Fukushima plant could remain a hazard for at least 30 years The second type is the penetrating gamma-radiation, which as the name implies can pass through some materials and clothing. The caesium isotopes are the main emitters of gamma-radiation which is dangerous at a distance, unlike the beta-emitters. Japan's nuclear watchdog said the radiation at the site is mainly beta radiation, which British experts suggest is probably from strontium-90, which will remain a biohazard until at least the middle of the century. Radioactive substances decay at different rates. For instance, the half life of iodine-131 - the time for radiactivity to diminish by 50 per cent - is relatively short, just eight days, which means that it has now disappeared, and so no longer the concern it once was when the crisis first erupted in March 2011. Strontium-90, however, has a half-life of 30 years and this probably accounts for a substantial portion of the beta-radiation being found at the site. The half lives of the caesium isotopes vary widely, from two years in the case of caesium-134 to 30 years in the case of caesium-137. The risk of radioactive caesium being taken up by fish in the ocean is the main reason why fishing was banned.