As a first try to assess the point that i listed above (3-), i mean the volume of the possibly melted fuel at N°1, we could start with the TMI corium data, especially densities:
"The bulk density of the samples varied between 7.45 and 9.4 g/cm3 (the densities of UO2 and ZrO2 are 10.4 and 5.6 g/cm3). The porosity of samples varied between 5.7 and 32%, averaging at 18±11%."
Of course it's a little bit difficult to be sure of the accuracy of this approach because:
-TMI was a pressurized reactor, so the core is substantially different.
-the TMI core was only around 50% damaged
-There can be some void/porosities in various combinations inside
-AND, last but not least, we still don't know if part of it melted, or just got damaged, and in which percentage. I even saw in an article (don't remember which sorry) that the fuel has been "sliding" below (i don't know how to interpret this!).
Anyway, the 100% corium hypothesis can be calculated to assess the minimum volume it would occupy assuming it's at the bottom of the RPV (which seems strange IF the temp of 100°C is confirmed, but i don't trust to much these readings now i must say...).
Any hypothesis with only a part of the core melted (partial corium) would result in a global lower density for the destroyed core, so a bigger volume. So let's see if the minimum volume hypothesis (100% corium) fits the actual volume at the bottom of he RPV, 1m below the bottom level of fuel rods.
1- we have some possible densities for corium between 7.45 and 9.4 g/cm3 (the densities of UO2 and ZrO2 are 10.4 and 5.6 g/cm3).
2- we need the core mass in unit 1, including all the "stuff" around, and the mass of the control rods.
3- we need the volume of this part of the RPV for Unit 1 (around 5m diameter I think, but we need more precise data). The key data is also how high were located the bottoms of fuel rods from the very bottom of RPV.
Any sources of infos?
Found this from tepco site:
--> So inner diameter of RPV is indicated "around 4,8m"
--> 400 fuel assemblies, 69 tons of uranium (but do they include the total weight of fuel rods or just uranium content?)
--> 97 control rods (which unit weight?)
i add this picture and sketch of BWR RPV:
This one was called BWR800 (800 MWatts?) on the page i found it; it shows the bottom of the RPV with the peripheral flange on which it seats over its concrete piedestal, and the many holes into which the control rods are entering (damn how can this thing not leaking???)
these sketches of RPV are from BWR4 and 6 designs, but i think the global layout is the same so it gives some ideas: