The NRC’s fire protection staff and graphic artists have worked together to create a new introduction to our website’s fire protection pages. The illustrations for the “Prevention,” “Suppression” and “Safe Shutdown” tabs highlight the details in each area of fire protection.
Prevention is a combination of training, NRC inspections and procedures to keep potential fire starters such as welding under control. U.S. reactors have improved their prevention efforts over time. In 1985 they reported 22 significant fires. By the late 1990s, even though more reactors were running, the annual reporting numbers had fallen by more than half. In 2011 U.S. plants reported only six significant fires – less than one fire for every 10 operating reactors.
The next layer of protection involves fighting fires if they occur at or near a reactor. Plants’ fire detection systems are a lot like the smoke detectors in your house. When these detectors go off, however, trained firefighters show up with extinguishers and fire hoses. Many key plant areas also have automatic sprinkler systems. Plants also have plenty of firefighting water available and can get that water onto a fire using onsite staff and equipment or fire engines from nearby communities.
Even with all these measures, U.S. plants must still be able to safely shut down if a fire breaks out. The fire protection approach puts barriers between each reactor’s multiple sets of shutdown equipment, so a fire can’t disable all the equipment at once. The power and control cables are separated to make sure that those systems are available to shut the plant down.
Plants also have alternate control stations if fires disrupt the control room’s ability to manage the situation. The plants have emergency power sources, both installed large diesel generators and portable equipment the NRC required after 9/11. These sources help ensure fires outside the reactor can’t deprive systems of the electricity they need.
Check out the new graphics and fire protection web pages. We hope this information makes the topic easier to understand and gives you a better sense of how layers of protection help ensure nuclear plants remain safe from fires.
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.