Published: March 7th, 2012 at 8:00 am ET
Blue Ribbon Commission on America’s Nuclear Future (BRC)
Date: May 10, 2011
Memorandum for: Commissioners
From: BRC Staff
Subject: Overview of the Accident at the Fukushima-Daiichi Nuclear Complex
The Unit 1 isolation condenser boiled dry at 4:36 pm on March 11, leading to the loss of ability to cool the reactor. Subsequently the Unit 3 RCIC failed at 5:10a on March 13, and the Unit 2 RCIC at 1:25p on March 14.
With the loss of the backup cooling systems, the water in the reactors heated up and ultimately boiled off in amounts sufficient to uncover at least part of the fuel in the reactor cores in Units 1, 2 and 3. This caused the pressure inside the reactor’s steel primary containment vessel to build up from the generation of steam and hydrogen gas. To control the pressure in the primary containments, operators were required to vent some gases. For an unknown reason, flammable gases from the venting, which should have been directed out through the plant stacks, instead accumulated in reactor buildings and subsequently exploded, damaging the reactor buildings at Units 1 and 3. Unit 2 also suffered a hydrogen explosion (“deflagration”) in its primary containment building, but the extent of the damage to its secondary containment was much less than at the other three units.
Unit 4, which was shutdown and defueled at the time of the tsunami, also suffered an explosion that severely damages its reactor building. The flammable gas that exploded in Unit 4 came from an unknown source. It may have been generated by fuel in the storage pool which might have become uncovered, but this has not been confirmed.
Ultimately, the damage to fuel in the Unit 1, 2 and 3 reactors was stopped by actions to connect portable pumps (fire trucks) to inject seawater into the reactor vessels to provide cooling. The action to initiate seawater injection took 27 hours to complete after Unit 1 lost backup cooling, and approximately 7 hours after Unit 2 and Unit 3 lost cooling. There is still considerable uncertainty regarding the precise cause and progress of events leading to explosions and radionuclide releases, in part because instrumentation was not available to measure key plant safety parameters such as water inventories in spent fuel pools and reactor and turbine building sumps.
As of late April 2011, the status of the reactors at the Fukushima Daiichi station is believed to be the following:
The reactor cores in Units 1, 2, and 3 have been severely damaged. Tokyo Electric Power Company (TEPCO) estimates that the cores have sustained damage (i.e., melting of the fuel elements) ranging from 55 percent (Unit 1) to 35 and 30 percent for Units 2 and 3. For purposes of comparison, the reactor meltdown at Three Mile Island was between 50 and 75 percent.
The cores also have extensive buildup of salt as the result of emergency injection of seawater into them in an effort to cool the reactors. Fresh water is now being used to cool the reactors, so no new salt is being added. Nitrogen is also being injected to reduce the potential for additional explosions. The initial addition of salt water is expected to accelerate corrosion processes, and will be one factor complicating long-term management of the accident.
High radiation levels have been detected in the containment buildings and around the reactor site. It is believed one or more of the building explosions ejected radioactive material around the site. Radiation readings in the buildings and around the site have been trending downward but are still hampering efforts to stabilize the site and begin mitigation efforts.
At Unit 4, as noted earlier, the reactor core had been unloaded about three months before the earthquake and tsunami occurred. The cooling system of the spent fuel pool at Unit 4 ceased to function, and makeup water needed to be added to the pool using a truck normally used for pumping concrete. Water continues to be added periodically to all the units’ spent fuel pools, most extensively at the Unit 4 pool.5 Recent video taken inside the Unit 4 pool shows that the fuel is in intact and largely or completely undamaged condition (Figure 1).
Units 5 and 6 at Fukushima Daiichi were shut down at the time and spent fuel pool cooling is functional at both units. These units are believed to be relatively undamaged.
There is separate central shared spent fuel pool at the plant site that contains 6,375 spent fuel assemblies, some 60 percent of the spent fuel on-site. That facility appears to have suffered little if any significant damage.
The plant also has nine dry storage casks containing 408 older spent fuel assemblies; initial reports indicate no significant damage has occurred to these casks.
The building housing the casks was flooded during the tsunami.
A great deal of attention has been paid to the spent fuel stored at the Fukushima Daiichi plant and the current state of that fuel. The following table shows the location, types (new or spent), and numbers of fuel assemblies, in the Fukushima Daiichi reactors, spent fuel pools, shared pool, and dry storage facility.
Please note that we have seen discrepancies in reports of the plant’s actual fuel storage inventories and capacities, so the numbers provided below must be considered preliminary at this time.
The reactor spent fuel pools have high density racking are relatively full, with 5,042 assemblies out of a total capacity of 8,310 assemblies.
The freshly offloaded fuel in the Unit 4 pool was apparently closely packed, rather than being distributed in a “checkerboard” pattern intermingled with older fuel as is required in the United States. The central storage facility at the site, which seems to have escaped serious damage, is nearly full—it only has additional space for some 465 assemblies, only a fraction of the assemblies in the reactor pools that will need to be removed. 408 assemblies are stored in dry casks.
The entire core of Unit 4—548 assemblies—had been unloaded into its spent fuel pool for reactor maintenance about three months before March 11th. There is no mixed oxide (MOX) fuel in any pools, although some had been loaded in the Unit 3 core. The pools’ inventory may have been relatively high in part because TEPCO had planned to ship SNF from this and other reactor sites to the Recyclable Fuel Storage Center in Mutsu beginning in 2012.13
The accident at Fukushima has already released far more radioactivity than was released at Three Mile Island (and an estimated ten percent of what was released during the Chernobyl accident), but the amounts and effects are still being assessed. Not surprisingly, workers at the site received the greatest radiation exposures, and will continue to be exposed as cleanup efforts continue. 22 workers have received doses over 100 milliseiverts (10 rem or 10,000 millirem), but to date none have reportedly reached 250 milliseiverts (25 rem, or 250,000 milllirem), the limit recently set for emergency workers.
An acute lethal dose is on the order of 5000 to 10000 miliseiverts. Three workers were killed at the site as a result of the earthquake and tsunami (not by radioactivity).
Following the hydrogen explosions, radioactive cesium and iodine were detected in the vicinity of the plant, a clear indication of fuel damage. These had been released via venting of gases from the reactors, including an apparent rupture of the suppression chamber at Unit 2.
The population within a 20-kilometer (12.4 mile) radius of the plant has been evacuated; this evacuation was made mandatory on April 21st. Residents in the 20-30 kilometer 12.4-18.6 mile) radius have been told to prepare to shelter in place or evacuate, depending on developments.15 Food grown in the region was banned from sale soon after the event, although restrictions on some products have since been lifted following extensive sampling. Rice in the evacuation and evacuation preparation zones will not be cultivated in 2011.16 France’s nuclear safety agency IRSN estimates the maximum external doses to people living around the plant are unlikely to exceed 30 milliseiverts (3 rem, or 3,000 millirem) in the first year. This is based on airborne measurements taken to date, and has not been confirmed by any other agency. Natural background levels between 2-3 milliseiverts (0.2-3 rem, or 200-300 millirem) would normally be expected in the region.
Gamma radiation measurements onsite close to the reactors decreased greatly when the Unit 3 fuel pool was replenished with water on March 19th. Some buildings continue to have very high radiation readings inside, which have been measured remotely using robots. TEPCO is continuing to remove radioactive rubble from the plant site using remotely-controlled equipment, and these efforts are reducing radiation levels at the plant.
Published: March 7th, 2012 at 8:00 am ET