Efforts to manage Fukushima Daiichi 3

13 March 2011

FIRST PUBLISHED: 4.03am GMT
UPDATE 1: 9.35am GMT. Information on seawater injection

UPDATE 2: 1.30pm GMT. Issues related to seawater injection, Change of headline from Venting Fukushima Daiichi 3
UPDATE 3: 8.24pm GMT. Status information supplied by JAIF.
UPDATE 4: 8.34pm GTM. Additional information on units 1 and 2

 

Operations to relieve pressure in the containment of Fukushima Daiichi 3 have taken place after the failure of a core coolant system. Seawater is being injected to make certain of core cooling. Malfunctions have hampered efforts but there are strong indications of stability. 

 

The intense focus on unit 3 comes one day after the plant's first reactor was effectively written off as a result of a hydrogen explosion and the move to inject seawater to make certain of cooling the reactor core. Two days ago were the earthquake and tsunami that have proven Japan's worst ever natural disaster.

Tepco reported it had not been able to restart unit 3's high pressure injection system after an automatic stop. This left the reactor without sufficient coolant and obligated Tepco to notify government of an emergency situation.

 

Preparations for potential pressure relief had already been underway for many hours and Tepco manually vented the containment between 8.41am and 9.20am on March 13.

 

Noriyuki Shikata, director of global communications in the prime minister's office, said the venting operation was expected to cool the containment, noting that "minute quantities of radioactive materials are released." When this occurred at unit 1, the International Atomic Energy Agency said the emission would be filtered to retain radioactive materials within the containment.

 

Mixed signals, but much relief 

 

Injection of fresh water mixed with boron to inhibit nuclear reactions was started as soon as venting had been completed. However, water levels continued to fall and Tepco began an operation to inject seawater into the reactor vessel. 

Unit 1 

 

Seawater injection continues and it is thought the reactor core is now sufficiently cool. Safety regulators consider reactor pressure of 353 kPa an indication of a stable condition. 
Unit 2
 

 
The normal reactor core isolation cooling system is in use. Fuel rods are covered by about 3.8 metres of water.
 
 

In a media briefing at 8.00pm, chief cabinet secretary Yukio Edano stated that while initially the water level had risen, a gauge indicated that this had leveled off, despite ongoing seawater injection.

 

The Japan Atomic Industry Forum (JAIF) reported back from a press conference given by the Nuclear and Industrial Safety Agency (NISA) at 11.30pm. The trade body said that a malfunctioning guage means that water levels cannot be confirmed. The gauge in question reads that water levels are around two metres below the top of the nuclear fuel assemblies, which would represent a very serious situation with the risk of fuel damage.

 

"It is unknown whether [the reading] is real or not," said JAIF. Other readings from the reactor system do not indicate that the reading - and the associated potential damage to fuel - are the true situation. Pressure levels stand at around 250 kPa, compared to reference levels of 400 kPa - and a high of 840 kPa recorded at unit 1 yesterday. Radiation levels have dropped during the seawater injection, said NISA.

 

Tepco was able to overcome difficulty with an air-driven pressure relief valve by connecting a compressor.

 

Measurements around the unit had not detected increased radiation levels. A twenty kilometer evacuation order is in effect and some 200,000 people have been moved from their homes so far. 
  

Earthquake impact on Fukushima Daiichi 
 
Reactors 1, 2 and 3 were in operation at Tokyo Electric Power Company's (Tepco's) east coast Fukushima Daiichi nuclear power plant when the magnitude 9.0 earthquake struck. Three other reactors were already shut for inspection but all three operating units underwent automatic shutdown as expected. Because plant power and grid power were unavailable during the earthquake, diesel generators started automatically to supply power for decay heat removal.
 
  

This situation continued for one hour until the plant was hit by the tsunami wave, which stopped the generators and left the plant in black-out conditions.
 
The tsunami wave that hit the plant measured at least 7 metres in height, compared to the maximum 6.5 metre case the plant was designed to cope with.
 
The loss of power meant inevitable rises in temperature within the reactor system as well increases in pressure. Engineers fought for many hours to install mobile power units to replace the diesels and managed to stabilise conditions at units 2 and 3. 
 

 
However, there was not enough power to provide sufficient coolant to unit 1, which came under greater and greater strain from falling water levels and steady pressure rises. Tepco found it necessary yesterday to vent steam from the reactor containment. Next, the world saw a sharp hydrogen explosion destroy a portion of the reactor building roof. The government ordered the situation brought under control by the injection of seawater to the reactor vessel.
 
 

 
Researched and written
by World Nuclear News
 
 

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