KANSAS CITY, Mo. — As record floodwaters along the Missouri River drench homes and businesses, concerns have grown about keeping a couple of notable structures dry: two riverside nuclear power plants in Nebraska. Though the plants have declared “unusual events,” the lowest level in the emergency taxonomy used by federal nuclear regulators, both were designed to withstand this level of flooding, and neither is viewed as being at risk for a disaster, said a spokesman for the Nuclear Regulatory Commission. “We think they’ve taken all the necessary precautions and made the appropriate arrangements to deal with the flooding conditions,” said...