Tokyo ( CNN ) -- Some plutonium found in soil on the grounds of the Fukushima Daiichi nuclear power plant may have come from its earthquake-damaged reactors , but it poses no human health risk , the plant 's owners reported Monday . The element was found in soil samples taken March 21-22 from five locations around the plant , the Tokyo Electric Power Company told CNN late Monday . The company said it was equivalent to the amounts that fell on Japan following aboveground nuclear weapons tests by other countries in past decades . `` It is not a health risk to humans , '' the company said . But it added , `` Just in case , TEPCO will increase the monitoring of the nuclear plant grounds and the surrounding environment . '' Plutonium is a byproduct of nuclear reactions that is also part of the fuel mix at the plant 's No . 3 reactor . It can be a serious health hazard if inhaled or ingested , but external exposure poses little health risk , according to the U.S. Environmental Protection Agency . Three plutonium isotopes -- Pu-238 , -239 and -240 -- were found in soil at five different points inside the plant grounds , Tokyo Electric reported . It said that plutonium found in two of the samples could have come out of the reactors that were damaged by the March 11 earthquake and tsunami that ravaged northern Japan . All three isotopes have long half-lives , with plutonium-239 taking 24,000 years to lose half its radioactivity . Plutonium-238 has an 87-year half-life , while plutonium-240 's is more than 6,500 years . The company said the discovery would not change efforts to bring an end to the crisis at the plant , where three of the six reactors are believed to have suffered damage to their radioactive cores . From CNN 's Whitney Hurst .
The amount found is comparable to fallout from old nuclear tests . Tokyo Electric says the discovery poses no risk to humans . But company says it will increase monitoring `` just in case ''