This partial meltdown may not have damaged the nuclear fuel itself. In the worst-case scenario of a full meltdown, the fuel pellets melt and fall through the bottom of the reactor vessel onto the floor of the conta inment vesse What is a containment vessel? A sealed un it made of steel and reinf orced concrete that contains the reactor vessel This is the last barrier to radiation leakage into the outer building In Fuk ushima, some of the outer build ings have been wrecked by explos ions so the containment vessels are exposed to the environment. At Chernobyl, there was no containment vessel What's the difference between radiation and radioactivity Radioactiv ity is the decay of the nucleus of an unstable atom, turning that atom into a new form or isotope, of the same element or a different element altogether Unsta ble atoms exist natura lly(as in granite rocks)and decay in their own time, but nuclear fission is an accelerated form of radioactive decay For example, decay ing uranium can turn into cesium or iod ine: radioactive particles of both these elements have been detected around the Fukushima nuclear powe Radioactive decay emits radiation, electromagnetic waves or sub-atomic particles like alpha particles, beta particles and gamma rays In Fukushima, the ultimate fear is that the radioactive products of fission escape and repeatedly give off radiation as they decay again and again over many years When is radioactivity dangerous to human health? When it is large doses of ionising alpha partic les, beta particles and gamma rays: this radiation can create DNA-disrupting ions within the body possibly leading to damage and mutation of cells Radiation is a fact of life arising from the earth's crust and from cosmic radiation. A typical human receives 2 millisieverts of this background radiation a year but it can be as high as 50 millisieverts a year in some parts of the world. A spinal X-ray delivers about 1 millisievert of radiation Only when radiation levels get to several hundred times the background radiationThis ‘partial meltdown’ may not have damaged the nuclear fuel itself . In the worst-case scenario of a full meltdown, the f uel pellets melt and fall through the bottom of the reactor vessel onto the floor of the containment vessel. What is a containment vessel? A sealed unit made of steel and reinforced concrete that contains the reactor vessel. This is the last barrier to radiation leakage into the outer building. In Fukushima, some of the outer buildings have been wrecked by explosions so the containment vessels are exposed to the environment. At Chernobyl, there was no containment vessel. What’s the difference between radiation and radioactivity? Radioactivity is the decay of the nucleus of an unstable atom, turning that atom into a new form, or isotope, of the same element or a dif ferent element altogether. Unstable atoms exist naturally (as in granite rocks) and decay in their own time, but nuclear fission is an accelerated form of radioactive decay. For example, decaying uranium can turn into cesium or iodine: radioactive particles of both these elements have been detected around the Fukushima nuclear power plant. Radioactive decay emits radiation, electromagnetic waves or sub-atomic particles like alpha particles, beta particles and gamma rays. In Fukushima, the ultimate fear is that the radioactive products of fission escape and repeatedly give of f radiation as they decay again and again over many years. When is radioactivity dangerous to human health? When it is large doses of ‘ionising’ alpha particles, beta particles and gamma rays: this radiation can create DNA-disrupting ions within the body possibly leading to damage and mutation of cells. Radiation is a fact of life, arising f rom the earth's crust and f rom cosmic radiation. A typical human receives 2 millisieverts of this ‘background radiation’ a year but it can be as high as 50 millisieverts a year in some parts of the world. A spinal X-ray delivers about 1 millisievert of radiation. Only when radiation levels get to several hundred times the background radiation