Problems for units 3 and 4

Problems for units 3 and 4

16 March 2011

FIRST PUBLISHED 0.25am GMT
UPDATE 1:14am GMT Information from TEPCO spokesman and video feed
UPDATE 2: 4:10am GMT Change title from 'Second fire reported at unit 4' and information on Unit 3 and 4 from Yukiyo Edano
UPDATE 3: 4.50pm GMT Status of fuel pond in unit 4, status of seawater pumping, radiological conditions
UPDATE 4: 10.01pm GMT Unit 3's pond status and the current focus of work
UPDATE 5: 22.47pm GMT Addition of pond temperatures

Chief cabinet secretary Yukio Edano has described problems that occurred on the morning of 16 March with Fukushima Daiichi 3 and 4, as well as plans to pump water into unit 4. Controlling the situation of the used fuel storage ponds is the urgent priority now.

At 8.34am white smoke was seen billowing out of Fukushima Daiichi 3. Efforts to determine the cause of this development were interrupted by the evacuation of workers after rising radiation readings. Readings from a sensor near the front gate had fluctuated for some time, although Edano said that on the whole there was no health hazard. Earlier in the morning readings had ranged between 0.6-0.8 millisieverts per hour, but at 10am readings rose to 1.0 millisieverts per hour before falling again just before 11am.

Edano said that one possibility being considered was that the unit 3 reactor had suffered a similar failure to that at unit 2 yesterday, where the torus pressure suppression structure is suspected of being damaged. However, there had been no reported blast or loud sound, which had been the case for unit 2. The immediate focus, said Edano, was on monitoring of water levels and checking pumping operations.

Fuel ponds

About 60% of the total used nuclear fuel from all six reactors is kept in a shared facility, while each of the units also has its own storage pool near the top of the reactor building. At unit 4, the reactor itself contains no fuel, this having been removed to the unit's own store over 100 days ago for maintenance work to take place.

Cooling pond temperatures As reported by the International Atomic Energy Agency: Unit 4 14 March, 10:08 UTC:  84 ˚C 15 March, 10:00 UTC:  84 ˚C 16 March, 05:00 UTC:  no data Unit 5 14 March, 10:08 UTC:  59.7 ˚C 15 March, 10:00 UTC:  60.4 ˚C 16 March, 05:00 UTC:  62.7 ˚C Unit 6 14 March, 10:08 UTC:  58.0 ˚C 15 March, 10:00 UTC:  58.5 ˚C 16 March, 05:00 UTC:  60.0 ˚C

There have now been two fires in the upper portion of unit 4's reactor building, thought to follow an explosion early on 15 March which left the building severely damaged and also damaged unit 3. The cause of the fires remains unknown due to radiation in the area, and they could in fact be one fire that died down before reigniting. The exact nature of the explosion is also unknown.

The Japan Atomic Industry Forum reports that the level of water in unit 4's fuel pond is low and damage to fuel stored there is suspected. Efforts are underway to refill the pool, including an abandoned attempt to douse the building with water from an army helicopter, hoping to get some to go through the damaged building. The temperature of the pond was last known to be 84ºC on 14 and 15 March, said the International Atomic Energy Agency. There was no data for today.
Unit 3's pond is also low and engineers are preparing to add water to both. This is the urgent focus of work for Tepco engineers. Without proper containment, the risk of serious radioactive release in the event of fuel damage or fire is very real. Tepco has not announced the original issue that allowed the water levels in the fuel ponds to drop so low.

Temperatures in the ponds of units 5 and 6 are also elevated. The IAEA gave figures showing a steady increase to around 60-63ºC.

Seawater pumping

Efforts to cool the partially exposed cores of units 1, 2 and 3 continue. So long as radiological conditions allow, a team of workers pumps seawater into the reactor vessels. This boils away, raising steam pressure which must later be vented. Fuel assemblies are exposed by between one and two metres at the top, but the high thermal conductivity of the zirconium alloy rod casings helps cooling with just the lower portion of the rods submerged. This process is set to continue until the heat produced by the core has reduced so that the entire core can be covered.

Radiation levels on site are fluctuating, which makes it difficult for engineers to work as well as to analyse the extent of possible damage to various functional systems in the reactor buildings. Venting steam from units 1, 2 and 3 contributes a certain amount which dissipates in a matter of hours. It is thought some emission is the result of damage to unit 2's torus. Any release due to damaged nuclear fuel is unknown.

The peak of radiation on site is near unit 3, where levels of 400 millisieverts per hour have been recorded. Dose limits to workers under emergency regulations have been raised to 250 millisieverts from 100 millisieverts after which they may not return. This compares to usual nuclear worker limits of 20 millisieverts per year.
    
Researched and written