While Japan has battled a series of food scares in the wake of the nuclear crisis, the latest revelation hits at the heart of the country's proud national staple: Radioactive cesium exceeding the government limit was detected for the first time in rice harvested in Fukushima prefecture.

About 630 becquerels of cesium per kilogram were found in a rice sample taken from a farm in Fukushima City, the prefectural government said Wednesday night. The levels measure above the government-mandated limit of 500 becquerels per kilogram...