Reactor Complex

The crisis has centered at the large Fukushima Daiichi nuclear power complex, home to six nuclear reactor units. [See "Potential Sources of Major Radiation Release" below. For an explanation of what let to this current situation, see "What Went Wrong"].

Fukushima_Daichi_Nuclear_plan.jpeg
Satellite image of the Fukushima Daiichi reactor complex before the earthquake. Shows all six reactors.

March_18_annotated_fukushima_damage.jpeg
Satellite image shows damage to Fukushima Daiichi complex as of March 18 local time (showing Units 1-4). Source: Digital Globe via ISIS.

Radiation Watch
See http://RadiationWatch.blogspot.com for a collection of publicly available radiation monitoring data from throughout Japan.

Making sense of radiation readings:
  • This table helps make sense of various radiation levels reported in the media, including four common units with which to report radiation exposure, and the biological damage that exposure incurs (click graphic to enlarge).

    RadiationInContext.jpg


    You can download the "Radiation Exposure Calculator," a spreadsheet tool, here, which allows you to convert easily between any of these four units and to interpret the likely impact on human health of radiation levels reported in the media.

    The U.S. Environmental Protection Agency has resources on radiation and public health here. For more 'fast facts' on radiation at Fukushima, see this Scientific American post.


Radiation levels inside Fukushima nuclear power complex
[Last updated: March 29 at 7:00pm Pacific Time, USA / March 30, 11:00am Fukushima local time, Japan]
  • Irradiated water has been found in trenches that run beneath the turbine buildings of each reactor unit at Fukushima Daiichi. The trenches carry cables and piping between the plants, and are outside the normal radiation containment areas of the reactor buildings. Radiation levels at the surface of the water in the trench at Unit 2 are the highest and were recorded at dangerous levels of over 1,000 mSv/hr as of 2:30am on March 27. At that level, workers in the area would receive the maximum dose allowed by the Japanese government of 250 mSv in just 15 minutes, and at 1,000 mSv (1 Sv) of received dosage, the beginnings of radiation sickness typically set in.

    Radiation levels in the water at the trench near Unit 1 are much lower, at 0.4 mSv/hr. Readings could not be gathered at Unit 3, where debris from the tsunami prevented a measurement. (Source: IAEA).

    Previously, radiation readings of 2.02 mSv/hr were recored at a monitor just north of the plant's service building at 3:00pm on March 21 (local time), and at 2.97 mSv/hr at 7:00pm on March 19.

    Radiation levels fall off precipitously as distance from the reactor buildings grows. At 6:30pm Fukushima local time on March 29, radiation levels at the main gate of Fukushima Daiichi Nuclear Power Station (about 3,280 feet from Unit 2 reactor building) were 0.18 mSv/hr, while radiation levels at the plant's west gate (approximately 3,610 feet from Unit 2) were 0.12 mSv/hr. (The Wall Street Journal now has good time series data displayed for the west gate at Fukushima Daiichi here)

    Dose limits to workers under emergency regulations have been raised to 250 mSv (up from 100 mSv previously) after which the workers may not return to the site. This compares to usual nuclear worker limits of 20 mSv per year.

    On March 28, TEPCO announced that plutonium 238, 239 and 240 were detected in the soil sampled on March 21st and 22nd at five spots inside the power station complex. Although the ratios of the particular isotopes present indicate the plutonium may originate from one of Fukushima Daiichi reactors themselves, indicating that very small amounts of plutonium may have been released at the site. However, the plutonium isotopes were found at the same concentration levels as are typically observed in Japanese soils due to the fallout from past atmospheric nuclear tests and poses no major impact on human health. (Source: IAEA)


Radiation levels in Japan inside of 20 kilometer evacuation zone around Fukushima plant:
[Last updated: March 29 at 7:00pm Pacific Time, USA / March 30, 11:00am Fukushima local time, Japan]
  • Radiation levels and long-term impact are likely to be greatest as vicinity to the plant increases, particularly within 20-30 kilometers from the Fukushima Daiichi plant. Long-term health impacts from the Chernobyl disaster, for example, were concentrated within the immediate vicinity of the plant (and Chernobyl released much more radiation than Fukushima is likely to do, even if events there escalate). However, there is limited data at this time on current radiation levels inside the 30 km evacuation zone set up by the Japanese government. We are working to find any data available and will update this site when we find it (please contact Jesse[at]theBreakthrough.org if you have information).


