On page 3.7-6 (first column) explains the term downcomer inside the reactor pressure vessel.
You mean this blurb - It should be noted that some water is trapped in the downcomer region surrounding the jet pumps. This occurs because the initial temperature of the water in the jet pump region is less then the temperature of the water in the core region. Hence, a lower portion of the water in the downcomer region is flashed during the rapid vessel depressurization.
How does that explain the term downcomer in the RPV ? And why are you even talking about the RPV ? In unit #1, the core has left the RPV, completely. In unit #2 and #3 at least 1/2 of the cores have left the RPV. In total, 2/3 of the cores in these three reactors have left the RPV's. That is my only concern right now and what my original one line post was referring too. Corium on the concrete dry well floor. The RPV's are pretty much toast now.
It can be partially visualized by looking at Figure 3.7-18 (page 3.7-42).
A drawing of the bottom of the RPV again. Hate to break this to you, but the RPV's are fairly irrelevant now. Elvis has left the RPV's. And the RPV has nothing to do with my initial point. Here is my initial point that set you off.
All that don't matter one bit if some of the corium flowed down the drywell sumps and out into the downcomers. And Tepco sure wont tell you if that actually occurred.
The drywell sumps are not located anywhere near the RPV. They are in the dry well floor located way below the bottom of the RPV's. Obviously I was referring to the corium flowing into the drywell sumps.
Page 4.1-2 (second column) explains the downcomer in the torus (suppression pool chamber). Also see Figure 4.1-7.
Now we are talking about what I was referring too. Here is the relevant text. If a LOCA occurs, steam flows from the drywell through a set of vent lines and downcomers into the suppression pool, where the steam is condensed. LOCA stands for Loss of Cooling Accident, which is exactly what happened to these reactors.
Your confusing post earlier is based on Page 4.2-3 in the first column that describes an issue with a Mark II containment that is substantially different from the Mark I. See a Mark II containment illustration given by Figure 4.1-8 (page 4.1-18).
Now we get to the meat of the entire argument. Here is the text you are referring too. Two paragraphs. One about Mark I and one about Mark II.
A phenomenon of importance primarily for Mark I BWR's is shell (liner) meltthough. At vessel breach, the molten material may flow out of the pedestal region, across the drywell floor and then directly contact the steel liner, causing failure. The likelihood of this event and potential means for its mitigation are discussed in more detail in Section 4.7.
A phenomenon of importance for Mark II BWR's is downcomer failure. While Mark II designs vary significantly, there is often the potential for molten material to flow across the floor and into the downcomers. This molten material may directly fail the downcomer or, possibly, lead to a steam explosion that fails the downcomer. Downcomer failure does not lead to immediate containment failure, however the suppression pool is bypassed, thus negating its heat removal and fission product scrubbing capabilities.
If you take just these paragraphs, you are correct about the Mark II. However, note that the first paragraph states, discussed in more detail in section 4.7. Apparently the first Mark I design required an entire section to describe what happens in a liner fail by melt attack. So if we go to section 4.7 (BWR Mark I Liner Failure by Melt Attack). The melt through can attack the containment shell and is more appropriately called a shell failure not a liner failure. So your point is more or less immaterial, since once the shell has failed, containment is lost and that corium will go wherever it wants, including the wet wells via vent lines and downcomers. This paragraph below from section 4.7.6.1 Extension to other BWR Facilities also illustrates how shell failure can occur.
One of the most important geometric parameters with respect to the shell failure issue is the height of the vent line entrance above the drywell floor. This height determines the maximum depth of water over the floor and, should debris enter a vent pipe, local failure will be virtually certain. As discussed in Part 1 of Reference 3, the location of the vent line openings is plant specific, but in general the shorter heights are associated with the facilities that have the smaller cores and hence the smaller potential debris flows.
So in a nut shell, we are arguing about calling vent pipes, downcomers. My post was assuming they are essentially one and the same for this circumstance. Now every reactor is going to have a different specific configuration of sump volume and vent pipe height. If the corium melt overflows the sump, it will attack the containment shell and could rise high enough to flow down the vent pipes and downcomers into the suppression pool. So you are wrong.
In the Peach Bottom (Mark I) design, there is a maximum water depth of approximately 2 feet on the pedestal and drywell floor before water would overflow into the suppression chamber via the downcomer.
So your terminology problem is with the NRC, not me.