A: It would appear that some of the nuclear fuel in FD1 has melted and released gaseous fission products to the interior of the reactor. These would include xenon, krypton, and iodine. There are about seven isotopes of xenon that are radioactive and would be released in such an incident, with atomic masses of 133, 135, 137, and 138, along with three isomers, 131m, 133m, and 135m. With the exception of the isomers, each of these xenon atoms will decay into cesium and some into other elements past cesium. There is also krypton but its radioactivity and decay products are of less concern.
Xenon itself is not particularly dangerous. It is a noble gas and is not concentrated in the body. Cesium is more of a concern. Here are the seven decay sequences:
Xenon-131m will decay to stable xenon-131.
Xenon-133m will decay to radioactive xenon-133.
Xenon-133 will decay to stable cesium-133.
Xenon-135m will decay to radioactive xenon-135.
Xenon-135 will decay to very mildly radioactive cesium-135.
Xenon-137 will decay to radioactive cesium-137.
Xenon-138 will decay to radioactive cesium-138 and then quickly to stable barium-138.
Of all of these, the decay of xenon-137 to cesium-137 is probably responsible for the most risk, but xenon-137 decays so quickly that the mobility of cesium-137 is limited. I do not think this poses much risk because it can’t get far. The xenon-135 has a longer half-life (9 hours) but decays to a nearly harmless form of cesium (135). I think this poses almost no risk due to the very long half life of cesium-135.