Radiation levels in Japan outside of 20 kilometer evacuation zone around Fukushima plant:
[Last updated: March 29 at 7:00pm Pacific Time, USA / March 30, 11:00am Fukushima local time, Japan]
  • Radiation levels reported by various monitoring sites at a distance of more than 20 kilometers from the Fukushima Daiichi plant are low and, according to the World Health Organization, do not pose a risk to public health. The following map displays recent radiation readings at monitoring sites collected by the Japanese government:

    Radiation Map Fukushima.png


    As you can see, most sites report values below 10 micro-Sieverts/hour, while the highest readings register at 73.5 micro-Sv/hr at a site about 20 km northwest of Fukushima Daiichi nuclear complex, and at 43.0 and 38.5 micro-Sv/hr at two sites about 30 km northwest of the plant (1,000 micro-Sv = 1 milli-sievert or mSv).

    For comparison, a person receives roughly 30 micro-Sv during a typical international plane flight, or about 100 to 200 micro-Sv when receiving a chest x-ray.

    For more, the Wall Street Journal now maintains a good interactive map and time series graphic showing radiation readings from various monitoring sites across Japan.

    Trace amounts of radioactive Iodine isotopes were detected on March 19 (Japan time) in tapwater in Tokyo and in Tochigi, Gunma, Niigata, Chiba and Saitama prefectures, each neighboring Fukushima. Trace amounts of Cesium have been detected in tapwater in Tochigi and Gunma prefectures, according to the Japanese Ministry of Education, Culture, Sports, Science and Technology. The ministry cautions however that Iodine and Cesium levels detected do not affect human health even if they are ingested.


Radiation levels reported in food products from Japan:
[Last updated: March 19 at 8:50am Pacific Time, USA / March 20, 12:50am Fukushima local time, Japan]
  • The presence of radioactive isotopes of Iodine at levels exceeding normal limits set by the Japanese government have been detected in samples of milk and spinach collected in the region of the Fukushima Daiichi nuclear complex, according to a statement from Japanese chief cabinet secretary Yukiyo Edano issued late in the evening on March 19, local Fukushima time. However, the levels measured are said to pose no immediate threat to health.

    According to Edano, the milk samples had been collected within Fukushima Prefecture at a location more than 30 kilometres from the damaged Fukushima Daiichi plant, while six samples of spinach showing elevated radiation levels were collected in Ibaraki Prefecture, neighbouring to the south of Fukushima Prefecture. Ibaraki Prefecture begins more than 65 kilometres south of the Fukushima Daiichi plant.

    Health risks from the radiated food are minor however. If a person were to consume the radiated milk for an entire year, for example, the radiation dose would be equivalent to receiving a single CT scan, Edano said. The levels found in the spinach were much lower, and if consumed for an entire year, would result in radiation exposure the equivalent of one-fifth of a single CT scan.

    The Ministry of Health, Labour and Welfare, as well as the prefectural governments, will continue to monitor and analyse food samples and determine the exact location where contaminated samples are found, Edano said. The Japanese government will consider taking necessary actions, depending on radiation levels found, potentially including a ban on the shipment of foodstuffs or setting limits to the intake of such foods.

    Residents leaving the 20 kilometre evacuation area around the plant have been instructed to ingest stable (non-radioactive) iodine pills (or syrup for children) to limit potential uptake of radioactive iodine that may be present in any food or water from the area around the plant.

