Three Mile Island Unit 1 Suffers Reactor Coolant Pressure Boundary Leakage

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The Three Mile Island nuclear power plant, operated by Exelon Nuclear Generation near Londonderry, Pennsylvania, was forced to shutdown and depressurize the Reactor Coolant System in order to stop a leak. The leak was not able to be isolated with valves or other devices because it was a through weld leak at a Pressurizer Heater penetration into the Pressurizer. The Pressurizer establishes the necessary Reactor Coolant System Pressure Control by forming and maintaining a steam bubble in the upper part of the Pressurizer during operation.

The Electric Heaters ensure that boiling does not occur in the Fuel Core because it keeps the Reactor Coolant System Pressurized greater than saturated Pressure for the normal operating Temperature range.  This is known as “Sub-Cooling Margin” which prevents Departure from Nucleate Boiling in Pressurized Water Reactors.  By balancing Pressurizer Spray and Pressurizer Heater operation, fine Pressure control is effected throughout all operating conditions.

The leakage was calculated at approximately .2 Gallons per Minute and was completely contained within the Reactor Containment Vessel.

The leakage apparently took place at a location where one of the Electric Heaters penetrates the Pressurizer.  Pressurizer Heater Weld Leaks occurred at the Calvert Cliffs Nuclear Power Plant in the late 1980s and kept the unit off line for several months.

Reactor Coolant System Pressure Boundary Leakage must be completely analyzed to determine the Root Cause and to see if other similar welds are affected. Repairs would then have to be completed.

On August 12, 2012, Palisades shutdown for Reactor Coolant System Pressure Boundary Leakage and is still off-line.

Event Number: 48220

Notification Date: 08/22/2012


“On August 22, 2012, it was determined, based on a remote video camera inspection inside the secondary shield wall, that a primary system leak at a Pressurizer Heater bundle exists and is not isolable. Technical Specification (TS) Limiting Condition for Operation (LCO) does not allow any leakage through a nonisolable RCS strength boundary. The discovery of leakage from the Pressurizer Heater bundle is considered a degradation of a principal safety barrier. This condition does not represent a reduction in the public health and safety.

“This is reportable as a 4-hour ENS notification under 10CFR50.72(b)(2)(i) due to the initiation of a nuclear plant shutdown required by the plant’s Technical Specifications.

“The licensee notified the NRC Resident Inspector.”

The licensee expects to also notify state and local government agencies of the shutdown. The current leak rate, as calculated by the plant process computer, is approximately .2 gallons per minute.

Source: NRC Event Notifications

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