Lepreau incident required a crew to enter reactor building to fix heavy water leak

A leak of heavy water at the Point Lepreau nuclear generating station last week required three plant employees to enter the reactor building to fix the problem manually, according to new information about the incident released by N.B. Power. 

In response to a series of questions about the heavy water spill, N.B. Power's Dominique Couture said in a statement "three authorized and qualified staff" were outfitted to enter the reactor containment building where an unknown amount of heavy water was escaping from a tube about half the width of a household garden hose.

"The leak was stopped manually by crimping the tubing, which took a little over 30 minutes," according to Couture.  

"All safety precautions were taken and safety protocols were followed."

A pair of events at the nuclear plant last week forced the generator offline indefinitely with no clear explanation yet of exactly what happened.

Official accounts from N.B. Power and the Canadian Nuclear Safety Commission have been light on details, and to date include no mention of a crew needing to enter the reactor building to manually fix a leaking pipe.

Volume of heavy water involved unclear

In the early morning hours of Dec. 14 N.B. Power reported that a "partial loss of power" to the station, which at the time was running at full capacity, initiated a complete shutdown. Several hours later it was discovered heavy water was also leaking from a pipe inside the reactor building.

According to Couture, it is still unclear whether the two events are connected to each other or coincidental.

"The investigation is still ongoing," she wrote.

Also unclear is how much heavy water was involved in the leak which occurred in what N.B. Power called a 3/8ths-inch diameter "instrument line" that is part of the station's "heat transport system."

Heavy water is similar to ordinary water, but is about 10 per cent more dense because of a difference in the makeup of its hydrogen atom. It is not radioactive, but can be contaminated with tritium after prolonged use in an operating nuclear plant. Tritium can be hazardous if ingested or inhaled.

When it is running, the Lepreau heat transport system circulates something under 200,000 litres of heavy water, at high pressure, through an elaborate piping system that allows it to take heat generated in the reactor and transport it to a set of steam boilers in a continuous loop. Normal water in the boilers is turned into steam, which is used to rotate the station's electricity-producing turbines.

Inside the transport system, heavy water reaches temperatures above 250 C and operates under pressures of 1,200 pounds per square inch, about 20 times greater than an average kitchen tap.

Radiation fields in 'normal range'  

Couture said the leak was fixed "within hours" of being identified, but says how much heavy water escaped before that happened has not been calculated. Complicating the measurement, she said, was that spilled heavy water has been collected and returned into the heat transport system.

"The specific quantity is not known, but an inventory will be done," she said 

Radiation fields inside the reactor building during the repair were "in the normal range," according to Couture, but there were elevated levels of  "airborne tritium" from the heavy water spill.  

She said the three employees sent to fix the leak were each wearing protective plastic suits with independent air supplies for safety and had radiation survey instrumentation with them.

"All radiation protection procedures and dose limits for staff were adhered to during this work." she said.

It is at least the second heavy water spill at Lepreau.

In 2011, "between four and six litres" of heavy water were dropped in the reactor building as it was being refilled.

The incident caused problems when an air lock door malfunctioned and could not be opened for two hours, with the crew cleaning the spill still inside. 

However, that spill occurred while the reactor was still being refurbished and had not been operational for more than three years.

N.B. Power has not said yet how long it expects the nuclear plant to be offline. It is normally the utility's largest generator of electricity and outside of N.B. Power's fleet of hydro electric dams is the cheapest to operate.   

Over the last week much of Lepreau's normal output has been replaced by more expensive power generated at the Coleson Cove oil-fired generating station in Saint John.   

According to figures filed with the New Brunswick Energy and Utilities Board in October, it is costing an estimated $121.75 for oil at Coleson Cove to  produce one megawatt hour of electricity this year, compared to $6.57 of nuclear fuel at Lepreau.