Radiation decreasing, fuel ponds warming

Radiation decreasing, fuel ponds warming

15 March 2011

FIRST PUBLISHED 1.22am GMT
UPDATE 1: 1.58am GMT Information from TV appearance, change of headline from 'Possible damage to Fukushima Daiichi 2'
CORRECTION: 1.58am GMT Removal of potential inaccuracy on torus' status in containment
UPDATE 2: 2.19am GMT Information from government, change of headline from 'Damage to Fukushima Daiichi 2'
UPDATE 3: 3.20am GMT Details on fire from PM's spokesman
UPDATE 4: 3.44am GMT Fire reported put out
UPDATE 5: 8.36am GMT More on torus, Containment shows 'no change'
UPDATE 6: 1.52pm GMT Radiation data from IAEA, status of fuel ponds
 
Loud noises were heard at Fukushima Daiichi 2 this morning and a major component beneath the reactor may be damaged. Evacuation to 20 kilometres is being completed, while radiation levels decrease from a high in the morning. Concern is growing over the status of fuel cooling ponds at units 4, 5 and 6.
  
Confirmation of loud sounds at unit 2 this morning came from the Nuclear and Industrial Safety Agency (NISA). It noted that "the suppression chamber may be damaged." It is not clear that the 6am sounds were explosions in the usual sense.

This cutaway diagram shows the central reactor vessel, thick concrete containment and lower torus structure in a typical boiling water reactor of the same era as Fukushima Daiichi 2

Also known as the torus, this large doughnut-shaped structure sits in the centre of the reactor building at a lower level than the reactor. It contains a very large body of water to which steam can be directed in emergency situations. The steam then condenses and reduces pressure in the reactor system. One effect of this is that the water and steam in the torus will exist alongside a range of gases produced by the nuclear processes in the reactor.
 
The pressure in the pool was seen to decrease from three atmospheres to one atmosphere after the noise, suggesting possible damage. Radiation levels on the edge of the plant compound briefly spiked at 8217 microsieverts per hour but later fell to about a third that.

In line with the theory that non-condensed gases in the torus will be released fairly promptly and not replenished at the same rate, it is possible that the radiation release - at least via this route - will dimininsh and stabilise.

Radiation

Japanese authorities told the International Atomic Energy Agency (IAEA) that radiation levels at the plant site between units 3 and 4 reached a peak of some 400 millisieverts per hour. "This is a high dose-level value," said the body, "but it is a local value at a single location and at a certain point in time."

Later readings were 11.9 millisieverts per hour, followed six hours later by 0.6 millisieverts, which the IAEA said "indicate the level of radioactivity has been decreasing."

Almost all people living within 20 kilometres of the plant have already been evacuated and supplied with potassium-iodide pills that will protect them effectively against the effects of iodine-131 that could conceivably be emitted in the future. The pills saturate the thyroid gland and prevent the radioactive iodine-131 from being absorbed, dramatically reducing the risk of thyroid cancers, which are the major potential health effect from the possible progression of the accident.

The IAEA said 150 people from around the site have been monitored. The results of some people have been reported and 23 have undergone decontamination.

Containment

A close watch is being kept on the radiation levels to ascertain the status of containment. As a precaution Tokyo Electric Power Company has evacuated all non-essential personnel from the unit. Fifty of the company's engineers remain to pump seawater into the reactor pressure vessel in an effort to cool it.

A message recieved from Tepco at about 5pm said the primary containment vessel around the reactor and secondary containment provided by the reactor building "show no significant change."

Fire at unit 4, concern for fuel ponds

Prime minister Naoto Kan confirmed a fire burning at unit 4, which, according to all official sources, had never been a safety concern since the earthquake. This reactor was closed for periodic inspections when the earthquake and tsunami hit, therefore did not undergo a rapid and sudden shutdown. It was of course violently shaken and subject to the tsunami.

Kan's spokesman Noriyuki Shikata said that there had been "a sign of leakage" while firefighters were at work, "but we have found out the fuel is not causing the fire." The fire is now reported extinguished.

The International Atomic Energy Agency did confirm that the fire had taken place in the used fuel storage pool. The Japan Atomic Industry Forum's status report said the water was being supplied to make up for low levels.

Similar to the need to cool fuel in the reactor core, used fuel assemblies in cooling ponds require a covering of water to remove decay heat. The main differences being the amount of decay heat to be removed decreases exponentially with time and that fuel ponds are much less of an enclosed space than a reactor vessel. At the same time, ponds may contain several years of fuel.

JAIF reported that temperatures in the cooling ponds at units 5 and 6 are increasing, but the reason for this is not yet available.