I'm no nuclear scientist, no nuclear engineer. I studied physics and mathematics, then computer science, and since then have worked in that field. I therefore claim no more than a modest command of nuclear physics, and may be mistaken. However, and I'm not happy to say so, I now believe that reactor 3 of the Fukushima-I (dai-ichi) plant is leaking. This has been a regular supposition in the last days ; some commentators have pointed out for instance that there is no other way to explain some low pressure readings. I'm however considering another clue here.

Some workers have had the misfortune of walking without proper protection in contaminated water in the machine room of that reactor, apparently suffering burns to the legs. This water has been analyzed, and the results of the analysis have been made public by the plant operator, TEPCO. Among the radionucleides are some fission products with short half-life. My attention was drawn to the mention of a quite non negligible amount of Technetium-99m. 99mTc is a radionucleide that does not occur naturally ; it is producted by desintegration of Molybdenum-99, which itself is produced by neutron irradiation of Uranium-235, the main fissile ingredient in the reactor fuel. I knew about 99mTc because it is used in some medical procedures, but there is currently a shortage of it as some of the research reactors where it is produced are shut down. The problem is that 99Mo and 99mTc decay very rapidly : if you have 1 gram of 99mTc, six hours later you only have 1/2 gram, twelve hours later 1/4 gram, one day later 1/16 gram — one says that 99mTc's half-life is 6 hours ; for 99Mo it is 2.7 days. This means that it is impossible to build up stocks of 99Mo, let alone 99mTc : 99Mo is shipped from research reactors to hospitals a matter of days before it is used, which partly explains the current shortages.

Medical matters aside, what's important is that this Technetium in the water can only come from Molybdenum produced as a fresh fission product from a reactor. It cannot come from the old fuel rods stored in the cooling pool above the reactor, which caused an emergency when it dried out, because the Molybdenum in these rods has almost disappeared during months of storage. This means that the water must have had contact with freshly used reactor fuel. In the normal operation of a boiling water reactor, cooling water never comes into contact with reactor fuel, which is enclosed in rods ; this means that overheated rods have melted and released some of their content into the water. Furthermore, this means that this water has come out of the reactor and the containment building.

One possibility is that this water condensed from steam which was released into the atmosphere in order to relieve pressure in the containment. This does not sound very realistic to me : such steam was very hot and released outside of the buildings, one does not see how it could have condensed in the machine room. The other, more likely possibilities, is that it leaked from the reactor vessel and containment (thus implying that they are breached), or that it leaked from cooling system pipes. Neither possibility bodes well for the future.