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Why Has It Become Impossible for Fukushima Dai-ichi Nuclear Power Station to Cool Reactor Core?

Mar. 18, 2011

TOKYO --Loss of cooling function at Fukushima Dai-ichi Nuclear Power Station of Tokyo Electric Power Co. (TEPCO) caused core damage and hydrogen explosion. How does a cooling system usually work? And, in what order was the cooling function lost this time?

When a nuclear reactor shuts down in the event of an emergency and falls into a state to cool its core, an emergency core cooling system (ECCS) works. ECCS has multiple systems, and if an external power source (grid-connected power source) can provide electricity, the core is cooled by an electric-driven high pressure core injection system. So, the decay heat of fuels can be sufficiently cooled. In addition to a high pressure core injection system, there is a condensate water make-up system that is also an electric-driven core injection system.

How was the case of TEPCO’s Kashiwazaki-Kariwa Nuclear Power Station that was damaged by the Niigata Chuetsu Earthquake on July 16, 2007?

With Unit 2 as an example, which was in the process of startup at the time of the earthquake, the unit scrammed and shut down at 10:30 a.m. on July 16, 2007. After control rods were fully inserted, cooling water was supplied by using a low pressure coolant injection system, in addition to condensate pumps and control rod drive pumps in the process of pressure reduction operation with main steam relief valves. The core was cooled by water injected from various cooling systems connected with the reactor, and the temperature of reactor water decreased to lower than 100℃ and the reactor was placed into cold shut down state at 7:40 p.m.

In the case of the Kashiwazaki-Kariwa, however, an emergency power supply system immediately worked, and its electric system was workable, too. These points are fundamentally different from the case of Fukushima Dai-ichi Nuclear Power Station.

If an external power source is lost, high pressure core injection system and others are operated by electricity from an emergency power supply system. In the case like Fukushima where even an emergency power supply system is lost and electric system does not operate, water is injected by using diesel-powered pumps among a reactor core isolation cooling system (RCIC) operated by steam from the reactor and a fire protection system connected to filtrate tanks.

However, any of these systems cannot be used unless either an external power source or an emergency power supply system is restored. They cannot keep their cooling functions for a long period of time. As the reactor is cooled, the quantity of steam becomes smaller. And, the diesel fuel runs out in about eight hours. It is desirable for fuel tanks to have sufficient fuels stored in them, but the tanks seem to have been damaged or destroyed because of tsunami.

As cooling functions were lost, Fukushima Dai-ichi Nuclear Power Station took an emergency measure to inject seawater. But, unless the pressure in the containment vessel decreases, water injection does not work well. For this reason, the work for pressure reduction was necessary, but the pressure did not sufficiently decline and the water injection work had rough going.

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