ET tackles a tall job
How can you saw a 250-foot contaminated stack into sections and not create airborne contamination? That’s the challenge that faced Humboldt Bay Power Plant – and Encapsulation Technologies had the answer.

Humboldt Bay Power Plant is a 65 MWe, natural circulation boiling water reactor that operated from 1963 to 1976. In 1976, the plant was shutdown for a normal refueling. During the shutdown, a number of unresolved seismic issues led Pacific Gas and Electric (PG&E) to keep the plant shutdown for an extended period of time. In 1984, the fuel was removed from the reactor vessel and the plant was placed in SAFSTOR, waiting decommissioning.

Because the plant experienced a number of fuel failures, particularly during the early years of operation, it has numerous systems contaminated with alpha-emitting isotopes. These isotopes, particularly Am-241 and Pu-239, account for more than 90% of the DAC dose commitment. The effective DAC value at the plant is 2.39E-12 uCi/cc.

The main plant stack, a 250-foot reinforced concrete structure, needed to be removed due to seismic considerations. The stack posed a missile threat to the fuel in the spent fuel pool during an earthquake. PG&E, working with the Nuclear Regulatory Commission (NRC) and local officials, agreed to remove the stack prior to licensing and installing dry cask storage for the spent fuel. In addition to the stack, a large portion of the main plant ventilation system was to be removed and replaced by a new high efficiency filtration system (HEPA).

PG&E contracted with Encapsulation Technologies (ET) to encapsulate the contamination within the plant’s ventilation systems and main plant stack prior to removal of these structures and components. The ventilation system ran throughout the plant, with elevation change of more than 300 feet – from 66 feet below sea level to 252 feet above sea level. The job was broken into two separate parts. First, the main plant ventilation system, the intake plenum, fans, dampers, plant registers and ducting were fogged to seal the contamination. Second, the main plant stack was fogged to seal the contamination within the stack. ET used the same fog injection point and an innovative system of air-filled balloons to seal off sections of the ventilation system. The fogging was broken into two tasks to allow PG&E officials to start removal of the ventilation system while fogging of the stack progressed. This approach saved time and money for PG&E.

PG&E radiation protection personnel had estimated that DAC values would exceed 180 DAC during the concrete cutting on the plant stack. The more than 150 air samples taken during the demolition of the ventilation system and the plant stack found no air samples above 0.3 DAC, with most less than the minimum detectable activity for the counting system. The Passive Aerosol Generator (PAG) and capture coating technology drastically reduced the hazards associated with working with alpha isotopes and, just as important, reduced the cost by eliminating the use of respirators. In a plant inspection report, the NRC complimented the plant for using this innovative technology to reduce the airborne potential.

Should you need to reduce the hazards to your employees and reduce the cost and time of your project, contact ET. We have proven technology to assist you.

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