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Addressing your technical response. Thanks for taking the time.

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.

I see this was a complete waste of time for you. Hopefully other FR readers will benefit. You used the term downcomer incorrectly and imprecisely in your post -- you even admit as much. I have worked on severe accident studies for all reactor types during the years since 1991. In addition, I worked at one of the American BWRs with a Mark I containment during construction so I was able to walk the entire reactor building and inside the RPV itself unimpeded for months. Skimming one NUREG makes you some sort of armchair nuclear quarterback rather than a nuclear engineer with a specialty in risk assessment.

The point of the original post I shared from NEI is that the corium is contained within the concrete structure of the reactor building. Corium cannot "go wherever it wants" -- you are absolutely wrong. And like Chernobyl, the world is offered more empirical evidence via Fukushima that there is no credible means to create the hypothetical "China Syndrome".

You noted that "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." That LOCA description is appropriate when there is a break in the pipe connected to the RPV. It is wrong to equate the Fukushima sequence to the design-basis-LOCA you cited. In the Fukushima sequence the long-term loss of AC-power led to a lack of water for injection and the water in the core barrel boiled away. The RPV and connected lines were intact. The steam lines are connected high on the RPV. Between the top-of-active-fuel there are large steel structures (steam separators, steam dryers). Corium forms because there is no water (LOCA) and the fuel rod cladding melts (like a candle burning down) letting the ceramic UO2 pellets (with a much higher melting temperature point) fall to the core support plate. In the Fukushima sequence, first water is driven out of the core barrel by boiling it away without replacement (LOCA). ADVs and SRVs dump the steam to the suppression pool -- a fairly routine occurrence. Then the corium forms. Corium will not flow up to the steam lines. Furthermore, the torus (steam suppression chamber) is considered part of containment in a Mark I (sometimes referred to as the wet-well). Thus your understanding of core melt progression, RPV design, and containment design (liner et al) are all seriously flawed.

Only pathetic anti-nukes who cannot tell reality from a Jane Fonda propaganda movie would believe "... containment is lost and that corium will go wherever it wants, including the wet wells via vent lines and downcomers." Even if 2 meters (about 6.5 feet) of that structure has been eroded, another 8.2 meters (almost 27 feet) of reinforced steel and concrete lies between the melted fuel and the external environment. It is sad to see that even FR has a virulent group of anti-nukes.