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On Jan. 7, Calvert Cliffs Nuclear Power Plant presented its plan for addressing the potential impact of debris blockage in an emergency recirculation situation to the U.S. Nuclear Regulatory Commission and the Nuclear Reactor Regulation.

NRC spokesman Neil Sheehan said Jan. 22 that a “great deal of the discussion” was determining where Calvert Cliffs’ current plans are in addressing this issue.

“They have some issues to resolve before we consider the matter closed,” Sheehan explained.

In the last decade, studies referenced in NRC documentation of the issue have indicated the possibility of debris blockage on sump screens and flow paths during emergency recirculation.

According to a 2007 NRC inspection manual regarding the issue, the safety issue was established to determine whether the transport and accumulation of debris in pressurized water reactor containments, like Calvert Cliffs, following a loss-of-coolant accident or other high-energy line breaks, if recirculation is credited, will impede the long-term operation of the emergency core cooling systems or containment spray systems.

In a PWR, such as Calvert Cliffs, pressurized water in a coolant loop carries heat generated in the core to the steam generator where heat from the primary coolant loop vaporizes the water in the secondary loop producing steam. The steam is then directed through the streamline to the main turbine, which causes the turbine generator to turn, resulting in electricity, according to the NRC.

The concern is that materials, such as thermal insulation, coatings and concrete, in the vicinity of a break during a loss-of-coolant accident would be damaged, dislodged and transported to the recirculation sump and accumulate on its screens. This would result in an increased head loss across the sump screens, which has the potential to exceed the net positive suction head margin required to assure the successful operation of the emergency core cooling systems or containment spray systems.

In 2004, in light of the concern, a letter was sent from the NRC to all holders of operating licenses for PWRs requesting an evaluation of the emergency core cooling system and containment spray system recirculation functions, and that additional actions be taken to ensure system function.

Kory Raftery, spokesman for Calvert Cliffs, said in a statement Thursday morning that the plant’s evaluations show that the safety and cooling systems can perform their functions.

“As a continuous learning and improving organization, we are actively identifying ways to increase our safety margin with respect to those systems,” he said.

Solutions for preventing or lessening the debris blockage possibility have included expanding the size of the screens around the pumps and replacing materials that could flow through the pipes as debris, Sheehan said prior to the Jan. 7 meeting. He added then that Calvert Cliffs is planning on enlarging the reactor drains and replacing some insulation by 2014 and then, by 2016, making other modifications and replacing additional insulation.

On Jan. 22, he said Calvert Cliffs is taking a risk-informed approach to solving the issue, which is “an approach in which insights from probabilistic risk assessments are considered with other engineering insights,” as defined in the NRC 2012-2013 Information Digest.

In an email from Sheehan on Wednesday, he wrote that Calvert Cliffs is still “reviewing the possible effects of a pipe rupture and how any resulting mixture of the released steam and water with chemicals inside the containment building might adversely impact the sump pumps at the bottom of the structure.” He said Tuesday the South Texas Project plant, also a PWR, located south of Houston, is the lead plant for studying these potential effects.

Raftery said Thursday, “Calvert Cliff’s No. 1 priority is the safety of our facility and maintaining the ability to respond to any unforeseen circumstances that could impact our neighbors near the facility.”

In the “not-too-distant future,” Sheehan said, NRC plans to meet and discuss the issue again with Calvert Cliffs.

aharrison@somdnews.com