I have been a little quiet recently as I have had a bit of writers block. I was not sure quite what you wanted to read, life would be better if my loyal readers would write in and say what they want to read more about but today my thoughts wandered onto the subject of silver chemistry.
Those of you who have been paying close attention to the Fukushima event will recall that some radioactive silver has been released by the accident, but to date no ruthenium has been released by the accident.
The boiling point of silver is about 2200 oC, while ruthenium metal has a melting point of about 4200 oC. As a result of this difference in a reducing environment the silver will boil out of the reactor and form aerosol particles at a much lower temperature than the ruthenium. But under oxidizing conditions (if air gets to the hot fuel) the ruthenium metal will be converted to ruthenium tetroxide (RuO4) which is very volatile. It will enter the air even at room temperature.
What the RuO4 will do is to coat the surfaces of dust particles which are formed from things like steel to form ruthenium rich hot particles which will then escape from the plant. The silver can not form a volatile oxide, above about 300 oC silver oxide will decompose to oxygen and silver metal.
These facts which we have from the radioisotope signature suggest to me that the fuel was at least 2200 oC but it was not exposed to air while it was hot. If we look at page 18 of the following set of slides, then you will see an Ellingham diagram which explains how tellurium is more noble than uranium. The lower down the diagram the more able an element is to react with oxygen, if the line for an oxide is higher than the yellow line for uranium dioxide then the element is likely to be in the zero valent state (elemental form).
Tellurium dioxide has a boiling point of 1245 °C, while tellurium has a boiling point of 988 °C. If the tellurium is in the form of the element then it will be able to diffuse and boil out of the fuel at a relatively low temperature. A paper was written by S.G. Prussin, D.R. Olander, W.K. Lau and L. Hansson, Journal of Nuclear Materials, 1988, volume 154, pages 25 to 37 which is all about how the tellurium, iodine and other fission products can diffuse out of hot fuel.