    Radioactive Iodine has a short half-life (about 8 days) and decays quickly from the environment, but does pose a short-term risk if ingested or inhaled in significant quantities. Iodine iostopes build up in and cause damage to the thyroid, particularly in children. Ingestion of stable Iodine in Potassium Iodine pills or syrups prevents the uptake by the thyroid of radioactive Iodine isotopes.

    To date, no other radioactive isotopes have been shown to increase in the analysis of food products around Fukushima. (Sources: World Nuclear News and IAEA)


Radiation levels in United States (and Canada):
[Last updated: March 19 at 8:10am Pacific Time, USA / March 20, 12:10am Fukushima local time, Japan]
  • "The United States Government has an extensive network of radiation monitors around the country and no radiation levels of concern have been detected," according to a joint statement issued by the U.S. Environmental Protection Agency and the Department of Energy on March 18th.

    Minute levels of radiation from the Fukushima plant are drifting westward over the Pacific Ocean towards the western coast of the United States. Radiation has reportedly been detected in Southern California as of mid-day, 12:00pm on March 18th (Pacific time), and the EPA and DOE report that trace quantities of the radioactive isotope Xenon-133 were detected by monitoring stations in Sacramento, CA and in Washington state. However, according to the federal agencies, the noble gas is present "in a dose rate approximately one-millionth of the dose rate that a person normally receives from rocks, bricks, the sun and other natural background sources."

    The Union of Concerned Scientists warns that pottasium iodine pills are needed most in Japan and U.S. residents should refrain from purchasing to limit impact on supplies.



Status Report: Unit-by-Unit
[Last updated: March 25 at 1:00pm Pacific Time, USA / March 26, 5:00am Fukushima local time, Japan; Sources: JAIF, TEPCO, or IAEA, unless otherwise noted; All dates and times below reported as local Fukushima time unless otherwise noted.]

BWR_schematic.jpeg
Schematic of GE Mark 1 Boiling Water Reactor, as at Fukushima Daiichi, showing reactor core, containment, spent fuel pool, and outer building. Source: Union of Concerned Scientists. Here is a very detailed schematic of the boiling water reactor design employed at Fukushima Daiichi

VIDEO: You can view video from a helicopter flight over the damaged reactor complex taken on March 17th here.

Reactor Unit 1:
  • Structural integrity and systems status:
    • Core and fuel integrity: damaged;
    • Loaded fuel assemblies in core: 400;
    • Pressure vessel integrity: unknown;
    • Containment integrity: not damaged;
    • Building integrity: severely damaged by hydrogen explosion on March 12;
    • Primary coolant system: not functioning;
    • Level of irradiated water pool found in turbine building: 40 cm (as of March 25);
  • Core and fuel status:
    • Water level in core: fuel exposed partially or in full;
    • Pressure and temperature in reactor pressure vessel: Remains high but stabilized, falling slightly now after brief rise earlier;
    • Pressure in containment vessel: stable, falling slightly now after brief rise earlier.
    • Temperature inside pressure vessel at feed-water injection nozzle: 243 C and decreasing as of March 25;
  • Emergency operations:
    • Sea water injected into core via feed water system continuously from 2:30 am on March 23rd to 3:37 pm on March 25th, when TEPCO switched to injecting fresh water into core to try to prevent build-up of salt crystals on fuel assemblies that could interfere with optimal coolant circulation. This follows earlier injections of sea water beginning 8:20pm on March 12 and continuing intermittently afterwards.
    • Venting of steam and pressure halted for now.
    • Power cable connected to distribute switchboard for Units 1 & 2 on March 20 (Fukushima time). Power and lights restored to Unit 1 control room as of 11:30am on March 24th. Workers are now checking the availability and operation of normal core cooling systems.
  • Used Fuel Pool at Unit 1:
    • Water level and temperatures: unreported;
    • Structural integrity of fuel pool: possible damage as hydrogen explosion on March 12 damaged the upper portion of Unit 1 outer containment building housing spent fuel pool, but no reports as to condition.
    • Fuel assemblies stored in spent fuel pool: 292
    • Integrity of used fuel assemblies: unknown;
    • Estimated rate of heat build-up in spent fuel pool from radioactive decay of spent fuel pools: 60,000 kilocalories per hour.
    • Emergency operations: Water injection into fuel pool to be considered. If power restored to coolant system (see above), it will restore normal coolant circulation to fuel pool as well (if system operational). No reports of ongoing emergency operations at #1 fuel pool at this time however.


