U.S. NRC Blog

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Unclogging a Long-Standing Concern

Scott Burnell
Public Affairs Officer
 

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.

The first pressurized water reactor site to completely fix the issue to our satisfaction is the Catawba Nuclear Station.

The first pressurized water reactor site to completely fix the issue to our satisfaction is the Catawba Nuclear Station.

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.

6 responses to “Unclogging a Long-Standing Concern

  1. Anonymous February 18, 2014 at 10:24 am

    Does Catawba Nuclear use “scratch and sniff” inspectors to inspect safety-related coatings inside containment so as to take credit for those tons of potential debris not adding to the clogging of ECCS strainers and pumps? These inspections are not reliable simply because they do not predict the physical changes to the aged and top-coatings after a LOCA. This is an ongoing dangerous path for the NRC to be allowing the plants to follow.

  2. Anonymous February 18, 2014 at 10:10 am

    If Catawba Nuclear is one of the many plants which rely on regular inspections of safety-related, or Level I, coatings inside containment to assume that they (the coatings) will remain intact and not add to the debris which could contribute to clogging ECCS pumps and strainers, they are taking a dangerous and misleading approach. There are tons of coatings which are over 30 years old and still in place or have been coated over and there is no inspection which can be done at this point to insure they will remain in place during and after a LOCA. These tons of debris must be included in any flow study and must be accounted for the same as insulation and other contributors. The “scratch and sniff” inspections are worthless, as they are performed before the LOCA damaging effects on the aged coatings.

  3. Ellen Hassett Cahill February 10, 2014 at 3:05 pm

    Hello; thank you for this post. Might you please address: how does this decade-long-and-counting safety gap tie in with additional concern related to recent natural disasters? (e.g. Sandy, Fukushima)

    • Moderator February 10, 2014 at 3:36 pm

      Natural events such as hurricanes or tsunami could deprive a reactor’s systems of electricity, which can quickly lead to problems. That’s why, after Fukushima, the NRC requires plants to have enough portable equipment to keep key systems running even if the installed power goes out.

      The sump issue by itself doesn’t immediately affect a plant’s response to an emergency. Plants only consider using the sump if an emergency lasts long enough to use up the considerable amount of water already stored at the site. The approach Catawba has taken ensures the sump provides an ongoing source of water for long-term use.

      Scott Burnell

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