The origin of xenon in the containment of Fukushima Daiichi 2 is currently considered to be spontaneous fission, a process of radioactive decay not involving any chain reaction.
Tokyo Electric Power Company (Tepco) was able to clarify the matter somewhat today, having been unsure of a previous trace detection of xenon. Subsequent work by the Japan Atomic Energy Agency confirmed the presence of the element, which is among a range of elements found after heavy atoms undergo fission.
The usual chain reaction of fission in a nuclear power reactor is initiated by a source of neutrons and sustained by a specific arrangement of fissile elements and moderating water. Spontaneous fission, however, occurs naturally from time to time in heavy elements of above 230 in atomic mass without any external stimulus and not usually causing any subsequent fissions.
Tepco said it considered the source of the xenon to be spontaneous fission on those grounds that it had injected boric acid to the reactor vessel to reduce the likelihood of chain fission reactions but was still able to detect xenon. Temperature and pressure data from the unit also showed no change around the time of the xenon's discovery in another indication that chain reactions were not taking place.
While spontaneous fission is infrequent, it nevertheless occurs continuously at a low level in all nuclear reactors. It is one of several possible forms of radioactive decay, albeit far less common than alpha and beta decay. The additional heat input from spontaneous fission is insignificant compared to the overall decay heat that must be removed continuously as a basic matter of nuclear safety.
Researched and written
by World Nuclear News