The World Nuclear Industry Status Report 2013
[link to www.worldnuclearreport.org]

snip



Seismometer signals tripped the Unit 1–3 reactors on 11 March 2011 at 14:47 JST. All of the five powerlines that had been transmitting electricity to and from Fukushima Daiichi were destroyed by the earthquake, which left the station without external power supply. Emergency diesel generators, two each for all of the reactor-turbine units, automatically started. Less than an hour later, the units were overrun by water by a tsunami wave exceeding 13 meters and all the diesel generators were flooded. [262]

All of the diesel generators, circuit boards, and DC [263] batteries stored in the basement of the turbine building went underwater. The doors of the turbine buildings were at 10 m above sea level, but the tsunami rose higher. The total loss of electric power, or station blackout (SBO), resulted in successive reactor core meltdowns and melt-through events. Large amounts of hydrogen gas were generated inside the pressure vessel as hot steam reacted with the zirconium alloy of the fuel cladding. This hydrogen exploded later, but how the gas moved out, and exactly where it deflagrated, remains unexplained.


Current Status of Fukushima Daiichi 1–4

After six different professional investigations (see hereunder), the real story of why and how the destruction of each reactor unit came about is still in the mist. This section reviews the current conditions and the specific problems of each reactor units.

Unit-1

Meltdown in the Unit-1 reactor proceeded much sooner than the other units. The fuel mostly melted through (thus the temperature at the pressure vessel bottom is now lower than other units; see Table 1) and dropped down onto the secondary container floor. The molten fuel is probably eroding the thick concrete base at the bottom of the container, although the depth of the erosion is hard to estimate. The quick meltdown and melt-through seem to have been accelerated by the operators’ failure to keep the isolation condenser running properly, and by a loss of coolant accident (LOCA) caused by the earthquake shocks. [273]

The radiation environment at Unit-1 is not so severe as at Unit-2, but the dose rate inside the Unit-1 container is more than twice as high as that of Unit-3 (see Table 5).

The Unit-1 building is currently covered with polyester sheets to minimize radioactive releases into the atmosphere. However, TEPCO plans to remove the cover early next year so that the debris scattered around on the top floor (Level-5) of the building can be removed and a crane to retrieve irradiated fuel from the spent fuel pool can be installed above the building. [274] The company says a new, larger sheet will be installed to cover both the building and the crane in 2017. In the meantime, radioactive releases will inevitably increase.

Unit-2

Unlike the other three units, the building of Unit-2 was not severely damaged by explosions, but most investigators agree that the radioactive release from this reactor was the worst (the largest in Bq). This was because the blow-out panel in the wall of the Unit-2 collapsed from the force of the Unit-1 explosion, leaving a large opening in the Unit-2 building. The good news was that explosive hydrogen escaped from the hole; the bad news was that radioactive gas found an easy way out as well.

The Reactor Core Isolation Cooling (RCIC), which had activated just after the main shock on 11 March 2011, ceased to function on 14 March and meltdown started in the Unit-2 reactor. Secondary containment pressure went as high as 750 kPa, but venting failed. On 15 March, the pressure was rapidly lost after the operators/workers heard an explosive sound somewhere around the torus (suppression chamber in the bottom part of the container).

The location of the explosion, its cause and nature—hydrogen or nitrogen explosion and where the gas came from—and the degrees of the damage in the torus and/or pipings are all unknown, and will remain so for some time given the high radiation environment (maximum of 72.9 Sv/h as monitored in March 2013) in and near the container vessel impeding even a cursory human inspection. What is known is that there is a large leak (supposedly a substantial rupture) somewhere and the cooling water poured into the reactor vessel is constantly escaping.