Reactor Unit 2:
  • Structural integrity and systems status:
    • Core and fuel integrity: damaged;
    • Loaded fuel assemblies in core: 548;
    • Pressure vessel integrity: unknown;
    • Containment integrity: damage suspected. Likely damage to suppression chamber/torus after explosion heard March 15. Atmospheric pressure dropped from three times normal to normal in an instant after the explosion, and radiation levels recorded at the plant peaked temporarily into dangerous levels.
    • Building integrity: slightly damaged by explosion on March 14 at nearby Unit 3;
    • Primary coolant system: not functioning;
    • Level of irradiated water pool found in turbine building: 1 meter (as of March 25);
  • Core and fuel status:
    • Water level in core: fuel exposed partially or in full (was exposed in full for at least a period of time before March 14);
    • Pressure in reactor pressure vessel: unknown;
    • Pressure in containment vessel: stable.
    • Temperature inside pressure vessel at feed-water injection nozzle: 102 C and stable as of March 25;
    • Other: Steam or smoke seen rising from crack in roof of Unit 2 outer building beginning around 6:20pm local time on March 21 from southeast corner above used fuel pool; As of 7:11am on March 22, smoke had "decreased to a level where we can hardly confirm," according to TEPCO. TEPCO said they do not believe it was from the reactor's core of spent fuel pool itself, but source of steam or smoke unconfirmed.
  • Emergency operations:
    • Injection of sea water into core has been continuous since March 20. TEPCO plans to switch to injecting fresh water on March 26.
    • Venting of steam and pressure halted for now.
    • Power cable connected to distribute switchboard for Units 1 & 2 on March 20 (Fukushima time). As of March 25, engineers were working to recover lighting and instrumentation in the main control room, as well as the main core cooling systems.
  • Used Fuel Pool at Unit 2:
    • Water level: unconfirmed, but TEPCO says water levels were presumed to be high enough to keep fuel rods fully covered through March 20 (local time);
    • Temperature: reported as 49 C on March 21, rising slightly to peak of 52 C at 1:00am on March 24 before falling to 47 C at 9:00am March 24.
    • Structural integrity of fuel pool: unconfirmed;
    • Fuel assemblies stored in spent fuel pool: 587
    • Integrity of used fuel assemblies: unknown;
    • Estimated rate of heat build-up in spent fuel pool from radioactive decay of spent fuel pools: 400,000 kilocalories per hour.
    • Emergency operations: Seawater injection began on March 20 via a water tank connected via hose to existing existing pool water circulation system. 40 tons of water had been added to the pool on March 20, estimated to have raised water levels 30 centimeters, and an additional 18 tons added on March 22. At 10:30am on March 25, TEPCO began injecting sea water directly into pool using the pool cleaning and filtering systems and finished pumping operation later that day. If power restored to coolant system (see above), it will restore normal coolant circulation to fuel pool as well (if system operational).


