Concrete degradation at ew hampshire, nuclear plant

Concrete is considered fairly durable. The alkali—silica reaction (ASR) is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline (amorphous) silica, which is found in many common aggregates. The Union of Concerned Scientists (UCS) released a report today about potentially serious concrete degradation possibly due to this reaction at the Seabrook nuclear power plant in Seabrook, New Hampshire. The report was written by Paul Brown, a professor of ceramic science and engineering at Penn State University. (An executive summary also is available on line.) After reviewing publicly available documents, Brown concluded that neither plant owner NextEra Energy nor the Nuclear Regulatory Commission (NRC) fully understand the scope or origins of the problem and therefore cannot adequately assess the plant’s structural status. The report provides a list of key questions the NRC and NextEra Energy need to address before they can assure nearby communities they are not at heightened risk. The problem was first officially noted a year ago, when NRC inspectors confirmed that widespread cracks in the plant’s concrete due to a process called alkali-silica reaction (ASR) resulted in a moderate to severe reduction in concrete strength in five buildings. ASR can occur 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. This 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 in the cement and compromise the concrete’s structural integrity. In his report, Brown said that NextEra and the NRC must determine to what extent ASR is occurring, what other negative reactions—including corrosion of embedded steel in the concrete—are occurring, and what basis is there for knowing which steps will be effective in addressing these problems. In 2010, NextEra Energy applied to the Nuclear Regulatory Commission (NRC) to renew Seabrook’s operating license for 20 more years, even though its license was issued in 1990 and does not expire until 2030. The license renewal would allow Seabrook to operate until 2050. ASR can be preventive mitigated in concrete by these three methods: Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na2O" content of cement. Limit the reactive silica content of the aggregate.    Add very fine siliceous materials to neutralize the excessive alkalinity of cement with silicic acid by voluntary provoking a controlled pozzolanic reaction at the early stage of the cement setting.