07/16/03

In the 17 months the Davis- Besse nuclear power plant has been down for repairs, engineers have found significant design shortcomings in the equipment that is supposed to work during and after accidents to help operators keep the reactor under control.

They include:

The emergency sump screen could clog after a coolant-loss accident, preventing operators from pumping water back into the reactor to keep the nuclear core from melting. Solution: The screen was enlarged from 50 square feet to nearly 1,200.

The high-pressure emergency pumps, which might have to take dirty water from the emergency sump, could clog with minute debris particles after an accident, allowing the core to melt. Solution: Engineers are now attempting to install a "self- cleaning" fine-mesh screen inside the pump.

The air conditioners for the building that contains the reactor have been badly corroded and possibly would have been inadequate following an accident when they would have to cool the building for weeks or even months. Solution: The coolers have been rebuilt, using all stainless-steel parts.

The gantry crane near the reactor's containment building could be blown off its track by a tornado and into the roof of a nearby service water pump room. Solution: Restraints were installed on the tracks.

Air-operated valves, which keep the radioactive coolant inside the containment building, were not closing all the way. Engineers also found that four valves were installed backward. Solution: The valves have been reinstalled, some with stronger springs.

The electrical distribution system might become overloaded after an accident and fail to provide sufficient power to crucial pumps and valves. Solution: Engineers are working a new "switching sequence" that would direct power to the most critical components first.

The exhaust pipes of the emergency diesel generators could be damaged by tornado-driven debris. A later, more sophisticated analysis showed the risk was low.

The emergency diesel generators, which switch on after an accident or if a connection to the external grid is cut, could overheat the room where they are installed, causing the electrical switches there to fail. Solution: Workers are providing new ventilation to the room.