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.