ORIGINALLY PUBLISHED: 3.48pm GMT
CORRECTION: 4.09pm GMT Correct size of tsunami below, plus altitude difference between Daini and Daiichi
Official notices of the accidents at the Fukushima nuclear power plants give insight into the challenges faced by power plant engineers in the aftermath of last week's natural disasters.
|Elena Buglova, acting head of the IAEA
Incident and Emergency Centre at a
press briefing yesterday
Eight separate ratings on the International Nuclear Event Scale (INES) have now been logged with the International Atomic Energy Agency by the Japanese government. One covers the effect of the earthquake on Japan's nuclear power plants as a whole, putting it as level 3 - a 'serious incident' - because of the eventual need to declare technical emergencies at Fukushima Daiichi and Daini.
The remainder of the nuclear crisis has its origins specifically in the tsunami that followed. Information accompanying the ratings gives insight into the starting point of the crisis and the technical challenges faced by Tokyo Electric Power Company (Tepco) in the immediate aftermath of the tsunami.
At the Daini plant, all three large boiling water reactors were operating at full power ratings of 1100 MWe when the earthquake hit. They shut down automatically and began using diesel generators to power core cooling systems.
One hour later the plant was inundated by a tsunami said to be seven metres in height, compared to design basis surge of 6.51 metres. Unit 3 was undamaged and continued to cold shutdown status, but the other units suffered flooding to pump rooms where equipment transfers heat from the reactor circuit to the sea - the ultimate heat sink.
With these out of action, the challenge for engineers was to manage the heat and pressure inevitably building within power plant systems until the seawater pumps could be brought back online.
Core cooling systems continued to work and these were complimented by a high pressure coolant injection (HPCI) system, powered directly by steam emerging from the main reactor system. Separate from the large steam turbine used to generate power, the HPCI is connected only to a pump which injects cooler water from the large torus suppression chamber beneath the reactor as well as a water storage tank.
|The boiling water reactor system
In addition, pressure can be controlled by venting steam from the reactor pressure vessel to the torus, which then condenses to reduce pressure in the main circuit.
However, all the systems rely on a finite source of water within the plant, which gradually heats up. The HPCI and torus venting systems also rely on a difference in temperature between the reactor system and the torus. When the temperature in the torus heats up to 100ºC, the system, including the injection from the HPCI, becomes ineffective.
When this occurred at Fukushima Daini 1, a technical emergency was declared. Today's INES ratings reveal that the same sequence of events happened at units 1, 2 and 4. These were each rated at Level 3 - 'serious incidents'.
Site workers were able to repair some equipment in pump rooms and restart operation. Going from one unit to the next they were able to achieve cold shutdown status for all of the units.
|In happier times: The Fukushima Daiichi plant
Similar problems occurred at the Daiichi plant. Units 1, 2 and 3 were operating at full power but shut down on the earthquake. They too were flooded by the tsunami and lost their sea water pumps - but this was exacerbated by the loss of emergency diesels as well. One factor in this could be that the Daiichi plant is at a slightly lower altitude than Daini, making the tsunami relatively more powerful.
This meant that heat was building up in the power plant in the same way as at Daini 1, 2 and 4, but that core cooling sprays could not be powered.
At Daiichi 1, 2 and 3, the steam-driven HPCIs were left as the only cooling system, which eventually heated the units' toruses to the point that they stopped working. Pressure from the reactor vessels built several times to the point that it required release. Separately, gas in the containment vessel was vented and this was enough to raise radiation levels at the site boundary to 0.5 millisieverts per hour.
Japanese officials reported that for each unit, "The behaviour of the pressure of the reactor vessel and the containment vessel, and the behaviour of the water level of the reactor were complicated. Some measurements were not possible because of failures of measuring equipment. As a result, a detailed estimate cannot be done." However, they said, the radiation signature of the releases matched a theory that a few percent of each reactor core had suffered damage.
Enough hydrogen was also produced within the reactor vessel by the interaction between water and hot fuel to cause an explosion at each unit when this was vented to the secondary containment. For units 1 and 3 this removed the top part of the reactor building. At unit 2 this may have taken place in the torus, causing damage there.
Core damage is rated at Level 5 on the INES scale, an 'accident with wider consequences'. This is applied to units 2 and 3, while unit 1's INES Level 5 rating is attributed to the abnormal rise of radiation dose at the site boundary.
After the total failure of plant cooling systems, seawater is being pumped into the reactor cores of units 1, 2 and 3 to prevent overheating and further core damage. This will likely continue for some time, although plant cooling systems may come back into operation once external power is restored.
Fukushima Daiichi 4
While Fukushima Daiichi 4 was fully defuelled for maintenance, it suffered the loss of seawater pumps and 'electricity rooms'. This led to the loss of cooling and water supply functions to the cooling ponds.
Eventually the water boiled, hydrogen was produced by the interaction of water and hot fuel and a hydrogen explosion wrecked the building. This event was rated at INES Level 3 - a 'serious incident'.
Researched and written
by World Nuclear News