The NRC is on the home stretch of a comprehensive decade-long process to help ensure U.S. reactors have reliable cooling water sources after an accident. So far, the first pressurized water reactor site to completely fix the issue to our satisfaction is the Catawba Nuclear Station in South Carolina.
Catawba, owned by Duke Energy Carolinas, has shown that pipe insulation or other debris from a coolant pipe break will not clog the “sump” at the bottom of the reactor building, and won’t block coolant flowing into the reactor. Importantly, water from a pipe break would collect in the sump and could then be used for long-term reactor and containment cooling after an accident.
The sump would only come into play after the plant’s other supplies of cooling water were not available.
The containment sump (also called the emergency or recirculation sump) is part of every U.S. reactor’s emergency core cooling system for dealing with serious accidents. After U.S. boiling-water reactors addressed this issue, further experience led us to ask pressurized-water reactors (in Generic Letter 2004-02) if debris could block their sump strainers during an accident response that needed the sump.
The NRC asked all pressurized-water reactor owners to thoroughly evaluate their sumps. The plants would then take any appropriate steps (including plant modifications) to ensure the system would work.
The plants started by significantly increasing the size of their sump strainers. Additional NRC and industry research, however, showed the combination of plant materials and the hot, chemical-laden coolant water could sometimes form a gooey mess. These “chemical effects,” together with fibers in debris, could still block a sump strainer. Many plants have made changes to reduce or eliminate the chemical effects problem. Catawba is the first pressurized-water reactor plant to answer all our questions on potential sump blockage after an accident.
We’ll continue reviewing the actions from the rest of the pressurized-water reactors until we’re satisfied they’ve all put the sump issue behind them. We expect most pressurized-water reactors will have finished the job by the end of 2016.