Tepco lists earthquake effects

Friday, 20 July 2007

Damage inflicted upon Kashiwazaki Kariwa by the earthquake of 16 July has been listed by owners Tokyo Electric Power Company.

The list of earthquake effects shows the strength of the plant in the face of a nearby earthquake of around twice the power of its design basis. The earthquake was still within the ultimate safe shutdown capability of the design, up to which environmental protection should be assured.

No damage was listed to reactor or turbine building structures or to major components.

The pattern of damage was consistent in that ducts were displaced on five of the seven units' exhaust stacks. The cause of these dislocations is under investigation, but this effect would not be unexpected as the stacks are the tallest structures of the plant, and therefore more susceptible to vibration.

Exhaust stacks are used for controlled emissions of, for example, noble gases extracted from reactor coolant water by purification systems.

Water from the reactor refuelling pools was spilled in six of seven units. Refuelling pools are located alongside the top of the reactor vessel, which in a boiling water reactor (BWR) is about 18m tall. That water was shaken from the pools is also consistent with the effects of the earthquake being greater higher up in the buildings.

The unit that did not spill refuelling water was unit 7 - a later-design Advanced Boiling Water Reactor (ABWR), in which the reactor core is located lower in the building. The most advanced BWR designs today place the reactor even lower.

At two of the units, the service platforms over the used nuclear fuel cooling pools fell down. Tepco said these fell onto the fuel storage racks, but did not cause damage to fuel. Typically, service platforms are lightweight movable structures and would not be expected to withstand violent shaking beyond their design limits.

Tepco reported widespread liquefaction of soil around the site, a small landslide and a break in a roadway which left a 50cm difference in ground level. Five breaks were found in fire protection water pipes around units 1 and 2.

The effects on administration buildings included cracking and many broken windows, but no structural damage. Over the entire site, seven workers were injured.

The listing also reveals that a "miniscule" quantity of radioactivity was found in 0.6 litres of water in the reactor floor of unit 3, and 0.9 litres on the mezzanine third floor. Another small amount of radioactitivity was discharged through the exhaust stack of unit 7. These are in addition to the 1.2 cubic metres of mildly radioactive water from the used nuclear fuel cooling pools that was discharged to sea.

Further information

Tokyo Electric Power Company (Tepco)
Plant Status of Kashiwazaki-Kariwa NPP after the Niigata earthquake

WNA's Nuclear Power in Japan information paper
WNA's Nuclear Power Plants and Earthquakes information paper

WNA's Earthquakes and Nuclear Safety microsite

WNN: Tepco battered by media storm
WNN: Kashiwazaki Kariwa nuclear units shut down on earthquake