Reactor Unit 3:
  • Structural integrity and systems status:
    • Core and fuel integrity: damaged;
    • Loaded fuel assemblies in core: 548;
    • Pressure vessel integrity: unknown;
    • Containment integrity: damage possible but now reported by Japan Atomic Industrial Forum as "not damaged";
    • Building integrity: severely damaged by hydrogen explosion on March 14;
    • Primary coolant system: not functioning;
    • Level of irradiated water pool found in turbine building: 1.5 cm (as of March 25);
  • Core and fuel status:
    • Water level in core: fuel exposed partially or in full;
    • Pressure in reactor pressure vessel: unkown;
    • Pressure in containment vessel: Stable after temporary increase on March 20;
    • Temperature inside pressure vessel at feed-water injection nozzle: 185 C and decreasing as of March 25;
    • Other: At 8:30am on March 16, fog like steam was confirmed arising from the
      reactor building. Steam or smoke seen rising from Unit 3 reactor building from roughly 3:55pm on March 21 from southeast corner above used fuel pool; by March 22nd, color of smoke changed to "somewhat white" according to TEPCO, and was dissipating. At around 4:20 pm on March 23, TEPCO staff confirmed light black smoke again "belching" from the Unit 3 building. At approximately 11:30 pm on March 23 and 4:50 am on March 24, visual inspections by TEPCO employees found no signs of smoke. The source of the steam or smoke has not been confirmed.
  • Emergency operations:
    • Injection of sea water into core continuing through March 25, when injection operations switched to fresh water to prevent build-up of salt crystals on fuel assemblies that could impact optimal coolant circulation.
    • Venting of steam and pressure halted for now.
    • Power connection restored to Unit 3 and 4 distribution box. Lighting restored in Unit 3 control room at 10:45pm on March 22. Work was underway to recover control room instruments and core cooling systems, but had to be suspended at least temporarily when three workers installing cabling were exposed to elevated levels of radiation while standing in irradiated water that has pooled inside the turbine building. The workers received doses of 170-180 millisieverts and seem to have suffered shallow burns to their skin from beta radiation; they are now being treated at a medical facility.
  • Used Fuel Pool at Unit 3:
    • Water level: low.
    • Temperature: unreported but assumed to be rising.
    • Structural integrity of fuel pool: possible damage as hydrogen explosion on March 14 damaged the upper portion of Unit 3 outer containment building housing spent fuel pool, but no reports as to condition.
    • Fuel assemblies stored in spent fuel pool: 514
    • Integrity of used fuel assemblies: damage possible;
    • Estimated rate of heat build-up in spent fuel pool from radioactive decay of spent fuel pools: 200,000 kilocalories per hour.
    • Emergency operations: According to TEPCO, spraying of water into spent fuel pool at Unit 3 continued on March 25 from 1:28pm to 4:00pm by fire trucks operated by Kawasaki City Fire Department. IAEA also reports that around 120 tonnes of sea water was directly injected in the spent fuel pool via the cooling and purification line on March 24 with pumping operations beginning on March 23; [editorial note it is unclear why less effective spraying operations would continue if direct injection possible through pond cooling system, so unclear if pumping system stopped working]. Previously, after unsuccessful attempts to dump water via helicopter on March 17, mobile pumping trucks worked during each day from March 17 onwards. If power restored to coolant system (see above), it will restore coolant to fuel pool as well (if system operational).


Reactor Unit 4:
  • Reactor status:
    • Shut for periodic inspection at time of quake. All fuel assemblies were removed from reactor core in November and transferred to used fuel pool.
    • Core condition: stable.
    • Building integrity: An explosion, thought to be due to hydrogen buildup at the area around the used fuel pool (see below) has severely damaged the outer containment building and two fires burned in upper floors of building near spent fuel pools on March 15 and 16 (local time). Only frame of outer building roof remains.
    • Level of irradiated water pool found in turbine building: 80 cm (as of March 25);
  • Used Fuel Pool at Unit 4:
    • Water level: low.
    • Temperature: IAEA measurements showed rise to 84 C on March 14 and 15, then measurements reported "no data" since March 16. Assumed to be rising.
    • Structural integrity of fuel pool: possible damage as hydrogen explosion on March 15 damaged the upper portion of Unit 2 outer containment building housing spent fuel pool, but no reports as to condition.
    • Fuel assemblies stored in spent fuel pool: 1,331
    • Integrity of used fuel assemblies: damage possible;
    • Estimated rate of heat build-up in spent fuel pool from radioactive decay of spent fuel pools: 1,600,000 kilocalories per hour.
    • Emergency operations: As of March 25, TEPCO began injecting water directly into fuel pool using fuel pool cooling and filtering system. Previously, Japanese Self Defence Forces began spraying water into the building on March 20 using mobile pumping trucks. This followed two attempts to dump water via helicopter on March 17 that were considered largely unsuccessful. As of March 21, workers had attempted to add 90 tons of fresh water into the Unit 4 pool, according to the Japanese Nuclear and Industrial Safety Agency (NISA). Spraying operations halted temporarily on March 21 after smoke seen rising from nearby Unit 3 around 3:55pm (see above). Pumping operations resumed March 22 with addition of a tall pumping tower normally used to spray concrete into construction sites, which has allowed for more precise targeting of the water. As of March 20, TEPCO was working to restore power to distribution switchboard for Units 3 and 4 "in a few days time." If power is restored to coolant system, it will restore coolant to fuel pool as well (if system operational).


