"There are three weapons against contamination -- evacuation, confinement and iodine," said Patrick Gourmelon, director of radioprotection at a French nuclear watchdog, the Institute for Radioprotection and Nuclear Safety (IRSN).
About 200,000 people have already been evacuated from residential areas around Fukushima, located 250 kilometers (120 miles) north of Tokyo.
Confinement is a highly effective tool pending evacuation to a safer area.
It consists of taking shelter in an enclosed space, preferably a basement room, whose doors and windows are then sealed tight with plastic sheets and adhesive tape.
"The point is to prevent radioactive dust from entering the lungs and the digestive tract," said Gourmelon.
"You take a good shower to remove any contact between the fallout and the skin, but you shouldn't scrub, because this helps particles to penetrate," he said. Nail-biting, smoking and sucking or licking one's fingers are also out.
In a nuclear alert, the authorities also hand out iodine pills to prevent cancers of the thyroid, which is a particular risk for babies, young children, teenagers and expectant or breast-feeding mothers.babies The goal is to saturate the thyroid with "healthy iodine," shielding it from radioactive iodine, said Gourmelon.