As U.S. officials scramble to improve security at the nation’s 103 operating nuclear power plants, a new safety concern has emerged: stockpiles of spent nuclear fuel rods. Many authorities believe that an attack on the fuel — which has been removed from reactors but is still radioactive — could be even more disastrous than an attack on a nuclear plant, largely because storage sites for spent fuel are generally less secure. Most of the nation’s 40,000 tons of the nuclear waste are kept in concrete-reinforced pools that were built as temporary storage solutions but remain in use. An attack on such pools, or theft of the spent waste for use in a bomb, could bring awful consequences.