Reactor Unit 5 & 6:
  • Reactor status:
    • Shut for periodic inspection at time of quake. 548 fuel assemblies in Unit 5 core and 764 in Unit 6 core, but undamaged.
    • Core condition: considered stable w/coolant systems restored. IAEA reports that "both reactors achieved cold shutdown on 20 March. The reactors are now in a safe mode, with cooling systems stable and under control, and with low temperature and pressure within the reactor."
    • Core, containment and building integrity: undamaged
  • Used Fuel Pool at Unit 5 & 6:
    • Condition: considered stable w/coolant systems restored.
    • Fuel assemblies stored in spent fuel pool: 946 in Unit 5 pool and 876 in Unit 6 pool.
    • Integrity of used fuel assemblies: undamaged;
    • Estimated rate of heat build-up in spent fuel pool from radioactive decay of spent fuel pools: 700,000 kilocalories per hour in Unit 5 and 600,000 kilocalories per hour in Unit 6.
    • According to IAEA: "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 generator came online on 18 March, and the next day, the higher-capability Residual Heat Removal system recovered full function," after which temperatures in the spent fuel pools of Units 5 and 6 fell from a high of 67 degrees C on March 18 to had gradually returnedbelow 35 degrees C by March 19th local time.


Shared Central Spent Fuel Storage
  • The Fukushima Daiichi site has a central spent fuel storage pool and dry cask storage site, for used fuel that has cooled sufficiently to be moved from the individual spent fuel pools in each reactor building. 60 percent of the spent fuel from the Fukushima Daiichi complex is stored in shared central storage, according to TEPCO (some 6375 fuel assemblies). After cooling further in the shared central spent fuel pool, fuel rods are moved to dry cask storage, which is more secure.
  • Shared fuel pool condition: TEPCO has released conclusions from visual inspection of pool on March 19 local time. Reports water levels are 'secured' and temperatures are 55 C. Detailed inspection being prepared.
  • Shared fuel pool coolant systems and temperature: External power supply restored as of March 24 at 15:37pm and cooling started again 28 minutes later. Work is now under way for the recovery of the lighting and instrumentation systems. As of March 24, 6:40pm, the water temperature of the pool was around 73 C and falling.
  • Dry cask storage: TEPCO has released conclusions from visual inspection of dry cask storage on March 19 local time. Reports 'no signs of abnormal situation.' Detailed inspection being prepared.
  • Source: World Nuclear News


Potential Sources of Major Radiation Release

Both the reactor units themselves and the spent fuel pools are a source of concern, as each could be a major source of radioactive material release, if they meltdown or catch fire. Both must be continuously bathed in water to be kept cool, although the recently-active fuel in the reactor cores generates much more heat than the used fuel in the pools, and so must receive more water to keep cool.

BWR_illustration.jpeg
The reactors: The active fuel rods loaded into the reactor core are cased in multiple layers of defense against release of nuclear materials. This "defense-in-depth" strategy means that the fuel rods themselves, the 6-8 inch thick pressure vessel surrounding the reactor core, as well as the reinforced concrete containment vessel surrounding all of that. A major release of radiation from the reactor core is thus only possible if both the pressure vessel and the containment vessel are breached.

