Earlier this week, more than 50 tons of radioactive cooling water leaked from the No. 2 nuclear reactor operated by Japan Atomic Power Co. (Genden) in Tsuruga, Fukui Prefecture. Operators had to shut down the system manually, but apparently no radioactivity leaked into the atmosphere. The leaked cooling water was safely restricted to the reactor's concrete containment building, but the incident once again raises the fundamental issue of the safety of nuclear energy in general, as well as Genden's ability to handle emergencies.

In Monday's incident, 141/2 hours passed before operators could pinpoint the leak in the cracked steel pipe and seal it. What makes the latest mishap all the more troubling is that a similar leak occurred in the same primary cooling system at the same power plant in 1996. Genden seems to have failed to learn its lesson. The company must get to the bottom of the whole affair so that the public will not have to suffer a similar scare again. Above all, the company must review its reactor-operating procedures and ensure that they are fail-safe.

The Tsuruga nuclear reactor is of the pressurized light-water type. The primary cooling water absorbs the heat released through nuclear reaction, circulates it to a secondary cooling system -- where the steam is used to generate electricity -- and flows back to the core of the reactor chamber. The 8-cm crack was found in the bend of an L-shape stainless-steel pipe located in the heat exchange -- a device to remove impurities from the high-pressure primary cooling water and regulate its temperature. The pipe in question is 1.1 cm thick and measures 9 cm in diameter.