Chubu Electric Power Company released the one-page announcement (in Japanese) with colorful diagrams and photos of the condenser unit of its Reactor 5, where 500 tons of seawater entered the Reactor Pressure Vessel the other day when the reactor was being shut down.
"Multiple" small-diameter (3 centimeters) pipes out of "about 21,000" that carry seawater to cool the steam that drives the turbine broke, probably having been hit by the end cap of the 20-centimeter diameter pipe for recirculating the water.
Chubu's press release also says that a "small amount" of cobalt-60 was detected in the water chamber of the condenser. Chubu doesn't say how much, and but says cobalt-60 is one of the nuclides regularly found in the water in the RPV. Rest assured it wasn't released into the ocean, says Chubu.
Why can't they say exact numbers, instead of "multiple", "about 21,000", and "small amount"?
The number of these small pipes that broke was "about 20", according to Yomiuri (in Japanese; 5/20/2011).
Yomiuri also says the end cap was welded, not threaded. During the regular inspection in February this year, there was no problem with either the small pipes or the end cap, according to Yomiuri. The Reactor 5 at Hamaoka started to operate in 2005.
Other than bad weld, what could make the end cap to drop off like that?