As warmer-than-average waters in Connecticut’s Long Island Sound last week prompted Dominion to shut down one unit at its Millstone Nuclear Plant, an ammonia release caused an evacuation of part of Tennessee Valley Authority’s Watts Bar Unit 1, and Constellation Energy shut down of its Calvert Cliffs Unit 1 reactor after a control rod unexpectedly dropped into the reactor’s core. Then, on Tuesday, Xcel Energy shut down its Monticello Nuclear Generating Plant and Unit 1 of its Prairie Island Nuclear Generating Plant for repairs.

Dominion Powers Down Millstone Reactor Due to Warm Cooling Water Temperatures

Dominion Power on Sunday shut down its 871-MW Unit 2 of the two-unit Millstone Nuclear Plant in Waterford, Conn., saying average water temperatures in the Niantic Bay in Long Island Sound, from which it draws cooling water for plant safety-related systems, exceeded limits of 75F.

The company alerted the Nuclear Regulatory Commission (NRC) early in the morning on Aug. 12 that it was shutting down the unit based on the NRC’s "Ultimate Heat Sink Technical Specification." The specification basically requires a nuclear unit to be in hot shutdown within 6 hours when the average temperature of an ultimate heat sink exceeds the design basis maximum.

In an emergency license amendment, the NRC had last week granted Millstone unit operators permission to take an average of three temperature measurements for the water at the intake pipes of Unit 2, rather than using the single highest measure.

According to an NRC event notification, Dominion was ramping down while monitoring the ultimate heat sink average temperature, waiting for water temperature conditions to drop below the limit. At about 5:30 a.m., it exited the specification, saying temperatures had dropped to just below 75F, and told the NRC that it would hold power at 65% to ensure temperatures were stable. Hours later, however, at about 10 a.m., the average ultimate heat sink temperatures on the unit had reportedly exceeded the limit again, prompting Dominion to commence shutdown. At 4 p.m., the unit was in hot standby even though temperatures were below 75F.

An estimate of when the unit will restart has not been provided. Millstone’s Unit 3, which draws water from a deeper, colder part of the bay, was not affected.

A Dominion spokesperson said the company had never had to shut the reactor due to overly warm water, though Unit 3 had shut down before when its ultimate heat sink temperatures dropped below the minimum of 33F.

Temperatures of waters in Long Island Sound were reportedly nearly 80F on Sunday, higher than the typical 74F. The temperature rise was pegged to extreme heat in July and a mild winter.

Accidental Drop of Control Rod into Core Prompts Shutdown of Calvert Cliffs Unit 1

Constellation Energy began shutting down its 825-MW Unit 1 at the Calvert Cliffs Nuclear Power Plant near Lusby, Md., on Sunday after a control rod unexpectedly dropped into the reactor’s core, a filing with the Nuclear Regulatory Commission shows.

The company told the regulator that a fully inserted control rod in one of the unit’s control element assemblies dropped into the core. Technical specifications require operators to fully realign the control element assembly within 2 hours, but because this did not happen, operators were forced to commence shutdown procedures. The unit was in full shutdown by Aug. 13, the NRC noted.

A Constellation spokesperson said the company was looking for the cause of the malfunction but cannot predict when the reactor will be returned to service. Unit 2 is running at full power.

Xcel Energy Shuts Down Monticello Plant and Prairie Island Unit for Repairs

Xcel Energy shut down its 600-MW Monticello Nuclear Generating Plant in Monticello, Minn., to repair a gasket on a pipe flange, while in an unrelated action on Tuesday, operators shut down the 548-MW Unit 1 at the company’s Prairie Island Nuclear Generating Plant in Red Wing, Minn., to repair two diesel generators.

The Monticello reactor had been operating at 10% power since the weekend, as workers investigated leakage to a collection point inside the plant’s steel and steel-reinforced concrete containment structure. “All of the leakage was contained within the containment structure. The investigation identified a gasket on a pipe flange needed repairs, and the plant was shut down to complete the work,” Xcel said in statement.

At Prairie Island, operators determined during testing that both Unit 1 emergency diesel generators had exhaust leaks, the company said. “The plant has additional layers of backup protection, including other diesel generators, turbine-driven pumps and portable pumps.”

Xcel said it would buy power from the Midwest Independent Transmission System Operator or other utilities, or increase production at its other generating plants to compensate for the outages and ensure an adequate power supply for customers.

The Monticello plant and Prairie Island unit would be restarted “as soon as repairs are complete,” Xcel said. “Neither outage is expected to be lengthy.”

Ammonia Release Causes Evacuation at Watts Bar I

An ammonia leak discovered in the turbine building of Tennessee Valley Authority’s (TVA’s) Watts Bar Unit 1 on Friday morning prompted the evacuation of part of the plant. Workers were allowed to return in the afternoon, after operators secured the leak.

A TVA filing with the NRC that declared a “Notification of Unusual Event” shows that the toxic gas release occurred at about 8:45 a.m. Operators in protective gear ventilated the residual gas before it could cause personnel injuries or equipment damage. The reactor continued to operate at 100% power.

The ammonia leak occurred because a valve in the fill line to the ammonia mixing tank was not fully closed, thereby causing the tank to overflow, the filing said.

Sources: POWERnews, NRC, Dominion, Constellation Energy, Xcel Energy

—Sonal Patel, Senior Writer (@POWERmagazine)