The reactor temperature (39ºC at the bottom of the pressure vessel) is relatively and constantly higher than at the other reactor units, indicating that the cooling is less effective in this reactor. It probably reflects the condition of meltdown. The hot molten fuel may have gone deeper into the concrete floor of the container vessel, and pouring in water has a limited cooling effect. If that’s the case, it means that retrieving the fuel would be more difficult and would take longer. TEPCO’s decommissioning roadmap shows that the company is planning on more time in the case of Unit-2 (2017–2023 for spent fuel pool evacuation and 2020–2024 to begin with fuel debris recovery from the reactor containment) than in the case of Unit-1 (2017 and 2020–22, respectively, for the same operations). [275]

Unit-3

Venting was carried out twice in Unit-3, but it could not prevent the explosion, which took place outside the containment seven hours after the second venting. As to the Unit-3 explosion, independent nuclear expert Arnie Gundersen claims a possibility of small nuclear detonation due to moderated prompt criticality in the spent fuel pool, probably due to distortion of fuel assembly racks in the pool caused by the impact of the hydrogen explosion that occurred close to the spent fuel pool. [276] So far, most other professional engineers remain doubtful or reject this scenario outright. However, findings of fragments of fuel material outside the Unit-3 building and the recent retrieval of extremely contaminated debris (540 mSv/h recorded at the surface) in the Level 5 (spent fuel pool floor) of the Unit-3 building [277] reflect the fact that the Unit-3 explosion was much more intense than that of Unit-1. The detonation theory will be ultimately challenged and resolved when the fuel assembly racks and the fuel rods are physically inspected years from now.

The temperature at the bottom of the Unit-3 reactor pressure vessel was around 36ºC in December 2012. It went down to 25ºC in late March 2013, but again it is around 37ºC in early June 2013. The fluctuation is much larger than at Unit-1 (low twenties) and Unit-2 (upper thirties). TEPCO plans to start retrieving fuel debris from the Unit-3 containment in 2021, one year later than Units-1 and -2, while the fuel retrieval from the Unit-3 spent fuel pool is planned for 2015, two years sooner than from pools at Units-1 and -2. [278] As the explosion of Unit-3 was rather large, the conditions of debris inside and outside the Unit-3 building are worse than in other units. This makes decommissioning work particularly hazardous in Unit-3.

Unit-4

At the time of the earthquake and tsunamiUnit-4 was not operational and was undergoing a periodic inspection. All fuel had been taken out of the reactor vessel and moved to the spent fuel pool, where older spent fuel assemblies were also stored. At approximately 6:10 am, 15 March 2011, the Unit-4 building exploded, blowing the upper-floor walls and the ceilings away, leaving the spent fuel pool filled with 1,535 assemblies (1,331 irradiated ones and 204 fresh ones) [279] in the open air without containment, though debris fell into the pond. The cause and mechanism of this explosion are still unclear. Unlike the earlier explosions at Unit-1 and -3, no video-recording is available; even the exact time of the explosion remains uncertain. A likely explanation is that hydrogen came from Unit-3 via the piping; there is also a probability that Unit-4 spent fuel pool temperature went high enough to cause radiolysis of the water, producing hydrogen gas.

What if the spent fuel pool gets cracked and loses its cooling water? What if the already severely-damaged (and, as it seems, slightly leaning) reactor building collapses and the spent fuel pool crashes down, perhaps triggering a spent fuel fire? This could lead to a worst case scenario that was drawn up in March 2011 by Prof. Kondo, Chairman of the Japan Atomic Energy Commission (JAEC), would still apply. Evacuation of over 10 million residents in the wider Tokyo megalopolis within a 250-km radius of Fukushima Daiichi, depending on wind direction, may be required. [280] Radioactivity of the Unit-4 spent fuel pool is more or less equivalent to three full reactor-loads; i.e. the quantity of the irradiated fuel rods kept in that single pool roughly equals those in Unit-1, -2 and -3 reactor cores combined. Thus, full release from the Unit-4 spent fuel pool, without any containment or control, could cause by far the most serious radiological disaster to date.

TEPCO claims that the Unit-4 building has been reinforced enough to survive further quakes, but retrieving the heat-generating spent fuel from the pool is nonetheless imperative and as quickly as possible. TEPCO puts a top priority to the construction of a crane-supporting iron framework over the Unit-4 building. Retrieval of the assemblies is scheduled to begin in November 2013 and to be completed by 2014. [281]