(Ottawa, Canada) — Decaying concrete at nuclear power plants is the latest concern for nuclear safety authorities.
At Quebec’s sole atomic power station, Gentilly-2, eroding concrete has prompted federal licensing officials to suggest that any provincial attempt to refurbish and re-license the 30-year-old plant must satisfy federal concerns over the aging concrete’s ability to stand up to another two or three decades of service.
The move comes as economic pressures force nuclear utilities to consider refurbishing their nuclear plants and operating them well past their 25- to 30-year initial lives.
With Gentilly-2 at the end of its service life, the Quebec government is under pressure to decide soon whether to order a refit or shut down the plant permanently. Refurbishment estimates range from $2 billion to $3 billion. A shutdown is pegged at $1.6 billion.
Of particular concern for any “life extension” is the dome-shaped containment building that encloses the 675-megawatt CANDU 6 reactor. The metre-thick, steel-reinforced concrete structure serves as the final physical barrier against radioactive contamination escaping into the atmosphere around Becancour, on the south shore of the St. Lawrence River across from Trois-Rivieres and an hour’s drive northeast of Montreal.
“Special attention is needed for the containment structure in the longer term since it has been identified that containment concrete suffers from” a common type of concrete decay called alkali-silica reaction (ASR), says a 2010 report by the Canadian Nuclear Safety Commission (CNSC) in Ottawa.
Despite those long-term concerns, the CNSC last year renewed the plant’s operating licence until 2016.
“There is no impact on the safety of any of Canada’s nuclear facilities,” the federal nuclear watchdog agency said in a brief written statement this week. “These facilities are licensed by the Commission because they continue to be safe.”
Meanwhile, concrete degradation has surfaced in the reactor containment buildings of three U.S. nuclear power stations. The Nuclear Regulatory Commission recently warned operators there that design strengths and assumptions used in original containment building design plans “may no longer hold true,” if ASR and its telltale cracks and fissures are present.
ASR can take years to develop and its chemistry is well understood. But its effect on the structural behaviour of nuclear reactor containment and other buildings is not. The issue is especially relevant to Gentilly-2, which sits on a seismic fault line.
“The potential mechanical consequences of the chemical reaction, in terms of ultimate resistance of structural elements and overall structural behaviour, are unknown,” according to the CNSC.
The agency says it is in the process of commissioning an independent research project to establish an aging-concrete regulatory standard for Canada’s fleet of nuclear power plants in general and “in particular for Gentilly-2 with the goal to have regulatory program in place to assess Gentilly-2 refurbishment program and to support licensing of Gentilly-2 life extension.”
Already, nuclear power plant operators in Canada are required to implement “aging management programs,” including for concrete containment buildings. The Canadian Standards Association also sets standards for concrete containment buildings, one of which calls for in-service examination and testing requirements.
The Charest government, accused of dithering on the fate of Gentilly-2, obviously has been watching developments in neighbouring New Brunswick, where the provincial power utility is mired in the refurbishment of the Point Lepreau nuclear generation station.
Originally scheduled to take 18 months and cost $1.4 billion, the project is expected to finish this fall, three years late and $1 billion over budget.
And that’s without having to meet any new regulatory standard for the life-extension of aging concrete.
Hydro-Quebec has been pushing for a Gentilly-2 refit since 2008. The utility told federal regulators in 2010 that the containment building’s concrete decay “does not pose any safety problems until the refurbishment outage,” which was then planned to begin this year. The building is equipped with embedded structural monitoring equipment.
A utility spokesman was unavailable to detail the extent of the ASR degradation at Gentilly-2, but previous Hydro-Quebec statements to the CNSC have characterized the situation as manageable.
ASR occurs when certain forms of silica in the bulk material in concrete, such as crushed rock and sand, react in the presence of water with such chemicals as sodium or potassium, which are commonly found in the cement paste. The reaction produces a gel that forms in the pores of the concrete and then expands, causing stress and cracking. Over time, those cracks can join together to form larger fissures and compromise the concrete’s structural integrity.
Hydro-Quebec has told the CNSC the reaction is present in all of its concrete structures, including power dams, and that it has developed the expertise to the combat the problem. ASR also is common in bridges, roads and airport runways.
By Ian MacLeod