There are concerns however that the GE Mark I containment vessel used at the Fukushima reactor units is not as strong as other designs and may be more likely to crack or be damaged in the case of a build-up of heat and pressure if coolant is lost to the core.

The used or spent fuel rods from the nuclear reactors are stored on site until they cool sufficiently in a used fuel pool. This is the second source of concern at Fukushima. The spent fuel rods must be kept bathed in water or they may heat up precipitously and potentially burn or melt down. This interactive graphic from the New York Times succinctly explains the danger posed by the used fuel pools.

Each fuel assembly consists of about 60 fuel rods and has a mass of about 170 kg.
The fuel assemblies are about 4 meters long and sit on racks in the pool, slightly off the pool's floor, with water levels typically kept 7 to 8 meters above the top of the fuel assemblies, according to the Union of Concerned Scientists.

The following graphic displays an estimate of the amount of fuel in both the spent fuel pools and in each reactor unit at Fukushima, based on media reports (source: Union of Concerned Scientists).

View image


The estimated rate of heat generation at the fuel pool for Unit 4 shown above clearly illustrates why this fuel pool is one of the greatest sites of concern at the Fukushima complex, although authorities consider the fuel pool at Unit 3 to also be of concern.

As can be seen from the table above, the majority of the fuel assemblies stored on site are kept in a shared central spent fuel storage pool, and several hundred are currently in dry cask storage. These are not currently a concern.

[See Situation Report above for current status of each source of potential major radiation release]


What Went Wrong? An Explanation of the Crisis

The Fukushima "Daiichi" (which means "first" in Japanese) complex has six separate boiling water nuclear reactor units built between 1971 and 1979; (a second Fukushima "Daini" nuclear power complex located several miles north has an additional four reactor units).

At the time of the massive 9.0 earthquake that hit Japan Friday, March 11th, Fukushima Daiichi reactors 1-3 were online, while units 4-6 were offline for scheduled maintenance. The earthquake triggered the automatic shutdown of all three operational units. But shortly after the quake, a major tsunami struck the site, located right on the shore along the coastal plane of Fukushima prefecture.

The tsunami flooded and destroyed the backup power generators that were providing power to the coolant systems at the reactor complex. Both the fuel in the reactor cores and the used fuel in cooling ponds near each reactor unit continue to generate heat due to radioactive decay and must be kept cool to avoid a surge in temperature that can generate steam, explosive hydrogen gas, and -- if temperatures rise sufficiently -- damage and melt the fuel rods themselves.

Without grid power or backup power to run the coolant systems at the Fukushima site, the coolant water bathing the nuclear fuel began to heat up and eventually boil off. This built up pressure in the cores which had to be released -- venting small amounts of radioactive iodine and cesium along with it. As the water boiled off, it also had to be replaced to avoid exposing the fuel rods to the air, which could lead to rapid build-up of heat.

Workers have battled continuously at the site since Friday, to keep each of the reactor units and the used fuel pools continuously covered in water, using emergency pumping systems powered by mobile generators that were hastily brought to the site to restore coolant circulation. Eventually, these efforts failed in turn at reactors 1, 2 and 3. Water levels dropped in the reactor core and heat and pressure built, escalating the challenging situation. [See Situation Report above]

World Nuclear News has a good explanation of the engineering challenges faced by workers at both Fukushima Daiichi and Daini complexes following the quake and tsunami here.

David Biello, associate editor of Scientific American, has written an excellent day-by-day chronology of events at Fukushima from March 11-20th, which we recommend.

The following timeline summarizing events at Japan's nuclear power stations impacted by the earthquake, including Fukushima Daiichi, was created by the Japan Atomic Industrial Forum and has been updated periodically [note: we'll do our best to keep most recent update posted here, but also check the english language JAIF site here for latest updates]:

Fukushima_Timeline_page1.jpg
Fukushima_Timeline_page2.jpg


More news and information to come soon...